Necrostatin: a potentially novel cardioprotective agent?

The in vivo evidence for regulated necrosis

Necrosis is a hallmark of several widespread diseases or their direct complications. In the past decade, we learned that necrosis can be a regulated process that is potentially druggable. RIPK3- and MLKL-mediated necroptosis represents by far the best studied pathway of regulated necrosis. During necroptosis, the release of damage-associated molecular patterns (DAMPs) drives a phenomenon referred to as necroinflammation, a common consequence of necrosis. However, most studies of regulated necrosis investigated cell lines in vitro in a cell autonomous manner, which represents a non-physiological situation. Conclusions based on such work might not necessarily be transferrable to disease states in which synchronized, non-cell autonomous effects occur. Here, we summarize the current knowledge of the pathophysiological relevance of necroptosis in vivo, and in light of this understanding, we reassess the morphological classification of necrosis that is generally used by pathologists. Along these lines, we discuss the paucity of data implicating necroptosis in human disease. Finally, the in vivo relevance of non-necroptotic forms of necrosis, such as ferroptosis, is addressed.

RGMb protects against acute kidney injury by inhibiting tubular cell necroptosis via an MLKL-dependent mechanism

Tubular cell necrosis is a key histological feature of acute kidney injury (AKI). Necroptosis is a type of programed necrosis, which is executed by mixed lineage kinase domain-like protein (MLKL) upon its binding to the plasma membrane. Emerging evidence indicates that necroptosis plays a critical role in the development of AKI. However, it is unclear whether renal tubular cells undergo necroptosis in vivo and how the necroptotic pathway is regulated during AKI. Repulsive guidance molecule (RGM)-b is a member of the RGM family. Our previous study demonstrated that RGMb is highly expressed in kidney tubular epithelial cells, but its biological role in the kidney has not been well characterized. In the present study, we found that RGMb reduced membrane-associated MLKL levels and inhibited necroptosis in cultured cells. During ischemia/reperfusion injury (IRI) or oxalate nephropathy, MLKL was induced to express on the apical membrane of proximal tubular (PT) cells. Specific knockout of Rgmb in tubular cells (Rgmb cKO) increased MLKL expression at the apical membrane of PT cells and induced more tubular cell death and more severe renal dysfunction compared with wild-type mice. Treatment with the necroptosis inhibitor Necrostatin-1 or GSK'963 reduced MLKL expression on the apical membrane of PT cells and ameliorated renal function impairment after IRI in both wild-type and Rgmb cKO mice. Taken together, our results suggest that proximal tubular cell necroptosis plays an important role in AKI, and that RGMb protects against AKI by inhibiting MLKL membrane association and necroptosis in proximal tubular cells.

Exogenous hydrogen sulfide protects human umbilical vein endothelial cells against high glucose‑induced injury by inhibiting the necroptosis pathway

Hyperglycemia is a key factor in the development of diabetic complications, including the processes of atherosclerosis. Receptor-interacting protein 3 (RIP3), a mediator of necroptosis, is implicated in atherosclerosis development. Additionally, hydrogen sulfide (H₂S) protects the vascular endothelium against hyperglycemia-induced injury and attenuates atherosclerosis. On the basis of these findings, the present study aimed to confirm the hypothesis that necroptosis mediates high glucose (HG)-induced injury in human umbilical vein endothelial cells (HUVECs), and that the inhibition of necroptosis contributes to the protective effect of exogenous H₂S against this injury. The results revealed that exposure of HUVECs to 40 mM HG markedly enhanced the expression level of RIP3, along with multiple injuries, including a decrease in cell viability, an increase in the number of apoptotic cells, an increase in the expression level of cleaved caspase-3, generation of reactive oxygen species (ROS), as well as dissipation of the mitochondrial membrane potential (MMP). Treatment of the cells with sodium hydrogen sulfide (NaHS; a donor of H₂S) prior to exposure to HG significantly attenuated the increased RIP3 expression and the aforementioned injuries by HG. Notably, treatment of cells with necrostatin-1 (Nec-1), an inhibitor of necroptosis, prior to exposure to HG ameliorated the HG-induced injuries, leading to a decrease in ROS generation and a loss of MMP. However, pre-treatment of the cells with Nec-1 enhanced the HG-induced increase in the expression levels of cleaved caspases-3 and -9. By contrast, pre-treatment with Z-VAD-FMK, a pan-caspase inhibitor, promoted the increased expression of RIP3 by HG. Taken together, the findings of the present study have demonstrated, to the best of our knowledge for the first time, that exogenous H₂S protects HUVECs against HG-induced injury through inhibiting necroptosis. The present study has also provided novel evidence that there is a negative interaction between necroptosis and apoptosis in the HG-treated HUVECs.

Necroptosis: A novel manner of cell death, associated with stroke (Review)

Cell death is indispensable in the physiology, pathology, growth, development, senility and death of an organism. In recent years, the identification of a highly regulated form of necrosis, known as necroptosis, has challenged the traditional concept of necrosis and apoptosis, which are two major modes of cell death. This novel manner of cell death is similar in form to necrosis in terms of morphological features, and it can also be regulated in a caspase‑independent manner. Therefore, necroptosis can be understood initially as a combination of necrosis and apoptosis. The mechanism of its regulation, induction and inhibition is complicated, and involves a range of molecular expression and regulation. According to the recent literature, necroptosis takes place in the physiological regulatory processes of an organism and is involved in the occurrence, development and prognosis of a variety of diseases that have a necrosis phenotype, including neurodegenerative diseases, ischemic disease, hemorrhagic disease, inflammation and viral infectious diseases. In the present review, the features, molecular mechanism and identification of necroptosis under pathological conditions are discussed, with particular emphasis on its association with stroke.

Neuroprotective Effect of β-Caryophyllene on Cerebral Ischemia-Reperfusion Injury via Regulation of Necroptotic Neuronal Death and Inflammation: In Vivo and in Vitro

Necrotic cell death is a hallmark feature of ischemic stroke and it may facilitate inflammation by releasing intracellular components after cell-membrane rupture. Previous studies reported that β-caryophyllene (BCP) mitigates cerebral ischemia-reperfusion (I/R) injury, but the underlying mechanism remains unclear. We explored whether BCP exerts a neuroprotective effect in cerebral I/R injury through inhibiting necroptotic cell death and inflammation. Primary neurons with and without BCP (0.2, 1, 5, 25 μM) treatment were exposed to oxygen-glucose deprivation and re-oxygenation (OGD/R). Neuron damage, neuronal death type and mixed lineage kinase domain-like (MLKL) protein expression were assessed 48 h after OGD/R. Furthermore, mice underwent I/R procedures with or without BCP (8, 24, 72 mg/kg, ip.). Neurologic dysfunction, cerebral infarct volumes, cell death, cytokine levels, necroptosis core molecules, and HMGB1-TLR4 signaling were determined at 48 h after I/R. BCP (5 μM) significantly reduced necroptotic neurons and MLKL protein expression following OGD/R. BCP (24, 72 mg/kg, ip.) reduced infarct volumes, neuronal necrosis, receptor-interaction protein kinase-1 (RIPK1), receptor-interaction protein kinase-3 (RIPK3) expression, and MLKL phosphorylation after I/R injury. BCP also decreased high-mobility group box 1 (HMGB1), toll-like receptor 4 (TLR4), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α) levels. Thus, BCP alleviates ischemic brain damage potentially by inhibiting necroptotic neuronal death and inflammatory response. This study suggests a novel application for BCP as a neuroprotective agent.

Ripk3 induces mitochondrial apoptosis via inhibition of FUNDC1 mitophagy in cardiac IR injury

Ripk3-required necroptosis and mitochondria-mediated apoptosis are the predominant types of cell death that largely account for the development of cardiac ischemia reperfusion injury (IRI). Here, we explored the effect of Ripk3 on mitochondrial apoptosis. Compared with wild-type mice, the infarcted area in Ripk3-deficient (Ripk3-/-) mice had a relatively low abundance of apoptotic cells. Moreover, the loss of Ripk3 protected the mitochondria against IRI and inhibited caspase9 apoptotic pathways. These protective effects of Ripk3 deficiency were relied on mitophagy activation. However, inhibition of mitophagy under Ripk3 deficiency enhanced cardiomyocyte and endothelia apoptosis, augmented infarcted area and induced microvascular dysfunction. Furthermore, ischemia activated mitophagy by modifying FUNDC1 dephosphorylation, which substantively engulfed mitochondria debris and cytochrome-c, thus blocking apoptosis signal. However, reperfusion injury elevated the expression of Ripk3 which disrupted FUNDC1 activation and abated mitophagy, increasing the likelihood of apoptosis. In summary, this study confirms the promotive effect of Ripk3 on mitochondria-mediated apoptosis via inhibition of FUNDC1-dependent mitophagy in cardiac IRI. These findings provide new insight into the roles of Ripk3-related necroptosis, mitochondria-mediated apoptosis and FUNDC1-required mitophagy in cardiac IRI.

Necroptosis: A Novel Cell Death Modality and Its Potential Relevance for Critical Care Medicine

Cell death is intertwined with life in development, homeostasis, pathology, and aging. Until recently, apoptosis was the best known form of programmed cell death, whereas necrosis was for a long time considered accidental owing to physicochemical injury. However, identification of crucial signaling and execution molecules, which are highly regulated, revealed that necrosis encompasses several cell death modalities that can be therapeutically targeted. The best understood form of regulated necrosis is necroptosis, which is transduced by the kinase activities of receptor interacting protein kinase-1 and receptor interacting protein kinase-3, eventually leading to the activation of mixed lineage kinase domain-like and plasma membrane permeabilization. We are only beginning to appreciate the role of necroptosis in different pathological conditions, including critical illnesses. In this review, we discuss the molecular mechanisms of necroptosis and analyze the effect of inhibiting necroptosis in experimental models of critical illnesses. In view of the identification of an increasing number of cell death modalities, we also briefly discuss the simultaneous targeting of multiple cell death modalities because, depending on the cell type and cellular conditions, various types of cell death may contribute to the pathology.

Initiation and execution mechanisms of necroptosis: an overview

Necroptosis is a form of regulated cell death, which is induced by ligand binding to TNF family death domain receptors, pattern recognizing receptors and virus sensors. The common feature of these receptor systems is the implication of proteins, which contain a receptor interaction protein kinase (RIPK) homology interaction motif (RHIM) mediating recruitment and activation of receptor-interacting protein kinase 3 (RIPK3), which ultimately activates the necroptosis executioner mixed lineage kinase domain-like (MLKL). In case of the TNF family members, the initiator is the survival- and cell death-regulating RIPK1 kinase, in the case of Toll-like receptor 3/4 (TLR3/4), a RHIM-containing adaptor, called TRIF, while in the case of Z-DNA-binding protein ZBP1/DAI, the cytosolic viral sensor itself contains a RHIM domain. In this review, we discuss the different protein complexes that serve as nucleation platforms for necroptosis and the mechanism of execution of necroptosis. Transgenic models (knockout, kinase-dead knock-in) and pharmacologic inhibition indicate that RIPK1, RIPK3 or MLKL are implicated in many inflammatory, degenerative and infectious diseases. However, the conclusion of necroptosis being solely involved in the etiology of diseases is blurred by the pleiotropic roles of RIPK1 and RIPK3 in other cellular processes such as apoptosis and inflammasome activation.

A novel damage mechanism: Contribution of the interaction between necroptosis and ROS to high glucose-induced injury and inflammation in H9c2 cardiac cells

Recently, a novel mechanism known as 'programmed necrosis' or necroptosis has been shown to be another important mechanism of cell death in the heart. In this study, we investigated the role of necroptosis in high glucose (HG)-induced injury and inflammation, as well as the underlying mechanisms. In particular, we focused on the interaction between necroptosis and reactive oxygen species (ROS) in H9c2 cardiac cells. Our results demonstrated that the exposure of H9c2 cardiac cells to 35 mM glucose (HG) markedly enhanced the expression level of receptor-interacting protein 3 (RIP3), a kinase which promotes necroptosis. Importantly, co-treatment of the cells with 100 µM necrostatin-1 (a specific inhibitor of necroptosis) and HG for 24 h attenuated not only the increased expression level of RIP3, but also the HG-induced injury and inflammation, as evidenced by an increase in cell viability, a decrease in ROS generation, the attenuation of the dissipation of mitochondrial membrane potential and a decrese in the secretion levels of inflammatory cytokines, i.e., interleukin (IL)-1β and tumor necrosis factor (TNF)-α. Furthermore, treatment of the cells with 1 mM N-acetyl‑L‑cysteine (a scavenger of ROS) for 60 min prior to exposure to HG significantly reduced the HG-induced increase in the RIP3 expression level, as well as the injury and inflammatory response described above. Taken together, the findings of this study clearly demonstrate a novel damage mechanism involving the positive interaction between necroptosis and ROS attributing to HG-induced injury and inflammation in H9c2 cardiac cells.

Suppression of autophagic flux contributes to cardiomyocyte death by activation of necroptotic pathways

BACKGROUND

The role of necroptosis in myocardial injury has not been fully characterized. Here we examined roles of mitochondrial permeability transition pore (mPTP) and autophagy in necroptosis of cardiomyocytes.

METHODS AND RESULTS

In H9c2 cells, necroptosis was induced by treatment with TNF-α (TNF) and z-VAD-fmk (zVAD) for 24h, and necroptotic death was determined by LDH release (as % of total). TNF/zVAD increased LDH release from 16.6±4.3% to 60.6±2.7%, and the LDH release was suppressed by necrostatin-1 (29.4±4.0%), a RIP1 inhibitor, and by siRNA-mediated knockdown of RIP3 (27.7±2.0%), confirming RIP1-RIP3-dependent necroptosis. TNF/zVAD-induced necroptosis was not attenuated by mPTP inhibitors or GSK-3β inhibitors. TNF/zVAD increased LC3-II level, but the change was not further enhanced by bafilomycin A1. The increase of LC3-II by TNF/zVAD was associated with suppression of both autophagic flux and LC3-LAMP1 co-localization. TNF/zVAD did not modify phosphorylation of Akt, p70s6K, AMPK, ULK1 or VASP but significantly increased RIP1-p62 binding and conversely reduced p62-LC3 binding. Rapamycin inhibited RIP1-p62 and RIP1-RIP3 interactions induced by TNF/zVAD and partly restored autophagic flux and suppressed LDH release in TNF/zVAD-treated cells. The effect of rapamycin on LDH release was reduced by knockdown of Atg5 expression. Knockdown of p62 by siRNA augmented LDH release by TNF/zVAD.

CONCLUSION

Suppression of autophagic flux contributes to RIP1-RIP3 interaction and necroptosis of cardiomyocytes, and sequestration of p62 from its interaction with LC3-II by p62-RIP1 interaction possibly underlies the suppressed autophagy. The mPTP is unlikely to play a major role in execution of necroptosis in cardiomyocytes.

Evidence of necroptosis in hearts subjected to various forms of ischemic insults

Long-lasting ischemia can result in cell loss; however, repeated episodes of brief ischemia increase the resistance of the heart against deleterious effects of subsequent prolonged ischemic insult and promote cell survival. Traditionally, it is believed that the supply of blood to the ischemic heart is associated with release of cytokines, activation of inflammatory response, and induction of necrotic cell death. In the past few years, this paradigm of passive necrosis as an uncontrolled cell death has been re-examined and the existence of a strictly regulated form of necrotic cell death, necroptosis, has been documented. This controlled cell death modality, resembling all morphological features of necrosis, has been investigated in different types of ischemia-associated heart injuries. The process of necroptosis has been found to be dependent on the activation of RIP1-RIP3-MLKL axis, which induces changes leading to the rupture of cell membrane. This pathway is activated by TNF-α, which has also been implicated in the cardioprotective signaling pathway of ischemic preconditioning. Thus, this review is intended to describe the TNF-α-mediated signaling leading to either cell survival or necroptotic cell death. In addition, some experimental data suggesting a link between heart dysfunction and the cellular loss due to necroptosis are discussed in various conditions of myocardial ischemia.

Pore-forming toxin-mediated ion dysregulation leads to death receptor-independent necroptosis of lung epithelial cells during bacterial pneumonia

We report that pore-forming toxins (PFTs) induce respiratory epithelial cell necroptosis independently of death receptor signaling during bacterial pneumonia. Instead, necroptosis was activated as a result of ion dysregulation arising from membrane permeabilization. PFT-induced necroptosis required RIP1, RIP3 and MLKL, and could be induced in the absence or inhibition of TNFR1, TNFR2 and TLR4 signaling. We detected activated MLKL in the lungs from mice and nonhuman primates experiencing Serratia marcescens and Streptococcus pneumoniae pneumonia, respectively. We subsequently identified calcium influx and potassium efflux as the key initiating signals responsible for necroptosis; also that mitochondrial damage was not required for necroptosis activation but was exacerbated by MLKL activation. PFT-induced necroptosis in respiratory epithelial cells did not involve CamKII or reactive oxygen species. KO mice deficient in MLKL or RIP3 had increased survival and reduced pulmonary injury during S. marcescens pneumonia. Our results establish necroptosis as a major cell death pathway active during bacterial pneumonia and that necroptosis can occur without death receptor signaling.

Effects of necrostatin-1, an inhibitor of necroptosis, and its inactive analogue Nec-1i on basal cardiovascular function

Inhibition of receptor-interacting serine/threonine-protein kinase 1 (RIP1) by necrostatin-1 (Nec-1) alleviates cardiac injury due to prevention of necroptotic cell death. Its inactive analogue necrostatin-1i (Nec-1i), lacking RIP1 activity, serves as a suitable control. It is unknown if these agents influence the heart function in the absence of damaging stimuli. For this purpose, we measured intraarterial blood pressure (systolic - sBP and diastolic - dBP) and ECG parameters after a bolus administration of Nec-1 and Nec-1i in rats during 30 min. Nec-1, unlike Nec-1i, increased sBP and dBP, as well as heart rate reaching the peak at 20 min. The P wave duration tended to be decreased and the duration of the PR interval was shortened by Nec-1 indicating faster conduction of the impulses through atria to the ventricles. The drugs did not influence the QTc interval duration and no episode of ventricular arrhythmia was observed. In summary, Nec-1 temporarily modulates blood pressure and electrical function of the healthy heart. These effects of Nec-1 are likely due to its off-target action or RIP1 has an important role in the regulation of cardiovascular function independently of its action on the necroptotic pathway.

Increased hepatic receptor interacting protein kinase 3 expression due to impaired proteasomal functions contributes to alcohol-induced steatosis and liver injury

Chronic alcohol exposure increased hepatic receptor-interacting protein kinase (RIP) 3 expression and necroptosis in the liver but its mechanisms are unclear. In the present study, we demonstrated that chronic alcohol feeding plus binge (Gao-binge) increased RIP3 but not RIP1 protein levels in mouse livers. RIP3 knockout mice had decreased serum alanine amino transferase activity and hepatic steatosis but had no effect on hepatic neutrophil infiltration compared with wild type mice after Gao-binge alcohol treatment. The hepatic mRNA levels of RIP3 did not change between Gao-binge and control mice, suggesting that alcohol-induced hepatic RIP3 proteins are regulated at the posttranslational level. We found that Gao-binge treatment decreased the levels of proteasome subunit alpha type-2 (PSMA2) and proteasome 26S subunit, ATPase 1 (PSMC1) and impaired hepatic proteasome function. Pharmacological or genetic inhibition of proteasome resulted in the accumulation of RIP3 in mouse livers. More importantly, human alcoholics had decreased expression of PSMA2 and PSMC1 but increased protein levels of RIP3 compared with healthy human livers. Moreover, pharmacological inhibition of RIP1 decreased Gao-binge-induced hepatic inflammation, neutrophil infiltration and NF-κB subunit (p65) nuclear translocation but failed to protect against steatosis and liver injury induced by Gao-binge alcohol. In conclusion, results from this study suggest that impaired hepatic proteasome function by alcohol exposure may contribute to hepatic accumulation of RIP3 resulting in necroptosis and steatosis while RIP1 kinase activity is important for alcohol-induced inflammation.

Active MLKL triggers the NLRP3 inflammasome in a cell-intrinsic manner

Necroptosis is a physiological cell suicide mechanism initiated by receptor-interacting protein kinase-3 (RIPK3) phosphorylation of mixed-lineage kinase domain-like protein (MLKL), which results in disruption of the plasma membrane. Necroptotic cell lysis, and resultant release of proinflammatory mediators, is thought to cause inflammation in necroptotic disease models. However, we previously showed that MLKL signaling can also promote inflammation by activating the nucleotide-binding oligomerization domain (NOD)-like receptor protein 3 (NLRP3) inflammasome to recruit the adaptor protein apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (ASC) and trigger caspase-1 processing of the proinflammatory cytokine IL-1β. Here, we provide evidence that MLKL-induced activation of NLRP3 requires (i) the death effector four-helical bundle of MLKL, (ii) oligomerization and association of MLKL with cellular membranes, and (iii) a reduction in intracellular potassium concentration. Although genetic or pharmacological targeting of NLRP3 or caspase-1 prevented MLKL-induced IL-1β secretion, they did not prevent necroptotic cell death. Gasdermin D (GSDMD), the pore-forming caspase-1 substrate required for efficient NLRP3-triggered pyroptosis and IL-1β release, was not essential for MLKL-dependent death or IL-1β secretion. Imaging of MLKL-dependent ASC speck formation demonstrated that necroptotic stimuli activate NLRP3 cell-intrinsically, indicating that MLKL-induced NLRP3 inflammasome formation and IL-1β cleavage occur before cell lysis. Furthermore, we show that necroptotic activation of NLRP3, but not necroptotic cell death alone, is necessary for the activation of NF-κB in healthy bystander cells. Collectively, these results demonstrate the potential importance of NLRP3 inflammasome activity as a driving force for inflammation in MLKL-dependent diseases.

Complex Pathologic Roles of RIPK1 and RIPK3: Moving Beyond Necroptosis

A process of regulated necrosis, termed necroptosis, has been recognized as a major contributor to cell death and inflammation occurring under a wide range of pathologic settings. The core event in necroptosis is the formation of the detergent-insoluble 'necrosome' complex of homologous Ser/Thr kinases, receptor protein interacting kinase 1 (RIPK1) and receptor interacting protein kinase 3 (RIPK3), which promotes phosphorylation of a key prodeath effector, mixed lineage kinase domain-like (MLKL), by RIPK3. Core necroptosis mediators are under multiple controls, which have been a subject of intense investigation. Additional, non-necroptotic functions of these factors, primarily in controlling apoptosis and inflammatory responses, have also begun to emerge. This review will provide an overview of the current understanding of the human disease relevance of this pathway, and potential therapeutic strategies, targeting necroptosis mediators in various pathologies.

Nonapoptotic cell death in acute kidney injury and transplantation

Acute tubular necrosis causes a loss of renal function, which clinically presents as acute kidney failure (AKI). The biochemical signaling pathways that trigger necrosis have been investigated in detail over the past 5 years. It is now clear that necrosis (regulated necrosis, RN) represents a genetically driven process that contributes to the pathophysiology of AKI. RN pathways such as necroptosis, ferroptosis, parthanatos, and mitochondrial permeability transition-induced regulated necrosis (MPT-RN) may be mechanistically distinct, and the relative contributions to overall organ damage during AKI in living organisms largely remain elusive. In a synchronized manner, some necrotic programs induce the breakdown of tubular segments and multicellular functional units, whereas others are limited to killing single cells in the tubular compartment. Importantly, the means by which a renal cell dies may have implications for the subsequent inflammatory response. In this review, the recent advances in the field of renal cell death in AKI and key enzymes that might serve as novel therapeutic targets will be discussed. As a consequence of the interference with RN, the immunogenicity of dying cells in AKI in renal transplants will be diminished, rendering inhibitors of RN indirect immunosuppressive agents.

Necroptotic cell death in failing heart: relevance and proposed mechanisms

As cardiomyocytes have a limited capability for proliferation, renewal, and repair, the loss of heart cells followed by replacement with fibrous tissue is considered to result in the development of ventricular dysfunction and progression to heart failure (HF). The loss of cardiac myocytes in HF has been traditionally believed to occur mainly due to programmed apoptosis or unregulated necrosis. While extensive research work is being carried out to define the exact significance and contribution of both these cell death modalities in the development of HF, recent knowledge has indicated the existence and importance of a different form of cell death called necroptosis in the failing heart. This new cell damaging process, resembling some of the morphological features of passive necrosis as well as maladaptive autophagy, is a programmed process and is orchestrated by a complex set of proteins involving receptor-interacting protein kinase 1 and 3 (RIP1, RIP3) and mixed lineage kinase domain-like protein (MLKL). Activation of the RIP1-RIP3-MLKL signaling pathway leads to disruption of cation homeostasis, plasma membrane rupture, and finally cell death. It seems likely that inhibition of any site in this pathway may prove as an effective pharmacological intervention for preventing the necroptotic cell death in the failing heart. This review is intended to describe general aspects of the signaling pathway associated with necroptosis, to describe its relationship with cardiac dysfunction in some models of cardiac injury and discuss its potential relevance in various types of HF with respect to the underlying pathologic mechanisms.

The role of inflammation and cell death in the pathogenesis, progression and treatment of heart failure

Chronic inflammation underlies a variety of seemingly unrelated conditions including coronary artery disease. The interest in exploring the role of inflammation in heart failure (CHF) arises from earlier observations that circulating pro-inflammatory biomarker levels are elevated in patients with both ischaemic and non-ischaemic cardiomyopathies and correlate with severity of disease and prognosis (McMurray et al. in Eur Heart J 33:1787-1847, 2012; Mosterd and Hoes in Heart 93:1137-1146, 2007; Owan et al. in New Engl J Med 355:251-259, 2006). In acute decompensated HF, pro-inflammatory biomarker levels have been associated with mortality and readmission rates (Cowie et al. in Heart 83:505-510, 2000). Similar to neurohormonal activation and inflammation, production of pro-inflammatory cytokines is a response to stress in an attempt to restore cellular function. However, sustained expression and exposure to cytokines can lead to left ventricular dysfunction, negative inotropic effects, altered cardiac metabolism, myocardial remodelling and HF progression. However, it is unclear whether elevated levels of pro-inflammatory biomarkers, such as high-sensitivity C-reactive protein, signify an ongoing inflammatory process that leads to HF progression, or are merely markers of advanced disease. Beta-blockers, renin-angiotensin-aldosterone axis antagonists, statins and immunosuppressants have been found to decrease the levels of cytokines in small clinical studies of patients with HF (Hobbs et al. in Heart J 28:1128-1134, 2007). However, 'immunomodulatory' approaches applied in the RECOVER, RENAISSANCE, ATTACH, IMAC and ACCLAIM double-blind, placebo-controlled studies had neutral or negative effects on outcomes of patients with HF. In the present review, we focus on the role of inflammation in pathogenesis and progression of the HF, the value of pro-inflammatory cytokines as biomarkers and the potential therapeutic applications of immunomodulation in HF patients.

Ischemia/Reperfusion

Ischemic disorders, such as myocardial infarction, stroke, and peripheral vascular disease, are the most common causes of debilitating disease and death in westernized cultures. The extent of tissue injury relates directly to the extent of blood flow reduction and to the length of the ischemic period, which influence the levels to which cellular ATP and intracellular pH are reduced. By impairing ATPase-dependent ion transport, ischemia causes intracellular and mitochondrial calcium levels to increase (calcium overload). Cell volume regulatory mechanisms are also disrupted by the lack of ATP, which can induce lysis of organelle and plasma membranes. Reperfusion, although required to salvage oxygen-starved tissues, produces paradoxical tissue responses that fuel the production of reactive oxygen species (oxygen paradox), sequestration of proinflammatory immunocytes in ischemic tissues, endoplasmic reticulum stress, and development of postischemic capillary no-reflow, which amplify tissue injury. These pathologic events culminate in opening of mitochondrial permeability transition pores as a common end-effector of ischemia/reperfusion (I/R)-induced cell lysis and death. Emerging concepts include the influence of the intestinal microbiome, fetal programming, epigenetic changes, and microparticles in the pathogenesis of I/R. The overall goal of this review is to describe these and other mechanisms that contribute to I/R injury. Because so many different deleterious events participate in I/R, it is clear that therapeutic approaches will be effective only when multiple pathologic processes are targeted. In addition, the translational significance of I/R research will be enhanced by much wider use of animal models that incorporate the complicating effects of risk factors for cardiovascular disease. © 2017 American Physiological Society. Compr Physiol 7:113-170, 2017.

Nuclear Magnetic Resonance-Based Metabolomics of Human Filtered Serum: A Great White Hope in Appraisal of Chronic Stable Angina and Myocardial Infarction

BACKGROUND

Biochemical detection of chronic stable angina (CSA) and myocardial infarction (MI) are challenging. To address the shortcomings of the conventional biochemical approach for detection of MI, we applied serum lacking proteins and lipoprotein-based metabolomics in an approach using proton nuclear magnetic resonance (1H NMR) spectroscopy for screening of coronary artery disease (CAD) and especially MI. Our aim was to discover differential biomarkers among subjects with normal coronary (NC), CSA, and MI.

METHODS

The study comprised serum samples from nondiabetic angiographically proven CAD [CSA (n = 88), MI (n = 90)] and NC (n = 55). 1H NMR spectroscopy was used to acquire metabolomics data. Clinical variables such as troponin I (TI), lactate dehydrogenase (LD), creatine kinase (CK, CK-MB, CK-MM), serum creatinine, and lipid profiles were also measured in all subjects. Metabolomic data and clinical measures were appraised separately using a chemometric approach and ROC analysis.

RESULTS

The screening outcomes revealed that the pattern of methylguanidine, lactate, creatinine, threonine, aspartate, and trimethylamine (TMA), and TI, LD, CK, and serum creatinine were changed in CAD compared to NC. Statistical analysis demonstrated high precision (93.6% by NMR and 67.4% by clinical measures) to distinguish CAD from NC. Further analysis indicated that methylguanidine, arginine, and threonine, and TI, LD, and serum creatinine were significantly changed in CSA compared to MI. Statistical analysis demonstrated high accuracy (88.2% by NMR and 92.1% by clinical measures) to discriminate CSA from MI.

CONCLUSIONS

In contrast to other laboratory methods, 1H NMR-based metabolomics of filtered sera appears to be a robust, rapid, and minimally invasive approach to probe CSA and MI.

Synergistic effect of apoptosis and necroptosis inhibitors in cisplatin-induced nephrotoxicity

Necroptosis is a nonapoptotic cell death pathway. We aim to study the effect of necrostatin-1 (a specific necroptosis inhibitor) in cisplatin-induced injury. We analyzed the effect of the combined use of inhibitors of apoptosis (z-vad) and necroptosis (necrostatin-1) in acute kidney injury by cisplatin in human proximal tubule cells. Our results showed moderate effectiveness in cytoprotection after treatment with z-vad. But the concomitant use of inhibitors (z-vad and necrostatin-1) presented synergistic and additive protection. The present study analyzed the caspase-3 activity and we observed a significant decrease in the group treated with z-vad and cisplatin. However we did not observe changes in the group treated with both inhibitors (z-vad and necrostatin-1) and cisplatin. Thus, demonstrating that necroptosis is a caspase-independent mechanism. We also analyzed the effect of necrostatin-1 in vivo model. C57BL/6 mice were treated with cisplatin and/or inhibitors. The concomitant use of inhibitors (z-vad and necrostatin-1) recovered renal function and decreased levels of urinary Ngal. Additionally, we analyzed the expression of RIP-1, a specific marker for necroptosis. In animals treated with cisplatin and z-VAD levels of RIP-1 were higher. This result reinforces that necroptosis occurs only in conditions where apoptosis was blocked. However, the use of both inhibitors (z-vad and necrostatin-1) provided additional protection. In conclusion, our study has a significant potential to show in vitro and in vivo protection obtained by necrostatin-1. Therefore, our results suggest that necroptosis may be an important mechanism of cell death after kidney injury.

Necroptosis is a key mediator of enterocytes loss in intestinal ischaemia/reperfusion injury

Cell death is an important biological process that is believed to have a central role in intestinal ischaemia/reperfusion (I/R) injury. While the apoptosis inhibition is pivotal in preventing intestinal I/R, how necrotic cell death is regulated remains unknown. Necroptosis represents a newly discovered form of programmed cell death that combines the features of both apoptosis and necrosis, and it has been implicated in the development of a range of inflammatory diseases. Here, we show that receptor‐interacting protein 1/3 (RIP1/3) kinase and mixed lineage kinase domain‐like protein recruitment mediates necroptosis in a rat model of ischaemic intestinal injury in vivo. Furthermore, necroptosis was specifically blocked by the RIP1 kinase inhibitor necrostatin‐1. In addition, the combined treatment of necrostatin‐1 and the pan‐caspase inhibitor Z‐VAD acted synergistically to protect against intestinal I/R injury, and these two pathways can be converted to one another when one is inhibited. In vitro, necrostatin‐1 pre‐treatment reduced the necroptotic death of oxygen‐glucose deprivation challenged intestinal epithelial cell‐6 cells, which in turn dampened the production of pro‐inflammatory cytokines (tumour necrosis factor‐α and interleukin‐1β), and suppressed high‐mobility group box‐1 (HMGB1) translocation from the nucleus to the cytoplasm and the subsequent release of HMGB1 into the supernatant, thus decreasing the activation of Toll‐like receptor 4 and the receptor for advanced glycation end products. Collectively, our study reveals a robust RIP1/RIP3‐dependent necroptosis pathway in intestinal I/R‐induced intestinal injury in vivo and in vitro and suggests that the HMGB1 signalling is highly involved in this process, making it a novel therapeutic target for acute ischaemic intestinal injury.

Necroptosis inhibitors as therapeutic targets in inflammation mediated disorders - a review of the current literature and patents

INTRODUCTION

Recent studies have shown substantial interplay between the apoptosis and necroptosis pathways. Necroptosis, a form of programmed cell death, has been found to stimulate the immune system contributing to the pathophysiology of several inflammation-mediated disorders. Determining the contribution of necroptotic signaling pathways to inflammation may lead to the development of selective and specific molecular target implicated necroptosis inhibitors. Areas covered: This review summarizes the recently published and patented necroptosis inhibitors as therapeutic targets in inflammation-mediated disorders. The role of several necroptosis inhibitors, focusing on specific signaling molecules, was discussed with particular attention to inflammation-mediated disorders. Data was obtained from Espacenet®, WIPO®, USPTO® patent websites, and other relevant sources (2006-2016). Expert opinion: Necroptosis inhibitors hold promise for treatment of inflammation-mediated clinical conditions in which necroptotic cell death plays a major role. Although necroptosis inhibitors reviewed in this survey showed inhibitory effects against several inflammation-mediated disorders, only a few have passed to the stage of clinical testing and need extensive research for therapeutic practice. Revisiting the existing drugs and developing novel necroptosis inhibiting agents as well as understanding their mechanism are essential. A detailed study of necroptosis function in animal models of inflammation may provide us an alternative strategy for the development of drug-like necroptosis inhibitors.

Inhibition of mitochondrial fission as a molecular target for cardioprotection: critical importance of the timing of treatment

Recent attention has focused on the concept that mitochondrial dynamics-that is, the balance between mitochondrial fusion and fission (fragmentation)-may play a pivotal role in determining cell fate in the setting of myocardial ischemia-reperfusion injury. In this regard, there is an emerging consensus that: (1) ischemia-reperfusion favors mitochondrial fragmentation and (2) strategies aimed at inhibiting the translocation of dynamin-related protein 1 (DRP1: the 'master regulator' of fission) from the cytosol to the mitochondria, when initiated as a pretreatment, are cardioprotective. However, direct molecular evidence of a cause-and-effect relationship between mitochondrial fission and cardiomyocyte death has not been established. To address this issue, we used a well-characterized in vitro, immortal cultured cardiomyocyte model to establish whether subcellular redistribution of DRP1 to mitochondria: (1) is triggered by hypoxia-reoxygenation; (2) plays a causal role in hypoxia-reoxygenation-induced cytochrome c release (harbinger of apoptosis) and cardiomyocyte death; and (3) represents a molecular mechanism that can be targeted in a clinically relevant time frame to render cells resistant to lethal hypoxia-reoxygenation injury. Our results provide direct evidence that the redistribution of DRP1 to mitochondria contributes to cardiomyocyte death, and corroborate the previous observations that the pre-ischemic inhibition of DRP1 translocation is cardioprotective. Moreover, we report the novel finding that-in marked contrast to the data obtained with pretreatment-inhibition of DRP1 translocation initiated at the time of reoxygenation had complex, unexpected and unfavorable consequences: i.e., attenuated cardiomyocyte apoptosis but exacerbated total cell death, possibly via concurrent upregulation of necroptosis.

The role of necroptosis in pulmonary diseases

By regulating the cell number and eliminating harmful cells, programmed cell death plays a critical role in development, homeostasis, and disease. While apoptosis is a recognized form of programmed cell death, necrosis was considered a type of uncontrolled cell death induced by extreme physical or chemical stress. However, recent studies have revealed the existence of a genetically programmed and regulated form of necrosis, termed necroptosis. Necroptosis is defined as necrotic cell death that is dependent on receptor-interacting protein kinase 3 (RIPK3). RIPK3, receptor-interacting protein kinase 1 (RIPK1), and a mixed-lineage kinase domain-like protein (MLKL) form a multiprotein complex called a necrosome. Although necroptosis generally provides a cell-autonomous host defense, on the other hand, cell rupture caused by necroptosis induces inflammation through the release of damage-associated molecular patterns, such as mitochondrial DNA, HMGB1, and IL-1. Previously, necroptosis was considered an alternative to apoptosis, but it is becoming increasingly clear that necroptosis itself is relevant to clinical disease, independent of apoptosis. According to some recent studies, autophagy, a cellular process for organelle and protein turnover, regulates necroptosis. This review outlines the principal components of necroptosis and provides an overview of the emerging importance of necroptosis in the pathogenesis of pulmonary disease, including chronic obstructive pulmonary disease, lung cancer, infection, and sepsis. We also discuss the molecular relationship between necroptosis and autophagy. Strategies targeting necroptosis may yield novel therapies for pulmonary diseases.

RIPK3 deficiency or catalytically inactive RIPK1 provides greater benefit than MLKL deficiency in mouse models of inflammation and tissue injury

Necroptosis is a caspase-independent form of cell death that is triggered by activation of the receptor interacting serine/threonine kinase 3 (RIPK3) and phosphorylation of its pseudokinase substrate mixed lineage kinase-like (MLKL), which then translocates to membranes and promotes cell lysis. Activation of RIPK3 is regulated by the kinase RIPK1. Here we analyze the contribution of RIPK1, RIPK3, or MLKL to several mouse disease models. Loss of RIPK3 had no effect on lipopolysaccharide-induced sepsis, dextran sodium sulfate-induced colitis, cerulein-induced pancreatitis, hypoxia-induced cerebral edema, or the major cerebral artery occlusion stroke model. However, kidney ischemia–reperfusion injury, myocardial infarction, and systemic inflammation associated with A20 deficiency or high-dose tumor necrosis factor (TNF) were ameliorated by RIPK3 deficiency. Catalytically inactive RIPK1 was also beneficial in the kidney ischemia–reperfusion injury model, the high-dose TNF model, and in A20^−/− mice. Interestingly, MLKL deficiency offered less protection in the kidney ischemia–reperfusion injury model and no benefit in A20^−/− mice, consistent with necroptosis-independent functions for RIPK1 and RIPK3. Combined loss of RIPK3 (or MLKL) and caspase-8 largely prevented the cytokine storm, hypothermia, and morbidity induced by TNF, suggesting that the triggering event in this model is a combination of apoptosis and necroptosis. Tissue-specific RIPK3 deletion identified intestinal epithelial cells as the major target organ. Together these data emphasize that MLKL deficiency rather than RIPK1 inactivation or RIPK3 deficiency must be examined to implicate a role for necroptosis in disease.

Necroptosis and Inflammation

Necroptosis is a regulated form of necrosis, with the dying cell rupturing and releasing intracellular components that can trigger an innate immune response. Toll-like receptor 3 and 4 agonists, tumor necrosis factor, certain viral infections, or the T cell receptor can trigger necroptosis if the activity of the protease caspase-8 is compromised. Necroptosis signaling is modulated by the kinase RIPK1 and requires the kinase RIPK3 and the pseudokinase MLKL. Either RIPK3 deficiency or RIPK1 inhibition confers resistance in various animal disease models, suggesting that inflammation caused by necroptosis contributes to tissue damage and that inhibitors of these kinases could have therapeutic potential. Recent studies have revealed unexpected complexity in the regulation of cell death programs by RIPK1 and RIPK3 with the possibility that necroptosis is but one mechanism by which these kinases promote inflammation.

CaMKII is a RIP3 substrate mediating ischemia- and oxidative stress-induced myocardial necroptosis

Regulated necrosis (necroptosis) and apoptosis are crucially involved in severe cardiac pathological conditions, including myocardial infarction, ischemia-reperfusion injury and heart failure. Whereas apoptotic signaling is well defined, the mechanisms that underlie cardiomyocyte necroptosis remain elusive. Here we show that receptor-interacting protein 3 (RIP3) triggers myocardial necroptosis, in addition to apoptosis and inflammation, through activation of Ca(2+)-calmodulin-dependent protein kinase (CaMKII) rather than through the well-established RIP3 partners RIP1 and MLKL. In mice, RIP3 deficiency or CaMKII inhibition ameliorates myocardial necroptosis and heart failure induced by ischemia-reperfusion or by doxorubicin treatment. RIP3-induced activation of CaMKII, via phosphorylation or oxidation or both, triggers opening of the mitochondrial permeability transition pore and myocardial necroptosis. These findings identify CaMKII as a new RIP3 substrate and delineate a RIP3-CaMKII-mPTP myocardial necroptosis pathway, a promising target for the treatment of ischemia- and oxidative stress-induced myocardial damage and heart failure.

Ensembling and filtering: an effective and rapid in silico multitarget drug-design strategy to identify RIPK1 and RIPK3 inhibitors

Necroptosis, a programmed necrosis pathway, is witnessed in diverse human diseases and is primarily regulated by receptor-interacting serine/threonine protein kinase 1 (RIPK1) and RIPK3. Ablation or inhibition of these individual proteins, or both, has been shown to be protective in various in vitro and in vivo disease models involving necroptosis. In this study, we propose an effective and rapid virtual screening strategy to identify multitarget inhibitors of both RIPK1 and RIPK3. It involves ensemble pharmacophore-based screening (EPS) of a compound database, post-EPS filtration (PEPSF) of the ligand hits, and multiple dockings. Structurally diverse inhibitors were identified through ensemble pharmacophore features, and the speed of this process was enhanced by filtering out the compounds containing cross-features. The stability of these inhibitors with both of the proteins was verified by means of molecular dynamics (MD) simulation. Graphical Abstract A generalized workflow employed in this study. Subsequent utilization of EPS and PEPSF might lead to reduced computational time and load.

Role of necroptosis in the pathogenesis of solid organ injury

Necroptosis is a type of regulated cell death dependent on the activity of receptor-interacting serine/threonine-protein (RIP) kinases. However, unlike apoptosis, it is caspase independent. Increasing evidence has implicated necroptosis in the pathogenesis of disease, including ischemic injury, neurodegeneration, viral infection and many others. Key players of the necroptosis signalling pathway are now widely recognized as therapeutic targets. Necrostatins may be developed as potent inhibitors of necroptosis, targeting the activity of RIPK1. Necrostatin-1, the first generation of necrostatins, has been shown to confer potent protective effects in different animal models. This review will summarize novel insights into the involvement of necroptosis in specific injury of different organs, and the therapeutic platform that it provides for treatment.

Necroptosis, a novel type of programmed cell death, contributes to early neural cells damage after spinal cord injury in adult mice

BACKGROUND

Necroptosis is an emerging programmed necrosis other than traditional necrosis and apoptosis. Until recently, there have not been studies that have investigated a relationship between necroptosis and pathogenesis of cell death after spinal cord injury (SCI).

OBJECTIVE

To investigate whether necroptosis takes part in the early pathophysiological processes of traumatic SCI in mice.

METHODS

Female ICR mice were randomized equally into three groups: the sham, the vehicle-treated + SCI group, and the Nec-1-treated + SCI group. To induce SCI, the mice were subjected to a laminectomy at T9 and compression with a vascular clip. After mice were sacrificed 24 hours post-SCI, propidium iodide (PI)-positive cells were detected using in vivo PI labeling. Morphological analyses were performed by hematoxylin and eosin staining and Nissl staining. The samples were evaluated for apoptosis by the in situ TUNEL assay. The expression of caspase-3 was assessed by western blot. Locomotor behavior of hindlimb was evaluated by BMS (Basso mouse scale) score at 1, 3, 5, 7, and 14 days post-injury.

RESULTS

Compared with dimethyl sulfoxide -treated mice, necrostatin-1-treated mice showed decreased PI-positive cells (P < 0.05), alleviated tissue damage, more surviving neuron at 24 hours after SCI (P < 0.05), and improved functional recovery from days 7 to 14 (P < 0.05). Necrostatin-1 did not reduce the expression of caspase-3 and the number of TUNEL-positive cells at 24 hours after SCI (P > 0.05).

CONCLUSIONS

Necroptosis contributes to necroptotic cell death and influences functional outcome after SCI in adult mice. The inhibition of necroptosis by necrostatin-1 may have therapeutic potential for patients with SCI.

The Role of Necroptosis in Burn Injury Progression in a Rat Comb Burn Model

OBJECTIVES

Progression of cell death after burn injury may occur by one of three mechanisms: passive necrosis, apoptosis, and programmed necroptosis that requires the receptor-interacting protein kinase-3 (RIP-3). The hypothesis was that RIP-3 is present in normal and burned skin; that necroptosis plays a role in burn injury progression; and that treatment with necrostatin-1, an inhibitor of necroptosis, would reduce burn progression.

METHODS

Skin specimens from rats were examined for the presence of RIP-3. Using a 150-g brass comb preheated to 100°C, we created two comb burns (one on each side) consisting of four rectangular burns, separated by three unburned interspaces, on both sides of the backs of anesthetized male Sprague-Dawley rats (240 to 300 g). The interspaces represent the ischemic zones surrounding the central necrotic core. Left untreated, these areas undergo necrosis. In the first experiment, 10 rats each were randomized to 1.65 mg/kg necrostatin-1 or control given by intraperitoneal injection 1 hour after injury. In the second experiment, 10 rats each were randomized to two intravenous injections of 1.65 mg/kg necrostatin-1 or its vehicle at 1 and 4 hours after injury. The primary outcome was the percentage of interspaces undergoing necrosis within 7 days of injury. Binary data were compared with chi-square or Fishers' exact tests.

RESULTS

All normal and burned skin specimens from rats stained positive for RIP-3. In the first experiment, nearly all unburned interspaces in both the experimental and the control rats underwent necrosis (47 of 48, 97.9% vs. 48 of 48, 100%; p = not significant [NS]). Similarly, in the second experiment, there was no difference in the percentage of unburned interspaces undergoing necrosis within 7 days of injury in rats treated with two doses of necrostatin-1 or the control vehicle (46 of 48, 95.8% vs. 48 of 48, 100%; p = NS). There were no wound infections noted in rats injected with necrostatin-1.

CONCLUSIONS

The skin of rats contains RIP-3 necessary for necroptosis. Injection of rats with either a single intraperitoneal dose or two intravenous doses of necrostatin-1 failed to reduce burn injury progression in a rat comb burn model. This may be due to inactivity of necrostatin-1 or the lack of a role of necroptosis in burn injury progression in the rat comb burn model.

Gadd45γ regulates cardiomyocyte death and post-myocardial infarction left ventricular remodelling

AIMS

Post-infarction remodelling is accompanied and influenced by perturbations in mitogen-activated protein kinase (MAPK) signalling. The growth arrest and DNA-damage-inducible 45 (Gadd45) proteins are small acidic proteins involved in DNA repair and modulation of MAPK activity. Little is known about the role of Gadd45 in the heart. Here, we explored the potential contribution of Gadd45 gamma (γ) isoform to the acute and late phase of heart failure (HF) after myocardial infarction (MI) and determined the mechanisms underlying Gadd45γ actions.

METHODS AND RESULTS

The Gadd45γ isoform is up-regulated in murine cardiomyocytes subjected to simulated ischaemia and in the mouse heart during MI. To mimic the situation observed during MI, we enhanced Gadd45γ content in cardiomyocytes with a single injection of an adeno-associated viral (AAV9) vector encoding Gadd45γ under the cTNT promoter. Gadd45γ overexpression induces cardiomyocyte apoptosis, fibrosis, left ventricular dysfunction, and HF. On the other hand, genetic deletion of Gadd45γ in knockout mice confers resistance to ischaemic injury, at least in part by limiting cardiomyocyte apoptosis. Mechanistically, Gadd45γ activates receptor-interacting protein 1 (RIP1) and caspase-8 in a p38 MAPK-dependent manner to promote cardiomyocyte death.

CONCLUSION

This work is the first to demonstrate that Gadd45γ accumulation during MI promotes the development and persistence of HF by inducing cardiomyocyte apoptosis in a p38 MAPK-dependent manner. We clearly identify Gadd45γ as a therapeutic target in the development of HF.

Necrostatin-1 protection of dopaminergic neurons

Necroptosis is characterized by programmed necrotic cell death and autophagic activation and might be involved in the death process of dopaminergic neurons in Parkinson's disease. We hypothesized that necrostatin-1 could block necroptosis and give protection to dopaminergic neurons. There is likely to be crosstalk between necroptosis and other cell death pathways, such as apoptosis and autophagy. PC12 cells were pretreated with necroststin-1 1 hour before exposure to 6-hydroxydopamine. We examined cell viability, mitochondrial membrane potential and expression patterns of apoptotic and necroptotic death signaling proteins. The results showed that the autophagy/lysosomal pathway is involved in the 6-hydroxydopamine-induced death process of PC12 cells. Mitochondrial disability induced overactive autophagy, increased cathepsin B expression, and diminished Bcl-2 expression. Necrostatin-1 within a certain concentration range (5–30 μM) elevated the viability of PC12 cells, stabilized mitochondrial membrane potential, inhibited excessive autophagy, reduced the expression of LC3-II and cathepsin B, and increased Bcl-2 expression. These findings suggest that necrostatin-1 exerted a protective effect against injury on dopaminergic neurons. Necrostatin-1 interacts with the apoptosis signaling pathway during this process. This pathway could be a new neuroprotective and therapeutic target in Parkinson's disease.

Melatonin attenuates carbon tetrachloride-induced liver fibrosis via inhibition of necroptosis

We investigated the protective mechanisms of melatonin (MLT) associated with necroptosis signaling and damage-associated molecular patterns, which are mediated by the activation of pattern recognition receptors in liver fibrosis. Rats were given an intraperitoneal injection of carbon tetrachloride (CCl4) dissolved in olive oil (1:3, vol/vol) twice a week (0.5 mL/kg) for 8 weeks. During this period, MLT was administered orally at 2.5, 5, and 10 mg/kg once a day. Chronic CCl4 administration increased hepatic hydroxyproline content and hepatocellular damage. MLT attenuated these increases. The expression levels of transforming growth factor β1 and α-smooth muscle actin that were increased by chronic CCl4 exposure were attenuated by MLT. CCl4 significantly increased receptor-interacting protein 1 (RIP1) expression, the formation of the RIP1 and RIP3 necrosome complex, and the level of mixed lineage kinase domain-like protein in liver tissue, which were attenuated by MLT. MLT also attenuated CCl4-induced increases in serum high-mobility group box 1 (HMGB1) and interleukin 1α, as well as the interaction between HMGB1 receptors for advanced glycation end products (RAGE). The increases in toll-like receptor 4 expression, p38, c-Jun N-terminal kinases phosphorylation, and nuclear factor κB translocation were suppressed by MLT. MLT attenuated the overexpression of RAGE, increased level of early growth response protein 1, and increased messenger RNA level of macrophage inflammatory protein 2. Our findings suggest MLT may prevent liver fibrosis by inhibiting necroptosis-associated inflammatory signaling.

Regulated necrotic cell death: the passive aggressive side of Bax and Bak

Although the molecular effectors of apoptotic cell death have been largely annotated over the past 30 years, leading to a strong biological understanding of this process and its importance in cell biology, cell death through necrosis has only recently been accepted as a similarly regulated process with definable molecular effectors. The mitochondria are important and central mediators of both apoptosis and regulated necrosis. In apoptosis, the B-cell leukemia/lymphoma 2 (Bcl-2) family members Bcl-2-associated protein x (Bax) and Bcl-2 homologues antagonist/killer (Bak) undergo oligomerization in the outer mitochondrial membrane resulting in the release of apoptosis inducing substrates and the activation of caspases and nucleases. In contrast, during necrosis the mitochondria become dysfunctional and maladaptive in conjunction with reactive oxygen species production and the loss of ATP production, in part through opening of the mitochondrial permeability transition pore. Although regulated necrosis is caspase-independent, recent evidence has shown that it still requires the apoptotic regulators Bax/Bak, which can regulate the permeability characteristics of the outer mitochondrial membrane in their nonoligomerized state. Here, we review the nonapoptotic side of Bcl-2 family, specifically the role of Bax/Bak in regulated necrotic cell death. We will also discuss how these Bcl-2 family member effectors could be part of a larger integrated network that ultimately decides the fate of a given cell somewhere within a molecular continuum between apoptosis and regulated necrosis.

TNF-α-induced programmed cell death in the pathogenesis of acquired aplastic anemia

The mechanism of acquired aplastic anemia (AA), a bone marrow hematopoiesis failure disease, has not been fully understood. TNF-α is a pleiotropic cytokine involved in cell proliferation, differentiation and death, and inflammation through binding to specific receptors on cell membranes. Aberrant secretion of TNF-α contributes to a number of human diseases, including tumor development and inflammation. TNF-α is also an important negative regulator of hematopoiesis. Over-expression of TNF-α not only directly inhibits the proliferation and differentiation of hematopoietic cells, but also initiates the intracellular death pathway to induce hematopoietic cell death, leading to bone marrow hematopoiesis failure. In this review, we summarize the mechanisms underlying extrinsic apoptosis and necroptosis of hematopoietic cells induced by TNF-α, and discuss the role of TNF-α-induced programmed cell death in the pathogenesis of acquired AA.

Programmed cell death with a necrotic-like phenotype

Programmed cell death is the process by which an individual cell in a multicellular organism commits cellular 'suicide' to provide a long-term benefit to the organism. Thus, programmed cell death is important for physiological processes such as development, cellular homeostasis, and immunity. Importantly, in this process, the cell is not eliminated in response to random events but in response to an intricate and genetically defined set of internal cellular molecular events or 'program'. Although the apoptotic process is generally very well understood, programmed cell death that occurs with a necrotic-like phenotype has been much less studied, and it is only within the past few years that the necrotic program has begun to be elucidated. Originally, programmed necrosis was somewhat dismissed as a nonphysiological phenomenon that occurs in vitro. Recent in vivo studies, however, suggest that regulated necrosis is an authentic classification of cell death that is important in mammalian development and other physiological processes, and programmed necrosis is now considered a significant therapeutic target in major pathological processes as well. Although the RIP1-RIP3-dependent necrosome complex is recognized as being essential for the execution of many instances of programmed necrosis, other downstream and related necrotic molecules and pathways are now being characterized. One of the current challenges is understanding how and under what conditions these pathways are linked together.

Necroptosis and parthanatos are involved in remote lung injury after receiving ischemic renal allografts in rats

Early renal graft injury could result in remote pulmonary injury due to kidney-lung cross talk. Here we studied the possible role of regulated necrosis in remote lung injury in a rat allogeneic transplantation model. In vitro, human lung epithelial cell A549 was challenged with TNF-α and conditioned medium from human kidney proximal tubular cells (HK-2) after hypothermia-hypoxia insults. In vivo, the Brown-Norway rat renal grafts were extracted and stored in 4 °C Soltran preserving solution for up to 24 h and transplanted into Lewis rat recipients, and the lungs were harvested on day 1 and day 4 after grafting for further analysis. Ischemia-reperfusion injury in the renal allograft caused pulmonary injury following engraftment. PARP-1 (marker for parthanatos) and receptor interacting protein kinase 1 (Rip1) and Rip3 (markers for necroptosis) expression was significantly enhanced in the lung. TUNEL assays showed increased cell death of lung cells. This was significantly reduced after treatment with necrostatin-1 (nec-1) or/and 3-aminobenzamide (3-AB). Acute immune rejection exacerbated the remote lung injury and 3-AB or/and Nec-1 combined with cyclosporine A conferred optimal lung protection. Thus, renal graft injury triggered remote lung injury, likely through regulated necrosis. This study could provide the molecular basis for combination therapy targeting both pathways of regulated necrosis to treat such complications after renal transplantation.

Activation of the NLRP3 inflammasome by proteins that signal for necroptosis

Necroptosis-a form of programmed necrotic cell death-and its resulting release of damage-associated molecular patterns (DAMPs) are believed to participate in the triggering of inflammatory processes. To assess the relative contribution of this cell death mode to inflammation, we need to know what other cellular effects can be exerted by molecules shown to trigger necrotic death, and the extent to which those effects might themselves contribute to inflammation. Here, we describe the technical approaches that have been applied to assess the impact of the main signaling molecules known to mediate activation of necroptosis upon generation of inflammatory cytokines in LPS-treated mouse bone marrow-derived dendritic cells. The findings obtained by this assessment indicated that signaling molecules known to initiate necroptosis can also initiate activation of the NLRP3 inflammasome, thereby inducing inflammation independently of cell death by triggering the generation of proinflammatory cytokines such as IL-1β.