Project description:A disappointingly small proportion (10-30%) of patients with cancer show lasting responses to immune checkpoint blockade (ICB)-based monotherapies. The RNA-editing enzyme ADAR1 is an emerging determinant of resistance to ICB therapy, and prevents ICB responsiveness by repressing immunogenic double-stranded (ds)RNAs, such as those arising from the dysregulated expression of endogenous retroelements (EREs). These dsRNAs trigger an interferon (IFN)-dependent antitumor response by activating the A-form dsRNA (A-RNA) sensing proteins MDA-5, PKR, and OAS1. Here, we show that ADAR1 also prevents accrual of endogenous Z-form dsRNA elements (Z-RNAs) which were enriched in the 3’UTRs of IFN-stimulated mRNAs. Depleting ADAR1 resulted in Z-RNA accumulation and activation of the Z-RNA sensor ZBP1, culminating in RIPK3-mediated necroptosis. As no clinically viable ADAR1 inhibitors currently exist, we searched for a compound that can override the requirement for ADAR1 inhibition and directly activate ZBP1. We identified a small molecule, the curaxin CBL0137, which potently activates ZBP1 by triggering Z-DNA formation in cells. CBL0137 induced ZBP1-dependent necroptosis in cancer-associated fibroblasts and strongly reversed ICB unresponsiveness in mouse models of melanoma. Collectively, these results demonstrate that ADAR1 represses endogenous Z-RNAs and identifies ZBP1-mediated necroptosis as a new determinant of tumor immunogenicity masked by ADAR1. Therapeutic activation of ZBP1-induced necroptosis provides a readily-translatable avenue for rekindling immune responsiveness of ICB-resistant human cancers.
Project description:A disappointingly small proportion (10-30%) of patients with cancer show lasting responses to immune checkpoint blockade (ICB)-based monotherapies. The RNA-editing enzyme ADAR1 is an emerging determinant of resistance to ICB therapy, and prevents ICB responsiveness by repressing immunogenic double-stranded (ds)RNAs, such as those arising from the dysregulated expression of endogenous retroelements (EREs). These dsRNAs trigger an interferon (IFN)-dependent antitumor response by activating the A-form dsRNA (A-RNA) sensing proteins MDA-5, PKR, and OAS1. Here, we show that ADAR1 also prevents accrual of endogenous Z-form dsRNA elements (Z-RNAs) which were enriched in the 3’UTRs of IFN-stimulated mRNAs. Depleting ADAR1 resulted in Z-RNA accumulation and activation of the Z-RNA sensor ZBP1, culminating in RIPK3-mediated necroptosis. As no clinically viable ADAR1 inhibitors currently exist, we searched for a compound that can override the requirement for ADAR1 inhibition and directly activate ZBP1. We identified a small molecule, the curaxin CBL0137, which potently activates ZBP1 by triggering Z-DNA formation in cells. CBL0137 induced ZBP1-dependent necroptosis in cancer-associated fibroblasts and strongly reversed ICB unresponsiveness in mouse models of melanoma. Collectively, these results demonstrate that ADAR1 represses endogenous Z-RNAs and identifies ZBP1-mediated necroptosis as a new determinant of tumor immunogenicity masked by ADAR1. Therapeutic activation of ZBP1-induced necroptosis provides a readily-translatable avenue for rekindling immune responsiveness of ICB-resistant human cancers.
Project description:A disappointingly small proportion (10-30%) of patients with cancer show lasting responses to immune checkpoint blockade (ICB)-based monotherapies. The RNA-editing enzyme ADAR1 is an emerging determinant of resistance to ICB therapy, and prevents ICB responsiveness by repressing immunogenic double-stranded (ds)RNAs, such as those arising from the dysregulated expression of endogenous retroelements (EREs). These dsRNAs trigger an interferon (IFN)-dependent antitumor response by activating the A-form dsRNA (A-RNA) sensing proteins MDA-5, PKR, and OAS1. Here, we show that ADAR1 also prevents accrual of endogenous Z-form dsRNA elements (Z-RNAs) which were enriched in the 3’UTRs of IFN-stimulated mRNAs. Depleting ADAR1 resulted in Z-RNA accumulation and activation of the Z-RNA sensor ZBP1, culminating in RIPK3-mediated necroptosis. As no clinically viable ADAR1 inhibitors currently exist, we searched for a compound that can override the requirement for ADAR1 inhibition and directly activate ZBP1. We identified a small molecule, the curaxin CBL0137, which potently activates ZBP1 by triggering Z-DNA formation in cells. CBL0137 induced ZBP1-dependent necroptosis in cancer-associated fibroblasts and strongly reversed ICB unresponsiveness in mouse models of melanoma. Collectively, these results demonstrate that ADAR1 represses endogenous Z-RNAs and identifies ZBP1-mediated necroptosis as a new determinant of tumor immunogenicity masked by ADAR1. Therapeutic activation of ZBP1-induced necroptosis provides a readily-translatable avenue for rekindling immune responsiveness of ICB-resistant human cancers.
Project description:Caspase-8 and FADD play key roles in the regulation of cell death by necroptosis. The absence of either protein results in early embryonic lethality due to the activation of the kinase RIPK3 and its phosphorylation of the necroptosis executioner, MLKL. We genetically engineered and characterized a mouse model to monitor MLKL phosphorylation in the absence of necroptosis in vivo. Ablation of caspase-8 or FADD resulted in the transcriptional upregulation in several tissues of ZBP1, a cytosolic nucleic acid sensor capable of activating RIPK3, and ZBP1 was required for spontaneous phosphorylation of MLKL. Our findings provide a novel mechanism by which the FADD-Caspase-8 complex prevents necroptosis.
Project description:Necroptosis is a lytic form of cell death that is mediated by the kinase RIPK3 and the pseudokinase MLKL when caspase-8 is inhibited downstream of death receptors, toll-like receptor 3 (TLR3), TLR4, and the intracellular Z-form nucleic acid sensor ZBP1. Oligomerization and activation of RIPK3 is driven by interactions with the kinase RIPK1, the TLR adaptor TRIF, or ZBP1. In this study, we use immunohistochemistry (IHC) and in situ hybridization (ISH) assays to generate a tissue atlas characterizing RIPK1, RIPK3, Mlkl, and ZBP1 expression in mouse tissues. RIPK1, RIPK3, and Mlkl were co-expressed in most immune cell populations, endothelial cells, and many mucosal epithelia. ZBP1 was expressed in many immune populations, but had more variable expression in epithelia compared to RIPK1, RIPK3, and Mlkl. Intriguingly, expression of ZBP1 was elevated in Casp8-/- Tnfr1-/- embryos prior to their succumbing to aberrant necroptosis around embryonic day 15. ZBP1 contributed to this embryonic lethality because rare Casp8-/- Tnfr1-/- Zbp1-/- mice survived until after birth. Necroptosis mediated by TRIF contributed to the demise of Casp8-/- Tnfr1-/- Zbp1-/- pups in the perinatal period. Of note, Casp8-/- Tnfr1-/- Trif-/- Zbp1-/- mice exhibited autoinflammation and morbidity, typically within 5-7 weeks of being born, which is not seen in Casp8-/- Ripk1-/- Trif-/- Zbp1-/-, Casp8-/- Ripk3-/-, or Casp8-/- Mlkl-/- mice. Therefore, after birth, loss of caspase-8 probably unleashes RIPK1-dependent necroptosis driven by death receptors other than TNFR1.
Project description:The innate immune system recognizes nucleic acids as a signature of microbial infection and initiates host-protective responses, including the production of type I IFN and proinflammatory cytokines. Z-DNA binding protein 1 (ZBP1, also known as DLM-1 or DAI) was previously identified as a dsDNA binding protein, triggering DNA-mediated activation of innate immune responses. However, mice or cells lacking ZBP1 produce normal levels of type I IFN in response to dsDNA. Therefore, the classification of ZBP1 as a true DNA sensor remains to be resolved. Here, we report that the single stranded RNA virus, influenza A virus (IAV) is a trigger of the cytosolic sensor ZBP1. Sensing of IAV infection by ZBP1 engages a novel NLRP3 inflammasome pathway that is not defined by the conventions of the canonical and non-canonical NLRP3 inflammasome pathways. Surprisingly, IAV-induced cell death was not prevented by the absence of the NLRP3 inflammasome. Instead, we identified parallel contributions from pyroptosis, necroptosis and apoptosis in the execution of ZBP1-dependent cell death, mediated by the kinase RIPK3. Overall, the ability of ZBP1 to sense IAV infection signifies a point of divergence for IAV-induced programmed cell death pathways and inflammasome activation. We used microarrays to explore the gene expression profiles differentially expressed in influenza-infected bone marrow derived macrophages (BMDM) isolated from Ifnar1-/- and wild-type mice.