Necroptosis: Molecular Signalling and Translational Implications

Necroptosis is a form of programmed necrosis whose molecular players are partially shared with apoptotic cell death. Here we summarize what is known about molecular signalling of necroptosis, particularly focusing on fine tuning of FLIP and IAP proteins in the apoptosis/necroptosis balance. We also emphasize necroptosis involvement in physiological and pathological conditions, particularly in the regulation of immune homeostasis. 1. Introduction In 1998 Vandenabeele’s group demonstrated that murine L929 fibrosarcoma cells treated with the pan-caspase inhibitor zVAD-FMK rapidly die in a necrotic way after tumor necrosis factor (TNF) incubation, indicating a possible involvement of caspases in protection against TNF-induced necrosis [1]. Additional works then described this particular form of cell death having many hallmarks of cellular necrosis and induced by death receptor stimulation [2, 3]. Further studies performed by introducing the cowpox virus serpin and caspase-8 inhibitor CrmA in the cells, confirmed that caspase-8 inhibition leads to this form of cell death [4]. Remarkably, while necrosis has been believed in the past to be a passive and accidental form of cell death, it is now considered a finely regulated process [5]. For such a reason it is called necroptosis or programmed necrosis. Necroptosis is characterized by cell swelling, mitochondria dysfunction, plasma membrane permeabilization, and release of cytoplasmic content to the extracellular space. This form of cell death is also associated with high mitochondrial reactive oxygen species (ROS) production and unlike apoptosis it does not involve DNA fragmentation [6]. 2. Necroptosis Activation and Signalling Necroptosis can be activated by members of the tumor necrosis factor (TNF) family (through TNFR1, TNFR2, TRAILR1, and TRAILR2), Fas ligand, toll-like receptors, lipopolysaccharides (LPS), and genotoxic stress [2, 7–9]. Also different kinds of physical-chemical stress stimuli can initiate necroptosis, including anticancer drugs, ionizing radiation, photodynamic therapy, glutamate, and calcium overload [10]. Under conditions that are insufficient to trigger apoptosis, TNFα activates TNFR1 and in turn induces the recruitment of receptor-interacting protein 1 (RIP1) kinase and other proteins to form complex I. Subsequently, these proteins dissociate from TNFR1 and RIP1 can be found in the cytosol in complex IIb, which includes RIP1, receptor-interacting protein 3 (RIP3) kinase, caspase-8 and FADD. The formation of complex IIb leads to necroptosis [11]. Complex I also includes TRADD which is