The Gβγ-Src signaling pathway regulates TNF-induced necroptosis via control of necrosome translocation

Anti-necroptosis and anti-ferroptosis compounds from the Deep-Sea-Derived fungus Aspergillus sp. MCCC 3A00392

Eight undescribed (1-8) and 46 known compounds (9-54) were isolated from the deep-sea-derived Aspergillus sp. MCCC 3A00392. Compounds 1-3 were three novel oxoindolo diterpenoids, 4-6 were three bisabolane sesquiterpenoids, while 7 and 8 were two monocyclic cyclopropanes. Their structures were established by exhaustive analyses of the HRESIMS, NMR, and theoretical calculations of the NMR data and ECD spectra. Compounds 10, 33, 38, and 39 were able to inhibit tumor necrosis factor (TNF)-induced necroptosis in murine L929 cell lines. Functional experiments verified that compounds 10 and 39 inhibited necroptosis by downregulating the phosphorylation of RIPK3 and MLKL. Moreover, compound 39 also reduced the phosphorylation of RIPK1. Compounds 10, 33, and 34 displayed potent inhibitory activities against RSL-3 induced ferroptosis with the EC50 value of 3.0 μM, 0.4 μM, and 0.1 μM, respectively. Compound 10 inhibited ferroptosis by the downregulation of HMOX1, while compounds 33 and 34 inhibited ferroptosis through regulation of NRF2/SLC7A11/GCLM axis. However, these compounds only showed weak effect in either the necroptosis or ferroptosis relative mouse disease models. Further studies of pharmacokinetics and pharmacodynamics might improve their in vivo bioactivities.

Saracatinib inhibits necroptosis and ameliorates psoriatic inflammation by targeting MLKL

Necroptosis is a kind of programmed cell death that causes the release of damage-associated molecular patterns and inflammatory disease including skin inflammation. Activation of receptor-interacting serine/threonine kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL) is the hallmark of tumour necrosis factor α (TNF)-induced necroptosis. Here, we screened a small-molecule compound library and found that saracatinib inhibited TNF-induced necroptosis. By targeting MLKL, Saracatinib interfered with the phosphorylation, translocation, and oligomerization of MLKL induced by TNF. Consistently, mutation of the saracatinib-binding site of MLKL reduced the inhibitory effect of saracatinib on TNF-induced necroptosis. In an imiquimod (IMQ)-induced psoriasis mouse model, saracatinib effectively blocked MLKL phosphorylation and inflammatory responses in vivo. Taken together, these findings indicate that saracatinib inhibits necroptosis by targeting MLKL, providing a potential therapeutic approach for skin inflammation-related diseases such as psoriasis.

Chemical Constituents of the Deep-sea Gammarid Shrimp-Derived Fungus Penicillium citrinum XIA-16

One new alkaloid, (S)-2-acetamido-4-(2-(methylamino)phenyl)-4-oxobutanoic acid (1), was isolated from the deep-sea-derived Penicillium citrinum XIA-16, together with 25 known compounds including ten polyketones (2-11), eight alkaloids (12-19), six steroids (20-25), and a fatty acid (26). Their planar and relative structures were determined by an analysis of 1D and 2D nuclear magnetic resonance (NMR) as well as high resolution electrospray ionization mass spectroscopy (HR-ESI-MS) data. The absolute configuration of 1 was determined by comparison of the experimental and calculated electronic circular dichroism (ECD) spectra. Penicitrinol B (6) significantly inhibited RSL3-induced ferroptosis (EC50 =2.0 μM) by reducing lipid peroxidation and heme oxygenase 1 (HMOX1) expression. Under the concentration of 10 μM, penicitrinol A (7) was able to inhibit cuproptosis with the cell viabilities of 68.2 % compared to the negative control (copper and elesclomol) with the cell viabilities of 14.8 %.

SIRPα maintains macrophage homeostasis by interacting with PTK2B kinase in Mycobacterium tuberculosis infection and through autophagy and necroptosis

Background

To determine whether SIRPα can be a diagnostic marker of pulmonary tuberculosis (PTB) and the molecular mechanism of SIRPα regulating macrophages to kill Mycobacterium tuberculosis (MTB).

Methods

Meta-analysis combined with subsequent qRT-PCR, western-blotting and flow cytometry assay were used to detect SIRPα expression in PTB patients. Cell-based assays were used to explore the regulation of macrophage function by SIRPα. SIRPα^−/- and wide type macrophages transplanted C57BL/6J mice were used to determine the function of SIRPα on MTB infection in vivo.

Findings

SIRPα levels are closely correlated with the treatment outcomes among PTB patients. Cell-based assay demonstrated that MTB significantly induces the expression of SIRPα on macrophages. SIRPα deficiency enhances the killing ability of macrophages against MTB through processes that involve enhanced autophagy and reduced necroptosis of macrophages. Mechanistically, SIRPα forms a direct interaction with PTK2B through its intracellular C-terminal domain, thus inhibiting PTK2B activation in macrophages. Necroptosis inhibition due to SIRPα deficiency requires PTK2B activity. The transfer of SIRPα-deficient bone marrow-derived macrophages (BMDMs) into wild type mice resulted in a drop of bacterial load in the lungs but an enhancement of inflammatory lung damage, and the combination of ulinastatin and SIRPα^−/−→WT treatment could decrease the inflammation and maintain the bactericidal capacity.

Interpretation

Our data define SIRPα a novel biomarker for tuberculosis infection and underlying mechanisms for maintaining macrophage homeostasis.

Funding

This work was financially supported by the Chinese National Natural Science Foundation project (No.81401635). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Regulation of survivin protein stability by USP35 is evolutionarily conserved

Survivin is the key component of the chromosomal passenger complex and plays important roles in the regulation of cell division. Survivin has also been implicated in the regulation of apoptosis and tumorigenesis. Although the survivin protein has been reported to be degraded by a ubiquitin/proteasome-dependent mechanism, whether there is a DUB that is involved in the regulation of its protein stability is largely unknown. Using an expression library containing 68 deubiquitinating enzymes, we found that ubiquitin-specific-processing protease 35 (USP35) regulates survivin protein stability in an enzymatic activity-dependent manner. USP35 interacted with and promoted the deubiquitination of the survivin protein. USP38, an ortholog of USP35 encoded by the human genome, is also able to regulate survivin protein stability. Moreover, we found that the deubiquitinating enzyme DUBAI, the Drosophila homolog of human USP35, is able to regulate the protein stability of Deterin, the Drosophila homolog of survivin. Interestingly, USP35 also regulated the protein stability of Aurora B and Borealin which are also the component of the chromosomal passenger complex. By regulating protein stabilities of chromosomal passenger complex components, USP35 regulated cancer cell proliferation. Taken together, our work uncovered an evolutionarily conserved relationship between USP35 and survivin that might play an important role in cell proliferation.

CAMK2/CaMKII activates MLKL in short-term starvation to facilitate autophagic flux

MLKL (mixed lineage kinase domain like pseudokinase) is a well-known core component of necrosome that executes necroptotic cell death upon phosphorylation by RIPK3 (receptor interacting serine/threonine kinase 3). Recent studies also implicate a role of MLKL in endosomal trafficking, which is not always dependent on RIPK3. Using mouse Neuro-2a and L929 as well as human HEK293 and HT29 cells, we show here that MLKL is phosphorylated in response to serum and amino acid deprivation from the culture medium, in a manner that depends on CAMK2/CaMKII (calcium/calmodulin dependent protein kinase II) but not RIPK3. The starvation-induced increase in MLKL phosphorylation was accompanied by decreases in levels of lipidated MAP1LC3B/LC3B (microtubule associated protein 1 light chain 3 beta; LC3-II) and SQSTM1/p62 (sequestosome 1), markers of autophagosomes. These changes were prevented by disrupting either MLKL or CAMK2 by pharmacology and genetic manipulations. Moreover, disrupting MLKL or CAMK2 also inhibited the incorporation of LC3-II into autolysosomes, demonstrating a role of the CAMK2-MLKL pathway in facilitating autophagic flux during short-term starvation, in contrast to necroptosis which suppressed autophagic flux. Furthermore, unlike the necroptotic pathway, the starvation-evoked CAMK2-mediated MLKL phosphorylation protected cells from starvation-induced death. We propose that upon nutrient deprivation, MLKL is activated by CAMK2, which in turn facilitates membrane scission needed for autophagosome maturation, allowing the proper fusion of the autophagosome with lysosome and the subsequent substance degradation. This novel function is independent of RIPK3 and is not involved in necroptosis, implicating new roles for this pseudokinase in cell survival, signaling and metabolism.

Abbreviations: CAMK2/CaMKII: calcium/calmodulin dependent protein kinase II; DIABLO/SMAC: direct inhibitor of apoptosis-binding protein with low pI/second mitochondria-derived activator of caspase; ECS: extracellular solution; ESCRT: endosomal sorting complexes required for transport; FBS: fetal bovine serum; GSK3B: glycogen synthase kinase 3 beta; HBSS: Hanks’ balanced salt solution; KO: knockout; LC3-II: lipidated microtubule associated protein 1 light chain 3 beta; LDH: lactate dehydrogenase; MLKL: mixed lineage kinase domain like pseudokinase; MTOR: mechanistic target of rapamycin kinase; MTORC1: MTOR complex 1; N2a: Neuro-2a neuroblastoma; Nec-1: necrostatin-1; NSA: necrosulfonamide; PBS: phosphate-buffered saline; PI: propidium iodide; PK-hLC3: pHluorin-mKate2-human LC3; RIPK1: receptor interacting serine/threonine kinase 1; RIPK3: receptor interacting serine/threonine kinase 3; ROS: reactive oxygen species; RPS6KB1/S6K: ribosomal protein S6 kinase B1; shRNA: short hairpin RNA; siRNA: small interference RNA; SQSTM1/p62: sequestosome 1; TBS: Tris-buffered saline; TNF/TNF-α: tumor necrosis factor; TSZ, treatment with TNF + DIABLO mimetics + z-VAD-FMK.

Self-stabilizing regulation of deubiquitinating enzymes in an enzymatic activity-dependent manner

Deubiquitinating enzymes (DUBs) play important roles in many physiological and pathological processes by modulating the ubiquitination of their substrates. DUBs undergo post-translational modifications including ubiquitination. However, whether DUBs can reverse their own ubiquitination and regulate their own protein stability requires further investigation. To answer this question, we screened an expression library of DUBs and their enzymatic activity mutants and found that some DUBs regulated their own protein stability in an enzymatic activity- and homomeric interaction-dependent manner. Taking Ubiquitin-specific-processing protease 29 (USP29) as an example, we found that USP29 deubiquitinates itself and protects itself from proteasomal degradation. We also revealed that the N-terminal region of USP29 is critical for its protein stability. Taken together, our work demonstrates that at least some DUBs regulate their own ubiquitination and protein stability. Our findings provide novel molecular insight into the diverse regulation of DUBs.

PTPRA facilitates cancer growth and migration via the TNF-α-mediated PTPRA-NF-κB pathway in MCF-7 breast cancer cells

Protein tyrosine phosphatase receptor type A (PTPRA), one of the classic protein tyrosine phosphatases, is crucial for modulating tumorigenesis and metastasis in breast cancer; however, its functional mechanism has not fully elucidated. The present study assessed PTPRA expression and estimated its clinical impact on survival using the Gene Expression Profiling Interactive Analysis database (GEPIA). Growth curves, colony formations and Transwell assays were utilized to examine cell proliferation and migration. Additionally, luciferase reporter assays were used to examine the potential tumor signaling pathways targeted by PTPRA in HEK293T cells. Furthermore, quantitative PCR (qPCR) was utilized to confirm the transcriptional regulation of PTPRA expression. Bioinformatic analyses of data from GEPIA identified PTPRA overexpression in patients with breast cancer. The growth curve, colony formation and transwell experiments demonstrated that PTPRA upregulation significantly promoted the cell proliferation and migration of MCF-7 breast cancer cells. In contrast, PTPRA knockdown significantly attenuated cell proliferation and migration. Mechanistic experiments revealed that the transcriptional activity of NF-κB was higher compared with other classic tumor pathways when they were activated by PTPRA in HEK293T cells. Furthermore, the transcriptional activity of NF-κB was altered in a PTPRA-dose-dependent manner. Additionally, following exposure to TNF-α, PTPRA-deficient MCF-7 cells exhibited lower NF-κB transcriptional activity compared with normal control cells. The results of the present study demonstrate that PTPRA overexpression accelerates inflammatory tumor phenotypes in breast cancer and that the TNF-α-mediated PTPRA-NF-κB pathway may offer novel insight into early diagnosis and optimum treatment for breast cancer.

Targeting RIPK3 oligomerization blocks necroptosis without inducing apoptosis

Receptor-interacting serine/threonine-protein kinase 3 (RIPK3) is a central protein in necroptosis with great potential as a target for treating necroptosis-associated diseases, such as Crohn's disease. However, blockade of RIPK3 kinase activity leads to unexpected RIPK3-initiated apoptosis. Herein, we found that PP2, a known SRC inhibitor, inhibits TNF-α-induced necroptosis without initiating apoptosis. Further investigation showed that PP2 acts as an inhibitor of not only SRC but also RIPK3. PP2 does not disturb the integrity of the RIPK1-RIPK3-mixed lineage kinase domain-like pseudokinase (MLKL) necroptosome or the autophosphorylation of RIPK3 at T231/S232 but disrupts RIPK3 oligomerization, thereby impairing the phosphorylation and oligomerization of MLKL. These results demonstrate the essential role of RIPK3 oligomerization in necroptosis and suggest a potential RIPK3 oligomerization-targeting strategy for therapeutic development.

Current translational potential and underlying molecular mechanisms of necroptosis

Cell death has a fundamental impact on the evolution of degenerative disorders, autoimmune processes, inflammatory diseases, tumor formation and immune surveillance. Over the past couple of decades extensive studies have uncovered novel cell death pathways, which are independent of apoptosis. Among these is necroptosis, a tightly regulated, inflammatory form of cell death. Necroptosis contribute to the pathogenesis of many diseases and in this review, we will focus exclusively on necroptosis in humans. Necroptosis is considered a backup mechanism of apoptosis, but the in vivo appearance of necroptosis indicates that both caspase-mediated and caspase-independent mechanisms control necroptosis. Necroptosis is regulated on multiple levels, from the transcription, to the stability and posttranslational modifications of the necrosome components, to the availability of molecular interaction partners and the localization of receptor-interacting serine/threonine-protein kinase 1 (RIPK1), receptor-interacting serine/threonine-protein kinase 3 (RIPK3) and mixed lineage kinase domain-like protein (MLKL). Accordingly, we classified the role of more than seventy molecules in necroptotic signaling based on consistent in vitro or in vivo evidence to understand the molecular background of necroptosis and to find opportunities where regulating the intensity and the modality of cell death could be exploited in clinical interventions. Necroptosis specific inhibitors are under development, but >20 drugs, already used in the treatment of various diseases, have the potential to regulate necroptosis. By listing necroptosis-modulated human diseases and cataloging the currently available drug-repertoire to modify necroptosis intensity, we hope to kick-start approaches with immediate translational potential. We also indicate where necroptosis regulating capacity should be considered in the current applications of these drugs.

Programmed necrosis and its role in management of breast cancer

Breast cancer is one of the major causes of cancer related deaths in women worldwide. A major factor responsible for treatment failure in breast cancer is the development of resistance to commonly used chemotherapeutic drugs leading to disease relapse. Several studies have shown dysregulation of molecular machinery of apoptosis, the major programmed cell death pathway in breast malignancies. Thus, there is an unmet need to search for an alternative cell death pathway which can work when apoptosis is compromised. Necroptosis or programmed necrosis is a relatively recently described entity which has attracted attention in this context. Classically, even in physiological conditions necroptosis is found to act if apoptosis is not functional due to some reason. Recently, more and more studies are being conducted in different malignancies to explore the possibility and utility of inducing cell death by necroptosis. The present review describes the key molecular players involved in necroptotic pathway and their status in breast cancer. In addition, the research done to utilize this pathway for treatment of breast cancer has also been highlighted.

The deubiquitinating enzyme OTUD1 antagonizes BH3-mimetic inhibitor induced cell death through regulating the stability of the MCL1 protein

Background

Myeloid cell leukaemia 1 (MCL1) is a pro-survival Bcl-2 family protein that plays important roles in cell survival, proliferation, differentiation and tumourigenesis. MCL1 is a fast-turnover protein that is degraded via an ubiquitination/proteasome-dependent mechanism. Although several E3 ligases have been discovered to promote the ubiquitination of MCL1, the deubiquitinating enzyme (DUB) that regulates its stability requires further investigation.

Methods

The immunoprecipitation was used to determine the interaction between OTUD1 and MCL1. The ubiquitination assays was performed to determine the regulation of MCL1 by OTUD1. The cell viability was used to determine the regulation of BH3-mimetic inhibitor induced cell death by OTUD1. The survival analysis was used to determine the relationship between OTUD1 expression levels and the survival rate of cancer patients.

Results

By screening a DUB expression library, we determined that the deubiquitinating enzyme OTUD1 regulates MCL1 protein stability in an enzymatic-activity dependent manner. OTUD1 interacts with MCL1 and promotes its deubiquitination. Knockdown of OTUD1 increases the sensitivity of tumour cells to the BH3-mimetic inhibitor ABT-263, while overexpression of OTUD1 increases tumour cell tolerance of ABT-263. Furthermore, bioinformatics analysis data reveal that OTUD1 is a negative prognostic factor for liver cancer, ovarian cancer and specific subtypes of breast and cervical cancer.

Conclusions

The deubiquitinating enzyme OTUD1 antagonizes BH3-mimetic inhibitor induced cell death through regulating the stability of the MCL1 protein. Thus, OTUD1 could be considered as a therapeutic target for curing these cancers.

Nuclear RIPK3 and MLKL contribute to cytosolic necrosome formation and necroptosis

Necroptotic signaling converges in the assembly of a cytosolic signaling platform, the necrosome, with the activation of its downstream effector, MLKL. RIPK1 and RIPK3, key components of the necrosome, act as signaling intermediates for the activation of MLKL. We report that RIPK3 and MLKL continuously shuttle between the nucleus and the cytoplasm, whereas RIPK1 is constitutively present in both compartments. During TNF-induced necroptosis, nuclear RIPK1 becomes ubiquitinated, after which nuclear MLKL becomes phosphorylated and oligomerized. Pharmacological inhibition of the nuclear export machinery leads to retention of RIPK3 and MLKL in the nucleus, prevents the nucleation of cytosolic RIPK3/MLKL oligomerization, and reduces cell death. Our results suggest that passage of necroptotic signaling components through the nucleus is a mechanism for regulating cytosolic necrosome formation and consequently necroptotic cell death.

Kathrin Weber et al. report that the necrosome components RIPK3 and MLKL constitutively shuttle between the nucleus and cytoplasm. They find that increasing ratios of nuclear:cytosolic RIPK3 and MLKL prevents necrotic cell death, suggesting a mechanism by which the cell regulates necrosome formation and death.

Ubiquitin-Specific Protease USP6 Regulates the Stability of the c-Jun Protein

The c-Jun gene encodes a transcription factor that has been implicated in many physiological and pathological processes. c-Jun is a highly unstable protein that is degraded through a ubiquitination/proteasome-dependent mechanism. However, the deubiquitinating enzyme (DUB) that regulates the stability of the c-Jun protein requires further investigation. Here, by screening a DUB expression library, we identified ubiquitin-specific protease 6 (USP6) and showed that it regulates the stability of the c-Jun protein in a manner depending on its enzyme activity. USP6 interacts with c-Jun and antagonizes its ubiquitination. USP6 overexpression upregulates the activity of the downstream signaling pathway mediated by c-Jun/AP-1 and promotes cell invasion. Moreover, many aberrant genes that are upregulated in USP6 translocated nodular fasciitis are great potential targets regulated by c-Jun. Based on our data, USP6 is an enzyme that deubiquitinates c-Jun and regulates its downstream cellular functions.

DEPDC7 inhibits cell proliferation, migration and invasion in hepatoma cells

DEP Domain Containing 7 (DEPDC7) is highly and specifically expressed in normal liver tissue, belonging to the class of genes of liver-selective cell communication. Although the function of DEPDC7 remains poorly understood, its expression is decreased in liver cancer compared with normal liver tissues. It has previously been demonstrated that knockdown of DEPDC7 promotes cell growth, S phase entry and cell mobility and invasion in HepG2 cells. In the present study, it was shown that DEPDC7 expression is downregulated in four hepatoma cell lines (SMMC-7721, Huh-7, SK-Hep-1 and HepG2) and 48 hepatoma tissues, determined using western blot and immunohistochemical analysis. When DEPDC7 is overexpressed in hepatoma cell lines (SK-Hep-1 and Huh-7), it inhibits cell proliferation and cell growth; inhibits cell cycle entry; and inhibits cell motility and invasion. These results, together with the results of knockdown experiments, demonstrate that DEPDC7 may have an important role in hepatoma cells growth and metastasis and suggest it could be a therapeutic target; however, in vitro studies are required to validate this hypothesis.

RIP1 autophosphorylation is promoted by mitochondrial ROS and is essential for RIP3 recruitment into necrosome

Necroptosis is a type of programmed cell death with great significance in many pathological processes. Tumour necrosis factor-α(TNF), a proinflammatory cytokine, is a prototypic trigger of necroptosis. It is known that mitochondrial reactive oxygen species (ROS) promote necroptosis, and that kinase activity of receptor interacting protein 1 (RIP1) is required for TNF-induced necroptosis. However, how ROS function and what RIP1 phosphorylates to promote necroptosis are largely unknown. Here we show that three crucial cysteines in RIP1 are required for sensing ROS, and ROS subsequently activates RIP1 autophosphorylation on serine residue 161 (S161). The major function of RIP1 kinase activity in TNF-induced necroptosis is to autophosphorylate S161. This specific phosphorylation then enables RIP1 to recruit RIP3 and form a functional necrosome, a central controller of necroptosis. Since ROS induction is known to require necrosomal RIP3, ROS therefore function in a positive feedback circuit that ensures effective induction of necroptosis.

Mitochondrial reactive oxygen species (ROS) promote necroptosis and the receptor interacting protein 1 (RIP1) is a key player in this form of cell death. Here, the authors show that cysteine residues in RIP1 sense ROS and oxidation of the cysteines triggers RIP1 autophosphorylation, which promotes functional necrosome formation.

Cisplatin-induced necroptosis in TNFα dependent and independent pathways

Cisplatin is a chemotherapeutic drug for treatment of many solid tumors. It has been shown to induce apoptosis and/or necrosis in different types of cancer cells. However, the underlying mechanisms remain elusive. In this study, we provide evidences that cisplatin induces necroptosis in receptor-interacting protein 3 (RIP3)-expressing cell lines, but not in cell lines lacking RIP3 protein expression. Deficiency of core components of necroptotic pathway, RIP1, RIP3, or mixed lineage kinase domain-like protein (MLKL) blocked cisplatin-induced cell death in L929 cells. This phenomenon is dependent on RIP1/RIP3/MLKL necrosome formation and translocation to mitochondria-associated membrane (MAM), but only partially via autocrine production of tumor necrosis factor α (TNFα). Moreover, we demonstrate that the mitochondrial permeability transition pore opening (mPTP) opening and reactive oxygen species (ROS) generation is a critical downstream event of the formation of necrosome in cisplatin-induced necroptosis, which is TNFα independent. Deficiency of cyclophilin-D (CypD) partially reduced cisplatin-induced cell death, indicating CypD mediated-mPTP opening plays an important role during cisplatin-induced necroptosis. Both deletion of CypD and TNFα completely blocked cisplatin-induced cell death, suggesting that cisplatin could induce necroptosis through TNFα dependent and independent pathway. These findings provide new insight into the molecular mechanisms underlying cisplatin-induced necroptosis.

Construction and Identification of the RNAi Recombinant Lentiviral Vector Targeting Human DEPDC7 Gene

Human DEP domain containing 7 (DEPDC7) gene was originally found expressing high in liver tissue and low in most other tissues, but its function was largely unknown. In this study, we construct an RNA interference (RNAi) recombinant lentiviral vector particle targeting DEPDC7 in order to knockdown its gene expression in human hepatocellular carcinoma cell line HepG2. We screened three RNAi sequences targeting DEPDC7 and a scramble sequence by the aid of short hairpin RNAs (shRNA) design tools. Then, these sequences were separately cloned into the pLV-H1-EF1α-puro vector to construct four lentiviral vectors (pshRNA-DEPDC7-NC, pshRNA-DEPDC7-RNAi1, pshRNA-DEPDC7-RNAi2 and pshRNA-DEPDC7-RNAi3). All of the recombinant plasmids were identified and confirmed by double digestion and DNA sequencing. After infecting HepG2 cells, the DEPDC7 mRNA and protein expression levels were examined by real-time PCR and western blot, respectively, and the gene expression was significantly down-regulated at both levels (P < 0.01). Cell motility and invasiveness were detected by Matrigel migration and invasion assay, and the results revealed that migration and invasion of HepG2 cells were significantly increased (P < 0.05). Our study showed successful construction of three lentiviral RNAi vectors targeting DEPDC7 gene and shRNA-mediated knockdown of DEPDC7 enable promotion of cell migration and invasion.

Post-translational control of RIPK3 and MLKL mediated necroptotic cell death

Several programmed lytic and necrotic-like cell death mechanisms have now been uncovered, including the recently described receptor interacting protein kinase-3 (RIPK3)-mixed lineage kinase domain-like (MLKL)-dependent necroptosis pathway. Genetic experiments have shown that programmed necrosis, including necroptosis, can play a pivotal role in regulating host-resistance against microbial infections. Alternatively, excess or unwarranted necroptosis may be pathological in autoimmune and autoinflammatory diseases. This review highlights the recent advances in our understanding of the post-translational control of RIPK3-MLKL necroptotic signaling. We discuss the critical function of phosphorylation in the execution of necroptosis, and highlight the emerging regulatory roles for several ubiquitin ligases and deubiquitinating enzymes. Finally, based on current evidence, we discuss the potential mechanisms by which the essential, and possibly terminal, necroptotic effector, MLKL, triggers the disruption of cellular membranes to cause cell lysis.

Ppm1b negatively regulates necroptosis through dephosphorylating Rip3

The auto-phosphorylation of murine receptor-interacting protein 3 (Rip3) on Thr 231 and Ser 232 in the necrosome is required to trigger necroptosis. However, how Rip3 phosphorylation is regulated is still largely unknown. Here we identified protein phosphatase 1B (Ppm1b) as a Rip3 phosphatase and found that Ppm1b restricts necroptosis in two settings: spontaneous necroptosis caused by Rip3 auto-phosphorylation in resting cells, and tumour necrosis factor-α (TNF)-induced necroptosis in cultured cells. We revealed that Ppm1b selectively suppresses necroptosis through the dephosphorylation of Rip3, which then prevents the recruitment of mixed lineage kinase domain-like protein (Mlkl) to the necrosome. We further showed that Ppm1b deficiency (Ppm1b(d/d)) in mice enhanced TNF-induced death in a Rip3-dependent manner, and the role of Ppm1b in inhibiting necroptosis was evidenced by elevated Rip3 phosphorylation and tissue damage in the caecum of TNF-treated Ppm1b(d/d) mice. These data indicate that Ppm1b negatively regulates necroptosis through dephosphorylating Rip3 in vitro and in vivo.

The role of necroptosis in neurosurgical diseases

Programmed necrosis or necroptosis is an alternative form of cell death that is executed through a caspase-independent pathway. Necroptosis has been implicated in many pathological conditions. Genetic or pharmacological inhibition of necroptotic signaling has been shown to confer neuroprotection after traumatic and ischemic brain injury. Therefore, the necroptotic pathway represents a potential target for neurological diseases that are managed by neurosurgeons. In this review, we summarize recent advances in the understanding of necroptotic signaling pathways and explore the role of necroptotic cell death in craniocerebral trauma, brain tumors, and cerebrovascular diseases.

Emerging Roles for RIPK1 and RIPK3 in Pathogen-Induced Cell Death and Host Immunity

Receptor-interacting protein kinases 1 and 3 (RIPK1 and RIPK3 ) are homologous serine-threonine kinases that were recognized for their roles in directing programmed necrotic cell death or necroptosis under a broad range of pathologic settings. Emerging evidence suggests new physiologic roles for RIPK1 and RIPK3 in mediating cell death of innate immune responses. Our review discusses current evidence on the mechanisms and the impact of RIPK1- and/or RIPK3-dependent cell death in responses to a variety of viral and bacterial pathogens. Furthermore, the discussion also summarizes emerging roles for RIPK1 and RIPK3 in other facets of host immunity, including the maintenance of epithelial barrier function and pro-inflammatory processes that may, in some cases, manifest independent of cell death. Finally, we briefly consider the therapeutic opportunities in targeting RIPK1- and RIPK3-dependent processes in infection and immunity.