A cytosolic heat shock protein 90 and co-chaperone p23 complex activates RIPK3/MLKL during necroptosis of endothelial cells in acute respiratory distress syndrome

Necrosis with inflammation plays a crucial role in acute respiratory distress syndrome (ARDS). Receptor-interacting protein 3 (RIPK3) regulates a newly discovered programmed form of necrosis called necroptosis. However, the underlying mechanism of necroptosis in ARDS remains unknown. Thus, the purpose of this study was to examine the possible involvement of RIPK3 in ARDS-associated necroptosis. RIPK3 protein levels were found to be significantly elevated in the plasma and bronchoalveolar lavage fluid of ARDS patients. Next, we utilised a mouse model of severe ARDS induced with high-dose lipopolysaccharide and found that lung injury was mainly due to RIPK3-mixed lineage kinase domain-like pseudokinase (MLKL)-mediated necroptosis and endothelial dysfunction. The activation of RIPK3-MLKL by tumour necrosis factor receptor 1 (TNFR1) and TNFR1-associated death domain protein (TRADD) required catalytically active RIPK1 and the inhibition of Fas-associated protein with death domain (FADD)/caspase-8 catalytic activity. We further showed that the molecular chaperone heat shock protein 90 (Hsp90)/p23, as a novel RIPK3- and MLKL-interacting complex, played an important role in RIP-MLKL-mediated necroptosis, inflammation and endothelial dysfunction in the pulmonary vasculature, which resulted in ARDS. Collectively, the results of our study indicate that necroptosis is an important mechanism of cell death in ARDS and the inhibition of necroptosis may be a therapeutic intervention for ARDS. KEY MESSAGES: Lung injury in high-dose LPS-induced severe ARDS is mainly due to RIP3-MLKL-mediated necroptosis and endothelial dysfunction. Chaperone HSP90/p23 is a novel RIP3- and MLKL-interacting complex in HPAECs. HSP90/p23 is a novel RIP3- and MLKL-interacting complex in RIP-MLKL-mediated necroptosis, inflammation and endothelial dysfunction.

EBV(LMP1)-induced metabolic reprogramming inhibits necroptosis through the hypermethylation of the RIP3 promoter

EBV infection is a recognized epigenetic driver of carcinogenesis. We previously showed that EBV could protect cancer cells from TNF-induced necroptosis. This study aims to explore the epigenetic mechanisms allowing cancer cells with EBV infection to escape from RIP3-dependent necroptosis.

Methods: Data from the TCGA database were used to evaluate the prognostic value of RIP3 promoter methylation and its expression. Western blotting, real-time PCR, and immunochemistry were conducted to investigate the relationship between LMP1 and RIP3 in cell lines and NPC tissues. BSP, MSP and hMeDIP assays were used to examine the methylation level. Induction of necroptosis was detected by cell viability assay, p-MLKL, and Sytox Green staining.

Results: RIP3 promoter hypermethylation is an independent prognostic factor of poorer disease-free and overall survival in HNSCC patients, respectively. RIP3 is down-regulated in NPC (a subtype of HNSCC). EBV(LMP1) suppresses RIP3 expression by hypermethylation of the RIP3 promoter. RIP3 protein expression was inversely correlated with LMP1 expression in NPC tissues. Restoring RIP3 expression in EBV(LMP1)-positive cells inhibits xenograft tumor growth. The accumulation of fumarate and reduction of α-KG in EBV(LMP1)-positive cells led to RIP3 silencing due to the inactivation of TETs. Decreased FH activity caused fumarate accumulation, which might be associated with its acetylation. Incubating cells with fumarate protected NPC cells from TNF-induced necroptosis.

Conclusion: These results demonstrate a pathway by which EBV(LMP1)-associated metabolite changes inhibited necroptosis signaling by DNA methylation, and shed light on the mechanism underlying EBV-related carcinogenesis, which may provide new options for cancer diagnosis and therapy.

The Role of Smurf1 in Neuronal Necroptosis after Lipopolysaccharide-Induced Neuroinflammation

The role of inflammation in neurological disorders such as Alzheimer's disease and Parkinson's disease is gradually recognized and leads to an urgent challenge. Smad ubiquitination regulatory factor 1 (Smurf1), one member of the HECT family, is up-regulated by proinflammatory cytokines and associated with apoptosis in acute spinal cord injury. However, the function of Smurf1 through promoting neuronal necroptosis is still limited in the central nervous system (CNS). Hence, we developed a neuroinflammatory model in adult rats following lipopolysaccharide (LPS) lateral ventral injection to elaborate whether Smurf1 is involved in necroptosis in CNS injury. The up-regulation of Smurf1 detected in the rat brain cortex was similar to the necroptotic marker RIP1 expression in a time-dependent manner after LPS-induced neuroinflammation. Meanwhile, Smurf1 knockdown with siRNA inhibited neuronal necroptosis following LPS-stimulated rat pheochromocytomal PC12 cells. Thus, it was indicated that LPS-induced necroptosis could be promoted by Smurf1. In short, these studies suggest that Smurf1 might promote neuronal necroptosis after LPS-induced neuroinflammation, which might act as a novel and potential molecular target for the treatment of neuroinflammation associated diseases.

Necrostatin-1 protects hippocampal neurons against ischemia/reperfusion injury via the RIP3/DAXX signaling pathway in rats

Global cerebral ischemia/reperfusion (I/R) induces selective neuronal injury in CA1 region of hippocampus, leading to severe impairment in behavior, learning and memory functions. However, the molecular mechanism underlying the processes was not elucidated clearly. RIP3 is a key molecular switch connecting apoptosis, necrosis and necroptosis. DAXX, as a novel substrate of RIP3, plays a vital role in ischemia-induced neuronal death. The aim of this study is to investigate the role and mechanism of RIP3/DAXX signaling pathway on neurons in CA1 region of the rat hippocampus after cerebral I/R. Global cerebral ischemia was induced by the method of four-vessel occlusion. RIP1 specific inhibitor Necrostatin-1 was administered by intracerebroventricular injection 1h before ischemia. Open-field, closed-field, and Morris water maze tests were performed respectively to examine the anxiety and cognitive behavior in each group. Hematoxylin and eosinstaining was used to examine the survival of hippocampal CA1 pyramidal neurons. Western blot or immunoprecipitation were carried to detect protein expression, phosphorylation, and interaction. We found that pre-treatment with Nec-1 protected locomotive ability, relieved anxiety behavior, and improved cognitive ability in the rats subjected to cerebral I/R. In addition Moreover, Nec-1 decreased significantly the dead rate of neurons in hippocampal CA1 region after cerebral I/R through suppressing RIP1-RIP3 interaction and RIP3 activation along with RIP3-DAXX interaction, and then blocked DAXX translocation from nucleaus to cytoplasm, which resulted in the inactiviation of DAXX. We concluded that pre-treatment with Nec-1 can protect neurons in the hippocampal CA1 region against ischemic damage through the RIP3-DAXX signaling pathway.

Protective mechanisms of CA074-me (other than cathepsin-B inhibition) against programmed necrosis induced by global cerebral ischemia/reperfusion injury in rats

Many studies have demonstrated the key role of lysosomes in ischemic cell death in the brain and have led to the "lysosomocentric" hypothesis. In this hypothesis, the release of cathepsin-B due to a change of lysosomal membrane permeabilization (LMP) or rupture is critical, and this can be prevented by its inhibitors CA074 and CA074-me. However, the role of CA074-me in neuronal death and its effect on the change of lysosomal membrane integrity after global cerebral ischemia/reperfusion (I/R) injury is not clear, so we investigated this here. Rat hippocampal CA1 neuronal death was evaluated after 20-min global cerebral I/R injury. CA074-me (1 μg, 10 μg) were given intracerebroventricularly 1h before ischemia or 1h post reperfusion. The changes of heat shock protein 70 (Hsp70), cathepsin-B, lysosomal-associated membrane protein 1 (LAMP-1), receptor-interacting protein 3 (RIP3), and the change of lysosomal pH were evaluated respectively. Hippocampal CA1 neuronal programmed necrosis induced by global cerebral I/R injury was prevented by CA074-me both pre-treatment and post-treatment. Diffuse cytoplasmic cathepsin-B and LAMP-1 immunostaining synchronized with the pyknotic nuclear changes 2 days post reperfusion, and a rise of lysosomal pH with the leakage of DND-153, a dye of lysosomes, after oxygen-glucose deprivation (OGD) was detected. Both of these changes demonstrated the rupture of lysosomal membrane and the leakage of cathepsin-B, and this was strongly inhibited by CA074-me pre-treatment. The overexpression and nuclear translocation of RIP3 and the reduction of NAD(+) level after I/R injury were also inhibited, while the upregulation of Hsp70 was strengthened by CA074-me pre-treatment. Delayed fulminant leakage of cathepsin-B due to lysosomal rupture is a critical harmful factor in neuronal programmed necrosis induced by 20-min global I/R injury. In addition to being an inhibitor of cathepsin-B, CA074-me may have an indirect neuroprotective effect by maintaining lysosomal membrane integrity and protecting against lysosomal rupture.

Inhibition of receptor-interacting protein 3 upregulation and nuclear translocation involved in Necrostatin-1 protection against hippocampal neuronal programmed necrosis induced by ischemia/reperfusion injury

Receptor-interacting protein 3 (RIP3) is a key molecular switch in tumor necrosis factor-induced necroptosis requiring the formation of an RIP3-RIP1 complex. We have recently shown that hippocampal cornu ammonis 1 (CA1) neuronal death induced by 20-min global cerebral ischemia/reperfusion (I/R) injury is a form of programmed necrosis. However, the mechanism behind this process is still unclear and was studied here. Global cerebral ischemia was induced by the four-vessel occlusion method and Necrostatin-1 (Nec-1), a specific inhibitor of necroptosis, was administered by intracerebroventricular injection 1h before ischemia. Normally, in the hippocampal CA1 neurons, RIP1 and RIP3 are located in the cytoplasm. However, after I/R injury, RIP3 was upregulated and translocated to the nucleus while RIP1 was not affected. Nec-1 pretreatment prevented hippocampal CA1 neuronal death and I/R induced changes in RIP3. Decreased level of NAD+ in hippocampus and the release of cathepsin-B from lysosomes after I/R injury were also inhibited by Nec-1. Our data demonstrate that Nec-1 inhibits neuronal death by preventing RIP3 upregulation and nuclear translocation, as well as NAD+ depletion and cathepsin-B release. The nuclear translocation of RIP3 has not been reported previously, so this may be an important role for RIP3 during ischemic injury.