The Neuronal Apoptosis Inhibitory Protein Is a Direct Inhibitor of Caspases 3 and 7

Abstract

The neuronal apoptosis inhibitory protein (NAIP) was identified as a candidate gene for the inherited neurodegenerative disorder spinal muscular atrophy. NAIP is the founding member of a human protein family that is characterized by highly conserved N-terminal motifs called baculovirus inhibitor of apoptosis repeats (BIR). Five members of the human family of inhibitor of apoptosis proteins including NAIP have been shown to be antiapoptotic in various systems. To date, a mechanism for the antiapoptotic effect of NAIP has not been elucidated. To investigate NAIP function, we found cytoprotection of NAIP-expressing primary cortical neurons treated to undergo caspase-3-dependent apoptosis. The additional treatment of these neurons with the pancaspase inhibitor boc-aspartyl(OMe)-fluoromethylketone did not result in increased survival. Similar cytoprotective effects were obtained using HeLa cells transiently transfected with a NAIP N-terminal construct and treated to undergo a caspase-3-dependent cell death. To examine whether NAIP inhibits caspases directly, recombinant N-terminal NAIP protein containing BIR domains was overexpressed, purified, and tested for caspase inhibition potential. Our results demonstrate that inhibition of caspases is selective and restricted to the effector group of caspases, with K i values as low as ϳ14 nM for caspase-3 and ϳ45 nM for caspase-7. Additional investigations with NAIP fragments containing either one or two NAIP BIRs revealed that the second BIR and to a lesser extent the third BIR alone are sufficient to mediate full caspase inhibition.

The inhibitors of apoptosis: there is more to life than Bcl2

“…The pattern of neuronal loss in SMA is consistent with the distribution of the NAIP protein within the central nervous system . Finally, NAIP's capacity to suppress cell death is compatible with its deletion playing a role in determining cell loss (Liston et al, 1996;Xu et al, 1997;Maier et al, 2002).…”

“…It was subsequently shown that the IAPs could directly inhibit the activity of several recombinant caspases in vitro Roy et al, 1997;Maier et al, 2002), and that isolated BIR domains possessed this activity (Takahashi et al, 1998). The BIR domains of the IAPs are the most fully characterized functional units of the IAPs.…”

Constitutively Activated Nuclear Factor-κB, but not Induced NF-κB, Leads to TRAIL Resistance by Up-Regulation of X-Linked Inhibitor of Apoptosis Protein in Human Cancer Cells

“…(28)(29)(30). At the caspase level, inhibitor of apoptosis protein (IAP) family members [c-IAP1, c-IAP2, survivin, NIAP, and X-linked IAP (XIAP)] can bind to caspase-9, caspase-7, and caspase-3 and prevent the activation of the caspase cascade (31)(32)(33)(34)(35). Among these IAP members, XIAP seems to be the most potent direct caspase inhibitor (31,34) and has been shown to be overexpressed in tumor cells (36)(37)(38)(39).…”

Epithelial NAIPs protect against colonic tumorigenesis

“…Some studies show direct binding and inhibition of caspase-3 and -9 (Maier et al, 2002;Davoodi et al, 2004Davoodi et al, , 2010, but others do not (Roy et al, 1997). Also, NAIPs lack certain caspase-interaction residues within the BIR domains that would be necessary for direct inhibition of caspases, raising concern about whether NAIP can inhibit caspases in physiological settings (Scott et al, 2005;.…”