Combinatorial treatment of CD95L and gemcitabine in pancreatic cancer cells induces apoptotic and RIP1-mediated necroptotic cell death network.

Combination therapy of cancer is based on the cumulative effects mediated by several drugs. Although molecular mechanisms of action of each particular drug are partially elucidated, understanding of the dynamic cross-talk between different cell death pathways at the quantitative level induced by combination therapy is still missing. Here, we exemplified this question for the death receptor (DR) networks in pancreatic cancer cells. We demonstrate that the combined action of CD95L and gemcitabine in pancreatic cancer cells leads to the simultaneous induction of caspase-dependent and caspase-independent cell death. The pro-apoptotic effects are mediated through down-regulation of the anti-apoptotic proteins c-FLIP and Mcl-1, while caspase-independent cell death was blocked by inhibition of the kinase activity of RIP1. Furthermore, gemcitabine co-treatment strongly increased the amount of cells undergoing CD95-induced RIP1-regulated necrosis. Imaging flow cytometry has enabled us to get the quantitative insights into the apoptosis-necroptosis network and reveal that the majority of the cells upon the CD95L/gemcitabine co-treatment undergoes necroptosis. Our data underlie the importance of the quantitative understanding of the interplay between different cell death modalities, which is essential for the development of anti-cancer therapies. Taken together, our results are important for combination therapy of pancreatic cancer comprising chemotherapeutics and DR-agonists and offer a possibility to sensitize cells with defects in the apoptotic machinery towards necroptosis-type-mediated death.