Editors' ChoiceCell death

Roads to Death City

Science Signaling  06 Jan 2009:
Vol. 2, Issue 52, pp. ec3
DOI: 10.1126/scisignal.252ec3

Under certain conditions, such as when apoptotic cell death is blocked, some cells will undergo an alternate form of cell death that has distinctive characteristics and is called necroptosis, a form of programmed cell death with characteristics of necrosis (see Galluzzi and Kroemer). L929 mouse fibrosarcoma cells die by necroptosis when caspase activity is inhibited by zVAD.fmk (benzoyl-Val-Ala-Asp-fluoromethyl ketone). Other cells, such as NIH3T3 cells, are resistant to this type of necrotic cell death. Necroptosis requires the serine-threonine kinase RIP1, and pharmacological inhibition of the kinase activity by small-molecule inhibitors called necrostatins, such as Nec-1, promotes cell survival. Hitomi et al. performed a genome-wide RNAi screen with L929 cells treated with zVAD.fmk to identify genes that when inhibited promoted cell survival. Inhibition of any of 666 genes increased cell survival, and 432 of these also passed a secondary validation screen. Analysis of expression of the 432 genes in tissues and cells from normal mice suggested that some of the genes clustered into groups that were increased in either immune cells or neuronal cells and tissues. Cell death induced by zVAD.fmk treatment of primary peritoneal macrophages was inhibited by Nec-1, which is consistent with a necroptotic cell death pathway in the immune system. Approximately 65% of the 432 genes could be assigned to a molecular function or biological process. Several genes were associated with tumor necrosis factor–α (TNF-α) receptor (TNFR) or Toll-like receptor (TLR) signaling or were associated with the glutathione metabolic pathway, the glycosylphosphatidylinositol pathway, or translation. Neutralization of TNF-α with an antibody protected L929 cells from zVAD.fmk-induced cell death, and survival was increased by Nec-1 in cells cotreated with interferon γ and a double-stranded RNA agonist of TLR3. To better understand the relationship between necroptosis and apoptosis, the authors screened the 666 genes that promoted survival of zVAD.fmk-treated L929 cells to find those that promoted survival of TNF-α–treated L929 cells, which also triggers necroptosis, or to find genes that when inhibited promoted the survival of NIH 3T3 cells in response to TNF-α and inhibition of protein translation, which triggers apoptosis. Fifty-seven genes from the zVAD.fmk screen were identified, and seven were found in both the necroptotic and apoptotic death pathways. These included three genes encoding proteins involved in TNFR signaling: the receptor (Tnfrsf1a), a deubiquitinating enzyme that is recruited to the TNFR and that has activity toward RIP1 (cyld), and a protein called TIPE1 (Tnfaip8l1). The work of Hitomi et al. reveals many potential new targets for manipulating necrotic cell death, suggests that necroptosis may play important roles in the immune system, and provides insight into the signaling networks that control different forms of cell death.

J. Hitomi, D. E. Christofferson, A. Ng, J. Yao, A. Degterev, R. J. Xavier, J. Yuan, Identification of a molecular signaling network that regulates a cellular necrotic cell death pathway. Cell 135, 1311–1323 (2008). [PubMed]

L. Galluzzi, G. Kroemer, Necroptosis: A specialized pathway of programmed necrosis. Cell 135, 1161–1163 (2008). [PubMed]