Identification of cancer stem cells (CSC) in solid tumors - with self-renewal, multipotency, tumorigenesis, and therapy resistance capacities - has opened path to new targeting therapeutic approaches. However, CSC targeting alone might not be sufficient to eradicate a tumor. Indeed, recent studies showed that cancer cells are plastic, and conventional therapies, such as radiotherapy, can lead to cancer cells (non-CSC) reprogramming into iCSC (induced-CSC). The goal of our work is to identify the molecular mechanisms responsible for treatment-induced CSC emergence. First, we have shown that conditioned media from irradiated non-CSC is sufficient to induce iCSC reprogramming. These results suggest that cell plasticity might be actively regulated by diffusible factors secreted by irradiated cells. By using proteins arrays and ELISA, we demonstrated that the secretion of a specific cocktail of chemokines is induced by ionizing radiation, such as CXCL1 and CCL5. Interestingly, recombinant CXCL1 and CCL5 treatments increase the sphere forming capacity (SFC) of isolated non-CSC treated population. Concomitantly, treatment with neutralizing antibodies targeting CXCL1 and CCL5 leads to a decreased CSC number (ALDH+ cells). We also studied the expression of the corresponding chemokines receptors, by flow cytometry. First, we saw that reprogrammable ALDH- cells are enriched for CXCL1 and CCL5 receptors expressing cells compare to unsorted population or ALDH+ population (CSC). We analysed the reprogramming potential of isolated ALDH-/receptor-positive cells versus ALDH-/receptor-negative cells. The ALDH-/receptor-positive-derived cell population is more able to form spheres and overcomes the receptor-negative-derived population when the two populations are mixed and tested for their sphere forming capacity. The use of pharmacological inhibitors against the receptors induce a slight decrease of CSC. Taken together, our results indicate the involvement of chemokines, in particular CXCL1 and CCL5, in the reprogramming mechanism. To validate the implication of CXCL1 and CCL5 and in a pre-clinical perspective, we are currently beginning an in vivo experiment to study the effects of CXCL1 and CCL5 inhibition on tumor development and on CSC enrichment in vivo.

Citation Format: Justine Bailleul-Dubois, Nadège Bidan, Raphaëlle Mouttet-Audouard, Mélanie Arcicasa, Karine Hannebicque, Yuki Takayama, Samuel Meignan, Xuefen Le Bourhis, Chann Lagadec. CXCL1 and CCL5, induced by ionizing radiation, reprogram nontumorigenic cancer cells into cancer stem cells in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2881. doi:10.1158/1538-7445.AM2017-2881