Pharmacologic downregulation of c-FLIPL restores juxtacrine death receptor-mediated apoptosis in cancer cells in a peritoneal carcinomatosis model

Metastasis occurs when circulating cancer cells implant in normal secondary tissues. Paradoxically, many cancer cells express death receptors while many normal tissues express the cognate death receptor ligands, suggesting that cancer cells possess mechanisms to inhibit death receptor signaling. Pharmacological restoration of juxtacrine-mediated death receptor signaling could prevent cancer cells from implanting in normal tissues such as the peritoneum. The results showed that BAY 11-7085 significantly inhibited peritoneal carcinomatosis in mice following the introduction of colon and pancreatic cancer cell lines into the intra-abdominal cavity. Treatment with BAY 11-7085 restored juxtacrine death receptor signaling during the adhesion of the cancer cells to mesothelial cells, which line the peritoneum. BAY 11-7085 rapidly inhibited c-FLIPL expression in colon and pancreatic cancer cell lines during adhesion to mesothelial cells. Pancreatic cancer cells sorted for high c-FLIPL expression formed peritoneal implants much more readily than cells with low c-FLIPL expression, and RNAi inhibition of c-FLIPL in colon cancer cells dramatically reduced peritoneal implantation. This is a novel demonstration that the restoration of death receptor-mediated apoptotic signaling in cancer cells through the pharmacological inhibition of c-FLIPL can inhibit tumor implantation in a clinically relevant model of peritoneal carcinomatosis, a fatal disease. Pharmacological inhibitors of FLIP hold promise as a way to curtail cancer cell colonization of secondary tissues.