X-chromosome-linked inhibitor of apoptosis protein (XIAP) controls cell survival in several regulated cell death pathways and coordinates a range of inflammatory signaling events. Initially ...identified as a caspase-binding protein, it was considered to be primarily involved in blocking apoptosis from both intrinsic as well as extrinsic triggers. However, XIAP also prevents TNF-mediated, receptor-interacting protein 3 (RIPK3)-dependent cell death, by controlling RIPK1 ubiquitylation and preventing inflammatory cell death. The identification of patients with germline mutations in XIAP (termed XLP-2 syndrome) pointed toward its role in inflammatory signaling. Indeed, XIAP also mediates nucleotide-binding oligomerization domain-containing 2 (NOD2) proinflammatory signaling by promoting RIPK2 ubiquitination within the NOD2 signaling complex leading to NF-κB and MAPK activation and production of inflammatory cytokines and chemokines. Overall, XIAP is a critical regulator of multiple cell death and inflammatory pathways making it an attractive drug target in tumors and inflammatory diseases.
Nucleotide-binding and oligomerization domain (NOD)-like receptors constitute a first line of defense against invading bacteria. X-linked Inhibitor of Apoptosis (XIAP) is implicated in the control of ...bacterial infections, and mutations in XIAP are causally linked to immunodeficiency in X-linked lymphoproliferative syndrome type-2 (XLP-2). Here, we demonstrate that the RING domain of XIAP is essential for NOD2 signaling and that XIAP contributes to exacerbation of inflammation-induced hepatitis in experimental mice. We find that XIAP ubiquitylates RIPK2 and recruits the linear ubiquitin chain assembly complex (LUBAC) to NOD2. We further show that LUBAC activity is required for efficient NF-κB activation and secretion of proinflammatory cytokines after NOD2 stimulation. Remarkably, XLP-2-derived XIAP variants have impaired ubiquitin ligase activity, fail to ubiquitylate RIPK2, and cannot facilitate NOD2 signaling. We conclude that XIAP and LUBAC constitute essential ubiquitin ligases in NOD2-mediated inflammatory signaling and propose that deregulation of NOD2 signaling contributes to XLP-2 pathogenesis.
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▸ XIAP is required for NOD2-dependent signaling and inflammation in vivo ▸ XIAP ubiquitylates RIPK2 after NOD2 stimulation to facilitate LUBAC recruitment ▸ LUBAC regulates NOD2-dependent immune signaling ▸ XLP-2-derived XIAP mutations impair ubiquitin ligase activity and NOD2 signaling
FAS belongs to the subgroup of the tumor necrosis factor receptor (TNF-R) family that contains an intracellular “death domain” and triggers apoptosis. Its physiological ligand FASL is a member of the ...TNF cytokine family. Studies with mutant mice and cells from human patients have shown that FAS plays critical roles in the immune system, including the killing of pathogen-infected cells and the death of obsolete and potentially dangerous lymphocytes. Fas thereby functions as a guardian against autoimmunity and tumor development. FAS triggers apoptosis through FADD-mediated recruitment and activation of caspase-8. In certain cells such as hepatocytes, albeit not lymphocytes, FAS-induced apoptosis requires amplification through proteolytic activation of the proapoptotic BCL-2 family member BID. Curiously, several components of the FAS signaling machinery have been implicated in nonapoptotic processes, including cellular activation, differentiation, and proliferation. This review describes current understanding of Fas-induced apoptosis signaling and proposes experimental strategies for future advances.
High-throughput methods to investigate tumour omic landscapes have quickly catapulted cancer specialists into the precision oncology era. The singular lesson of precision oncology might be that, for ...it to be precise, treatment must be personalized, as each cancer's complex molecular and immune landscape differs from patient to patient. Transformative therapies include those that are targeted at the sequelae of molecular abnormalities or at immune mechanisms, and, increasingly, pathways previously thought to be undruggable have become druggable. Critical to applying precision medicine is the concept that the right combination of drugs must be chosen for each patient and used at the right stage of the disease. Multiple puzzles remain that complicate therapy choice, including evidence that deleterious mutations are common in normal tissues and non-malignant conditions. The host's role is also likely to be key in determining treatment response, especially for immunotherapy. Indeed, maximizing the impact of immunotherapy will require omic analyses to match the right immune-targeted drugs to the individualized patient and tumour setting. In this Perspective, we discuss six key riddles that must be solved to optimize the application of precision oncology to otherwise lethal malignancies.
X-linked inhibitor of apoptosis protein (XIAP) has been identified as a potent regulator of innate immune responses, and loss-of-function mutations in XIAP cause the development of the X-linked ...lymphoproliferative syndrome type 2 (XLP-2) in humans. Using gene-targeted mice, we show that loss of XIAP or deletion of its RING domain lead to excessive cell death and IL-1β secretion from dendritic cells triggered by diverse Toll-like receptor stimuli. Aberrant IL-1β secretion is TNF dependent and requires RIP3 but is independent of cIAP1/cIAP2. The observed cell death also requires TNF and RIP3 but proceeds independently of caspase-1/caspase-11 or caspase-8 function. Loss of XIAP results in aberrantly elevated ubiquitylation of RIP1 outside of TNFR complex I. Virally infected Xiap−/− mice present with symptoms reminiscent of XLP-2. Our data show that XIAP controls RIP3-dependent cell death and IL-1β secretion in response to TNF, which might contribute to hyperinflammation in patients with XLP-2.
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•XIAP protects against TLR- and TNF-driven inflammasome formation and cell death•XIAP contributes to the regulation of RIP1 ubiquitylation in response to TNF•Exaggerated IL-1β secretion and cell death in Xiap−/− dendritic cells are RIP3 dependent•Upon infection, Xiap−/− mice show signs of hyperinflammation similar to XLP-2 patients
Loss of X-linked inhibitor of apoptosis protein (XIAP) function causes the development of the X-linked lymphoproliferative syndrome type 2 (XLP-2) in humans. Yabal et al. now show that in the absence of XIAP, activation of immune cells by TNF leads to the induction of RIP3-dependent cell death and, independently, to aberrant inflammasome activation. In this process, XIAP controls the correct ubiquitylation of RIP1. These findings might hold promise for new therapeutic opportunities for patients with XLP-2.
Innate immune signaling relies on the deposition of non-degradative polyubiquitin at receptor-signaling complexes, but how these ubiquitin modifications are regulated by deubiquitinases remains ...incompletely understood. Met1-linked ubiquitin (Met1-Ub) is assembled by the linear ubiquitin assembly complex (LUBAC), and this is counteracted by the Met1-Ub-specific deubiquitinase OTULIN, which binds to the catalytic LUBAC subunit HOIP. In this study, we report that HOIP also interacts with the deubiquitinase CYLD but that CYLD does not regulate ubiquitination of LUBAC components. Instead, CYLD limits extension of Lys63-Ub and Met1-Ub conjugated to RIPK2 to restrict signaling and cytokine production. Accordingly, Met1-Ub and Lys63-Ub were individually required for productive NOD2 signaling. Our study thus suggests that LUBAC, through its associated deubiquitinases, coordinates the deposition of not only Met1-Ub but also Lys63-Ub to ensure an appropriate response to innate immune receptor activation.
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•CYLD associates with LUBAC via HOIP and limits signaling by NOD2•RIPK2 ubiquitination is regulated by CYLD and OTULIN•CYLD trims Lys63 and Met1 linkages conjugated to RIPK2•Productive NOD2 signaling requires Lys63 and Met1 linkages
Hrdinka et al. show that productive signaling by the bacterial sensor NOD2 requires formation of Lys63- and Met1-linked ubiquitin. Both ubiquitin linkages are targeted by the deubiquitinase CYLD, which together with OTULIN regulates RIPK2 ubiquitination to restrict NOD2 signaling and cytokine production.
Since acute myeloid leukemia (AML) is characterized by the blockade of hematopoietic differentiation and cell death, we interrogated RIPK3 signaling in AML development. Genetic loss of Ripk3 ...converted murine FLT3-ITD-driven myeloproliferation into an overt AML by enhancing the accumulation of leukemia-initiating cells (LIC). Failed inflammasome activation and cell death mediated by tumor necrosis factor receptor caused this accumulation of LIC exemplified by accelerated leukemia onset in Il1r1−/−, Pycard–/–, and Tnfr1/2−/− mice. RIPK3 signaling was partly mediated by mixed lineage kinase domain-like. This link between suppression of RIPK3, failed interleukin-1β release, and blocked cell death was supported by significantly reduced RIPK3 in primary AML patient cohorts. Our data identify RIPK3 and the inflammasome as key tumor suppressors in AML.
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•RIPK3 suppresses HSPC transformation by promoting cell death and differentiation•Cell death in leukemia-initiating cells is driven by TNFR-driven RIPK3 activation•RIPK3 promotes differentiation of LIC by activating the inflammasome•RIPK3 expression is reduced in several subtypes of primary de novo AML
Höckendorf et al. demonstrate that RIPK3 restricts malignant myeloproliferation by activating the inflammasome, which promotes differentiation and cell death, and that loss of RIPK3 increases leukemic burden in mice. Reduced RIPK3 expression is observed across several human acute myeloid leukemia subtypes.
Ferroptosis is a recently defined form of regulated cell death, which is biochemically and morphologically distinct from traditional forms of programmed cell death such as apoptosis or necrosis. It ...is driven by iron, reactive oxygen species, and phospholipids that are oxidatively damaged, ultimately resulting in mitochondrial damage and breakdown of membrane integrity. Numerous cellular signaling pathways and molecules are involved in the regulation of ferroptosis, including enzymes that control the cellular redox status. Alterations in the ferroptosis-regulating network can contribute to the development of various diseases, including cancer. Evidence suggests that ferroptosis is commonly suppressed in cancer cells, allowing them to survive and progress. However, cancer cells which are resistant to common chemotherapeutic drugs seem to be highly susceptible to ferroptosis inducers, highlighting the great potential of pharmacologic modulation of ferroptosis for cancer treatment. Non-coding RNAs (ncRNAs) are considered master regulators of various cellular processes, particularly in cancer where they have been implicated in all hallmarks of cancer. Recent work also demonstrated their involvement in the molecular control of ferroptosis. Hence, ncRNA-based therapeutics represent an exciting alternative to modulate ferroptosis for cancer therapy. This review summarizes the ncRNAs implicated in the regulation of ferroptosis in cancer and highlights their underlying molecular mechanisms in the light of potential therapeutic applications.
The hematopoietic system represents an organ system with an exceptional capacity for the production of mature blood cells from a small and mostly quiescent pool of hematopoietic stem cells (HSCs). ...This extraordinary capacity includes self-renewal but also the propensity to rapidly respond to extrinsic needs, such as acute infections, severe inflammation, and wound healing. In recent years, it became clear that inflammatory signals such as cytokines, chemokine and danger signals from pathogens (PAMPs) or dying cells (DAMPs) impact on HSCs, shaping their proliferation status, lineage bias, and repopulating ability and subsequently increasing the output of mature effector cells. However, inflammatory danger signals negatively impact on the capacity of HSCs to self-renew and to maintain their stem cell capabilities. This is evidenced in conditions of chronic inflammation where bone marrow failure may originate from HSC exhaustion. Even in hematopoietic cancers, inflammatory signals shape the phenotype of the malignant clone as exemplified by necrosome-dependent inflammation elicited during malignant transformation in acute myeloid leukemia. Accordingly, understanding the contribution of inflammatory signals, and specifically necroinflammation, to HSC integrity, HSC long-term functionality, and malignant transformation has attracted substantial research and clinical interest. In this review, we highlight recent developments and open questions at the interplay between inflammation, regulated necrosis, and HSC biology in the context of blood cell development, acute and chronic inflammation, and hematopoietic cancer.
The transcription factor NF-κB is a tightly regulated positive mediator of T- and B-cell development, proliferation, and survival. The controlled activity of NF-κB is required for the coordination of ...physiologic immune responses. However, constitutive NF-κB activation can promote continuous lymphocyte proliferation and survival and has recently been recognized as a critical pathogenetic factor in lymphoma. Various molecular events lead to deregulation of NF-κB signaling in Hodgkin disease and a variety of T- and B-cell non-Hodgkin lymphomas either upstream or downstream of the central IκB kinase. These alterations are prerequisites for lymphoma cell cycling and blockage of apoptosis. This review provides an overview of the NF-κB pathway and discusses the mechanisms of NF-κB deregulation in distinct lymphoma entities with defined aberrant pathways: Hodgkin lymphoma (HL), diffuse large B-cell lymphoma (DLBCL), mucosa-associated lymphoid tissue (MALT) lymphoma, primary effusion lymphoma (PEL), and adult T-cell lymphoma/leukemia (ATL). In addition, we summarize recent data that validates the NF-κB signaling pathway as an attractive therapeutic target in T- and B-cell malignancies.
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