Extracellular ATP contributes to the reactive oxygen species burst and exaggerated mitochondrial damage in D-galactosamine and lipopolysaccharide-induced fulminant hepatitis.

Fulminant hepatitis (FH) is a severe clinical syndrome leading to hepatic failure and even mortality. D-galactosamine (D-GalN) plus lipopolysaccharide (LPS) challenge is commonly used to establish an FH mouse model, but the mechanism underlying D-GalN/LPS-induced liver injury is incompletely understood. Previously, it has been reported that extracellular ATP that can be released under cytotoxic and inflammatory stresses serves as a damage signal to induce potassium ion efflux and trigger the NACHT, LRR and PYD domains-containing protein 3 (NLRP3) inflammasome activation through binding to P2X7 receptor. In this study, we tried to investigate whether it contributed to the fulminant hepatitis (FH) induced by D-GalN plus LPS. In an in vitro cellular model, D-GalN plus extracellular ATP, instead of D-GalN alone, induced pyroptosis and apoptosis, accompanied by mitochondrial reactive oxygen species (ROS) burst, and the oligomerization of Drp1, Bcl-2, and Bak, as well as the loss of mitochondrial membrane potential in LPS-primed macrophages, well reproducing the events induced by D-GalN and LPS in vivo. Moreover, these events in the cellular model were markedly suppressed by both A-804598 (an ATP receptor P2X7R inhibitor) and glibenclamide (an ATP-sensitive potassium ion channel inhibitor); in the FH mouse model, administration of A-804598 significantly mitigated D-GalN/LPS-induced hepatic injury, mitochondrial damage, and the activation of apoptosis and pyroptosis signaling, corroborating the contribution of extracellular ATP to the cell death. Collectively, our data suggest that extracellular ATP acts as an autologous damage-associated molecular pattern to augment mitochondrial damage, hepatic cell death, and liver injury in D-GalN/LPS-induced FH mouse model.

Anti-Necroptotic Effects of Itaconate and its Derivatives.

Itaconate is an unsaturated dicarboxylic acid that is derived from the decarboxylation of the Krebs cycle intermediate cis-aconitate and has been shown to exhibit anti-inflammatory and anti-bacterial/viral properties. But the mechanisms underlying itaconate's anti-inflammatory activities are not fully understood. Necroptosis, a lytic form of regulated cell death (RCD), is mediated by receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL) signaling. It has been involved in the pathogenesis of organ injury in many inflammatory diseases. In this study, we aimed to explore whether itaconate and its derivatives can inhibit necroptosis in murine macrophages, a mouse MPC-5 cell line and a human HT-29 cell line in response to different necroptotic activators. Our results showed that itaconate and its derivatives dose-dependently inhibited necroptosis, among which dimethyl itaconate (DMI) was the most effective one. Mechanistically, itaconate and its derivatives inhibited necroptosis by suppressing the RIPK1/RIPK3/MLKL signaling and the oligomerization of MLKL. Furthermore, DMI promoted the nuclear translocation of Nrf2 that is a critical regulator of intracellular redox homeostasis, and reduced the levels of intracellular reactive oxygen species (ROS) and mitochondrial superoxide (mtROS) that were induced by necroptotic activators. Consistently, DMI prevented the loss of mitochondrial membrane potential induced by the necroptotic activators. In addition, DMI mitigated caerulein-induced acute pancreatitis in mice accompanied by reduced activation of the necroptotic signaling in vivo. Collectively, our study demonstrates that itaconate and its derivatives can inhibit necroptosis by suppressing the RIPK1/RIPK3/MLKL signaling, highlighting their potential applications for treating necroptosis-associated diseases.

Potassium ion efflux induces exaggerated mitochondrial damage and non-pyroptotic necrosis when energy metabolism is blocked.

Damage-associated molecular patterns (DAMPs) such as extracellular ATP and nigericin (a bacterial toxin) not only act as potassium ion (K+) efflux inducers to activate NLRP3 inflammasome, leading to pyroptosis, but also induce cell death independently of NLRP3 expression. However, the roles of energy metabolism in determining NLRP3-dependent pyroptosis and -independent necrosis upon K+ efflux are incompletely understood. Here we established cellular models by pharmacological blockade of energy metabolism, followed by stimulation with a K+ efflux inducer (ATP or nigericin). Two energy metabolic inhibitors, namely CPI-613 that targets α-ketoglutarate dehydrogenase and pyruvate dehydrogenase (a rate-limiting enzyme) and 2-deoxy-d-glucose (2-DG) that targets hexokinase, are recruited in this study, and Nlrp3 gene knockout macrophages were used. Our data showed that CPI-613 and 2-DG dose-dependently inhibited NLRP3 inflammasome activation, but profoundly increased cell death in the presence of ATP or nigericin. The cell death was K+ efflux-induced but NLRP3-independent, which was associated with abrupt reactive oxygen species (ROS) production, reduction of mitochondrial membrane potential, and oligomerization of mitochondrial proteins, all indicating mitochondrial damage. Notably, the cell death induced by K+ efflux and blockade of energy metabolism was distinct from pyroptosis, apoptosis, necroptosis or ferroptosis. Furthermore, fructose 1,6-bisphosphate, a high-energy intermediate of glycolysis, significantly suppressed CPI-613+nigericin-induced mitochondrial damage and cell death. Collectively, our data show that energy deficiency diverts NLRP3 inflammasome activation-dependent pyroptosis to Nlrp3-independent necrosis upon K+ efflux inducers, which can be dampened by high-energy intermediate, highlighting a critical role of energy metabolism in cell survival and death under inflammatory conditions.