Abstract IA09: Targeting tumor-initiating cells in xenograft models of human breast cancer

Historically, investigation of treatment resistance in breast cancer has focused on the identification of acquired genetic or epigenetic alterations in tumor epithelial cells that confer resistance to specific agents, or to multiple agents. Our recent data indicate that a unique subpopulation of tumor-initiating cells (TIC), or “cancer stem cells” present in the original tumors appear to possess a form of intrinsic (de novo) resistance to conventional systemic therapies including radiation and chemotherapies, and are therefore likely responsible for tumor initiation and cancer re-growth. Agents that may target TIC cells specifically are becoming attractive experimental therapeutic candidates. Current markers that allow enrichment of breast cancer TIC by fluorescence activated cell sorting (FACS) (e.g. CD44+/CD24low/neg and ALDH activity) are not universally expressed, cannot be monitored in vivo, and typically identify a much larger percentage of cells than the percentage of cells estimated to serve as TIC. Thus, to evaluate TIC therapeutic targeting effectively, new markers must be identified. To investigate the location and function of TIC, and to evaluate agents that may specifically target them therapeutically, we are developing a series of lentiviral fluorescent signaling reporters to identify cells responsive to Wnt, Hedgehog, Notch, and Stat3-mediated signaling – four pathways implicated in TIC self-renewal or regulation. These reporters have been introduced into a recently developed bank of patient-derived human breast cancer xenograft (PDX) models established in our laboratory, as well as traditional cell line xenograft models. Results thus far indicate that both Wnt responsiveness and STAT3-mediated signaling allows significant enrichment for the TIC population by FACS, and has allowed localization of responsive cells in vivo. Given that these pathways have been implicated in TIC regulation, we have begun to target these pathways using a battery of experimental therapeutics both in the preclinical setting, and in the case of Notch signaling, also in the clinical setting. Current data suggest that inhibition of Notch, STAT3, and Hedgehog signaling can enhance the efficacy of systemic chemotherapy, and can impair or alter the function of TIC in limiting dilution transplantation assays. However, in the case of Hedgehog signaling inhibition, the transplantation behavior of the TIC fraction is biphasic suggesting a novel form of inhibitory cell-cell interaction is present in at least some tumors. Overall, results are consistent with the concept that TIC targeted therapeutics may ultimately prove useful clinically. However, our results also indicate that identification and use of additional agents that actually kill TIC will likely be required for eradication of TIC in patients. Citation Format: Michael T. Lewis. Targeting tumor-initiating cells in xenograft models of human breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr IA09.