Multiple roles for nuclear localization signal (NLS, aa 442–472) of receptor interacting protein 3 (RIP3)

SPOP-mediated RIPK3 destabilization desensitizes LPS/sMAC/zVAD-induced necroptotic cell death

RIPK1/RIPK3-MLKL signaling molecules are fundamental in initiating necroptotic cell death, but their roles in the development of colon cancer are unclear. This study reports that RIPK3 interacted with SPOP, a component of the E3 ligase within the Cul3 complex. This interaction leads to K48-linked ubiquitination and subsequent proteasomal degradation of RIPK3. Two distinct degron motifs, PETST and SPTST, were identified within the linker domain of RIPK3 for SPOP. RIPK3 phosphorylations at Thr403 by PIM2 and at Thr412/Ser413 by ERK2 are essential to facilitate its interaction with SPOP. Computational docking studies and immunoprecipitation analyses showed that these PIM2 and ERK2 phosphorylations bolster the stability of the RIPK3-SPOP interaction. In particular, mutations of RIPK3 at the degron motifs extended the half-life of RIPK3 by preventing its phosphorylation and subsequent ubiquitination. The deletion of SPOP, which led to increased stability of the RIPK3 protein, intensified LPS/sMAC/zVAD-induced necroptotic cell death in colon cancer cells. These findings underscore the critical role of the SPOP-mediated RIPK3 stability regulation pathway in controlling necroptotic cell death.

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.

The Pathogenesis of Necroptosis-Dependent Signaling Pathway in Cerebral Ischemic Disease

Necroptosis is the best-described form of regulated necrosis at present, which is widely recognized as a component of caspase-independent cell death mediated by the concerted action of receptor-interacting protein kinase 1 (RIPK1) and receptor-interacting protein kinase 3 (RIPK3). Mixed-lineage kinase domain-like (MLKL) was phosphorylated by RIPK3 at the threonine 357 and serine 358 residues and then formed tetramers and translocated onto the plasma membrane, which destabilizes plasma membrane integrity leading to cell swelling and membrane rupture. Necroptosis is downstream of the tumor necrosis factor (TNF) receptor family, and also interaction with NOD-like receptor pyrin 3 (NLRP3) induced inflammasome activation. Multiple inhibitors of RIPK1 and MLKL have been developed to block the cascade of signal pathways for procedural necrosis and represent potential leads for drug development. In this review, we highlight recent progress in the study of roles for necroptosis in cerebral ischemic disease and discuss how these modifications delicately control necroptosis.

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.

Necroptosis-independent signaling by the RIP kinases in inflammation

Recent advances have identified a signaling cascade involving receptor interacting protein kinase 1 (RIPK1), RIPK3 and the pseudokinase mixed lineage kinase domain-like (MLKL) that is crucial for induction of necroptosis, a non-apoptotic form of cell death. RIPK1-RIPK3-MLKL-mediated necroptosis has been attributed to cause many inflammatory diseases through the release of cellular damage-associated molecular patterns (DAMPs). In addition to necroptosis, emerging evidence suggests that these necroptosis signal adaptors can also facilitate inflammation independent of cell death. In particular, the RIP kinases can drive NF-κB and inflammasome activation independent of cell death. In this review, we will discuss recent discoveries that led to this realization and present arguments why cell death-independent signaling by the RIP kinases may have a more important role in inflammation than necroptosis.

RIP3 overexpression sensitizes human breast cancer cells to parthenolide in vitro via intracellular ROS accumulation

AIM

Receptor-interacting protein 3 (RIP3) is involved in tumor necrosis factor receptor signaling, and results in NF-κB-mediated prosurvival signaling and programmed cell death. The aim of this study was to determine whether overexpression of the RIP3 gene could sensitize human breast cancer cells to parthenolide in vitro.

METHODS

The expression of RIP3 mRNA in human breast cancer cell lines (MCF-7, MDA-MB-231, MDA-MB-435 and T47D) was detected using RT-PCR. Both MDA-MB-231 and MCF-7 cells were transfected with RIP3 expression or blank vectors via lentivirus. Cell viability was measured with MTT assay; intracellular ROS level and cell apoptosis were analyzed using flow cytometry.

RESULTS

RIP3 mRNA expression was not detected in the four human breast cancer cell lines tested. However, the transfection induced higher levels of RIP3 protein in MCF-7 and MDA-MB-231 cells. Furthermore, overexpression of RIP3 decreased the IC50 values of parthenolide from 17.6 to 12.6 μmol/L in MCF-7 cells, and from 16.6 to 9.9 μmol/L in MDA-MB-231 cells. Moreover, overexpression of RIP3 significantly increased parthenolide-induced apoptosis and ROS accumulation in MCF-7 and MDA-MB-231 cells. Pretreatment with N-acetyl-cysteine abrogated the increased sensitivity of RIP3-transfected MCF-7 and MDA-MB-231 cells to parthenolide.

CONCLUSION

Overexpression of RIP3 sensitizes MCF-7 and MDA-MB-231 breast cancer cells to parthenolide in vitro via intracellular ROS accumulation.

RIP3 expression induces a death profile change in U2OS osteosarcoma cells after 5-ALA-PDT

BACKGROUND AND OBJECTIVE

The receptor-interacting protein 3 (RIP3) has recently been outlined as a key necrosis mediator but is also thought to participate in the regulation of apoptosis. The aim of this study is to compare the cell death profile induced by 5-aminolevulic acid (5-ALA)-mediated photodynamic therapy (PDT) in the RIP3-deficient cell line U2OS and in U2OS cells in which the expression of RIP3 was restored.

MATERIALS AND METHODS

RIP3-expressing U2OS cells (RIP3-U2OS) were obtained after transfection and antibiotic selection. Wild type and RIP3-U2OS cells were treated by 5-ALA-PDT. Overall cell viability was evaluated and different parameters characteristic of apoptosis, autophagy, and necrosis were studied.

RESULTS

Surprisingly, the survival of RIP3-U2OS cells was higher compared to that of the wild type cells. In addition, RIP3-U2OS cell death was decreased by a zVAD-fmk pre-treatment. A higher cleavage of caspase-3, 7, 8, 9, and PARP was also detected in these cells, pointing out to the activation of caspase-dependent apoptosis. In parallel, a thrust of autophagy was clearly identified in the RIP3-U2OS cells. Conversely, RIP3-U2OS exhibited a lower level of necrosis than the wild types. Interestingly, necrostatin-1 efficiently decreased necrosis level in RIP3-U2OS but not in wild type cells.

CONCLUSION

Expression of RIP3 in U2OS cells led to a better survival but also to a death profile change in response to PDT. The apoptotic and autophagic pathways were clearly up-regulated compared to the RIP3-deficient wild type cells. However, induction of necrosis was weaker in the RIP3-U2OS cells. In this context, autophagy is likely to play a protective role against PDT-induced cell death and to allow a better survival of RIP3-U2OS cells. This work also highlights the important role played by RIP3 in the apoptotic pathway, although the modalities are still widely unknown.

Alternatively Expressed Genes Identified in the CD4+ T Cells of Allograft Rejection Mice

Allograft rejection is a leading cause for the failure of allotransplantation. CD4+ T cells play critical roles in this process. The identification of genes that alternatively expressed in CD4+ T cells during allograft rejection will provide critical information for studying the mechanism of allograft rejection, finding specific gene markers for monitoring, predicting allograft rejection, and opening new ways to regulate and prevent allograft rejection. Here, we established allograft and isograft transplantation models by adoptively transferring wild-type BALB/c mouse CD4+ T cells into severe combined immunodeficient (SCID) mice with a C57BL/6 or BALB/c mouse skin graft. Using the whole transcriptome sequencing-based serial analysis of gene expression (SAGE) technology, we identified 97 increasingly and 88 decreasingly expressed genes that may play important roles in allograft rejection and tolerance. Functional classification of these genes shows that apoptosis, transcription regulation, cell growth and maintenance, and signal transduction are among the frequently changed functional groups. This study provides a genome-wide view for the candidate genes of CD4+ T cells related to allotransplantation, and this report is a good resource for further microarray studies and for identifying the specific markers that are associated with clinical organ transplantations.

Identification of a self-association domain in the Ewing's sarcoma protein: a novel function for arginine-glycine-glycine rich motifs?

The Ewing's sarcoma (EWS) protein is a ubiquitously expressed RNA chaperone. The EWS protein localizes predominantly to the nucleus. Previous reports have suggested that the EWS protein is capable of dimerizing. However, to date this has not been confirmed. Here, using a novel panel of recombinant proteins, we have performed an in vitro biomolecular interaction analysis of the EWS protein. We have demonstrated that all three arginine-glycine-glycine (RGG) motifs are capable of binding directly to the survival motor neuron protein, a Tudor domain containing EWS binding partner. We have also confirmed EWS is capable of self-associating, and we have mapped this binding domain to the RGG motifs. We have also found that self-association may be required for EWS nuclear import. This is the first direct evidence of RGG domains being involved in self-association and has implications on all RGG-containing proteins.