When cell death goes wrong: inflammatory outcomes of failed apoptosis and mitotic cell death.

Apoptosis is a regulated cellular pathway that ensures that a cell dies in a structured fashion to prevent negative consequences for the tissue or the organism. Dysfunctional apoptosis is a hallmark of numerous pathologies, and treatments for various diseases are successful based on the induction of apoptosis. Under homeostatic conditions, apoptosis is a non-inflammatory event, as the activation of caspases ensures that inflammatory pathways are disabled. However, there is an increasing understanding that under specific conditions, such as caspase inhibition, apoptosis and the apoptotic machinery can be re-wired into a process which is inflammatory. In this review we discuss how the death receptor and mitochondrial pathways of apoptosis can activate inflammation. Furthermore, we will highlight how cell death due to mitotic stress might be a special case when it comes to cell death and the induction of inflammation.

Voices from beyond the grave: The impact of apoptosis on the microenvironment.

Programmed cell death, in particular apoptosis, has vital functions in every healthy organism. In a highly regulated manner cells which are no longer needed or are harmful to the organism undergo suicide. More than just the mere elimination of a cell, apoptosis is increasingly being recognized performing important roles in cellular communication with the microenvironment. These interactions with surrounding cells can have various, and sometimes competing outcomes. Apoptotic cells can promote survival, proliferation and inflammation, but depending on the context also prevent survival and inflammation. In this review, we will summarize the emerging literature on how dying cells can transfer information to their neighbours, and which outcomes this communication has for the whole tissue.

Increased apoptotic sensitivity of glioblastoma enables therapeutic targeting by BH3-mimetics.

Glioblastoma (GBM) is the most prevalent malignant primary brain tumour in adults. GBM typically has a poor prognosis, mainly due to a lack of effective treatment options leading to tumour persistence or recurrence. We investigated the therapeutic potential of targeting anti-apoptotic BCL-2 proteins in GBM. Levels of anti-apoptotic BCL-xL and MCL-1 were consistently increased in GBM compared with non-malignant cells and tissue. Moreover, we found that relative to their differentiated counterparts, patient-derived GBM stem-like cells also displayed higher expression of anti-apoptotic BCL-2 family members. High anti-apoptotic BCL-xL and MCL-1 expression correlated with heightened susceptibility of GBM to BCL-2 family protein-targeting BH3-mimetics. This is indicative of increased apoptotic priming. Indeed, GBM displayed an obligate requirement for MCL-1 expression in both tumour development and maintenance. Investigating this apoptotic sensitivity, we found that sequential inhibition of BCL-xL and MCL-1 led to robust anti-tumour responses in vivo, in the absence of overt toxicity. These data demonstrate that BCL-xL and MCL-1 pro-survival function is a fundamental prerequisite for GBM survival that can be therapeutically exploited by BH3-mimetics.

Apoptotic stress-induced FGF signalling promotes non-cell autonomous resistance to cell death.

Damaged or superfluous cells are typically eliminated by apoptosis. Although apoptosis is a cell-autonomous process, apoptotic cells communicate with their environment in different ways. Here we describe a mechanism whereby cells under apoptotic stress can promote survival of neighbouring cells. We find that upon apoptotic stress, cells release the growth factor FGF2, leading to MEK-ERK-dependent transcriptional upregulation of pro-survival BCL-2 proteins in a non-cell autonomous manner. This transient upregulation of pro-survival BCL-2 proteins protects neighbouring cells from apoptosis. Accordingly, we find in certain cancer types a correlation between FGF-signalling, BCL-2 expression and worse prognosis. In vivo, upregulation of MCL-1 occurs in an FGF-dependent manner during skin repair, which regulates healing dynamics. Importantly, either co-treatment with FGF-receptor inhibitors or removal of apoptotic stress restores apoptotic sensitivity to cytotoxic therapy and delays wound healing. These data reveal a pathway by which cells under apoptotic stress can increase resistance to cell death in surrounding cells. Beyond mediating cytotoxic drug resistance, this process also provides a potential link between tissue damage and repair.

Mitochondria as multifaceted regulators of cell death.

Through their many and varied metabolic functions, mitochondria power life. Paradoxically, mitochondria also have a central role in apoptotic cell death. Upon induction of mitochondrial apoptosis, mitochondrial outer membrane permeabilization (MOMP) usually commits a cell to die. Apoptotic signalling downstream of MOMP involves cytochrome c release from mitochondria and subsequent caspase activation. As such, targeting MOMP in order to manipulate cell death holds tremendous therapeutic potential across different diseases, including neurodegenerative diseases, autoimmune disorders and cancer. In this Review, we discuss new insights into how mitochondria regulate apoptotic cell death. Surprisingly, recent data demonstrate that besides eliciting caspase activation, MOMP engages various pro-inflammatory signalling functions. As we highlight, together with new findings demonstrating cell survival following MOMP, this pro-inflammatory role suggests that mitochondria-derived signalling downstream of pro-apoptotic cues may also have non-lethal functions. Finally, we discuss the importance and roles of mitochondria in other forms of regulated cell death, including necroptosis, ferroptosis and pyroptosis. Collectively, these new findings offer exciting, unexplored opportunities to target mitochondrial regulation of cell death for clinical benefit.

BAX/BAK-Induced Apoptosis Results in Caspase-8-Dependent IL-1ß Maturation in Macrophages.

IL-1ß is a cytokine of pivotal importance to the orchestration of inflammatory responses. Synthesized as an inactive pro-cytokine, IL-1ß requires proteolytic maturation to gain biological activity. Here, we identify intrinsic apoptosis as a non-canonical trigger of IL-1ß maturation. Guided by the discovery of the immunomodulatory activity of vioprolides, cyclic peptides isolated from myxobacteria, we observe IL-1ß maturation independent of canonical inflammasome pathways, yet dependent on intrinsic apoptosis. Mechanistically, vioprolides inhibit MCL-1 and BCL2, which in turn triggers BAX/BAK-dependent mitochondrial outer membrane permeabilization (MOMP). Induction of MOMP results in the release of pro-apoptotic factors initiating intrinsic apoptosis, as well as the depletion of IAPs (inhibitors of apoptosis proteins). IAP depletion, in turn, operates upstream of ripoptosome complex formation, subsequently resulting in caspase-8-dependent IL-1ß maturation. These results establish the ripoptosome/caspase-8 complex as a pro-inflammatory checkpoint that senses the perturbation of mitochondrial integrity.

Identifying Target RNAs of PARPs.

Posttranscriptional regulation of RNA is an important component of gene expression by controlling the total amount of mRNA available for translation into protein. It involves multiple pathways including nuclear processing of mRNA and its precursors, RNA silencing, and regulation of RNA decay. Poly(ADP-ribose) polymerases (PARPs), enzymes that modify target proteins with ADP-ribose, play important roles in several RNA-regulatory pathways. RNA-binding PARPs target specific transcripts for regulation, and multiple PARPs ADP-ribosylate RNA-regulatory proteins to alter their localization, activity, or RNA binding. Additionally, RNA-binding proteins can bind directly to poly(ADP-ribose) with various effects on their function. Here we describe methods to identify and confirm specific transcripts that are regulated by PARPs.

Tight Sequestration of BH3 Proteins by BCL-xL at Subcellular Membranes Contributes to Apoptotic Resistance.

Anti-apoptotic BCL-2 family members bind to BH3-only proteins and multidomain BAX/BAK to preserve mitochondrial integrity and maintain survival. Whereas inhibition of these interactions is the biological basis of BH3-mimetic anti-cancer therapy, the actual response of membrane-bound protein complexes to these compounds is currently ill-defined. Here, we find that treatment with BH3 mimetics targeting BCL-xL spares subsets of cells with the highest levels of this protein. In intact cells, sequestration of some pro-apoptotic activators (including PUMA and BIM) by full-length BCL-xL is much more resistant to derepression than previously described in cell-free systems. Alterations in the BCL-xL C-terminal anchor that impacts subcellular membrane-targeting and localization dynamics restore sensitivity. Thus, the membrane localization of BCL-xL enforces its control over cell survival and, importantly, limits the pro-apoptotic effects of BH3 mimetics by selectively influencing BCL-xL binding to key pro-apoptotic effectors.

New directions in poly(ADP-ribose) polymerase biology.

Poly(ADP-ribose) polymerases (PARPs) regulate the function of target proteins by modifying them with ADP-ribose, a large and unique post-translational modification. Humans express 17 PARPs; however, historically, much of the focus has been on PARP1 and its function in DNA damage repair. Recent work has uncovered an amazing diversity of function for these enzymes including the regulation of fundamental physiological processes in the cell and at the organismal level, as well as new roles in regulating cellular stress responses. In this review, we discuss recent advancements in our understanding of this important protein family, and technological developments that have been critical for moving the field forward. Finally, we discuss new directions that we feel are important areas of further scientific exploration.

The p53 binding protein PDCD5 is not rate-limiting in DNA damage induced cell death.

The tumour suppressor p53 is an important mediator of cell cycle arrest and apoptosis in response to DNA damage, acting mainly by transcriptional regulation of specific target genes. The exact details how p53 modulates this decision on a molecular basis is still incompletely understood. One mechanism of regulation is acetylation of p53 on lysine K120 by the histone-acetyltransferase Tip60, resulting in preferential transcription of proapoptotic target genes. PDCD5, a protein with reported pro-apoptotic function, has recently been identified as regulator of Tip60-dependent p53-acetylation. In an effort to clarify the role of PDCD5 upon DNA damage, we generated cell lines in which PDCD5 expression was conditionally ablated by shRNAs and investigated their response to genotoxic stress. Surprisingly, we failed to note a rate-limiting role of PDCD5 in the DNA damage response. PDCD5 was dispensable for DNA damage induced apoptosis and cell cycle arrest and we observed no significant changes in p53 target gene transcription. While we were able to confirm interaction of PDCD5 with p53, we failed to do so for Tip60. Altogether, our results suggest a role of PDCD5 in the regulation of p53 function but unrelated to cell cycle arrest or apoptosis, at least in the cell types investigated.

RNA Regulation by Poly(ADP-Ribose) Polymerases.

Posttranscriptional regulation of RNA facilitates the fine-tuning of gene expression. It occurs through multiple pathways that include the nuclear processing of mRNA and its precursors, mRNA silencing, regulation of mRNA decay, and regulation of translation. Poly(ADP-ribose) polymerases (PARPs), enzymes that modify target proteins with ADP-ribose, play important roles in many of the RNA regulatory pathways through multiple mechanisms. For example, RNA-binding PARPs can target specific transcripts for regulation; ADP-ribosylation of RNA-regulatory proteins can alter their localization, activity, or RNA binding; and noncovalent interactions of RNA-binding proteins with poly(ADP-ribose) can affect their function. In addition to regulating RNA during non-stress conditions, PARPs regulate RNA function during cellular stress conditions that are critical for the proper execution of a stress response. In this review, we summarize the current knowledge regarding PARP-dependent regulation of RNAs, and describe how by modulating RNA processing, translation, and decay PARPs impact multiple processes in the cell.

Poly(ADP-ribose) polymerase-13 and RNA regulation in immunity and cancer.

Post-transcriptional regulation of RNA is an important mechanism for activating and resolving cellular stress responses. Poly(ADP-ribose) polymerase-13 (PARP13), also known as ZC3HAV1 and zinc-finger antiviral protein (ZAP), is an RNA-binding protein that regulates the stability and translation of specific mRNAs, and modulates the miRNA silencing pathway to globally affect miRNA targets. These functions of PARP13 are important components of the cellular response to stress. In addition, the ability of PARP13 to restrict oncogenic viruses and to repress the prosurvival cytokine receptor tumor necrosis factor (TNF)-related apoptosis-inducing ligand receptor 4 (TRAILR4) suggests that it can be protective against malignant transformation and cancer development. The relevance of PARP13 to human health and disease make it a promising therapeutic target.

Macrophage activation: on par with LPS.

The inflammatory response is a critical component of the immune system that is activated by stimuli such as cytokines, foreign DNA, RNA, or other harmful substances. Krukenberg et al. (2015) identify poly(ADP-ribose) as a new signaling molecule that activates inflammation, thus providing yet another mechanism by which PARPs are involved in cellular stress responses.

PARP13 regulates cellular mRNA post-transcriptionally and functions as a pro-apoptotic factor by destabilizing TRAILR4 transcript.

Poly(ADP-ribose) polymerase-13 (PARP13/ZAP/ZC3HAV1) is an antiviral factor, active against specific RNA viruses such as murine leukaemia virus, Sindbis virus and human immunodeficiency virus. During infection, PARP13 binds viral RNA via its four CCCH-type zinc-finger domains and targets it for degradation by recruiting cellular messenger RNA (mRNA) decay factors such as the exosome complex and XRN1. Here we show that PARP13 binds to and regulates cellular mRNAs in the absence of viral infection. Knockdown of PARP13 results in the misregulation of hundreds of transcripts. Among the most upregulated transcripts is TRAILR4 that encodes a decoy receptor for TRAIL-a pro-apoptotic cytokine that is a promising target for the therapeutic inhibition of cancers. PARP13 destabilizes TRAILR4 mRNA post-transcriptionally in an exosome-dependent manner by binding to a region in its 3' untranslated region. As a consequence, PARP13 represses TRAILR4 expression and increases cell sensitivity to TRAIL-mediated apoptosis, acting as a key regulator of the cellular response to TRAIL.

Haematopoietic stem cell survival and transplantation efficacy is limited by the BH3-only proteins Bim and Bmf.

Anti-apoptotic Bcl-2 family members are critical for the regulation of haematopoietic stem and progenitor cell (HSPC) survival. Little is known about the role of their pro-apoptotic antagonists, i.e. 'BH3-only' proteins, in this cell compartment. Based on the analysis of cytokine deprivation-induced changes in mRNA expression levels of Bcl-2 family proteins, we determined the consequences of BH3-only protein depletion on HSPC survival in culture and, for selected candidates, on engraftment in vivo. Thereby, we revealed a critical role for Bim and Bmf as regulators of HSPC dynamics both during early engraftment and long-term reconstitution. HSPCs derived from wild-type donors were readily displaced by Bim- or Bmf-deficient or Bcl-2-overexpressing HSPCs as early as 10 days after engraftment. Moreover, in the absence of Bim, significantly lower numbers of transplanted HSPCs were able to fully engraft radio-depleted recipients. Finally, we provide proof of principle that RNAi-based reduction of BIM or BMF, or overexpression of BCL-2 in human CD34(+) cord blood cells may be an attractive therapeutic option to increase stem cell survival and transplantation efficacy.

GSK3 TIPping off p53 to unleash PUMA.

Knowledge about "life vs. death" decisions made by p53 after DNA damage is limited but critical to preventing side effects during therapeutic application and to improve anticancer activity. Here, Charvet et al. define a signaling network that explains the protective effects of cytokines on cells exposed to γ-radiation.