The dual functions of receptor interacting protein 1 in fas-induced hepatocyte death during sepsis

In examining the liver's response to sepsis, our laboratory has found that septic hepatocytes exhibit a higher degree of necrosis when compared with septic thymocytes, which typically die through the canonical apoptotic pathway. Recently, an adaptor protein associated with the Fas/TNF death receptor pathway, receptor interacting protein 1 (RIP1), has been shown to be critical for determining whether a cell's death is apoptotic or necrotic. We propose to test the central hypothesis that RIP1 activation by death receptor (Fas) during sepsis determines whether the hepatocytes' fate is apoptotic versus necrotic. We approached this problem by delivering RIP1 siRNA in vivo to C57BL/6 mice and observing changes in mortality after septic challenge. Contrary to our hypothesis, RIP1-silenced mice did not survive as long as scrambled sequence injected controls (22.2% vs. 50.0% 14 days after cecal ligation and puncture, respectively). When we used a pharmacological/synthetic antagonist of RIP1 kinase, necrostatin 1 (Nec1), and examined the mortality of Nec1-treated mice, there was no difference from the RIP1 siRNA-treated mice (20.0% vs. 22.2%, respectively). Furthermore, we carried out a series of comparative histological studies, which indicated that septic mice pretreated with Nec1 exhibited a preservation of liver glycogen stores (represented by periodic acid Schiff stain) versus siRNA-treated mice, which exhibit lower glycogen stores as well as altered morphology. Furthermore, the histological studies also revealed that Nec1 treatment in septic mice increases caspase 3 activity. We speculate that these contradictatory findings are due to the dual-signaling responsibilities of RIP1, where the RIP1 kinase domain can induce death through Fas ligation while also initiating prosurvival signaling through nuclear factor κB (NF-κB).