S100A8/9-mediated inflammasome formation and pyroptosis defines MDS phenotype. (A) S100A8/A9 heterodimers bind both CD33 and TLR4, resulting in NLRP3 inflammasome assembly. Ligation of S100A8/A9 to TLR4 results in NF-κB–mediated transcription and subsequent production of proinflammatory cytokines such as pro-IL-1β and pro-IL-18, along with inflammasome components. (B) S100A8/A9 promotes activation of NADPH oxidase (NOX), which results in dual processes. First, NOX proteins generate ROS, with increasing levels leading to thioredoxin-interacting protein (TXNIP) dissociation from thioredoxin (TRX) with consequent binding to NLPR3 and inflammasome assembly. Second, NOX-derived ROS results in oxidation of nuceloredoxin (NRX), leading to its dissociation from Dishevelled (Dvl). Once dissociated, Dvl suppress the β-catenin destruction complex (glycogen synthase kinase 3 [GSKβ]/casein kinase 1 [CK1]/adenomatous polyposis coli [APC]/Axin/protein phosphatase 2A [PP2A]), resulting in stabilization of β-catenin. This allows β-catenin to enter the nucleus and induce transcription of T-cell factor (TCF) controlled genes, including cyclin-D1 and c-Myc. (C) Inflammasome assembly occurs through activation of NLRP3, triggering the recruitment of the apoptosis-associated speck like protein containing a caspase-recruitment domain (ASC), leading to the formation of cytosolic, heptameric complexes which serve as a platform for activation of pro-caspase-1. Once activated, inflammasomes mediate conversion of pro-caspase-1 to its mature and catalytically active form. Active caspase-1 cleaves pro-IL-1β and pro-IL-18 to their mature forms. (D) MDS-related gene mutations activate NF-κB and NLRP3 via NOX-generated ROS. Dotted line highlights potential mechanism that has not been definitively proven in MDS. β-Cat, β-Catenin; BCL, B-cell lymphoma; CARD, caspase activation and recruitment domain; CBP, CREB-binding protein; l-Arg, l-arginine; LRR, leucine-rich repeat; NOS, nitric oxide synthase; P, phosphate; PYD, pyrin domain.