Abstract 5667: Exposure of tumor-associated macrophages to apoptotic pancreatic cancer cells promotes cancer stem cell chemoresistance

Pancreatic ductal adenocarcinoma (PDAC) is the fourth leading cause of cancer related deaths worldwide. This is largely due to the existence of a subpopulation of stem-like cells present within the tumor, known as cancer stem cells (CSCs) that drive metastasis and chemoresistance. In addition, we have now come to realize that the tumor-associated microenvironment not only provides structural support for tumor development, but more importantly the microenvironment provides cues to CSCs that regulate their biological properties. Chemotherapy often leads to apoptosis of cancer cells, and in previous studies we have shown that tumor-associated macrophages (TAMs) exponentially increase following chemotherapy. We hypothesized that TAMs, in response to chemotherapy-induced apoptosis, secrete factors that potentiate PDAC chemoresistance. In line with this hypothesis, we show that monocyte-derived macrophages cultured in the presence of apoptotic PDAC cells polarize towards an M2 pro-tumor phenotype and secrete factors that render naive PDAC cells, specifically CSCs, resistant to Gemcitabine- or Abraxane-induced apoptosis, irrespective of mutations in p53. Importantly, chemoresistant cells showed increased sphere formation capacity and increased tumorigenesis as measured in an extreme limiting dilution assay in nude mice, confirming an enrichment in CSCs. Moreover, using a syngeneic orthotropic in vivo model of PDAC, we were able to significantly augment the anti-tumor potential of Gemcitabine by pharmacologically eliminating TAMs using clodronate liposomes. To determine the mechanism by which TAMs promote PDAC chemoresistance, we performed proteomic analyses on macrophage conditioned media and identified several proteins specifically induced and secreted when macrophages were co-cultured with apoptotic PDAC cells, including 14-3-3 protein zeta/delta (14-3-3ζ), a major regulator of apoptotic cellular pathways. We present additional data to show that TAM-seceretd 14-3-3ζ promotes CSC chemoresistance. Taken together, the sum of these data highlight a unique regulatory mechanism by which chemotherapy-induced apoptosis acts as a switch to initiate a pro-tumor/anti-apoptotic mechanism in PDAC, challenging the idea that apoptosis of tumor cell is therapeutically beneficial, at least when immune sensor cells, such as macropahges, are present. Citation Format: Gabriele D9Errico, Mireia Vallespinos, Sonia Alcala, Sandra Valle, Laura Martin-Hijano, Bruno Sainz. Exposure of tumor-associated macrophages to apoptotic pancreatic cancer cells promotes cancer stem cell chemoresistance [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 5667. doi:10.1158/1538-7445.AM2017-5667