Distinctive roles of receptor-interacting protein kinases 1 and 3 in caspase-independent cell death of L929.

Upon tumour necrosis factor alpha (TNFα) stimulation, cells respond actively by way of cell survival, apoptosis or programmed necrosis. The receptor-interacting proteins 1 (RIP1) and 3 (RIP3) are responsible for TNFα-mediated programmed necrosis. To delineate the differential contributions of RIP3 and RIP1 to programmed necrosis, L929 cells were stimulated with TNFα, carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone (zVAD) or zVAD along with TNFα following RNA interference against RIP1 and RIP3, respectively. RIP1 silencing did not protect cells from TNFα-mediated cell death, while RIP3 down-regulation made them refractory to TNFα. The heat shock protein 90 inhibitor geldanamycin (GA) down-regulated both RIP1 and RIP3 expression, which rendered cells resistant to zVAD/TNFα-mediated cell death but not to TNFα-mediated cell death alone. Therefore, the protective effect of GA on zVAD/TNFα-stimulated necrosis might be attributed to RIP3, not RIP1, down-regulation. Pretreatment of L929 cells with rapamycin mitigated zVAD-mediated cell death, while the autophagy inhibitor chloroquine did not affect necrotic cell death. Meanwhile, necrotic cell death by zVAD and TNFα was caused by reactive oxygen species generation and effectively diminished by lipid-soluble butylated hydroxyanisole. Taken together, the results indicate that RIP1 and RIP3 can independently mediate death signals being transduced by two different death stimuli, zVAD and TNFα.