Gemcitabine triggers a pro-survival response in pancreatic cancer cells through activation of the MNK2/eIF4E pathway

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive neoplastic disease. Gemcitabine, the currently used chemotherapeutic drug for PDAC, elicits only minor benefits, because of the development of escape pathways leading to chemoresistance. Herein, we aimed at investigating the involvement of the mitogen activating protein kinase interacting kinase (MNK)/eIF4E pathway in the acquired drug resistance of PDAC cells. Screening of a cohort of PDAC patients by immunohistochemistry showed that eIF4E phosphorylation correlated with disease grade, early onset of disease and worse prognosis. In PDAC cell lines, chemotherapeutic drugs induced MNK-dependent phosphorylation of eIF4E. Importantly, pharmacological inhibition of MNK activity synergistically enhanced the cytostatic effect of gemcitabine, by promoting apoptosis. RNA interference (RNAi) experiments indicated that MNK2 is mainly responsible for eIF4E phosphorylation and gemcitabine resistance in PDAC cells. Furthermore, we found that gemcitabine induced the expression of the oncogenic splicing factor SRSF1 and splicing of MNK2b, a splice variant that overrides upstream regulatory pathways and confers increased resistance to the drug. Silencing of SRSF1 by RNAi abolished this splicing event and recapitulated the effects of MNK pharmacological or genetic inhibition on eIF4E phosphorylation and apoptosis in gemcitabine-treated cells. Our results highlight a novel pro-survival pathway triggered by gemcitabine in PDAC cells, which leads to MNK2-dependent phosphorylation of eIF4E, suggesting that the MNK/eIF4E pathway represents an escape route utilized by PDAC cells to withstand chemotherapeutic treatments.