Tumor-associated macrophage-secreted 14-3-3ζ signals via AXL to promote pancreatic cancer chemoresistance

Pancreatic ductal adenocarcinoma (PDAC) is an inherently chemoresistant tumor. Chemotherapy leads to apoptosis of cancer cells, and in previous studies we have shown that tumor-associated macrophage (TAM) infiltration increases following chemotherapy in PDAC. Since one of the main functions of macrophages is to eliminate apoptotic cells, we hypothesized that TAMs phagocytose chemotherapy-induced apoptotic cells and secrete factors, which favor PDAC chemoresistance. To test this hypothesis, primary human PDAC cultures were treated with conditioned media (CM) from monocyte-derived macrophage cultures incubated with apoptotic PDAC cells (Mempty set(Apop)CM). Mempty set(Apop)CM pretreatment rendered naive PDAC cells resistant to Gemcitabine- or Abraxane-induced apoptosis. Proteomic analysis of Mempty set(Apop)CM identified YWHAZ/14-3-3 protein zeta/delta (14-3-3 zeta), a major regulator of apoptotic cellular pathways, as a potential mediator of chemoresistance, which was subsequently validated in patient transcriptional datasets, serum samples from PDAC patients and using recombinant 14-3-3 zeta and inhibitors thereof Moreover, in mice bearing orthotopic PDAC tumors, the antitumor potential of Gemcitabine was significantly enhanced by elimination of TAMs using clodronate liposomes or by pharmacological inhibition of the Axl receptor tyrosine kinase, a 14-3-3 zeta interacting partner. These data highlight a unique regulatory mechanism by which chemotherapy-induced apoptosis acts as a switch to initiate a protumor/antiapoptotic mechanism in PDAC via 14-3-3 zeta/Axl signaling, leading to phosphorylation of Akt and activation of cellular prosurvival mechanisms The data presented therefore challenge the idea that apoptosis of tumor cells is therapeutically beneficial, at least when immune sensor cells, such as macrophages, are present.