Research On The Role Of EIF4E And Sox2 In Pancreatic Cancer Cell Repopulation And The Underlying Mechanism

Pancreatic cancer is one of the most lethal cancers.In spite of decades of effort,the 5-year survival rate remains at only 7%.Due to the lack of effective early diagnostic approaches and typical symptoms or signs,most patients with pancreatic cancer are usually diagnosed at advanced stages.For patients with advanced pancreatic cancer,radiation is an effective therapy.However,even though radiation may kill the great majority of tumor cells,the accelerated repopulation of surviving tumor cells could reestablish the tumor.Tumor repopulation,of which the molecular mechanisms are poorly understood,plays a significant role in tumor recurrence after radiotherapy.Our previous study has demonstrated that caspase 3,which is a master executioner during cell apoptosis,promotes tumor repopulation in breast cancer through activating calcium-independent phospholipase A2(iPLA2)and increasing subsequent release of prostaglandin E2(PGE2)from apoptotic cells.Another recent study in our laboratory has manifested that PKC? in apoptotic pancreatic cancer cells,which can be cleaved and activated by activated caspase 3,stimulates tumor repopulation by activating p38,an important kinase mediating growth factor production.These studies indicate that caspase 3/PKC?/p38 axis is an initiating signaling pathway for tumor repopulation,but the downstream factors of this pathway is still unclear.Translational control of malignancy-related mRNAs has been considered as a major part in malignant progression.In most cases,translational control is conducted at the initiation step.eIF4E(eukaryotic initiation factor 4E)is thought to be the rate-limiting factor in translation initiation progression.Accumulating evidence suggested that eIF4 E plays a significant role in radiotherapy resistance and chemotherapy resistance.MNK1 regulates eIF4 E activity by phosphorylating Ser209 of eIF4 E.MNK1 is directly phosphorylated by mitogen-activated protein kinases p38 and ERK.We hypothesized that eIF4 E may be the downstream factor of caspase 3/PKC?/p38 pathway.Our results demonstrated that caspase 3/PKC?/p38/MNK1 signaling pathway leads to elevated phosphorylation eIF4 E.We subsequently investigated whether inhibition of eIF4 E activity mitigates tumor cell repopulation after irradiation.We used two drugs to inhibit eIF4 E activity through different mechanisms,results manifested that elevated eIF4 E activity is required for the stimulatory effect of irradiated dying cells on living tumor cells.Sox2 has been identified as a key transcription factor required for lineage specification,morphogenesis and differentiation in development of mouse embryos.Recently,accumulating evidence has suggested that dysregulation of Sox2 plays an important role in epithelial cancers.Sox2 regulates cancer stem cell(CSCs)function in skin squamous-cell carcinoma by directly promoting transcription of genes associated with stemness,proliferation and survival.As stem cell-like gene signature identified in ionizing radiation-treated cancer cells,we wondered whether IR induces the upregulation of some “stemness” related transcription factors in irradiated cancer cells.Results manifested that Sox2 expression significantly increased at 24 hours after irradiation in pancreatic cancer cells.Unexpectedly,our data from western blot and qPCR manifested that eIF4 E phosphorylation controls Sox2 expression in a translational manner.In order to find our the specific role of Sox2 in tumor cell repopulation after irradiation,we constructed pancreatic cancer cell lines stably overexpressing Sox2.We found that over-expression of Sox2 strongly enhanced the growthstimulating effect of irradiated dying tumor cells on living tumor cells through a paracrine modality.In conclusion,we have shown that activation of caspase 3/PKC?/p38/MNK1 signaling induces eIF4 E phosphorylation,which mediates upregulation of Sox2 in irradiated tumor cells.Importantly,Sox2 upregulation in irradiated dying tumor cells promotes proliferation of living tumor cells through a paracrine modality,indicating that Sox2 may be a target to reduce tumor cell repopulation after radiotherapy.