STING Signaling Promotes Apoptosis, Necrosis, and Cell Death: An Overview and Update

Author: Song Liu and Wenxian Guan

STING is a newly identified intracellular sensor of foreign and endogenous DNA. STING has been recognized as an activator of immune responses by TBK1/IRF3 and NF-κB pathways, and it is suggested to play critical roles in host defense, autoimmune diseases, and tumor immunity. Recent studies have revealed that the outcome of STING activation could vary between distinct cell types and scenarios. STING activation in certain cell types triggered cell death including apoptosis and necrosis. This effect could be critical for preventing unnecessary or excessive inflammatory events and maintaining host immune homeostasis. This review is dedicated to summarize recent evidences in the field of STING-mediated cell death and to demonstrate dual outcomes of STING signaling. Besides canonical immune responses represented by IFN and TNF productions, STING signaling can also induce cell death events in a variety of cell types. The double-faced characteristics of STING signaling requires further exploration and precious regulation before tailoring clinical strategies for associated diseases.

STING是胞内感知胞内外DNA的sensor，通常与TBK1/IRF3 and NF-κB通路有关，在天然免疫、自身免疫疾病以及肿瘤免疫中有重要作用。

STING的激活可以促使RCD，包括apoptosis and necrosis（凋亡以及坏死），这就使得STING可以预防不必要的或者过多的免疫活动，并且维持细胞稳态。

通常有IFN和TNF（tumor necrosis factor）的产物。

1.INTRODUCTION

STING (stimulator of interferon genes) has been recently recognized as a central part of the recognition of bacterial and viral DNA as well as endogenous DNA (e.g., mitochondrial DNA). Aseries of studies in the recent decade has demonstrated the critical role of STING signaling in host immune responses and therefore in autoimmune diseases and tumor immunity [1, 2].

STING作为细菌和病毒DNA以及内源DNA（如线粒体DNA）识别的中心部分。STING在宿主免疫中的作用十分重要，尤其天然免疫以及肿瘤免疫。

In antigen-presenting cells (e.g., macrophages and dendritic cells), STING can cooperate with other molecules (e.g., cGAS) to recognize DNA for TBK1/IRF3 and NF-κB pathway activation and subsequent IFN and TNF production, respectively [3, 4]. Also, STING is capable of directly sensing bacterial and viral messengers under certain conditions [3, 5, 6]. The cellular and molecular process of STING signaling in innate immunity has been well reviewed elsewhere [7–9]. The potential application of targeting the STING pathway for cancer immunotherapy has been reviewed as well [10]. The complex of STING agonist and nanoparticles has been tested in antitumor therapy. Following membrane rupture and oxidative stress of tumor cells by cytotoxic nanoparticles, STING activation can enhance antitumor immunity by increasing expansion of tumor-infiltrating antigen-presenting cells and CD8+ T cells [11].

在抗原呈递细胞（如巨噬细胞及树状突细胞）中，STING可以与其他分子（如cGAS）合作去识别DNA，并激活TBK1/IRF3 以及NF-κB信号通路，从而分别引导IFN以及TNF的合成。另外，STING可以在特定条件下直接识别细菌和病毒。在天然免疫中的细胞和分子水平的STING信号也被研究了很多。对STING信号的靶向药物的肿瘤研究也有。STING受体激动剂以及纳米粒子的复合物也在肿瘤治疗中使用过。通过细胞毒性的纳米粒子（cytotoxic nanoparticles）对肿瘤细胞产生膜破裂以及氧化压力，STING活性可以通过增加肿瘤浸润抗原呈递细胞和CD8+ T细胞，从而增强抗肿瘤的免疫。

Recent evidences are emerging to show that the outcome of STING activation could vary between distinct cell types (which will be discussed below). In contrast to significant immune responses by STING in cells of the innate immune system,STING activation in other cell types leads to contrary outcomes. This review will summarize recent evidences in the field of STING-mediated cell death (including apoptosis and necrosis) and discuss relevant clinical significance.

最近的研究表明STING活性的结果在不同细胞类型中有不同的体现（下面会讨论）。STING在天然免疫细胞中有很重要的免疫响应，在其他细胞中也有不同的结果。这个综述会总结最近的STING介导的细胞死亡（包括凋亡和坏死）研究结果并讨论相关的医学重要性。

2.Overview of STING in Apoptosis

The interaction between STING signaling and apoptosis was firstly and independently reported by White et al. [12] and Rongvaux et al. [13] in 2014. They reported that STING is involved in Bak/Bax-mediated apoptosis. They discovered that Bak/Bax could induce mtDNA efflux that triggers the cGAS-STING pathway and subsequent IFN production. More importantly, they found that activated caspases are capable of suppressing this Bak/Bax-induced STING-mediated apoptosis and consequently preventing dying cells from triggering inflammatory events to maintain host immune homeostasis. 

最先报道STING信号与细胞凋亡的关系的是2014年的White等人。他们报道STING参与Bak/Bax介导的细胞凋亡。他们发现Bak/Bax可以促进线粒体DNA（mtDNA）的排出，从而启动cGAS-STING信号通路，随后引导IFN的生成。更重要的是，他们发现活化的caspases（胱天蛋白酶，参与细胞凋亡及坏死及发育）可以抑制Bak/Bax诱导的STING介导的细胞凋亡，因此可以预防冗余的炎症反应从而维持机体稳定性。

Recently, McArthur et al. have successfully identified the mechanism of mtDNA efflux during apoptosis [14]. They utilized live-cell lattice light-sheet microscopy to observe real-time mtDNA escape from the mitochondria during intrinsic apoptosis. They discovered that this process requires prodeath protein Bak/Bax to form macropores in the mitochondrial outer membrane that allows mtDNA efflux into the cytoplasm [15].

最近，McArthur等人成功鉴别了在细胞凋亡过程中线粒体DNA外排的机制。他们利用live-cell lattice light-sheet microscopy（活细胞晶格光片显微镜）观察实时线粒体DNA在细胞内源性凋亡过程（intrinsic apoptosis）中从线粒体逸出的过程。他们发现这个过程中需要预示死亡蛋白（prodeath protein）Bak/Bax来形成在线粒体外膜上的大孔隙，从而允许mtDNA逃出线粒体来到细胞质中。

3.STING Promotes Apoptosis of T Lymphocytes

Larkin et al. provided the first evidence of STING activation in T cells [16]. They found that STING activation in T cells triggers canonical inflammatory IFN production and simultaneous T cell ER stress and death events. Consistently, Gulen et al. reported that cGAS-STING activation in T cells results in a different gene expression profile compared to that in dendritic cells and—more importantly—the induction of T cell apoptosis [17]. This finding could be especially helpful for developing new treatment strategies in the context of T cell-associated human tumors. 

Larkin等人首先发现了T细胞中STING的活性。他们发现STING在T细胞中的活性可以激活典型的炎症IFN合成，同时增加T细胞内质网的压力并且造成其死亡。Gulen等人报告说T细胞中的cGAS-STING活性会导致不同的基因表达——这与在树状突细胞中不同。更重要的是，STING会导致T细胞凋亡。这个结果可以被利用在与T细胞相关的人类肿瘤治疗。

In addition to accelerating cell death, STING activationin T lymphocytes could also prevent cell proliferation. Cerboni et al. discovered that the antiproliferative capacity of STING requires STING relocalization to the Golgi apparatus and is dependent on a C-terminal subdomain that activates NF-κB but is distinct from TBK1/IRF3 recruitment domains. They also confirmed that patients carrying constitutive mutations in the STING-encoding gene presented a reduced number of T memory cells and impaired T cell proliferation[18].This clinical relevance could help to understand the pathogenesis of T cell-deficiency or T cell-dysfunction diseases.

除了促进T细胞凋亡，STING在T细胞中的活性还可以阻止细胞增殖。Cerboni等人发现STING的防增殖活性使得STING重定位到高尔基体中，并且依赖于C端的一个亚区域可以激活NF-κB，但这与TBK1/IRF3募集区域不同。他们也发现携带STING编码基因的突变的病人，体内T记忆细胞数量下降，并且T细胞的增殖受损。这可以帮助理解T细胞缺陷以及T细胞功能障碍疾病的发病机制。

4. STING Promotes Apoptosis of Myeloid Cells

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Interestingly, T lymphocytes and myeloid cells can cooperate to induce tumor cell death upon the activation by the STING agonist. Weiss et al. demonstrated that the STING agonist causes breast tumor vasculature disruption and then recruits massive immune cells (including neutrophils, CD8+ T cells, and monocytes) into the tumor site. Subsequent tumor cell death relies on the cooperation between myeloid and T cell subsets [22]. This finding provides a new insight into the translational application of STING-targeted therapy in cancer.

有趣的是，在STING激动剂作用下，T细胞与骨髓细胞可以合作去促进肿瘤细胞死亡。Weiss等人报道STING激动剂可以促使乳腺肿瘤血管系统的破裂，并且募集大量免疫细胞（包括中性粒细胞，CD8+ T细胞以及单核白细胞）到肿瘤位点。随后肿瘤细胞的死亡依赖于骨髓细胞与T细胞的合作。这个发现可以帮助STING靶向的肿瘤治疗。

5. STING Promotes Apoptosis of Other Cell Types

STING has been shown to be critical in tumor immunity largely due to its capacity for boosting host antitumor immune responses, such as IFN production. Recently, Tang et al. reported that STING agonists can directly eradicate malignant B cells by promoting B cell apoptosis that also contributes to host antitumor activity [23]. This finding offers a theoretic support of STING agonists being used as adjuvants for vaccinations and cancer therapy. Nevertheless, their data simultaneously showed the cytotoxic effect of STING on normal B cells. Therefore, clinical awareness is still required when using STING agonists in treating B cell-associated diseases (e.g., chronic lymphocytic leukemia and multiple myeloma).

STING在宿主免疫中的大量响应使STING在肿瘤免疫中有很好的作用，比如IFN的合成。最近，Tang等人报道STING激动剂可以通过促进B细胞凋亡直接消灭恶性B细胞，这也可以促进宿主抗肿瘤活动。这个发现可以支持STING激动剂被用于疫苗以及肿瘤治疗。然而，他们的数据同时显示STING也会对普通B细胞产生细胞毒素影响。因此，当使用STING激动剂去治疗B细胞相关疾病（如慢性淋巴细胞白血病和多发性骨髓瘤）时需要有一定的医疗意识。