Unraveling the Pathogenesis of MDS: The NLRP3 Inflammasome and Pyroptosis Drive the MDS Phenotype

Abstract

Myelodysplastic syndromes (MDS) are characterized by bone marrow cytological dysplasia and ineffective hematopoiesis in the setting of recurrent somatic gene mutations and chromosomal abnormalities. The underlying pathogenic mechanisms that drive a common clinical phenotype from a diverse array of genetic abnormalities have only recently begun to emerge. Accumulating evidence has highlighted the integral role of the innate immune system in upregulating inflammatory cytokines via NF-κB activation in the pathogenesis of MDS. Recent investigations implicate activation of the NLRP3 inflammasome in hematopoietic stem/progenitor cells as a critical convergence signal in MDS with consequent clonal expansion and pyroptotic cell death though caspase-1 maturation. Specifically, the alarmin S100A9 and/or founder gene mutations trigger pyroptosis through the generation of reactive oxygen species leading to assembly and activation of the redox-sensitive NLRP3 inflammasome and β-catenin, assuring propagation of the MDS clone. More importantly, targeted inhibition of varied steps in this pathway restore effective hematopoiesis. Together, delineation of the role of pyroptosis in the clinical phenotype of MDS patients has identified novel therapeutic strategies that offer significant promise in the treatment of MDS.

Keywords: MDS; NLRP3; S100A9; TLR; inflammasome; pyroptosis.

Figure 1

TLR signaling governs multiple cellular processes through a complex signaling network. A simplified schematic of TLR signaling is presented. (A) The majority of the TLRs reside in the plasma membrane (with the exception of TLR3, TLR7–9). Following receptor ligation, a number of signaling adaptor proteins will be recruited, including MyD88, TIRAP, TRAM, and TRIF. Depending on the particular stimulus, certain TLRs will become activated, resulting in recruitment of specific adaptor proteins, kinases, and ubiquitin ligases that will help propagate signaling. (B) Through interaction with MyD88, the serine/threonine kinase IRAK4 will be recruited. Subsequently, IRAK1 and IRAK2 will be activated. (C) Autoubiquitination of TRAF6 allows for recruitment of the IKK complex (IKKα, IKKβ, and IKKγ/NEMO). Phosphorylation of IKKβ by TAK1 allows for phosphorylation of NF-κB, thereby targeting Ikβ proteins for degradation. Liberated NF-κB can enter into the nucleus and direct transcription. (D) MyD88 and IRAK join to form a large multiprotein complex including the E3 ubiquitin ligase TRAF6. (E,F) Activation of TAK1 and MEKK1 result in a signaling cascade, leading to the activation of MAPK signaling and modulation of gene expression.

Figure 2

A S100A9-pyroptosis circuit provokes phenotypes manifest in MDS. (A) Myeloid-derived suppressor cells (MDSCs) are markedly expanded in the BM of MDS patients. MDSCs produce and secrete S100A9, which functions to mediate progenitor cell death and also activates MDSCs in an autocrine manner. (B) S100A8/A9 binds both CD33 and TLR4, resulting in NLRP3 inflammasome assembly. Ligation of S100A8/A9 to TLR4 through the IRAK–TRAF6 axis results in NF-κB-mediated transcription and subsequent production of pro-inflammatory cytokines, like pro-IL-1β and -IL-18, along with inflammasome components. (C) S100A8/A9 promotes activation of NAPDH oxidase (NOX), which results in a dual function. First, NOX proteins generate ROS, which serve to active NLRs and trigger inflammasome assembly. Second, NOX-derived ROS result in the oxidation of nucleoredoxin (NRX), leading to its dissociation from disheveled (Dvl). Once dissociated, Dvl suppresses the β-catenin destruction complex, resulting in stabilization of β-catenin. This allows β-catenin to enter the nucleus and induce transcript of TCF/LEF-controlled genes, including cyclin-D1 and c-Myc, which are essential to self-renewal. (D) MDS-related gene mutations activate NF-κB and NLRP3 via NOX-generated ROS. (E) Formation of the NLRP3 inflammasome complex occurs as a consequence of ROS activation and DAMP signaling. 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. (F) Pyroptosis ensues with loss of membrane integrity resulting in the release of pro-inflammatory cytokines, ROS, and other intracellular contents into the extracellular milieu.

Figure 3

Mechanisms of cell death involved in the pathogenesis of myelodysplastic syndromes (MDS). Apoptosis, pyroptosis, and autophagy all contribute to cell death in MDS. Apoptosis, a non-inflammatory cell death, can be triggered through an extrinsic or intrinsic pathway leading to effector caspase activation and apoptotic body formation. Pyroptosis is an inflammatory cell death mechanism, which is triggered by damage-associated molecular patterns (DAMPs), in particular S100A8/S100A9, leading to ROS production and inflammasome activation resulting in production of pro-inflammatory cytokines (i.e., IL-1β and IL-18) and caspase-1 activation with consequent cell lysis. Lastly, mitophagy, or the selective degradation of mitochondria through lysosomal recycling and an autophagic mechanism, leads to ROS production and can induce both apoptosis and pyroptosis.