Caspase-7 mediates caspase-1-induced apoptosis independently of Bid.

Inflammasomes are innate immune mechanisms that activate caspase-1 in response to a variety of stimuli, including Salmonella infection. Active caspase-1 has a potential to induce two different types of cell death, depending on the expression of the pyroptosis mediator gasdermin D (GSDMD); following caspase-1 activation, GSDMD-sufficient and GSDMD-null/low cells undergo pyroptosis and apoptosis, respectively. Although Bid, a caspase-1 substrate, plays a critical role in caspase-1 induction of apoptosis in GSDMD-null/low cells, an additional mechanism that mediates this cell death independently of Bid has also been suggested. This study investigated the Bid-independent pathway of caspase-1-induced apoptosis. Caspase-1 has been reported to process caspase-6 and caspase-7. Silencing of caspase-7, but not caspase-6, significantly reduced the activation of caspase-3 induced by caspase-1, which was activated by chemical dimerization, in GSDMD/Bid-deficient cells. CRISPR/Cas9-mediated depletion of caspase-7 had the same effect on the caspase-3 activation. Moreover, in the absence of GSDMD and Bid, caspase-7 depletion reduced apoptosis induced by caspase-1 activation. Caspase-7 was activated following caspase-1 activation independently of caspase-3, suggesting that caspase-7 acts downstream of caspase-1 and upstream of caspase-3. Salmonella induced the activation of caspase-3 in GSDMD-deficient macrophages, which relied partly on Bid and largely on caspase-1. The caspase-3 activation and apoptotic morphological changes seen in Salmonella-infected GSDMD/Bid-deficient macrophages were attenuated by caspase-7 knockdown. These results suggest that in addition to Bid, caspase-7 can also mediate caspase-1-induced apoptosis and provide mechanistic insights into inflammasome-associated cell death that is one major effector mechanism of inflammasomes.

Vitamin B6 Prevents IL-1ß Protein Production by Inhibiting NLRP3 Inflammasome Activation.

Vitamin B6 includes six water-soluble vitamers: pyridoxal (PL), pyridoxamine (PM), pyridoxine (PN), and their phosphorylated forms. Pyridoxal 5'-phosphate (PLP) is an important cofactor for many metabolic enzymes. Several lines of evidence demonstrate that blood levels of PLP are significantly lower in patients with inflammation than in control subjects and that vitamin B6 has anti-inflammatory effects, with therapeutic potential for a variety of inflammatory diseases. Although one of our group previously demonstrated that PL inhibits the NF-κB pathway, the molecular mechanism by which vitamin B6 suppresses inflammation is not well understood. Here, we showed that both PL and PLP suppressed the expression of cytokine genes in macrophages by inhibiting Toll-like receptor (TLR)-mediated TAK1 phosphorylation and the subsequent NF-κB and JNK activation. Furthermore, PL and PLP abolished NLRP3-dependent caspase-1 processing and the subsequent secretion of mature IL-1ß and IL-18 in LPS-primed macrophages. In contrast, PM and PN had little effect on IL-1ß production. PLP, but not PL, markedly reduced the production of mitochondrial reactive oxygen species (ROS) in peritoneal macrophages. Importantly, PL and PLP reduced IL-1ß production induced by LPS and ATP, or by LPS alone, in mice. Moreover, PL and PLP protected mice from lethal endotoxic shock. Collectively, these findings reveal novel anti-inflammatory activities for vitamin B6 and suggest its potential for preventing inflammatory diseases driven by the NLRP3 inflammasome.

Pyroptotic cells externalize eat-me and release find-me signals and are efficiently engulfed by macrophages.

Pathogenic intracellular bacteria often hijack macrophages for their propagation. The infected macrophages release IL-1ß and IL-18 and simultaneously commit suicide, which is called pyroptosis; both responses require caspase-1. Here, we found that pyroptotic cells induced by microbial infection were efficiently engulfed by human monocytic THP-1-cell-derived macrophages or mouse peritoneal macrophages. This engulfment was inhibited by the D89E mutant of milk fat globule (MFG) epidermal growth factor (EGF) factor 8 (MFG-E8; a phosphatidylserine-binding protein) that has been shown previously to inhibit phosphatidylserine-dependent engulfment of apoptotic cells by macrophages, suggesting that the engulfment of pyroptotic cells by macrophages was also phosphatidylserine dependent. Using a pair of cell lines that respectively exhibited pyroptosis or apoptosis after muramyl dipeptide treatment, we showed that both pyroptotic and apoptotic cells bound to a T-cell immunoglobulin and mucin domain-containing 4 (Tim4; another phosphatidylserine-binding protein)-coated plate, whereas heat-killed necrotic cells did not, indicating that phosphatidylserine was externalized in pyroptosis and apoptosis but not in accidental necrosis. Macrophages engulfed apoptotic cells most efficiently, followed by pyroptotic and then heat-killed necrotic cells. Pyroptotic cells also released a macrophage attractant(s), 'find-me' signal, whose activity was diminished by apyrase that degrades nucleoside triphosphate to nucleoside monophosphate. Heat-killed necrotic cells and pyroptotic cells released ATP much more efficiently than apoptotic cells. These results suggest that pyroptotic cells, like apoptotic cells, actively induce phagocytosis by macrophages using 'eat-me' and find-me signals. Based on these results, a possible role of coordinated induction of pyroptosis and inflammatory cytokine production is discussed.

Caspase-1 protein induces apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC)-mediated necrosis independently of its catalytic activity.

The adaptor protein, apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), connects pathogen/danger sensors such as NLRP3 and NLRC4 with caspases and is involved in inflammation and cell death. We have found that ASC activation induced caspase-8-dependent apoptosis or CA-074Me (cathepsin B inhibitor)-inhibitable necrosis depending on the cell type. Unlike necroptosis, another necrotic cell death, ASC-mediated necrosis, was neither RIP3-dependent nor necrostatin-1-inhibitable. Although acetyl-YVAD-chloromethylketone (Ac-YVAD-CMK) (caspase-1 inhibitor) did not inhibit ASC-mediated necrosis, comprehensive gene expression analyses indicated that caspase-1 expression coincided with the necrosis type. Furthermore, caspase-1 knockdown converted necrosis-type cells to apoptosis-type cells, whereas exogenous expression of either wild-type or catalytically inactive caspase-1 did the opposite. Knockdown of caspase-1, but not Ac-YVAD-CMK, suppressed the monocyte necrosis induced by Staphylococcus and Pseudomonas infection. Thus, the catalytic activity of caspase-1 is dispensable for necrosis induction. Intriguingly, a short period of caspase-1 knockdown inhibited IL-1ß production but not necrosis, although longer knockdown suppressed both responses. Possible explanations of this phenomenon are discussed.

Gasdermin D mediates the maturation and release of IL-1α downstream of inflammasomes.

IL-1α serves as a pro-inflammatory cytokine. Although pro-IL-1α has cytokine activity, proteolytic maturation increases its potency and release from cells. IL-1α maturation occurs in a caspase-1-dependent manner following inflammasome activation. However, pro-IL-1α is not a substrate of caspase-1, and it remains unclear what mediates the maturation of this cytokine downstream of inflammasomes. Here, we show that gasdermin D (GSDMD), an executor of pyroptosis, is required for the rapid induction of IL-1α maturation by non-particulate inflammasome activators. Ablation of GSDMD abrogates the maturation of IL-1α, but not of IL-1ß. Inflammasome-induced maturation of IL-1α relies on extracellular Ca2+ and calpains. Ca2+ influx and calpain activation are induced in a GSDMD-dependent manner. Glycine, which inhibits cell lysis, but not GSDMD pore formation, does not affect IL-1α maturation. These results suggest that during inflammasome activation, GSDMD processed by caspase-1 forms plasma membrane pores that mediate Ca2+ influx, resulting in the calpain-dependent maturation of IL-1α.

Activation of ASC induces apoptosis or necrosis, depending on the cell type, and causes tumor eradication.

The adaptor protein ASC (also called TMS1) links certain NLR proteins (e.g., NLRC4, NLRP3) and caspases. It is involved in the chemosensitivity of tumor cells and inflammation. Here, we found that ASC activation using NLRC4 mimicry or an autoinflammatory disease-associated NLRP3 mutant induced necrosis in COLO205 colon adenocarcinoma cells, but induced caspase-8-dependent apoptosis in NUGC-4 stomach cancer cells. As the Fas ligand induced caspase-8-dependent apoptosis in COLO205 cells, caspase-8 was intact in this cell line. ASC-mediated necrosis was preceded by lysosomal leakage, and diminished by inhibitors for vacuolar H(+)-ATPase, cathepsins, and calpains but not by inhibitors for caspase-8, or aspartic proteases, suggesting that lysosomes and certain proteases were involved in this process. Finally, growing tumors of transplanted human cancer cells in nude mice were eradicated by the activation of endogenous ASC in the tumor cells, irrespective of the form of cell death. Thus, ASC mediates distinct forms of cell death in different cell types, and is a promising target for cancer therapy.

Caspase-1 initiates apoptosis in the absence of gasdermin D.

Caspase-1 activated in inflammasomes triggers a programmed necrosis called pyroptosis, which is mediated by gasdermin D (GSDMD). However, GSDMD-deficient cells are still susceptible to caspase-1-mediated cell death. Therefore, here, we investigate the mechanism of caspase-1-initiated cell death in GSDMD-deficient cells. Inflammasome stimuli induce apoptosis accompanied by caspase-3 activation in GSDMD-deficient macrophages, which largely relies on caspase-1. Chemical dimerization of caspase-1 induces pyroptosis in GSDMD-sufficient cells, but apoptosis in GSDMD-deficient cells. Caspase-1-induced apoptosis involves the Bid-caspase-9-caspase-3 axis, which can be followed by GSDME-dependent secondary necrosis/pyroptosis. However, Bid ablation does not completely abolish the cell death, suggesting the existence of an additional mechanism. Furthermore, cortical neurons and mast cells exhibit little or low GSDMD expression and undergo apoptosis after oxygen glucose deprivation and nigericin stimulation, respectively, in a caspase-1- and Bid-dependent manner. This study clarifies the molecular mechanism and biological roles of caspase-1-induced apoptosis in GSDMD-low/null cell types.

NLRP3 mediates NF-κB activation and cytokine induction in microbially induced and sterile inflammation.

Nucleotide-binding domain and leucine-rich repeat-containing family, pyrin domain containing 3 (NLRP3) has recently emerged as a central regulator of innate immunity and inflammation in response to both sterile inflammatory and microbial invasion signals. Although its ability to drive proteolytic procaspase-1 processing has drawn more attention, NLPR3 can also activate NF-κB. To clarify the physiological relevance of this latter function, we examined the effect of NLRP3 on NF-κB activation and cytokine induction in RNA-interference-based NLRP3-knockdown cell lines generated from the human monocytic cell line THP-1. Knocking down NLRP3 reduced NF-κB activation and cytokine induction in the early stages of Staphylococcus aureus infection. Expression of cytokine genes induced by Staphylococcus aureus was not inhibited by a caspase-1 inhibitor, and did not occur through an autocrine mechanism in response to newly synthesized cytokines. We also demonstrated that NLRP3 could activate NF-κB and induce cytokines in response to sterile signals, monosodium urate crystals and aluminum adjuvant. Thus, NLRP3 mediates NF-κB activation in both sterile and microbially induced inflammation. Our findings show that not only does NLRP3 activate caspase-1 post-translationally, but it also induces multiple cytokine genes in the innate immune system.