RIP1 post-translational modifications

The RIPK1 death domain restrains ZBP1- and TRIF-mediated cell death and inflammation

RIPK1 is a multi-functional kinase that regulates cell death and inflammation and has been implicated in the pathogenesis of inflammatory diseases. RIPK1 acts in a kinase-dependent and kinase-independent manner to promote or suppress apoptosis and necroptosis, but the underlying mechanisms remain poorly understood. Here, we show that a mutation (R588E) disrupting the RIPK1 death domain (DD) caused perinatal lethality induced by ZBP1-mediated necroptosis. Additionally, these mice developed postnatal inflammatory pathology, which was mediated by necroptosis-independent TNFR1, TRADD, and TRIF signaling, partially requiring RIPK3. Our biochemical mechanistic studies revealed that ZBP1- and TRIF-mediated activation of RIPK3 required RIPK1 kinase activity in wild-type cells but not in Ripk1R588E/R588E cells, suggesting that DD-dependent oligomerization of RIPK1 and its interaction with FADD determine the mechanisms of RIPK3 activation by ZBP1 and TRIF. Collectively, these findings revealed a critical physiological role of DD-dependent RIPK1 signaling that is important for the regulation of tissue homeostasis and inflammation.

The emerging roles of non-canonical ubiquitination in proteostasis and beyond

Ubiquitin regulates various cellular functions by posttranslationally modifying substrates with diverse ubiquitin codes. Recent discoveries of new ubiquitin chain topologies, types of bonds, and non-protein substrates have substantially expanded the complexity of the ubiquitin code. Here, we describe the ubiquitin system covering the basic principles and recent discoveries related to mechanisms, technologies, and biological importance.

Mediators of necroptosis: from cell death to metabolic regulation

Necroptosis, a programmed cell death mechanism distinct from apoptosis, has garnered attention for its role in various pathological conditions. While initially recognized for its involvement in cell death, recent research has revealed that key necroptotic mediators, including receptor-interacting protein kinases (RIPKs) and mixed lineage kinase domain-like protein (MLKL), possess additional functions that go beyond inducing cell demise. These functions encompass influencing critical aspects of metabolic regulation, such as energy metabolism, glucose homeostasis, and lipid metabolism. Dysregulated necroptosis has been implicated in metabolic diseases, including obesity, diabetes, metabolic dysfunction-associated steatotic liver disease (MASLD) and alcohol-associated liver disease (ALD), contributing to chronic inflammation and tissue damage. This review provides insight into the multifaceted role of necroptosis, encompassing both cell death and these extra-necroptotic functions, in the context of metabolic diseases. Understanding this intricate interplay is crucial for developing targeted therapeutic strategies in diseases that currently lack effective treatments.

Inhibition of RIP1 improves immune reconstitution and reduces GVHD mortality while preserving graft-versus-leukemia effects

Graft-versus-host disease (GVHD) remains the major cause of morbidity and nonrelapse mortality (NRM) after hematopoietic cell transplantation (HCT). Inflammatory cytokines mediate damage to key GVHD targets such as intestinal stem cells (ISCs) and also activate receptor interacting protein kinase 1 (RIP1; RIPK1), a critical regulator of apoptosis and necroptosis. We therefore investigated the role of RIP1 in acute GVHD using samples from HCT patients, modeling GVHD damage in vitro with both human and mouse gastrointestinal (GI) organoids, and blocking RIP1 activation in vivo using several well-characterized mouse HCT models. Increased phospho-RIP1 expression in GI biopsies from patients with acute GVHD correlated with tissue damage and predicted NRM. Both the genetic inactivation of RIP1 and the RIP1 inhibitor GNE684 prevented GVHD-induced apoptosis of ISCs in vivo and in vitro. Daily administration of GNE684 for 14 days reduced inflammatory infiltrates in three GVHD target organs (intestine, liver, and spleen) in mice. Unexpectedly, GNE684 administration also reversed the marked loss of regulatory T cells in the intestines and liver during GVHD and reduced splenic T cell exhaustion, thus improving immune reconstitution. Pharmacological and genetic inhibition of RIP1 improved long-term survival without compromising the graft-versus-leukemia (GVL) effect in lymphocytic and myeloid leukemia mouse models. Thus, RIP1inhibition may represent a nonimmunosuppressive treatment for GVHD.

cIAPs control RIPK1 kinase activity-dependent and -independent cell death and tissue inflammation

Cellular inhibitor of apoptosis proteins (cIAPs) are RING-containing E3 ubiquitin ligases that ubiquitylate receptor-interacting protein kinase 1 (RIPK1) to regulate TNF signalling. Here, we established mice simultaneously expressing enzymatically inactive cIAP1/2 variants, bearing mutations in the RING domains of cIAP1/2 (cIAP1/2 mutant RING, cIAP1/2MutR ). cIap1/2MutR/MutR mice died during embryonic development due to RIPK1-mediated apoptosis. While expression of kinase-inactive RIPK1D138N rescued embryonic development, Ripk1D138N/D138N /cIap1/2MutR/MutR mice developed systemic inflammation and died postweaning. Cells expressing cIAP1/2MutR and RIPK1D138N were still susceptible to TNF-induced apoptosis and necroptosis, implying additional kinase-independent RIPK1 activities in regulating TNF signalling. Although further ablation of Ripk3 did not lead to any phenotypic improvement, Tnfr1 gene knock-out prevented early onset of systemic inflammation and premature mortality, indicating that cIAPs control TNFR1-mediated toxicity independent of RIPK1 and RIPK3. Beyond providing novel molecular insights into TNF-signalling, the mouse model established in this study can serve as a useful tool to further evaluate ongoing therapeutic protocols using inhibitors of TNF, cIAPs and RIPK1.

Proximity proteomics reveals role of Abelson interactor 1 in the regulation of TAK1/RIPK1 signaling

Dysregulation of the adaptor protein Abelson interactor 1 (ABI1) is linked to malignant transformation. To interrogate the role of ABI1 in cancer development, we mapped the ABI1 interactome using proximity-dependent labeling (PDL) with biotin followed by mass spectrometry. Using a novel PDL data filtering strategy, considering both peptide spectral matches and peak areas of detected peptides, we identified 212 ABI1 proximal interactors. These included WAVE2 complex components such as CYFIP1, NCKAP1, or WASF1, confirming the known role of ABI1 in the regulation of actin-polymerization-dependent processes. We also identified proteins associated with the TAK1-IKK pathway, including TAK1, TAB2, and RIPK1, denoting a newly identified function of ABI1 in TAK1-NF-κB inflammatory signaling. Functional assays using TNFα-stimulated, ABI1-overexpressing or ABI1-deficient cells showed effects on the TAK1-NF-kB pathway-dependent signaling to RIPK1, with ABI1-knockout cells being less susceptible to TNFα-induced, RIPK1-mediated, TAK1-dependent apoptosis. In sum, our PDL-based strategy enabled mapping of the ABI1 proximal interactome, thus revealing a previously unknown role of this adaptor protein in TAK1/RIPK1-based regulation of cell death and survival.

A novel RIP1-mediated canonical WNT signaling pathway that promotes colorectal cancer metastasis via β -catenin stabilization-induced EMT

RIP1 (receptor-interacting protein kinase 1) is an important component of TNF-α signaling that contributes to various pathological effects. Here, we revealed new potential roles of RIP1 in controlling WNT/β-catenin canonical signaling to enhance metastasis of colorectal cancer (CRC). First, we showed that WNT3A treatment sequentially increased the expression of RIP1 and β-catenin. Immunohistochemical analyses of human CRC tissue arrays consisting of normal, primary, and metastatic cancers indicated that elevated RIP1 expression might be related to β-catenin expression, carcinogenesis, and metastasis. Intravenous injection of RIP1 over-expressed CRC cells into mice has demonstrated that RIP1 may promote metastasis. Immunoprecipitation (IP) results indicated that WNT3A treatment induces direct binding between RIP1 and β-catenin, and that this stabilizes the β-catenin protein in a manner that depends on the regulation of RIP1 ubiquitination via downregulation of the E3 ligase, cIAP1/2. Elimination of cIAP1/2 expression and inhibition of its ubiquitinase activity enhance WNT3A-induced RIP1 and β-catenin protein expression and binding, which stimulates endothelial-mesenchymal transition (EMT) induction to enhance the migration and invasion of CRC cells in vitro. The results of the in vitro binding assay and IP of exogenous RIP1-containing CRC cells additionally verified the direct binding of RIP1 and β-catenin. RIP1 expression can destroy the β-catenin-β-TrCP complex. Taken together, these results suggest a novel EMT-enhancing role of RIP1 in the WNT pathway and suggest a new canonical WNT3A-RIP1-β-catenin pathway that contributes to CRC malignancy by promoting EMT.

5-Iodotubercidin sensitizes cells to RIPK1-dependent necroptosis by interfering with NFκB signaling

Receptor-interacting protein kinases (RIPK)-1 and -3 play crucial roles in cell fate decisions and are regulated by multiple checkpoint controls. Previous studies have identified IKK1/2- and p38/MK2-dependent checkpoints that phosphorylate RIPK1 at different residues to inhibit its activation. In this study, we investigated TNF-induced death in MAPK-activated protein kinase 2 (MK2)-deficient cells and found that MK2 deficiency or inactivation predominantly leads to necroptotic cell death, even without caspase inhibition. While RIPK1 inhibitors can rescue MK2-deficient cells from necroptosis, inhibiting RIPK3 seems to switch the process to apoptosis. To understand the underlying mechanism of this switch, we screened a library of 149 kinase inhibitors and identified the adenosine analog 5-Iodotubercidin (5-ITu) as the most potent compound that sensitizes MK2-deficient MEFs to TNF-induced cell death. 5-ITu also enhances LPS-induced necroptosis when combined with MK2 inhibition in RAW264.7 macrophages. Further mechanistic studies revealed that 5-ITu induces RIPK1-dependent necroptosis by suppressing IKK signaling in the absence of MK2 activity. These findings highlight the role for the multitarget kinase inhibitor 5-ITu in TNF-, LPS- and chemotherapeutics-induced necroptosis and its potential implications in RIPK1-targeted therapies.

RIPK1 is aberrantly expressed in multiple B-cell cancers and implicated in the underlying pathogenesis

According to the latest epidemiology of the US, B-cell cancers account for > 3% of all new cancer cases and > 80% of non-Hodgkin lymphomas. However, the disease-modifying small molecular drug suitable for most B-cell cancers is still lacking. RIPK1 (receptor-interacting serine/threonine-protein kinase 1) has been observed to be dysregulated and implicated in the pathogenesis of multiple solid cancers, of which, however, the roles in blood cancers are quite unclear. In our study, to identify multi-function targets for B-cell cancer treatment, we reanalyzed a public transcriptomic dataset from the database of Gene Expression Omnibus, which includes CD19⁺ B-cell populations from 6 normal donors and patients of 5 CLL, 10 FL, and 8 DLBCL. After overlapping three groups (CLL vs. normal, FL vs. normal, and DLBCL vs. normal) of differentially expressed genes (DEGs), we obtained 69 common DEGs, of which 3 were validated by real-time quantitative PCR, including RIPK3, IGSF3, TGFBI. Interestingly, we found that the loss function of RIPK1 significantly increases the proliferation and viability of GM12878 cells (a normal human B lymphocyte cell line). Consistently, overexpression of RIPK1 in TMD8 and U2932 cells effectively inhibited cell proliferation and growth. More importantly, modifying RIPK1 kinase activity by a small molecule (such as necrostain-1, HOIPIN-1, etc.) alters the cell growth status of B-cell lymphoma, showing that RIPK1 exhibits anti-tumor activity in the context of B-cell lymphoma. Taken together, we consider that RIPK1 may be a potential target in the clinical application of B-cell lymphoma (including CLL, DLBCL, and FL) treatment.

Targeting RIPK1 kinase for modulating inflammation in human diseases

Receptor-Interacting Serine/Threonine-Protein Kinase 1 (RIPK1) is a master regulator of TNFR1 signaling in controlling cell death and survival. While the scaffold of RIPK1 participates in the canonical NF-κB pathway, the activation of RIPK1 kinase promotes not only necroptosis and apoptosis, but also inflammation by mediating the transcriptional induction of inflammatory cytokines. The nuclear translocation of activated RIPK1 has been shown to interact BAF-complex to promote chromatin remodeling and transcription. This review will highlight the proinflammatory role of RIPK1 kinase with focus on human neurodegenerative diseases. We will discuss the possibility of targeting RIPK1 kinase for the treatment of inflammatory pathology in human diseases.

Pore-forming proteins as drivers of membrane permeabilization in cell death pathways

Regulated cell death (RCD) relies on activation and recruitment of pore-forming proteins (PFPs) that function as executioners of specific cell death pathways: apoptosis regulator BAX (BAX), BCL-2 homologous antagonist/killer (BAK) and BCL-2-related ovarian killer protein (BOK) for apoptosis, gasdermins (GSDMs) for pyroptosis and mixed lineage kinase domain-like protein (MLKL) for necroptosis. Inactive precursors of PFPs are converted into pore-forming entities through activation, membrane recruitment, membrane insertion and oligomerization. These mechanisms involve protein-protein and protein-lipid interactions, proteolytic processing and phosphorylation. In this Review, we discuss the structural rearrangements incurred by RCD-related PFPs and describe the mechanisms that manifest conversion from autoinhibited to membrane-embedded molecular states. We further discuss the formation and maturation of membrane pores formed by BAX/BAK/BOK, GSDMs and MLKL, leading to diverse pore architectures. Lastly, we highlight commonalities and differences of PFP mechanisms involving BAX/BAK/BOK, GSDMs and MLKL and conclude with a discussion on how, in a population of challenged cells, the coexistence of cell death modalities may have profound physiological and pathophysiological implications.

Progress in Research on TLR4-Mediated Inflammatory Response Mechanisms in Brain Injury after Subarachnoid Hemorrhage

Subarachnoid hemorrhage (SAH) is one of the common clinical neurological emergencies. Its incidence accounts for about 5-9% of cerebral stroke patients. Even surviving patients often suffer from severe adverse prognoses such as hemiplegia, aphasia, cognitive dysfunction and even death. Inflammatory response plays an important role during early nerve injury in SAH. Toll-like receptors (TLRs), pattern recognition receptors, are important components of the body's innate immune system, and they are usually activated by damage-associated molecular pattern molecules. Studies have shown that with TLR 4 as an essential member of the TLRs family, the inflammatory transduction pathway mediated by it plays a vital role in brain injury after SAH. After SAH occurrence, large amounts of blood enter the subarachnoid space. This can produce massive damage-associated molecular pattern molecules that bind to TLR4, which activates inflammatory response and causes early brain injury, thus resulting in serious adverse prognoses. In this paper, the process in research on TLR4-mediated inflammatory response mechanism in brain injury after SAH was reviewed to provide a new thought for clinical treatment.