Abstract SY27-03: Breast cancer stem cell state transitions mediate therapeutic resistance
Authors: Wicha Max S. 
Published: August 2015

Abstract
Tumor cellular heterogeneity represents one of the greatest challenges to the development of molecular targeted therapeutics. A component of this heterogeneity is generated through stochastic mutation and subsequent clonal selection. However, in addition to genetic mechanisms, there is increasing evidence for a role of epigenetic regulation in generating cellular heterogeneity and therapeutic resistance. Epigenetic regulation of gene expression occurs during normal cellular differentiation and during carcinogenesis generates tumors that display a hierarchical organization. At the apex of this hierarchy are “cancer stem cells” (CSCs) that are characterized by their ability to self-renew, as well as to generate the populations constituting the tumor bulk. CSCs mediate tumor metastasis as well as resistance to cytotoxic chemotherapy and radiation therapy. A number of molecular mechanisms accounting for therapeutic resistance of CSCs have been described, including altered cell cycle kinetics, increased DNA repair, antioxidant defense, expression of antiapoptotic proteins, expression of cellular transporters and chemotherapy metabolizing enzymes.
Despite the fact that CSCs and more differentiated progeny derived from these cells share the same mutation profiles, they may be driven by and depend upon different signal transduction pathways. As a result, therapies capable of targeting differentiated cells may be ineffective at eliminating CSC populations. For example, the BCR-abl inhibitor, Imatinib, effectively targets differentiated cells in chronic myelogenous leukemia (CML) resulting in substantial clinical benefit. However, CML stem cells are resistant to Imatinib and as a consequence, patients relapse following discontinuation of Imatinib. This suggests that specific targeting of CSC regulatory pathways may be necessary to eliminate this cell population.
CSCs themselves may exhibit intratumor molecular heterogeneity generated through both genetic and epigenetic mechanisms. The genetic evolution of cancer stem cells during carcinogenics and therapy highlights that both clonal evolution and hierarchical models of carcinogenesis are not mutually exclusive and in fact both may contribute to cancer progression and treatment resistance. In addition to genetic heterogeneity of CSCs they also display epigenetic heterogeneity which is regulated by the tumor microenvironment. We have demonstrated that in human breast cancer CSCs exist in alternative states characterized by expression of different molecular markers (1). Mesenchymal EMT-like CSCs characterized as CD44+/CD24- are largely quiescent, but highly invasive. In contrast, epithelial (MET) CSCs are characterized by expression of aldehyde dehydrogenase (ALDH) are proliferative and capable of generating more differentiated cells constituting the tumor bulk. Breast CSCs maintain a cellular plasticity allowing them to transition between EMT and MET states in a process regulated by the tumor microenvironment. This plasticity may be required for successful metastatic colonization. The existence of EMT and MET-like CSC states suggests that elimination of CSCs may require successful targeting of both CSC populations.
The molecular targeting of HER2-positive breast cancer is illustrative of the importance of targeting alternative CSC states. We have shown that HER2 is an important regulator of breast CSCs and suggested that the remarkable clinical efficacy of HER2-targeting agents may be due to ability of these agents to target breast CSCs (2). In fact, in a subset of breast tumors, HER2 may be selectively expressed in the CSC population in the absence of HER2 gene amplification (3). We have found that resistance to HER2-targeted agents may be mediated by activation of an inflammatory loop involving IL-6, STAT3 and NFκB resulting in the generation of EMT-like stem cells. This is accompanied by downregulation of HER2 expression rendering these cells resistant to HER2 targeting agents. In pre-clinical models, simultaneous targeting of HER2 and IL-6 signaling results in maximal elimination of breast CSCs (4). We plan to test the concept of targeting alternative CSC states in a clinical trial involving simultaneous IL-6 and HER2 blockade, in women with trastuzumab resistant HER2+ breast cancer.
In summary, our studies and those of others suggest that both genetic and epigenetic mechanisms contribute to tumor heterogeneity and treatment resistance. CSCs also display heterogeneity mediated in part by cell state transitions regulated by the tumor microenvironment. Successful targeting of these cell populations may be required to eliminate these cells which mediate tumor growth and metastasis.
1. Liu, S., et al. Breast Cancer Stem Cells Transition between Epithelial and Mesenchymal States Reflective of their Normal Counterparts. Stem Cell Reports 2, 78-91 (2014).
2. Korkaya, H., Paulson, A., Iovino, F. & Wicha, M.S. HER2 regulates the mammary stem/progenitor cell population driving tumorigenesis and invasion. Oncogene 27, 6120-6130 (2008).
3. Ithimakin, S., et al. HER2 drives luminal breast cancer stem cells in the absence of HER2 amplification: implications for efficacy of adjuvant trastuzumab. Cancer Res 73, 1635-1646 (2013).
4. Korkaya, H., et al. Activation of an IL6 inflammatory loop mediates trastuzumab resistance in HER2+ breast cancer by expanding the cancer stem cell population. Mol Cell 47, 570-584 (2012).
Citation Format: Max S. Wicha. Breast cancer stem cell state transitions mediate therapeutic resistance. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr SY27-03. doi:10.1158/1538-7445.AM2015-SY27-03