A Disease-associated Mutant of NLRC4 Shows Enhanced Interaction with SUG1 Leading to Constitutive FADD-dependent Caspase-8 Activation and Cell Death.

Nod-like receptor family card containing 4 (NLRC4)/Ipaf is involved in recognition of pathogen-associated molecular patterns leading to caspase-1 activation and cytokine release, which mediate protective innate immune response. Point mutations in NLRC4 cause autoinflammatory syndromes. Although all the mutations result in constitutive caspase-1 activation, their phenotypic presentations are different, implying that these mutations cause different alterations in properties of NLRC4. NLRC4 interacts with SUG1 and induces caspase-8-mediated cell death. Here, we show that one of the autoinflammatory syndrome-causing mutants of NLRC4, H443P, but not T337A and V341A, constitutively activates caspase-8 and induces apoptotic cell death in human lung epithelial cells. Compared with wild type NLRC4, the H443P mutant shows stronger interaction with SUG1 and with ubiquitinated cellular proteins. Phosphorylation of NLRC4 at Ser533 plays a crucial role in caspase-8 activation and cell death. However, H443P mutant does not require Ser533 phosphorylation for caspase-8 activation and cell death. Caspase-8 activation by NLRC4 and its H443P mutant are dependent on the adaptor protein FADD. A phosphomimicking mutant of NLRC4, S533D does not require SUG1 activity for inducing cell death. Ubiquitin-tagged NLRC4 could induce cell death and activate caspase-8 independent of Ser533 phosphorylation. Our work suggests that SUG1-mediated signaling results in enhanced ubiquitination and regulates FADD-dependent caspase-8 activation by NLRC4. We show that the autoinflammation-associated H443P mutant is altered in interaction with SUG1 and ubiquitinated proteins, triggering constitutive caspase-8-mediated cell death dependent on FADD but independent of Ser533 phosphorylation.

PKC activation contributes to caspase-mediated eIF2alpha phosphorylation and cell death.

BACKGROUND: Stress-induced phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha), involved in translation, promotes cell suicide or survival. Since multiple signaling pathways are implicated in cell death, the present study has analyzed the importance of PKC activation in the stress-induced eIF2alpha phosphorylation, caspase activation and cell death in the ovarian cells of Spodoptera frugiperda (Sf9) and in their extracts. METHODS: Cell death is analyzed by flow cytometry. Caspase activation is measured by Ac-DEVD-AFC hydrolysis and also by the cleavage of purified recombinant PERK, an endoplasmic reticulum-resident eIF2alpha kinase. Status of eIF2alpha phosphorylation and cytochrome c levels are analyzed by western blots. RESULTS: PMA, an activator of PKC, does not promote cell death or affect eIF2alpha phosphorylation. However, PMA enhances late stages of UV-irradiation or cycloheximide-induced caspase activation, eIF2alpha phosphorylation and apoptosis in Sf9 cells. PMA also enhances cytochrome c-induced caspase activation and eIF2alpha phosphorylation in cell extracts. These changes are mitigated more efficiently by caspase inhibitor, z-VAD-fmk, than by calphostin, an inhibitor of PKC. In contrast, tunicamycin-induced eIF2alpha phosphorylation that does not lead to caspase activation or cell death is unaffected by PMA, z-VAD-fmk or by calphostin. CONCLUSIONS: While caspase activation is a cause and consequence of eIF2alpha phosphorylation, PKC activation that follows caspase activation further enhances caspase activation, eIF2alpha phosphorylation, and cell death in Sf9 cells. GENERAL SIGNIFICANCE: Caspases can activate multiple signaling pathways to enhance cell death.