Programmed necrosis and autophagy in immune function

It has long been known that apoptosis is vital to the generation and maintenance of proper adaptive immune function. An example is the essential requirement for apoptotic signaling during the generation of self-tolerant lymphocytes: the apoptotic death of B and T cells with overt autoreactivity is essential to central tolerance. More recently, the contributions of additional processes including cellular autophagy and programmed necrosis have been implicated in controlling both innate and adaptive immune functions. Evidence has been provided to demonstrate that the death of cells following ligation of death receptors (DRs), a subfamily of cell surface molecules related to tumor necrosis factor receptor 1, is not exclusively the domain of caspase-dependent apoptosis. In cells lacking the capacity to activate caspase-8 following DR ligation, cell death instead occurs via programmed necrosis, or as it has been recently termed, 'necroptosis'. This death process depends on RIP1 and RIP3, serine/threonine kinases that are recruited by DRs, and likely by other cellular signals including DNA damage and antigen receptor ligation. The generation of RIP1/RIP3 containing 'necrosomes' activates downstream necroptotic signaling that ultimately targets cellular energetic metabolism. Also related to cellular metabolic regulation, cellular autophagy has also been found to play unique and important roles in immunity. In this review, we describe the roles of necroptosis and autophagy in innate and adaptive immunity and speculate on the intriguing interplay between these two cellular processes.     

RIP1在细胞程序性死亡中作用的研究进展

近期研究发现受体相互作用蛋白（receptor—interacting protein,RIP）是细胞生存和死亡的重要交叉点,在细胞的凋亡与存活、程序性坏死等过程中发挥着关键性的作用。RIP1为RIP家族中的第一个成员,是一种重要的细胞信号转导调控分子。RIP1的结构与生物学功能及在细胞程序性死亡中的作用具有重要意义。     

中枢神经系统损伤疾病的坏死性凋亡机制研究进展

坏死性凋亡是新近发现的一种程序性坏死途径，在死亡受体信号激活后由RIP1和RIP3调控，并可被化合物necrostatin-1特异性抑制。目前研究证实坏死性凋亡涉及多种中枢神经系统损伤疾病的发生机制，并且通过干预坏死性凋亡信号通路，对诸多因素引起的中枢神经系统损伤具有一定的保护作用。深入研究坏死性凋亡的分子调控机制，有望为中枢神经系统损伤疾病治疗提供更多的潜在新靶点。     

Caspase-8: regulating life and death

Roles for cell death in development, homeostasis, and the control of infections and cancer have long been recognized. Although excessive cell damage results in passive necrosis, cells can be triggered to engage molecular programs that result in cell death. Such triggers include cellular stress, oncogenic signals that engage tumor suppressor mechanisms, pathogen insults, and immune mechanisms. The best-known forms of programmed cell death are apoptosis and a recently recognized regulated necrosis termed necroptosis. Of the two best understood pathways of apoptosis, the extrinsic and intrinsic (mitochondrial) pathways, the former is induced by the ligation of death receptors, a subset of the TNF receptor (TNFR) superfamily. Ligation of these death receptors can also induce necroptosis. The extrinsic apoptosis and necroptosis pathways regulate each other and their balance determines whether cells live. Integral in the regulation and initiation of death receptor-mediated activation of programmed cell death is the aspartate-specific cysteine protease (caspase)-8. This review describes the role of caspase-8 in the initiation of extrinsic apoptosis execution and the mechanism by which caspase-8 inhibits necroptosis. The importance of caspase-8 in the development and homeostasis and the way that dysfunctional caspase-8 may contribute to the development of malignancies in mice and humans are also explored.     

Necroptosis-一种新的程序性死亡的研究进展

细胞的死亡机制一直是生物医学研究的核心热点之一.目前公认的主要细胞死亡类型有3种.第1种是"坏死",第2种是"凋亡",第3种是"自噬".凋亡和自噬均需要能量和合成新的蛋白质,是一个细胞自我调控的主动过程,因此也被称为"程序性死亡" [1-3].     

RIPK3-driven cell death during virus infections

The programmed self-destruction of infected cells is a powerful antimicrobial strategy in metazoans. For decades, apoptosis represented the dominant mechanism by which the virus-infected cell was thought to undergo programmed cell death. More recently, however, new mechanisms of cell death have been described that are also key to host defense. One such mechanism in vertebrates is programmed necrosis, or “necroptosis”, driven by receptor-interacting protein kinase 3 (RIPK3). Once activated by innate immune stimuli, including virus infections, RIPK3 phosphorylates the mixed lineage kinase domain-like protein (MLKL), which then disrupts cellular membranes to effect necroptosis. Emerging evidence demonstrates that RIPK3 can also mediate apoptosis and regulate inflammasomes. Here, we review studies on the mechanisms by which viruses activate RIPK3 and the pathways engaged by RIPK3 that drive cell death.     

RIP1-mediated regulation of lymphocyte survival and death responses

RIP1 is an adaptor serine/threonine kinase associated with the signaling complex of death receptors (DRs) including Fas, TNFR1, and TRAIL-Rs which can initiate apoptosis. While DRs are dispensable throughout development, RIP1 deletion results in perinatal lethality. The developmental defect caused by absence of RIP1 remains unexplained. In previous studies, RIP1-deficient hematopoietic progenitors failed to reconstitute the T cell compartment and our recent data indicate a new role for RIP1 in TCR-induced activation of the pro-survival NF-κB pathway. Here, we show that RIP1 is also critical for B cell development. In addition, RIP1(-/-) B cells stimulated through LPS/TLR4 are impaired in NF-κB activation but have no major defect in the Akt pathway. Recently, RIP1 has also emerged as a critical player in necrosis-like death, necroptosis, in various cell lines. We have demonstrated that RIP1 deficiency can reverse the embryonic and T cell proliferation defects in mice lacking FADD, a caspase adaptor protein, which indicates a potential role for RIP1 in mediating in vivo necroptosis. We provide an overview and discussion of the accumulating data revealing insights into the diverse functions of RIP1 in survival and death signaling in lymphocytes.     

RIP3: a molecular switch for necrosis and inflammation

The receptor-interacting protein kinase 3 (RIP3/RIPK3) has emerged as a critical regulator of programmed necrosis/necroptosis, an inflammatory form of cell death with important functions in pathogen-induced and sterile inflammation. RIP3 activation is tightly regulated by phosphorylation, ubiquitination, and caspase-mediated cleavage. These post-translational modifications coordinately regulate the assembly of a macromolecular signaling complex termed the necrosome. Recently, several reports indicate that RIP3 can promote inflammation independent of its pronecrotic activity. Here, we review our current understanding of the mechanisms that drive RIP3-dependent necrosis and its role in different inflammatory diseases.     

小鼠胚胎肾脏细胞坏死性凋亡的超微结构

目的 观察小鼠胚胎肾脏发育过程中细胞坏死性凋亡的超微结构变化.方法 应用电子显微镜技术对不同胚龄(E12、14、16、18d)胎鼠(各3例)的肾脏的细胞坏死性凋亡进行系统观察.结果 细胞坏死性凋亡确实出现在胚胎肾脏中,它同时具有坏死的细胞质和凋亡的细胞核.细胞肿胀,细胞膜破溃,细胞器也肿胀空泡化.核染色质高密度固缩,边聚,多呈月牙状.细胞凋亡被发现的机会大于坏死性凋亡和细胞胀亡.结论 细胞坏死性凋亡与细胞凋亡及细胞胀亡共同构成了小鼠胚胎肾脏发育过程中的程序性细胞死亡,但坏死性凋亡和胀亡为辅,而以细胞凋亡为主.     

Role of RIP1 in physiological enterocyte turnover in mouse small intestine via nonapoptotic death

Enterocyte shedding in the small intestine is often referred as an example of programmed cell death. However, little is known about the underlying mechanisms, although both apoptotic and nonapoptotic cell death have been suggested to play an important role. Here, we show by electron microscope that the majority of cells dying in the mouse small intestine do not display apoptotic characteristics. Chemical biological approach in vivo and in an organ culture showed that necrostatin‐1 (Nec‐1), an inhibitor of receptor‐interacting protein 1 (RIP1, also called RIPK1), inhibited the shedding/nonapoptotic death of enterocyte, resulting in suppression of physiological enterocyte turnover. Moreover, RIP1 knockdown in vivo and RIP1 haploinsufficiency significantly suppressed physiological enterocyte turnover. Unlike Nec‐1‐sensitive (RIP1‐dependent) cell death, so called necroptosis, which is also dependent on RIP3, physiological enterocyte turnover in RIP3‐deficient mice was executed normally and still inhibited by Nec‐1. As inhibition of the shedding/nonapoptotic death of enterocyte by Nec‐1 resulted in suppression of crypt cell proliferation, the shedding process plays a dominant role over cell proliferation in maintaining homeostasis of enterocyte turnover. These results indicate that RIP1 plays a major role in physiological enterocyte turnover through a RIP3‐independent nonapoptotic death mechanism in the mouse small intestine.     

Necroptosis的调控机制及其在组织及细胞缺血损伤中的作用和意义

Necroptosis不同于坏死和凋亡,具有坏死的细胞形态特点和自噬的活化,并且是主动耗能的,是被一系列信号传导通路所调控的细胞死亡机制.Necroptosis的发现和确认为细胞死亡的逆转和治疗开创了一个新的研究和应用途经.RIP1激酶是调控Necroptosis 形成的关键酶,Necrostatins则是一类小分子化合物,它通过特异性地抑制细胞RIP1激酶而抑制Necroptosis 的形成.     

Mechanisms of CD4 T-cell depletion triggered by HIV-1 viral proteins

Infection with HIV-1 leads to progressive CD4 T-cell death, resulting in AIDS development. The mechanisms that trigger this CD4 T-cell death are still not fully understood, but a lot of data indicates that apoptosis plays a major role in this cell demise. Both infected and uninfected CD4 T-cells can die during HIV-1 infection by different cell-death pathways, but HIV-1-induced, bystander, CD4 T-cell killing is now recognized as central to immunodeficiency. The HIV-1 directly modulates CD4 T-cell death using multiple different strategies in which several viral proteins have an essential role. Recent data demonstrate that relationships can exist between the three main types of programmed cell death, i.e. apoptosis, autophagic programmed cell death, and necrosis-like programmed cell death. Almost nothing is currently known about the role of necrosis-like programmed cell death in CD4 T-cell death induced by the viral proteins, but a very recent study demonstrates that autophagy is needed to trigger apoptosis of bystander CD4 T-cells, further increasing the level of complexity of this pathology. This review presents an overview of the major types of programmed cell death and details the mechanisms by which the HIV-1 viral proteins control both infected and uninfected CD4 T-cell death.     

The adaptor protein FADD protects epidermal keratinocytes from necroptosis in vivo and prevents skin inflammation

Epidermal keratinocytes provide an essential structural and immunological barrier forming the first line of defense against potentially pathogenic microorganisms. Mechanisms regulating barrier integrity and innate immune responses in the epidermis are important for the maintenance of skin immune homeostasis and the pathogenesis of inflammatory skin diseases. Here, we show that epidermal keratinocyte-restricted deficiency of the adaptor protein FADD (FADD(E-KO)) induced severe inflammatory skin lesions in mice. The development of skin inflammation in FADD(E-KO) mice was triggered by RIP kinase 3 (RIP3)-mediated programmed necrosis (termed necroptosis) of FADD-deficient keratinocytes, which was partly dependent on the deubiquitinating enzyme CYLD and tumor necrosis factor (TNF)-TNF receptor 1 signaling. Collectively, our findings provide an in?vivo experimental paradigm that regulation of necroptosis in keratinocytes is important for the maintenance of immune homeostasis and the prevention of chronic inflammation in the skin.     

坏死性凋亡与肿瘤

坏死性凋亡是不依赖于caspase激活的一种细胞程序性死亡方式,其激活主要依赖于坏死性小体的形成。坏死性凋亡的调控受到多种因素响,RIPK1既可启动坏死性凋亡,也可抑制坏死性凋亡;caspase-8是坏死性凋亡的重要负反馈调节蛋白;CHIP是新发现的坏死性凋亡调控蛋白。坏死性凋亡的触发为对经典凋亡途径抵抗的肿瘤提供了新的治疗策略。     

Programmed necrosis and its role in management of breast cancer

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

程序性细胞坏死及细胞焦亡信号通路研究进展概述

细胞死亡是生命活动中一个非常重要的事件,真核生物可因物理损伤刺激导致细胞死亡,因特异信号通路介导的程序性细胞死亡近年来得到越来越多的关注。目前程序性细胞死亡主要存在以下3种方式:凋亡(apoptosis)、程序性坏死(necroptosis)、细胞焦亡(pyroptosis)。程序性坏死和细胞焦亡是近年来才发现的新的细胞程序性死亡方式,在细菌或病毒感染宿主细胞过程中起着关键作用。这两种细胞死亡都是细胞裂解型死亡,但其信号通路存在明显差异。本文就这两种程序性细胞死亡的形态学特征、信号转导通路及其在病原体感染过程中的作用等方面的研究进展作一综述。     

RSK3 mediates necroptosis by regulating phosphorylation of RIP3 in rat retinal ganglion cells

Receptor-interacting protein 3 (RIP3) plays an important role in the necroptosis signaling pathway. Our previous studies have shown that the RIP3/mixed lineage kinase domain-like protein (MLKL)-mediated necroptosis occurs in retinal ganglion cell line 5 (RGC-5) following oxygen-glucose deprivation (OGD). However, upstream regulatory pathways of RIP3 are yet to be uncovered. The purpose of the present study was to investigate the role of p90 ribosomal protein S6 kinase 3 (RSK3) in the phosphorylation of RIP3 in RGC-5 cell necroptosis following OGD. Our results showed that expression of RSK3, RIP3, and MLKL was upregulated in necroptosis of RGC-5 after OGD. A computer simulation based on our preliminary results indicated that RSK3 might interact with RIP3, which was subsequently confirmed by co-immunoprecipitation. Further, we found that the application of a specific RSK inhibitor, LJH685, or rsk3 small interfering RNA (siRNA), downregulated the phosphorylation of RIP3. However, the overexpression of rip3 did not affect the expression of RSK3, thereby indicating that RSK3 could be a possible upstream regulator of RIP3 phosphorylation in OGD-induced necroptosis of RGC-5 cells. Moreover, our in vivo results showed that pretreatment with LJH685 before acute high intraocular pressure episodes could reduce the necroptosis of retinal neurons and improve recovery of impaired visual function. Taken together, our findings suggested that RSK3 might work as an upstream regulator of RIP3 phosphorylation during RGC-5 necroptosis.     

RIP kinase-dependent necrosis drives lethal systemic inflammatory response syndrome

Engagement of tumor necrosis factor receptor 1 signals two diametrically opposed pathways: survival-inflammation and cell death. An additional switch decides, depending on the cellular context, between caspase-dependent apoptosis and RIP kinase (RIPK)-mediated necrosis, also termed necroptosis. We explored the contribution of both cell death pathways in TNF-induced systemic inflammatory response syndrome (SIRS). Deletion of apoptotic executioner caspases (caspase-3 or -7) or inflammatory caspase-1 had no impact on lethal SIRS. However, deletion of RIPK3 conferred complete protection against lethal SIRS and reduced the amounts of circulating damage-associated molecular patterns. Pretreatment with the RIPK1 kinase inhibitor, necrostatin-1, provided a similar effect. These results suggest that RIPK1-RIPK3-mediated cellular damage by necrosis drives mortality during TNF-induced SIRS. RIPK3 deficiency also protected against cecal ligation and puncture, underscoring the clinical relevance of RIPK kinase inhibition in sepsis and identifying components of the necroptotic pathway that are potential therapeutic targets for treatment of SIRS and sepsis.