RIPK3 activation promotes DAXX-dependent neuronal necroptosis after intracerebral hemorrhage in mice

BACKGROUND

Necroptosis induced by receptor-interacting protein kinase 3 (RIPK3) is engaged in intracerebral hemorrhage (ICH) pathology. In this study, we explored the impact of RIPK3 activation on neuronal necroptosis and the mechanism of the death domain-associated protein (DAXX)-mediated nuclear necroptosis pathway after ICH.

METHODS

Potential molecules linked to the progression of ICH were discovered using RNA sequencing. The level of DAXX was assessed by quantitative real-time PCR, ELISA, and western blotting. DAXX localization was determined by immunofluorescence and immunoprecipitation assays. The RIPK3 inhibitor GSK872 and DAXX knockdown with shRNA-DAXX were used to examine the nuclear necroptosis pathway associated with ICH. Neurobehavioral deficit assessments were performed.

RESULTS

DAXX was increased in patients and mice after ICH. In an ICH mouse model, shRNA-DAXX reduced brain water content and alleviated neurologic impairments. GSK872 administration reduced the expression of DAXX. shRNA-DAXX inhibited the expression of p-MLKL. Immunofluorescence and immunoprecipitation assays showed that RIPK3 and AIF translocated into the nucleus and then bound with nuclear DAXX.

CONCLUSIONS

RIPK3 revitalization promoted neuronal necroptosis in ICH mice, partially through the DAXX signaling pathway. RIPK3 and AIF interacted with nuclear DAXX to aggravate ICH injury.

Reovirus Type 3 Dearing Variants Do Not Induce Necroptosis in RIPK3-Expressing Human Tumor Cell Lines

Reoviruses are used as oncolytic viruses to destroy tumor cells. The concomitant induction of anti-tumor immune responses enhances the efficacy of therapy in tumors with low amounts of immune infiltrates before treatment. The reoviruses should provoke immunogenic cell death (ICD) to stimulate a tumor cell-directed immune response. Necroptosis is considered a major form of ICD, and involves receptor-interacting protein kinase 1 (RIPK1), RIPK3 and phosphorylation of mixed-lineage kinase domain-like protein (MLKL). This leads to cell membrane disintegration and the release of damage-associated molecular patterns that can activate immune responses. Reovirus Type 3 Dearing (T3D) can induce necroptosis in mouse L929 fibroblast cells and mouse embryonic fibroblasts. Most human tumor cell lines have a defect in RIPK3 expression and consequently fail to induce necroptosis as measured by MLKL phosphorylation. We used the human colorectal adenocarcinoma HT29 cell line as a model to study necroptosis in human cells since this cell line has frequently been described in necroptosis-related studies. To stimulate MLKL phosphorylation and induce necroptosis, HT29 cells were treated with a cocktail consisting of TNFα, the SMAC mimetic BV6, and the caspase inhibitor Z-VAD-FMK. While this treatment induced necroptosis, three different reovirus T3D variants, i.e., the plasmid-based reverse genetics generated virus (T3D^K), the wild-type reovirus T3D isolate R124, and the junction adhesion molecule-A-independent reovirus mutant (jin-1) failed to induce necroptosis in HT29 cells. In contrast, these viruses induced MLKL phosphorylation in murine L929 cells, albeit with varying efficiencies. Our study shows that while reoviruses efficiently induce necroptosis in L929 cells, this is not a common phenotype in human cell lines. This study emphasizes the difficulties of translating the results of ICD studies from murine cells to human cells.

[Tanshinone IIA alleviates lipopolysaccharide-induced renal tubular epithelial cell apoptosis by inhibiting RIP3/FUNDC1 signaling pathway]

OBJECTIVE

To investigate the effect of tanshinone IIA pretreatment on acute renal injury in lipopolysaccharide (LPS)-induced septic mice and explore the possible mechanism.

METHODS

Thirty C57BL/6 mice were randomized for treatment with saline (control), 10 mg/kg LPS for 24 h, or 10 mg/kg tanshinone IIA 15 min before LPS treatment. After the treatments, serum creatinine and blood urea nitrogen levels of the mice were detected, renal pathologies were observed with PAS staining, and renal expressions of RIP3, cleaved caspase-3 and p¹⁸-FUNDC1 were detected with Western blotting. In the cell experiment, cultured normal human renal tubular epithelial cells (HK-2) were treated with LPS (10 mg/mL), LPS+ siNC, LPS+ siRIP3, or LPS+tanshinone IIA (10 mg/L), and the changes in cell apoptosis were examined with TUNEL staining; Western blotting was performed to detect the expression levels of RIP3, cleaved caspase-3 and p¹⁸-FUNDC1, and qRT-PCR was used to detect the expression of RIP3 mRNA.

RESULTS

LPS challenge for 24 h significantly increased serum creatinine and blood urea nitrogen levels in the mice, caused obviously damages in the proximal renal tubules, and increased renal expressions of RIP3, cleaved caspase-3 and p¹⁸-FUNDC1 proteins. Tanshinone IIA pretreatment significantly improved LPS-induced renal injury in the mice, alleviated apoptosis of the renal cells, and inhibited the expressions of RIP3, cleaved caspase-3 and p¹⁸-FUNDC1 proteins. In HK-2 cells, LPS stimulation significantly increased the protein expressions of RIP3, cleaved caspase-3 and p¹⁸-FUNDC1 and induced obvious cell apoptosis. Pretreatment with tanshinone IIA strongly inhibited the expression of RIP3 and p¹⁸-FUNDC1 and reduced LPS-induced apoptosis of HK-2 cells.

CONCLUSION

Tanshinone IIA can reduce LPS-induced apoptosis of renal tubular epithelial cells by inhibiting RIP3/FUNDC1 signal pathway.

Receptor-Interacting Protein Kinase 3 Inhibition Relieves Mechanical Allodynia and Suppresses NLRP3 Inflammasome and NF-κB in a Rat Model of Spinal Cord Injury

Background

Neuroinflammation is critical in developing and maintaining neuropathic pain after spinal cord injury (SCI). The receptor-interacting protein kinase 3 (RIPK3) has been shown to promote inflammatory response by exerting its non-necroptotic functions. In this study, we explored the involvement of RIPK3 in neuropathic pain after SCI.

Methods

Thoracic (T10) SCI rat model was conducted, and the mechanical threshold in rats was measured. The expressions of RIPK3, nod-like receptor family pyrin domain-containing protein 3 (NLRP3), caspase-1, and nuclear factor-κB (NF-κB) were measured with western blotting analysis or quantitative real-time polymerase chain reaction (qRT-PCR). Double immunofluorescence staining was used to explore the colabeled NLRP3 with NeuN, glial fibrillary acidic protein (GFAP), and ionized calcium-binding adapter molecule 1 (IBA1). In addition, enzyme-linked immunosorbent assay (ELISA) was applied to analyze the levels of proinflammatory factors interleukin 1 beta (IL-1β), interleukin 18 (IL-18), and tumor necrosis factor alpha (TNF-α).

Results

The expression of RIPK3 was elevated from postoperative days 7–21, which was consistent with the development of mechanical allodynia. Intrathecal administration of RIPK3 inhibitor GSK872 could alleviate the mechanical allodynia in SCI rats and reduce the expression levels of RIPK3. The activation of NLRP3 inflammasome and NF-κB was attenuated by GSK872 treatment. Furthermore, immunofluorescence suggested that NLRP3 had colocalization with glial cells and neurons in the L4–L6 spinal dorsal horns. In addition, GSK872 treatment reduced the production of inflammatory cytokines.

Conclusion

Our findings indicated that RIPK3 was an important facilitated factor for SCI-induced mechanical allodynia. RIPK3 inhibition might relieve mechanical allodynia by inhibiting NLRP3 inflammasome, NF-κB, and the associated inflammation.

Analysis of necroptosis and its association with pyroptosis in organ damage in experimental pulmonary arterial hypertension

In this study, a role of cell loss due to necroptosis and its linkage with pyroptosis in organ damage under the conditions of pulmonary arterial hypertension (PAH) was examined. Monocrotaline (MCT) was used to induce PAH in Wistar rats, and depending on the severity of the disease progression, they were further divided into two subgroups: MCT group-sacrificed 4 weeks after MCT administration and ptMCT group-prematurely sacrificed due to rapid deterioration in vital functions (on Day 24,11 ± 0,7). The elevation of respiratory rate and right ventricular (RV) hypertrophy were more evident in ptMCT group, while the heart rate and cardiac haemodynamic stress markers were comparably higher in both diseased groups. Detailed immunoblotting analysis revealed that the upregulation of pThr²³¹ /Ser²³² -RIP3 proceeded into necroptosis execution in the RVs, unlike in the lungs of both PAH stages. The elevated pulmonary pThr²³¹ /Ser²³² -RIP3 levels in both PAH subgroups were associated rather with GSDMD-mediated pyroptosis. On the contrary, other inflammasome forms, such as AIM2 and NLRC4, were higher in the RV, unlike in the lungs, of diseased groups. The PAH-induced increase in the plasma RIP3 levels was more pronounced in ptMCT group, and positively correlated with RV hypertrophy, but not with haemodynamic stress. Taken together, we indicated for the first time that pThr²³¹ /Ser²³² -RIP3 upregulation resulting in two different necrosis-like cell death modes might underlie the pathomechanisms of PAH and that the plasma RIP3 might serve as an additional diagnostic and prognostic marker of cardiac injury under these conditions.

RIP3 blockade prevents immune-mediated hepatitis through a myeloid-derived suppressor cell dependent mechanism

Autoimmune hepatitis (AIH) is an immune-mediated chronic inflammatory liver disease, and its pathogenesis is not fully understood. Our previous study discovered that receptor interacting protein kinase 3 (RIP3) is correlated with serum transaminase levels in AIH patients. However, its role and underlying mechanism in AIH are poorly understood. Here, we detected the increased expression and activation of RIP3 in livers of patients and animal models with AIH. The inhibition of RIP3 kinase by GSK872 prevented concanavalin A (ConA)-induced immune-mediated hepatitis (IMH) by reduced hepatic proinflammatory cytokines and immune cells including Th17 cells and macrophages. Further experiments revealed that RIP3 inhibition resulted in an increase in CD11b⁺Gr1⁺ myeloid-derived suppressor cells (MDSCs) with immunoregulatory properties in the liver, spleen, and peripheral blood. Moreover, the depletion of Gr-1⁺ MDSCs abrogated the protective effect and immune suppression function of GSK872 in ConA-induced IMH. Altogether, our data demonstrate that RIP3 blockade prevents ConA-induced IMH through promoting MDSCs infiltration. Inhibition of RIP3 kinase may be a novel therapeutic avenue for AIH treatment.

[RIP1/RIP3-MLKL signaling pathway correlates with occurrence, progression and prognosis of chronic heart failure]

OBJECTIVE

To detect plasma levels of receptor-interacting protein kinase 1 (RIP1), RIP3 and mixed lineage kinase domain-like protein (MLKL) in patients with chronic heart failure and explore the expression pattern of programmed necrosis signaling pathway RIP1/RIP3-MLKL in the progression of heart failure.

METHODS

The patients with chronic heart failure (NYHA class Ⅱ-Ⅳ) admitted in our hospital between February, 2020 and March, 2021 were prospectively enrolled in this study, with 21 healthy volunteers as the control group. The enrolled patients included 20 with grade Ⅱ, 33 with grade Ⅲ, and 43 with grade Ⅳ cardiac function. Fasting venous blood was collected from all the participants for detecting plasma levels of RIP1, RIP3, and MLKL and protein expressions of RIP1/RIP3-MLKL pathway using enzyme-linked immunosorbent assay (ELISA) and Western blotting. The patients with grade Ⅳ cardiac function were followed up for 5 months to evaluate the clinical prognostic indicators.

RESULTS

Compared with the healthy volunteers, the patients with grade Ⅱ, Ⅲ and Ⅳ cardiac function had significantly increased plasma levels of RIP1, RIP3, and MLKL (P < 0.01), and their levels were significantly higher in grade Ⅲ/Ⅳ patients than in those with grade Ⅱ cardiac function (P < 0.01); the plasma levels of RIP1 and MLKL were significantly higher in grade Ⅳ patients than in grade Ⅲ patients (P < 0.05). The results of Western blotting also showed increased expressions of the proteins in the RIP1/RIP3-MLKL pathway in patients with heart failure. Pearson correlation analysis suggested that in patients with heart failure, the expression levels of RIP1, RIP3, and MLKL were positively correlated with SCR, AST, LVEDD and NT-proBNP (P < 0.05). Follow-up study of the patients with grade Ⅳ cardiac function showed that higher expression levels of RIP1/RIP3-MLKL were associated with a poorer prognosis of the patients.

CONCLUSION

The expressions of RIP1, RIP3 and MLKL are significantly upregulated in patients with heart failure in positive correlation with the severity of the disease condition, and the activation of the RIP1/RIP3-MLKL signaling pathway may contribute to the occurrence, development and prognosis of chronic heart failure.

RIPK3 Activates MLKL-mediated Necroptosis and Inflammasome Signaling during Streptococcus Infection

Community-acquired pneumonia is the most common type of pneumonia and remains a leading cause of morbidity and mortality worldwide. Although many different pathogens can contribute to pneumonia, Streptococcus pneumoniae is one of the common bacterial pathogens that underlie community-acquired pneumonia. RIPK3 (receptor-interacting protein kinase 3) is widely recognized as a key modulator of inflammation and cell death. To elucidate a potential role of RIPK3 in pneumonia, we examined plasma from healthy control subjects and patients positive for streptococcal pneumonia. In human studies, RIPK3 protein concentrations were significantly elevated and were identified as a potential plasma marker of pneumococcal pneumonia. To expand these findings, we used an in vivo murine model of pneumococcal pneumonia to demonstrate that RIPK3 deficiency leads to reduced bacterial clearance, severe pathological damage, and high mortality. Our results illustrated that RIPK3 forms a complex with RIPK1, MLKL (mixed-lineage kinase domain-like protein), and MCU (mitochondrial calcium uniporter) to induce mitochondrial calcium uptake and mitochondrial reactive oxygen species(mROS) production during S. pneumoniae infection. In macrophages, RIPK3 initiated necroptosis via the mROS-mediated mitochondrial permeability transition pore opening and NLRP3 inflammasome activation via the mROS-AKT pathway to protect against S. pneumoniae. In conclusion, our study demonstrated a mechanism by which RIPK3-initiated necroptosis is essential for host defense against S. pneumoniae.

Inhibition of Cardiac RIP3 Mitigates Early Reperfusion Injury and Calcium-Induced Mitochondrial Swelling without Altering Necroptotic Signalling

Receptor-interacting protein kinase 3 (RIP3) is a convergence point of multiple signalling pathways, including necroptosis, inflammation and oxidative stress; however, it is completely unknown whether it underlies acute myocardial ischemia/reperfusion (I/R) injury. Langendorff-perfused rat hearts subjected to 30 min ischemia followed by 10 min reperfusion exhibited compromised cardiac function which was not abrogated by pharmacological intervention of RIP3 inhibition. An immunoblotting analysis revealed that the detrimental effects of I/R were unlikely mediated by necroptotic cell death, since neither the canonical RIP3-MLKL pathway (mixed lineage kinase-like pseudokinase) nor the proposed non-canonical molecular axes involving CaMKIIδ-mPTP (calcium/calmodulin-dependent protein kinase IIδ-mitochondrial permeability transition pore), PGAM5-Drp1 (phosphoglycerate mutase 5-dynamin-related protein 1) and JNK-BNIP3 (c-Jun N-terminal kinase-BCL2-interacting protein 3) were activated. Similarly, we found no evidence of the involvement of NLRP3 inflammasome signalling (NOD-, LRR- and pyrin domain-containing protein 3) in such injury. RIP3 inhibition prevented the plasma membrane rupture and delayed mPTP opening which was associated with the modulation of xanthin oxidase (XO) and manganese superoxide dismutase (MnSOD). Taken together, this is the first study indicating that RIP3 regulates early reperfusion injury via oxidative stress- and mitochondrial activity-related effects, rather than cell loss due to necroptosis.

Changes in plasma levels of RIPK1, RIPK3, and MLKL in patients with coronary atherosclerotic heart disease and its clinical predictive value

OBJECTIVES

Coronary atherosclerotic heart disease (CHD) is caused by coronary atherosclerosis, which leads to stenosis and even occlusion of the lumen, resulting in myocardial ischemia, and necrosis subsequently. Its prevalence has been high for a long time. The prevention and treatment of CHD are important. The study aimed to investigate the role of plasma levels of receptor-interacting protein kinase 1 (RIPK1), receptor-interacting protein kinase 3 (RIPK3), and mixed-lineage kinase domain-like protein (MLKL) in patients with CHD and its clinical predictive value.

METHODS

A total of 190 patients with CHD who were diagnosed by coronary angiography and 70 healthy subjects in cardiovascular department from September 2015 to May 2017 were enrolled in this study. Patients with CHD were assigned into 4 groups: Patients with stable angina pectoris (SAP, n=46), patients with unstable angina pectoris (UAP, n=56), patients with non-ST-segment elevation myocardial infarction (NSTEMI, n=42), and patients with ST-segment elevation myocardial infarction (STEMI, n=46). Patients with CHD were assigned into a single-vessel lesion group, a double-vessel lesion group, and a multi-vessel lesion group according to the results of coronary angiography, and the severity of coronary artery stenosis was determined by Gensini score. Plasma levels of RIPK1, RIPK3, and MLKL were measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

The plasma levels of RIPK1, RIPK3, and MLKL in patients with CHD were significantly higher than those in the controls (P<0.05). The plasma levels of RIPK1, RIPK3, and MLKL in the UAP group were significantly higher than those in the SAP group (P<0.05). The plasma levels of RIPK1, RIPK3, and MLKL in NSTEMI and STEMI group were significantly higher than those in the UAP group (P<0.05). There was no significant difference between the NSTEMI group and STEMI group (P>0.05). The plasma levels of RIPK1, RIPK3 and MLKL were significantly increased with numbers of coronary artery lesions (P<0.05), which were positively correlated with Gensini scores. The multivariate logistic regression analysis showed that plasma levels of RIPK1, RIPK3, and MLKL were independent risk factors for severe coronary artery stenosis.The average period of follow-up was 24 months after hospital discharge. The patients were divided into 2 groups according to whether they had major adverse cardiovascular events (MACE). Compared with patient without MACE, patient with MACE had higher levels of RIPK1, RIPK3, and MLKL (P<0.05). Receiver operator characteristic (ROC) curve analysis showed that the area under curve of RIPK1 was 0.72 (P<0.001), the area under curve of RIPK3 was 0.83 (P<0.001), and the area under curve of MLKL was 0.75 (P<0.001).

CONCLUSIONS

Plasma levels of RIPK1, RIPK3, and MLKL are closely related to CHD, and they have predictive value for the prognosis evaluation for patients with CHD.

Receptor-Interacting Protein Kinase 3 (RIPK3) mRNA Levels Are Elevated in Blood Mononuclear Cells of Patients with Poor Prognosis of Acute-on-Chronic Hepatitis B Liver Failure

Necroptosis refers to a programmed form of necrosis, which involves the receptor-interacting protein kinase 3 (RIPK3) and mixed lineage kinase domain-like protein (MLKL). In this study, to investigate the role of necroptosis in the pathogenesis of acute-on-chronic hepatitis B liver failure (ACHBLF), we retrospectively analyzed 122 patients with ACHBLF, 131 patients with chronic hepatitis B (CHB), and 35 healthy controls (HCs). Using quantitative real-time polymerase chain reaction (RT-qPCR), we measured RIPK3 mRNA levels in peripheral blood mononuclear cells (PBMCs). ELISA was performed to measure the serum levels of MLKL, TNF-α and caspase-8. We found that RIPK3 mRNA levels were significantly higher in patients with ACHBLF than those with CHB or HCs. RIPK3 mRNA levels in patients with ACHBLF were positively correlated with serum levels of TNF-α or MLKL and negatively correlated with caspase-8 levels. Univariate and multivariate analysis revealed that RIPK3 mRNA level was predictive of 3-month mortality of ACHBLF. The area under receiver operating characteristic curve (AUC) of RIPK3 mRNA levels was 0.810 (95% CI: 0.729-0.876), which was higher than that of MELD scores (0.766, 95% CI: 0.681-0.838). The optimal cut-off point of 8.81 was determined for RIPK3 mRNA levels, which showed a sensitivity of 80.7% and a negative predictive value of 80.4%. These results indicate that elevated RIPK3 mRNA levels in PBMCs are associated with poor prognosis of ACHBLF. We thus propose that necroptosis may play an important role in pathogenesis of ACHBLF.

A common variant of RIP3 promoter region is associated with poor prognosis in heart failure patients by influencing SOX17 binding

Receptor‐interacting protein kinase 3 (RIP3) is a key determinant of necroptosis and participates in ischaemia—and oxidative stress‐induced necroptosis, myocardial remodelling and heart failure (HF). In this study, we tested the hypothesis that common variants in RIP3 gene were associated with the risk and prognosis of HF in the Chinese Han population. By re‐sequencing and luciferase assays, we identified a common functional variant in the RIP3 promoter region. The rs3212247‐T allele suppressed RIP3 promoter activity by facilitating transcription factor SOX17 binding, but not the C allele. We further recruited 2961 control participants and 3194 HF patients who underwent a mean follow‐up of 19 months (6‐31 months) for this study. Rs3212247 and another missense variant rs3212254 were genotyped. Although rs3212247 did not significantly associate with increased risk of HF (odds ratio = 1.00, 95% CI = 0.92‐1.08, P = 0.91), it raised the risk for cardiovascular death and cardiac transplantation (hazard ratio = 1.47, 95% CI = 1.13‐1.91, P = 0.004). Moreover, participants carrying the rs3212247 CC genotype had higher plasma levels of RIP3 than those carrying the TT or TC genotype (p for trend = 0.02) in New York Heart Association class III HF group. No association was found between the RIP3 missense variant rs3212254 and risk or prognosis of HF after adjustment for traditional risk factors. In conclusion, genetic variant in RIP3 promoter region is associated with increased RIP3 transcription, thus contributed to the poor prognosis of HF patients. Clinical Trial Registration: https://www.clinicaltrials.gov/ct2/show/NCT03461107?term=03461107&cond=Heart+Failure&cntry=CN&rank=1. Unique identifier: NCT03461107.

Frontline Science: Staphylococcus aureus promotes receptor-interacting protein kinase 3- and protease-dependent production of IL-1β in human neutrophils

Microbial infection elicits robust immune responses that initially depend on polymorphonuclear neutrophils (PMN), which ingest and kill invading bacteria. However, community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) remain viable within PMN and prompt their lysis with concomitant release of damage-associated molecular patterns and proinflammatory cytokines that promote additional inflammation. Here, we show that ultrapure human PMN (>99.8% pure) that have ingested CA-MRSA released interleukin (IL)-1β but not IL-18. The ingested CA-MRSA needed to be viable, and phagocytosis alone was insufficient to stimulate IL-1β secretion from PMN fed CA-MRSA. In contrast to PMN response to the canonical NLRP3 inflammasome agonist nigericin, IL-1β secretion by PMN fed CA-MRSA occurred independently of NLRP3 inflammasome or caspase-1 activation and required instead active receptor-interacting protein kinase 3 (RIPK3) but not RIPK1. Furthermore, inhibition of neutrophil serine proteases blocked pro-IL-1β cleavage in PMN fed CA-MRSA. Taken together, our data suggest that with respect to secretion of IL-1β and IL-18, PMN differ from human macrophages and exhibit agonist-specific responses. After phagocytosis of CA-MRSA, human PMN secreted IL-1β through a previously unrecognized mechanism dependent on RIPK3 and serine proteases but independent of canonical NLRP3 inflammasome and caspase-1 activation.

Is Receptor-Interacting Protein Kinase 3 a Viable Therapeutic Target for Mycobacterium tuberculosis Infection?

The dwindling list of antimicrobial agents exhibiting broad efficacy against clinical strains of Mycobacterium tuberculosis (Mtb) has forced the medical community to redefine current approaches to the treatment of tuberculosis (TB). Host receptor-interacting protein kinase 3 (RIPK3) has been flagged recently as a potential target, given that it is believed to regulate necroptosis-independent signaling pathways, which have been implicated in exacerbating several inflammatory conditions and which reportedly play a role in the necrosis of Mtb-infected macrophages. To examine the therapeutic potential of inhibiting RIPK3, we infected RIPK3-deficient mice with aerosolized Mtb. We found that the loss of RIPK3 did not alter overall disease outcomes, with deficient animals harboring similar bacterial numbers in the lungs and spleens compared to their wild-type counterparts. Mtb-infected macrophages were not rescued from dying by Ripk3 deletion, nor did this affect production of the pro-inflammatory cytokine IL-1β, both in vitro and in vivo. Infiltration of immune cells into the lungs, as well as the activation of adaptive immunity, similarly was not overtly affected by the loss of RIPK3 signaling. Collectively, our data argue against a role of RIPK3 in mediating pathological inflammation or macrophage necrosis during Mtb disease pathogenesis and thus suggest that this host protein is unlikely to be an attractive therapeutic target for TB.

Receptor-Interacting Protein Kinases 1 and 3, and Mixed Lineage Kinase Domain-Like Protein Are Activated by Sublytic Complement and Participate in Complement-Dependent Cytotoxicity

The complement system participates in the pathogenesis of many diseases. Complement activation produces several active protein complexes and peptides, including the terminal C5b-9 complexes. It was reported that C5b-9 complexes insert into the plasma membrane and cause membrane perturbation, intracellular calcium surge, metabolic depletion, and osmotic lysis. Previously, we showed that complement-dependent cytotoxicity (CDC) is regulated by JNK and Bid. Here, we demonstrate that three mediators in TNFα-induced necroptosis (regulated necrosis), the receptor-interacting protein kinases, receptor-interacting protein kinase 1 (RIPK1) and receptor-interacting protein kinase 3 (RIPK3), and mixed-lineage kinase domain-like protein (MLKL), are activated by complement and contribute to CDC. Cell treatment with necrostatin-1 (Nec-1), a RIPK1 inhibitor, GSK’872, a RIPK3 inhibitor, or necrosulfonamide and GW806742X, MLKL inhibitors, restrain CDC. These findings were confirmed by using specific siRNAs targeting the synthesis of these proteins. Mouse fibroblasts lacking RIPK3 or MLKL were found to be less sensitive to C5b-9 than were wild-type (WT) fibroblasts. Enhanced CDC was achieved by RIPK1 or RIPK3 overexpression but not by the overexpression of a RHIM-RIPK1 mutant nor by a kinase-dead RIPK3 mutant. Nec-1 reduces the CDC of WT but not of RIPK3-knockout fibroblasts. Cells treated with a sublytic dose of complement exhibit co-localization of RIPK3 with RIPK1 in the cytoplasm and co-localization of RIPK3 and MLKL with C5b-9 at the plasma membrane. Data supporting cooperation among the RIP kinases, MLKL, JNK, and Bid in CDC are presented. These results provide a deeper insight into the cell death process activated by complement and identify potential points of cross talk between complement and other inducers of inflammation and regulated necrosis.

Receptor-interacting protein kinase 3 mediates macrophage/monocyte activation in autoimmune hepatitis and regulates interleukin-6 production

Background

The mechanisms of macrophages/monocytes in autoimmune hepatitis (AIH) remain unclear. We investigated the role of receptor-interacting protein kinase 3 (RIP3), a key inflammatory signal adapter, in macrophage/monocyte activation in AIH.

Methods

Liver tissues and monocytes from patients were collected to evaluate the relationship between macrophage activation and RIP3 by double-immunofluorescence and Western blotting. RAW264.7 macrophages were used to study the regulation of RIP3 signaling on inflammatory cytokines.

Results

Compared to the hepatic cyst, the majority of accumulated macrophages expressed RIP3 in AIH liver tissues. Moreover, RIP3 expression of monocytes was correlated with the levels of serum hepatic enzyme in AIH. Furthermore, RIP3 signaling was activated by lipopolysaccharide in RAW264.7 macrophages, which was accompanied with upregulated interleukin (IL)-1β, IL-6, and IL-10 and downregulated IL-4 and transforming growth factor-β. Notably, necrostatin-1, the specific inhibitor of the RIP3 signaling pathway, and 6-thioguanine (6-TG), the active metabolite of azathioprine, predominantly reduced IL-6 production compared to other cytokines. Moreover, the gene level of IL-6 was dramatically increased in AIH liver tissues.

Conclusions

RIP3 signaling is involved in macrophage/monocyte activation in AIH and mediates IL-6 production, and is a novel molecular mechanism of 6-TG, indicating that it might be a promising therapeutic target for AIH treatment.

Receptor-interacting protein kinase 3 promotes platelet activation and thrombosis

Previous studies have shown that receptor-interacting protein kinase 3 (RIP3) is involved in many important biological processes, including necroptosis, apoptosis, and inflammation. Here we show that RIP3 plays a critical role in regulating platelet functions and in vivo thrombosis and hemostasis. Tail bleeding times were significantly longer in RIP3-knockout (RIP3^-/-) mice compared with their wild-type (WT) littermates. In an in vivo model of arteriole thrombosis, mice lacking RIP3 exhibited prolonged occlusion times. WT mice repopulated with RIP3^-/- bone marrow-derived cells had longer occlusion times than RIP3^-/- mice repopulated with WT bone marrow-derived cells, suggesting a role for RIP3-deficient platelets in arterial thrombosis. Consistent with these findings, we observed that RIP3 was expressed in both human and mice platelets. Deletion of RIP3 in mouse platelets caused a marked defect in aggregation and attenuated dense granule secretion in response to low doses of thrombin or a thromboxane A₂ analog, U46619. Phosphorylation of Akt induced by U46619 or thrombin was diminished in RIP3^-/- platelets. Moreover, RIP3 interacted with Gα₁₃ Platelet spreading on fibrinogen and clot retraction were impaired in the absence of RIP3. RIP3 inhibitor dose-dependently inhibited platelet aggregation in vitro and prevented arterial thrombus formation in vivo. These data demonstrate a role for RIP3 in promoting in vivo thrombosis and hemostasis by amplifying platelet activation. RIP3 may represent a novel promising therapeutic target for thrombotic diseases.

Receptor-interacting protein kinase 3-mediated programmed cell necrosis in rats subjected to focal cerebral ischemia-reperfusion injury

In the current study, the activation of tumor necrosis factor-α receptor 1 (TNFR1) and receptor-interacting protein kinase 3 (RIP3) were investigated following cerebral ischemia-reperfusion injury (CIRI). Healthy SD rats were randomly divided into 3 groups: Sham operation group, model group and inhibitor group. The model group and inhibitor group were further divided into 4 subgroups of 6, 12, 24 and 72 h following CIRI. Using right middle cerebral artery embolization, the CIRI model was generated. To confirm that the CIRI model was established, neurological scores, TTC staining and brain water content measurements were conducted. Immunohistochemistry and western blotting were conducted to investigate the expression of TNFR1 and RIP3 in the cerebral cortex. It was observed that nerve cell necrosis occurred following 6 h of CIRI. The appearance of necrotic cells was gradually increased with increasing CIRI duration. TNFR1 and RIP3 were positively expressed following 6 h of CIRI. With increasing durations of CIRI, the protein expression levels of TNFR1 and RIP3 were significantly increased. Pre‑administration with Z-VAD-FMK (zVAD) significantly increased the protein level of RIP3, however, had no effect on the levels of TNFR1, and was accompanied by a reduction in necrosis. In conclusion, RIP3‑mediated cell necrosis was enhanced by caspase blockade zVAD and the function of zVAD was independent of TNFR1 signaling following IR.