Alkylating DNA damage induces a necrotic type of programmed cell death through the poly(ADP-ribose) polymerases (PARP) and apoptosis-inducing factor (AIF). Following PARP activation, AIF is released from mitochondria and translocates to the nucleus, where it causes chromatin condensation and DNA fragmentation. By employing a large panel of gene knockout cells, we identified and describe here two essential molecular links between PARP and AIF: calpains and Bax. Alkylating DNA damage initiated a p53-independent form of death involving PARP-1 but not PARP-2. Once activated, PARP-1 mediated mitochondrial AIF release and necrosis through a mechanism requiring calpains but not cathepsins or caspases. Importantly, single ablation of the proapoptotic Bcl-2 family member Bax, but not Bak, prevented both AIF release and alkylating DNA damage-induced death. Thus, Bax is indispensable for this type of necrosis. Our data also revealed that Bcl-2 regulates N-methyl-N-nitro-N-nitrosoguanidine-induced necrosis. Finally, we established the molecular ordering of PARP-1, calpains, Bax, and AIF activation, and we showed that AIF downregulation confers resistance to alkylating DNA damage-induced necrosis. Our data shed new light on the mechanisms regulating AIF-dependent necrosis and support the notion that, like apoptosis, necrosis could be a highly regulated cell death program.

Programmed necrosis induced by DNA alkylating agents, such as MNNG, is a caspase-independent mode of cell death mediated by apoptosis-inducing factor (AIF). After poly(ADP-ribose) polymerase 1, calpain, and Bax activation, AIF moves from the mitochondria to the nucleus where it induces chromatinolysis and cell death. The mechanisms underlying the nuclear action of AIF are, however, largely unknown. We show here that, through its C-terminal proline-rich binding domain (PBD, residues 543-559), AIF associates in the nucleus with histone H2AX. This interaction regulates chromatinolysis and programmed necrosis by generating an active DNA-degrading complex with cyclophilin A (CypA). Deletion or directed mutagenesis in the AIF C-terminal PBD abolishes AIF/H2AX interaction and AIF-mediated chromatinolysis. H2AX genetic ablation or CypA downregulation confers resistance to programmed necrosis. AIF fails to induce chromatinolysis in H2AX or CypA-deficient nuclei. We also establish that H2AX is phosphorylated at Ser139 after MNNG treatment and that this phosphorylation is critical for caspase-independent programmed necrosis. Overall, our data shed new light in the mechanisms regulating programmed necrosis, elucidate a key nuclear partner of AIF, and uncover an AIF apoptogenic motif.

Ligation of CD47 triggers caspase-independent programmed cell death (PCD) in normal and leukemic cells. Here, we characterize the morphological and biochemical features of this type of death and show that it displays the hallmarks of type III PCD. A molecular and biochemical approach has led us to identify a key mediator of this type of death, dynamin-related protein 1 (Drp1). CD47 ligation induces Drp1 translocation from cytosol to mitochondria, a process controlled by chymotrypsin-like serine proteases. Once in mitochondria, Drp1 provokes an impairment of the mitochondrial electron transport chain, which results in dissipation of mitochondrial transmembrane potential, reactive oxygen species generation, and a drop in ATP levels. Surprisingly, neither the activation of the most representative proapoptotic members of the Bcl-2 family, such as Bax or Bak, nor the release of apoptogenic proteins AIF (apoptosis-inducing factor), cytochrome c, endonuclease G (EndoG), Omi/HtrA2, or Smac/DIABLO from mitochondria to cytosol is observed. Responsiveness of cells to CD47 ligation increases following Drp1 overexpression, while Drp1 downregulation confers resistance to CD47-mediated death. Importantly, in B-cell chronic lymphocytic leukemia cells, mRNA levels of Drp1 strongly correlate with death sensitivity. Thus, this previously unknown mechanism controlling caspase-independent type III PCD may provide the basis for novel therapeutic approaches to overcome apoptotic avoidance in malignant cells.