51. Mesothelin Activates Akt Signaling Pathway Which Leads To NF-κB Activation and Resistance To TNF-α Induced Apoptosis in Pancreatic Cancer Cells

Abstract

Introduction: Mesothelin (MSLN), a cell-surface glycoprotein, is up-regulated in pancreatic cancer. We have shown that over-expressed MSLN in pancreatic cancer cells contributes to the increased cell proliferation through faster cell cycle progression which gives rise to vigorous tumor growth in both subcutaneous and orthotopic pancreatic cancer mouse models. To further delineate the mechanisms of MSLN-induced tumor progression, we examined the roles of MSLN on Akt signaling pathway, NF-κB activation and resistance to TNF-α induced apoptosis in pancreatic cancer cells. Methods: MSLN stably over-expressed cell line MIA PaCa-2 (MIA-MSLN) and vector control cell line MIA-V were created for the study. Expression and activation/deactivation of various regulators of Akt/NF-κB pathways were examined by western blot for the total and phosphorylated forms of these proteins. Results: MIA-MSLN cells exhibited increased expression and phosphorylation of Akt at both the activating sites, Ser473 and Thr308. PDK1, the upstream kinase responsible for Akt phosphorylation at Thr308 site, was activated (p-Ser241) in MIA-MSLN cells. PTEN, a PIP3 phosphatase, was inhibited through increased phosphorylation at the deactivating site Ser380 in MIA-MSLN cells. In addition, c-Raf, a key mediator of cell apoptosis and downstream molecule of Akt, was inhibited via increased phosphorylation at deactivating site Ser250 in MIA-MSLN cells, which consequently showed increased resistance to TNF-α-induced apoptosis. Furthermore, MSLN over-expression increased nuclear translocation of p65 and p50 subunits of NF-κB in MIA-MSLN cells compared with the control MIA-V cells. Abrogation of MSLN over-expression using siRNA led to a decrease in this NF-κB activity in the MIA-MSLN cells. Phosphorylation of IκBα, a major inhibitor of NF-κB, was also increased in MIA-MSLN cells indicating the involvement of classical IKK-IκBα-phosphorylation-degradation pathway of NF-κB activation. Conclusions: Over-expression of MSLN in pancreatic cancer cells leads to the activation of Akt signaling pathway, which is responsible for the activation of NF-κB, inhibition of c-Raf, and resistance to TNF-α-induced apoptosis. This study deciphers the possible molecular mechanisms involved in MSLN-induced pancreatic cancer progression.