Regulation of death receptor-induced apoptosis by c-FLIP in pancreatic carcinoma

Pancreatic adenocarcinoma is one of the leading causes of cancer death and despite aggressive surgical or medical management, the mean life expectancy is only 3 - 6 months for patients with metastatic disease. Combination of surgery and current chemo- or radiotherapy on long-term survival is minimal, since escape from apoptosis (programmed cell death) is a characteristic feature of pancreatic cancer cells. Targeting the tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) apoptotic pathway is considered as a promising candidate for cancer therapy, since TRAIL has been shown to induce apoptosis specifically in tumor cells without harming most normal cells. However, many cancers including pancreatic cancer, exhibit resistance towards TRAIL, e.g. due to high expression of anti-apoptotic proteins such as cellular FLICE-inhibitory protein (c-FLIP) proteins. Here, we identified both the long (c-FLIPL) and the short isoform (c-FLIPS) of c-FLIP as promising targets to overcome resistance of pancreatic carcinoma cells towards TRAIL. Simultaneous downregulation of c-FLIPL and c-FLIPS as well as individual knockdowns of either isoform by RNA interference cooperates with TRAIL to trigger caspase activation and caspase-mediated apoptosis. Furthermore, pre-treatment with chemotherapeutic drugs, like 5-Fluorouracil (5-FU) or Cisplatin at subapoptotic concentrations causes downregulation of c-FLIP proteins and renders resistant cells sensitive to death receptor-induced apoptosis. Mechanistic studies revealed that 5-FU-mediated suppression of c-FLIP proteins results in increased recruitment to and activation of caspase-8 at the death inducing signaling complex (DISC), leading to caspase-3 activation and caspase-dependent cell death. Overexpression of c-FLIPL rescues cells from sensitization for TRAIL-induced apoptosis by 5-FU or Cisplatin, indicating that c-FLIP suppression is a key event in this chemotherapy-mediated sensitization to TRAIL