Calpain activation is not required for AIF translocation in PARP-1-dependent cell death (parthanatos)

Apoptosis-inducing factor (AIF) is critical for poly(ADP-ribose) polymerase-1 (PARP-1)-dependent cell death (parthanatos). The molecular mechanism of mitochondrial AIF release to the nucleus remains obscure, although a possible role of calpain I has been suggested. Here we show that calpain is not required for mitochondrial AIF release in parthanatos. Although calpain I cleaved recombinant AIF in a cell-free system in intact cells under conditions where endogenous calpain was activated by either NMDA or N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) administration, AIF was not cleaved, and it was released from mitochondria to the nucleus in its 62-kDa uncleaved form. Moreover, NMDA administration under conditions that failed to activate calpain still robustly induced AIF nuclear translocation. Inhibition of calpain with calpastatin or genetic knockout of the regulatory subunit of calpain failed to prevent NMDA- or MNNG-induced AIF nuclear translocation and subsequent cell death, respectively, which was markedly prevented by the PARP-1 inhibitor, 3,4-dihydro-5-[4-(1-piperidinyl)butoxyl]-1(2H)-iso-quinolinone. Our study clearly shows that calpain activation is not required for AIF release during parthanatos, suggesting that other mechanisms rather than calpain are involved in mitochondrial AIF release in parthanatos.