Advertisement

cGAS-STING Activation in the Tumor Microenvironment and Its Role in Cancer Immunity

  • Geneviève Pépin
  • Michael P. Gantier
Chapter
Part of the Advances in Experimental Medicine and Biology book series (AEMB, volume 1024)

Abstract

Stimulator of interferon (IFN) genes (STING) is a key mediator in the immune response to cytoplasmic DNA sensed by cyclic GMP-AMP (cGAMP) synthase (cGAS). After synthesis by cGAS, cGAMP acts as a second messenger activating STING in the cell harboring cytoplasmic DNA but also in adjacent cells through gap junction transfer. While the role of the cGAS-STING pathway in pathogen detection is now well established, its importance in cancer immunity has only recently started to emerge. Nonetheless, STING appears to be an essential component in the recruitment of immune cells to the tumor microenvironment, which is paramount to immune clearance of the tumor. This review presents an overview of the growing literature around the role of the cGAS-STING pathway in the tumor microenvironment, with a specific focus on the role that cancer cells may play in the direct activation of this pathway, and its amplification through cell-cell transfer of cGAMP.

Keywords

cGAS cGAMP STING Interferon Connexin Cancer 

8.1 Introduction

Detection of pathogen-associated molecular patterns (PAMPs) by the immune system is the foundation of innate immunity and is one of the first lines of defense against infections. Viral nucleic acids and bacterial cell wall components (such as lipopolysaccharides or flagellar proteins) are PAMPs selectively detected by different types of innate immune receptors. These include membrane-bound Toll-like receptors (TLRs)—located at the surface of the cells or in the endosomes (e.g., TLR3, TLR4, and TLR7/8/9)—or cytoplasmic sensors such as retinoic acid-inducible gene I (RIG-I)-like helicases, which detect foreign RNAs [1, 2]. Activation of such innate immune sensors results in the production of antiviral and antibacterial proteins, including cytokines such as TNF-α or type I interferons (IFNs). Type I IFNs exert a strong antiviral effect through transcriptional induction of more than 2000 genes [3].

8.1.1 Stimulator of Interferon Genes: STING

Stimulator of interferon genes (STING, also known as MITA, ERIS, MPYS, or TMEM173) was independently discovered by four different groups in 2008 in an attempt to characterize mechanisms of DNA recognition resulting in the production of type I interferon (IFN), independently of TLR9 [4, 5, 6, 7]. STING is an ER-localized protein containing N-terminal transmembrane helices and a large C-terminal cytosolic domain. Its activation promotes signal transduction through the TBK1-IRF3 axis and production of type I IFNs [8, 9]. STING is directly involved in intracellular bacterial detection through sensing of cyclic dinucleotides (CDNs). During infection, CDNs are produced by bacteria such as Listeria monocytogenes and Mycobacterium tuberculosis [10, 11], which activate STING to promote strong immune responses to these pathogens.

8.1.2 Cyclic GMP-AMP Synthase: cGAS

Despite the initial demonstration of STING’s involvement in intracellular DNA sensing [12], a lack of evidence for a direct dsDNA-STING interaction prompted the community to suggest the existence of an upstream DNA sensor. Several potential cytoplasmic DNA sensors had been described (e.g., DAI, IFI16, DDX41, DNA-PK, MRE11, Sox2, and PQBP1, reviewed in [13]), before the discovery of cyclic GMP-AMP synthase (cGAS) by the Chen laboratory in 2013 [14]. Following cytoplasmic DNA detection, cGAS produces an endogenous second messenger, 2′3′-cyclic GMP-AMP (2′3′-cGAMP), that binds directly to STING to promote its activation [15, 16, 17, 18, 19]. It is now clear that cGAS is an important immune receptor for DNA viruses and retroviruses. As such, its capacity to sense cytoplasmic DNA is not restricted to standard Watson-Crick DNA, and it can also sense DNA-RNA hybrids [20]. The DNA cGAS senses can originate from various sites; in addition to bacterial and viral DNA, there is now good evidence that it senses self-DNA leaked from the nucleus [21] or mitochondria [2, 22].

By activating STING, DNA detection by cGAS culminates in the production of pro-inflammatory cytokines and the recruitment of the IFN response. Soon after the discovery of this pathway, the role of cGAS-STING in sterile inflammation (i.e., in the absence of any pathogen) was investigated. Mutations promoting a gain-of-function of STING have been shown to result in chilblain lupus [23], a rare type of cutaneous auto-inflammation. In addition, cGas and/or Sting genetic deletion was found to rescue animals from lupus-like diseases [24, 25, 26].

8.1.3 cGAS-STING in Tumor Development

Beyond its critical role in innate immunity, the cGAS-STING pathway is rapidly emerging as a critical player in the control of tumor development. Clearance of tumorigenic cells by the immune system initially relies on type I IFN production by dendritic cells (DCs) and the recruitment of CD8+ T cells, which promote the targeted death of such aberrant cells. Accordingly, chemically induced tumors develop better in mice lacking type I IFN signaling compared to their wild-type (WT) counterparts; similarly, a deficiency in type I IFN signaling results in poorer rejection of transplanted immunogenic tumors [27, 28].

Recruitment of CD8+ T cells in the tumor microenvironment is the key step in antitumor immunity and is directly dependent on type I IFN production by DCs [29, 30]. Critically, STING appears to play a unique, non-redundant role in the recruitment of CD8+ T cells to the tumor microenvironment [31, 32]. Indeed, genetic loss of a number of the most important signaling molecules known to date in innate immunity did not have any impact on CD8+ T cell infiltration in the tumor microenvironment in a syngeneic melanoma model [31, 32]. Conversely, the loss of STING abolished the spontaneous infiltration of CD8+ T cells in this melanoma model [32, 33].

8.1.4 STING Agonists as Antitumoral Adjuvants

The use of pathogen-associated molecular pattern (PAMP)-like molecules to potentiate DC activation upon radiation therapy and help tumor clearance is well established, with several clinical trials currently underway [34]. Specifically, there has been a great deal of enthusiasm around the concept that synthetic STING ligands could be designed to facilitate tumor clearance [35, 36]. One STING ligand that showed early promise was DMXAA, a flavonoid compound comprising two phenyl rings and a heterocyclic ring. DMXAA was found to exert strong antitumoral activities in mice and to act as a direct ligand for mouse STING [37]. However, DMXAA failed in human clinical trials [38] and was found not to be a direct ligand for human STING [39]. Other research has focused on cGAMP, the second messenger that activates STING. Although cGAMP can be synthesized in vitro, its application as an adjuvant is partially limited by the fact that it is not membrane permeable. Nonetheless, cGAMP injection has been found to synergize with radiation therapy to control local and distant tumors in pancreatic cancer [40] and to decrease chemotherapy toxicity in colon cancer [41].

To remedy the limitations of natural cGAMP, various types of synthetic STING ligand based on the structures of CDNs have already been synthesized and tested in various cancer models (reviewed in [39]). Most of these ligands have been designed to target human STING and have the potential to be used in clinical trials, as exemplified by compound MIW815 (ADU-S100). In line with a role for STING in CD8+ T cell recruitment and considering prior evidence with DMXAA, intratumoral injection of these CDN molecules into different types of cancer (melanoma, colon, glioma, and breast carcinomas) caused rapid tumor regression and mediated lasting and systemic antigen-specific T cell immunity [37, 35, 39]. The action of STING agonists may not be limited to antitumor immunity, and they could also promote direct apoptosis of cancer cells, as suggested by observations in malignant B cell leukemia [36].

In addition to designing synthetic ligands, loading of otherwise non-permeable cyclic di-GMP into liposomes was found to stimulate clearance of murine melanoma [42]. This suggests that experimentation with packaging and delivery of STING agonists may help enhance the therapeutic potential of these molecules.

8.1.5 STING and Immunotherapy

The tumor microenvironment plays a critical role in the control of CD8+ T cells to kill tumor cells. The specificity of CD8+ T cells relies on the surface expression of different receptors and ligands, creating an immune checkpoint to limit aberrant killing by CD8+ T cells. Programmed cell death 1 (PD-1) expression on CD8+ T cells inhibits their activation upon binding of its ligand, PD-L1. Expression of PD-L1 in the tumor microenvironment promotes inhibition of CD8+ T cell clearance. Consequently, strategies to block PD-L1 or its binding to CD8+ T, ultimately aimed at reigniting immune clearance of tumor cells, have shown great therapeutic potential in several types of cancers [43]. Critically, the cGAS-STING axis appears to be very important in the capacity of PD-L1 blocking strategies to reactivate CD8+ T cells, as evidenced by cGAS- and STING-deficient animal models [44], and in studies using co-administration of PD-L1 with STING ligands [45, 46].

Conversely, there has been evidence that STING activation can dampen immune activity through induction of the immune checkpoint indoleamine 2,3-dioxygenase (IDO) [47, 48, 49]. Further, in a subtype of head and neck cancer where the tumor was in a repressed immune state, STING activation could not mediate recruitment of CD8+ T cells [45].

With these lines of evidence taken together, the role of STING in immunotherapy is context dependent, and further studies are needed to better understand the signaling pathways initiating the different outcomes of its activity. How STING is activated within tumor cells is not fully understood either and is a subject for further study, although the role of STING in immune cells during tumor immunity and tumor clearance is well documented. In this review, we will focus on the role of cGAS and STING within tumor cells. We will also address the role of cell-cell communication in STING activation within the tumor microenvironment.

8.2 Mechanisms Underlying IFN Production in Immune Cells Within the Tumor Microenvironment

8.2.1 STING Activation by Tumor DNA

The current model of IFN production and CD8+ T cell priming by tumor cells indicates a possible role for the activation of antigen-presenting cells (APCs) by DNA released from the tumor as a result of apoptosis and taken up by APC via phagocytosis. This is supported by the demonstration that DNA from the tumor could be observed in APCs where STING was activated and type I IFN produced [32, 31, 50]. In this model, sensing of apoptotic cell-derived nuclear DNA by DCs recruits the STING-IRF3 axis to promote type I IFN production, thereby enhancing the functionality of DCs in an autocrine loop. Activated DCs further mediate the activation and the clonal expansion of CD8+ T cells favoring tumor clearance [31].

However appealing, this model does not account for how the tumor DNA would be released from the endosome/phagolysosome of APCs to reach the cytoplasm, where cGAS is located. In fact, previous work indicates that phagocytosis of apoptotic cells does not result in IFN-I activation, due to sequestration of DNA in the phagosome [51]. Similarly, despite tumor DNA being found in APC cells in vivo, dead tumor cells incubated with APCs did not engage an IFN response in vitro [32]. Another limit of the phagocytosis model is that it restricts the role of tumor cells in initiating an immune response to only dying/apoptotic cells.

There is also emerging evidence that the growth and spread of tumors, such as melanoma tumors, which display very little cell death, are regulated by STING. Indeed, mouse melanoma cells transplanted in mice lacking STING were found to develop significantly more lung metastases than WT mice [31]. Collectively, these examples indicate that the essential role of STING in tumor restriction is probably not limited to the activation of cGAS-STING in APCs upon phagocytosis of dead/apoptotic tumor cells.

8.2.2 Cytosolic DNA Activation of cGAS-STING

STING is indirectly activated by cytosolic DNA, through upstream cGAS engagement. Following cytoplasmic DNA recognition, cGAS produces cGAMP which acts as a second messenger between cGAS and STING. Critically, once synthesized, cGAMP has the capacity to be transferred between cells and mediate STING activation in adjacent cells [52], provided they form gap junctions with the cell making cGAMP. In homeostasis, physical restriction of DNA in the nucleus or phagosome (when the DNA is phagocytosed) ensures that self-DNA is not accessible to cytosolic cGAS. However, it is now apparent that self-DNA can escape from its original localization and activate the cGAS-STING pathway in certain contexts. To prevent aberrant cGAS recruitment, several key nucleases are at play to degrade DNA molecules escaping nuclear retention. Accordingly, defects in such DNases—e.g., TREX1 [53], DNase2a [54], or the nuclease-like SamHD1 [55]—lead to cytoplasmic DNA accumulation and cGAS-STING activation by self-DNA. In addition, certain chemical modifications of DNA, such as 8-OHG, protect DNA from DNase degradation (such as by TREX1) thereby favoring cGAS activation [56, 25]. Decreases in genome stability or faults in DNA repair pathways can also lead to STING activation. Loss of ATM, which results in an impaired DNA damage response, increases genomic instability and causes spontaneous type I IFN production through STING activation [57]. In line with this, a lack of RPA and/or RAD51, which normally bind to damaged DNA and sequester it in the nucleus, results in cGAS detection of leaked cytoplasmic DNA [58]. Therefore, the cGAS-STING pathway is indirectly involved in the response activated upon DNA damage.

Genome stability is directly dependent on the capacity of the cell to stop and repair its DNA. During each cell division, multiple mutations are made but are also constantly being repaired. Oncogenes and tumor suppressors regulate cell cycle and proliferation. In cancer, these genes are often mutated to cause sustained proliferation. This constant pressure to proliferate leads to genomic instability by forcing the cells to divide even in the presence of damaged DNA [59]. Accordingly, cytosolic DNA can be observed in B cell lymphomas, but not in normal B cells [60]. Despite being predominantly produced by infiltrating immune cells, some growing tumors can also produce type I IFNs [50]. Such type I IFN production can directly relate to cGAS-STING activation as recently suggested using a model of breast cancer with genomic instability [61]. Although a detailed understanding about how nuclear DNA can be leaked to the cytoplasm remains elusive, there is data to suggest that such leakage results from an active process. Overexpression of RNaseH1 (that degrades the R-loops of DNA-RNA hybrids) reduces the level of cytosolic DNA and type I IFN production and hampers the rejection of the lymphoma tumors [62]. Conversely, in prostate cancer cells, the MUS81 endonuclease cleaves DNA at stalled replication forks, promoting export of DNA products into the cytoplasm [63]. This results in type I IFN production through STING activation by the cancer cells, which consequently enhances the rejection of the tumor in vivo [63]. All together, these data strongly suggest that the cGAS-STING can be activated in select tumor cells, independently of APC phagocytosis.

8.2.3 cGAS-STING Activation in Tumor Cells

To date, most reports addressing the in vivo role of the cGAS-STING pathway in the tumor environment have relied on the use of Sting −/− tumor-bearing mice compared to WT mice [33, 32, 35]. These models, although supporting a critical role for STING in the tumor microenvironment, are unable to define the source of STING activation in the tumor itself. Given that about 50% of tumor cell lines express cGAS (e.g., 5/11 colorectal cancer cell lines [64] and 7/11 melanoma cell lines [65]), cGAMP production by tumors is likely to be a frequent occurrence, assuming they also have defective DNA repair capacity. Whether such tumor-derived cGAMP activates type I IFN production by cancer cells is dependent on the presence of STING [64, 65], but this cGAMP can also modify the tumor microenvironment through horizontal transfer of the second messenger to adjacent cells [52].

With this in mind, at least two different scenarios involving cGAMP expression in tumor cells can be envisioned as illustrated in Fig. 8.1. First, tumor-derived cGAMP production could lead to type I IFN production by the tumor cells or STING-competent adjacent tumor cells and mediate the subsequent recruitment of immune cells. Alternatively, tumor-derived cGAMP could directly engage STING in immune cells during phagocytosis or during immune synapse formation. These scenarios are not mutually exclusive and could possibly happen simultaneously; they place the focus on tumor-derived cGAMP, rather than immune-derived cGAMP, and illustrate how cGAMP transfer to immune cells better equipped for type I IFN production could be used to amplify the local detection of cytoplasmic DNA, indicative of aberrant cellular replication in this case.
Fig. 8.1

Engagement of cGAS in tumor cells can amplify type I IFN production

Genomic instability, DNA damage, and accelerated cell proliferation can mediate leakage of DNA into the cytoplasm in tumor cells. The released DNA can then be detected by cytosolic cGAS to result in cGAMP production. cGAMP acts as a second messenger, which can directly activate STING in the cGAS engaged cells (if present—in this schematic the cells with cytosolic DNA do not express STING). Independent of intracellular STING activation, cGAMP can transfer to adjacent cells through gap junctions and activate STING in these recipient cells (1). Such cells, although not exhibiting cytosolic DNA, respond to cGAMP through STING engagement and produce type I IFN. This amplification of type I IFN production and associated cytokines by adjacent cells promotes the recruitment and activation of dendritic cells (2). Recruited dendritic cells scan tumor cells and can be further activated by cGAMP transfer through immune synapse or phagocytosis (3). Ultimately, activation of dendritic cells results in CD8+ T activation and tumor immunity

In support of the origin of non-hematopoietic-derived cGAMP, it should be noted that Trex1 −/− mice initially produce type I IFN in their non-hematopoietic compartment. This primary induction mediates recruitment of inflammatory cells that produce even more type–I IFN [66]. Importantly, cGAMP may also potentiate local type I IFN production through nonimmune adjacent tissues. As such, in an in vivo model of melanoma engraftment, cGAMP passage from the tumor to the vasculature may explain the observation that the primary source of IFN was endothelial cells [33].

It is tempting to speculate that cGAS activation in tumor cells could explain why some patients develop spontaneous leukocyte infiltration and antitumor T cell responses. Such spontaneous T cell tumor infiltration has, for instance, been reported in melanoma [67] and ovarian [68], breast [69, 61], and colorectal cancers [70]. Whether such infiltration depends on cGAS activation in the tumor and how such activation would take place are not currently defined. Future studies investigating the correlation between tumor-derived cGAMP and leukocyte infiltration may help refine disease prognosis—leukocyte infiltration is already a powerful prognostic factor in colorectal cancer patients [71]—while being informative about the best therapeutic approach to be selected.

8.2.4 Loss of cGAS-STING Expression in Tumor Cells

Many bacteria and viruses have evolved to block innate immune pathways, thereby facilitating their intracellular survival. Similar selection pressure constantly operates on tumor cells, which attempt to evade clearance by the immune system. Type I IFN is crucial to immune cell recruitment into the tumor microenvironment, and STING is a key factor in such type I IFN production, as discussed previously. One could speculate that selective pressure operates to block the cGAS-STING pathway in tumor cells to facilitate immune evasion. Accordingly, cGAS-STING inhibition has been recently described in human cancers.

The first extensive study on the loss of the cGAS-STING pathway comes from the field of colorectal adenocarcinoma (CA). In that research, cGAS-STING activity was decreased in the vast majority of CA cell lines, which lacked the capacity to produce type-I IFN in response to cytoplasmic DNA [64]. In some cell lines such as HT29 cells, the pathway was altered but still functional [64]. In support of this in vitro data, one third of 48 clinical samples of adenocarcinoma analyzed showed a loss of cGAS expression [64]. Interestingly, upregulation of cGAS expression in the early stages of cancer and disruption of the STING pathway in advanced stages were reported in a similar study [72]. These data indicate that engagement of the pathway during early stages of tumor development, but inhibition of the pathway later in disease, favored tumor growth. Similarly to CA, melanoma cells displayed recurrent loss of cGAS-STING expression, ultimately inhibiting type I IFN production [65]. Epigenetic repression was found to relate to the cGAS and STING inhibition proposed in these studies [64, 65], but other factors are most likely at play. In another study, ovarian cancer cells (serous, clear cell, and endometriosis) had lost responsiveness to DNA transfection via STING-IRF3 activation [73]. These reports collectively suggest that loss of expression or activity of the cGAS-STING pathway may favor the development of the tumor cells.

Therapeutically, such a loss of the cGAS-STING response can be harnessed to obtain clinical benefit. Indeed, cGAS-STING-deficient tumors are more susceptible to viral oncolysis—where modified DNA viruses like HSV-1 have been used to target and kill cancer cells [65, 64].

8.2.5 cGAS-STING in Pathogen-Driven Carcinogenesis

Whether the cGAS-STING pathway is involved in pathogen-driven carcinogenesis is a question that remains elusive. Human papillomavirus (HPV) is the causative agent of cervical cancer and other types of cancer. HPV is a DNA virus, making it a potential target of cGAS sensing [74]. Accordingly, and although not detailed in the current literature, the cGAS-STING pathway may play a role in HPV infection and cancer development. In line with this hypothesis, a single nucleotide polymorphism (SNP) in cGAS (rs311678) has been recently associated with a reduced risk of cervical precancerous lesions. This SNP was found to modulate cGAS expression in vitro, and higher cGAS was associated with a reduced risk of HPV infection [75]. As described for other viruses, HPV can also counteract the activity of the innate immune system. In vitro expression of the HPV E2 protein in human primary keratinocytes downregulates the expression of STING and several other innate immune genes [76]. In addition, STING repression has been shown in HPV+ low-grade squamous intraepithelial lesions, when compared with HPV controls. Furthermore, viral oncogenes such as HPV E7 and E1A of human adenovirus A5 were proposed to bind to the N-terminal region of STING to reduce its downstream signaling [77].

In line with a role for the cGAS-STING pathway in reducing the risk of early stages of pathogen-driven cancer, STING expression is significantly decreased in gastric cancers when compared to non-tumor tissues [78]. Helicobacter pylori, the main causative agent of gastric cancer, can activate STING and promote inflammation [78]. These data collectively suggest a role for cGAS and STING in pathogen-mediated carcinogenesis.

8.3 Connexin Expression in Tumor Cells and Its Impact on Tumor Development

Beyond the inhibition of tumor-derived type I IFN production by cGAS, one could argue that loss of cGAS expression by a significant number of tumor cells is also important for stopping the propagation of cGAMP within the tumor microenvironment. As mentioned previously, cGAMP transfers horizontally through gap junctions to activate STING in adjacent cells [52]. With this in mind, it is tempting to revisit previous works on the role of gap junctions in tumorigenesis.

8.3.1 Connexins and Gap Junctions

Gap junctions are formed through the interaction of connexins from both interacting cells to promote the intercellular circulation of ions and small molecules such as cGAMP. Among the family of connexins, connexin (CX) 43 and CX45 were found to be essential in human embryonic kidney 293T (HEK293T) cells for cGAMP horizontal transfer [52]. Critically, transfer of cGAMP to HEK293T CX43/45-deficient cells could be restored through the expression of human CX26, CX31, CX32, CX40, CX43, and CX62 and mouse CX43 and CX45, suggesting that most connexins are able to transfer cGAMP (noting that human CX50 overexpression did not restore transfer) [52].

8.3.2 Loss of Intercellular Cell Communication in Early-Stage Tumors

Loss of intercellular communication by cancer cells was first described over half a century ago [79]. Since their discovery, connexins have been shown to exert both pro- and antitumoral activities, making it a controversial field of research [80]. The overall view is that retention of connexin expression benefits antitumoral activities at early stages but can later favor metastasis. Arguably, loss of connexin expression early in tumor development would be expected to reduce cGAMP horizontal transfer to adjacent cells and inhibit type I IFN induction—thereby favoring tumor initiation and immune evasion. Accordingly, CX43 expression is decreased in prostate cancer patients compared to controls. In line with what is observed with the loss of cGAS, the reduction of CX43 expression was found to correlate with advanced stages of cancer [81]. Similar trends were observed in breast cancers and head and neck squamous cell carcinomas (HNSCC), where low expression of CX43 correlated with a negative prognosis [82]. Given that HPV is also often detected in HNSCC tumors [83], it is of interest to note that HPV-E6 protein expression was associated with a reduction of gap junction formation [84].

Further supporting an antitumoral effect of connexins in early tumor growth, overexpression of CX43 was found to reduce melanoma tumor growth in vivo [85], while its downregulation stimulated the growth of prostate cancer cells [86]. Re-expression of connexins in breast cancer cell lines implanted in vivo also reduced tumor growth [87]. Qigesan, a molecule that increases the expression of connexins, reduced cell migration and invasion in esophageal cancer cells [88]. Critically, in a model of chemically induced mammary tumors, enforced expression of connexin decreased the incidence of tumor formation, while no difference was observed on the tumor growth [89]. These findings suggest that connexins may play a greater role in tumor initiation than tumor development [89]. It should, however, be noted that the latter tumor model relied on tumoral expression of the Cre recombinase—which we linked recently to cGAS-STING engagement [90]—opening the possibility for a direct role for the cGAS-STING pathway in the initiation of tumors in this model.

8.3.3 The Role of Connexin in Metastasis

Conversely, there are instances where increased expression of connexins was found to enhance metastasis. Indeed, in a mouse melanoma model, the metastatic capacity of the cancer cells was found to be dependent on CX26 expression [91]. Furthermore, CX43 expression is induced in CA cell lines that display greater metastatic potential, while CX43 levels are almost absent in other tumor cell lines [92].

Critically, direct evidence for the role of cGAMP and gap junction in metastasis was recently reported. When breast and lung cancer metastatic cells migrated to the brain, they increased connexin expression, allowing passage of tumor cell-derived cGAMP to adjacent astrocytes [93]. cGAMP transfer to astrocytes promoted astrocyte activation and the subsequent secretion of pro-inflammatory cytokines favoring tumor growth and chemotherapeutic resistance [93].

8.4 cGAS-cGAMP: Connexins-STING in Chemotherapy

The role played by intercellular cell communication during chemotherapy is also controversial: there are reports of both positive and negative roles for connexins on the outcome of chemotherapy. For example, increased expression of connexins enhanced the sensitivity of RKO colon cancer cells to diverse chemotherapeutic agents such as fluorouracil, oxaliplatin, and irinotecan [94]. Critically, increased sensitivity was observed in vitro when the cells were more confluent or when they were treated with retinoic acid, which induces the expression of connexins [94]. In addition, low expression of connexins correlated with reduced sensitivity of hepatocarcinoma cells to oxaliplatin [95]. On the other hand, inhibition of connexins was reported to sensitize glioblastoma cells previously shown to be resistant to chemotherapy [96].

Chemotherapy, which mostly relies on the greater sensitivity of tumor cells to DNA damage, often induces type–I IFN production [97]. We recently discovered that the DNA intercalating agent acriflavine could promote cGAMP synthesis in SV40T immortalized mouse embryonic fibroblasts (MEFs), in association with increased cytoplasmic DNA levels [21]. Despite previous evidence of cGAS and STING activation following DNA damage and cytoplasmic DNA leakage [ 56, 57, 98], there had been no prior demonstration of the direct engagement of cGAS and cGAMP production in these contexts. Critically, the capacity of the topoisomerase I inhibitor, topotecan (TPT), to restrict breast cancer cell proliferation in vivo was abrogated in mice lacking STING [99]. This, however, does not tease out whether cGAMP was generated by the tumor after chemotherapy treatment or by phagocytes. Similar results were found using irradiation of tumors [100]. Interestingly, single-stranded DNA (ssDNA) leaking in the cytoplasm of the tumor cells following chemotherapy was shown to be a by-product of BLOOM syndrome helicase (BLM) and the exonuclease-RNase EXO1 [101]. Along with the discovery that RPA and RAD51 normally work to retain ssDNA generated by BLM and EXO1 during DNA repair [58], these findings suggest that cytoplasmic DNA leakage is likely related to a saturation of the cell’s capacity to retain it in the nucleus [101]. Although ssDNA is not supposed to activate cGAS, it can bind to it weakly, and it is likely that its modification upon DNA damage (such as by 8-OHG) somewhat favors cGAS activation [56, 101]. Further work is clearly warranted to better define the modalities of cytoplasmic DNA leakage and cGAS engagement upon DNA damage by chemotherapy.

8.5 DNA Damage Engages cGAS Activity and Horizontal STING Amplification

We have previously demonstrated that DNA damage can mediate cGAMP production [90, 21]. When using inducible Cre recombinase-mediated DNA damage, we observed that only a proportion of cells displayed the hallmarks of DNA damage (through γH2AX staining) [90]. Critically, we demonstrated that the capacity of damaged cells to generate a widespread type I IFN production in the cell monolayer was strongly dependent on cell density—which we attribute to a connexin-dependent transfer of cGAMP [90]. This illustrates the capacity of healthy adjacent cells to amplify the signal of selected damaged cells and suggests that a similar feedback loop could operate in the tumor microenvironment upon induction of DNA damage by chemotherapy. Surprisingly, cGAS-depleted bone marrow-derived DCs can be activated after co-incubation with irradiated tumor cells, albeit modestly, while STING depletion completely thwarted the effect. This residual activation in cGAS-deficient cells suggests that engagement of cGAS in phagocytes is not essential for STING activation by irradiated tumor cells [100]. From this point of view, the capacity of tumors cells to generate cGAMP and transfer it to adjacent cells may play an important role in the outcome of chemotherapy.

8.6 Conclusion

In summary, the cGAS-STING axis is crucial to cancer immunity. There is accumulating evidence for a role for STING in tumor DCs and cross priming of CD8+ T cells. These findings clearly suggest that STING ligands have strong therapeutic potential. Nonetheless, therapeutic STING activation may also contribute to the tumor expressing interferon-stimulated genes previously linked with chemoresistance [97, 101, 102]. As such, critical questions regarding the modalities of activation of the pathway in the tumor microenvironment and its impact on chemotherapies are still to be answered. For instance, while DCs have a central role in mediating the recruitment of an antitumor immune response through STING, how other cell types like macrophages and neutrophils contribute to this pathway should also be addressed. Critically, the source of cGAMP activating STING in APCs and its putative modalities of transfer to APCs are not defined. In light of the current literature reviewed herein and our own experiments, we propose a model, illustrated in Fig. 8.1, in which cGAMP can be synthesized by tumor cells to play a role in immune cell activation through immune gap junctions. While this is supported by the demonstration that metastatic breast cancer cells could transfer cGAMP to astrocytes in the brain [93], direct evidence of cGAMP production by tumor cells and its transfer to APCs remains to be found. Defining if and how cGAMP can be made by tumors has the potential to help understand why select tumors are devoid of infiltrating immune cells. Given that the efficacy of preferred chemotherapies has been associated with STING signaling in vivo (e.g., irinotecan), these may be particularly effective when used in patients with active cGAS and functional connexin tumors.

Notes

Acknowledgment

We thank Rebecca Smith, Jonathan Ferrand, and Lise Boursinhac for the assistance in the preparation of the manuscript. This work was funded in part by the Australian NHMRC (1062683 and 1081167 to M.P.G.), the Australian Research Council (140100594 Future Fellowship to M.P.G.), and the Victorian Government’s Operational Infrastructure Support Program. G.P. is a fellow from Fonds de Recherche du Québec – Santé (FRQS), Canada (35071).

References

  1. 1.
    He S, Mao X, Sun H, Shirakawa T, Zhang H, Wang X (2015) Potential therapeutic targets in the process of nucleic acid recognition: opportunities and challenges. Trends Pharmacol Sci 36(1):51–64. doi: 10.1016/j.tips.2014.10.013 CrossRefPubMedGoogle Scholar
  2. 2.
    Luecke S, Paludan SR (2016) Molecular requirements for sensing of intracellular microbial nucleic acids by the innate immune system. Cytokine. doi: 10.1016/j.cyto.2016.10.003 CrossRefPubMedGoogle Scholar
  3. 3.
    Rusinova I, Forster S, Yu S, Kannan A, Masse M, Cumming H, Chapman R, Hertzog PJ (2013) Interferome v2.0: an updated database of annotated interferon-regulated genes. Nucleic Acids Res 41(Database issue):D1040–D1046. doi: 10.1093/nar/gks1215 CrossRefPubMedGoogle Scholar
  4. 4.
    Ishikawa H, Barber GN (2008) STING is an endoplasmic reticulum adaptor that facilitates innate immune signalling. Nature 455(7213):674–678. doi: 10.1038/nature07317 CrossRefPubMedPubMedCentralGoogle Scholar
  5. 5.
    Sun W, Li Y, Chen L, Chen H, You F, Zhou X, Zhou Y, Zhai Z, Chen D, Jiang Z (2009) ERIS, an endoplasmic reticulum IFN stimulator, activates innate immune signaling through dimerization. Proc Natl Acad Sci U S A 106(21):8653–8658. doi: 10.1073/pnas.0900850106 CrossRefPubMedPubMedCentralGoogle Scholar
  6. 6.
    Zhong B, Yang Y, Li S, Wang YY, Li Y, Diao F, Lei C, He X, Zhang L, Tien P, Shu HB (2008) The adaptor protein MITA links virus-sensing receptors to IRF3 transcription factor activation. Immunity 29(4):538–550. doi: 10.1016/j.immuni.2008.09.003 CrossRefGoogle Scholar
  7. 7.
    Jin L, Waterman PM, Jonscher KR, Short CM, Reisdorph NA, Cambier JC (2008) MPYS, a novel membrane tetraspanner, is associated with major histocompatibility complex class II and mediates transduction of apoptotic signals. Mol Cell Biol 28(16):5014–5026. doi: 10.1128/MCB.00640-08 CrossRefPubMedPubMedCentralGoogle Scholar
  8. 8.
    Tanaka Y, Chen ZJ (2012) STING specifies IRF3 phosphorylation by TBK1 in the cytosolic DNA signaling pathway. Sci Signal 5(214):ra20. doi: 10.1126/scisignal.2002521 CrossRefPubMedPubMedCentralGoogle Scholar
  9. 9.
    Liu S, Cai X, Wu J, Cong Q, Chen X, Li T, Du F, Ren J, Wu YT, Grishin NV, Chen ZJ (2015) Phosphorylation of innate immune adaptor proteins MAVS, STING, and TRIF induces IRF3 activation. Science 347(6227):aaa2630. doi: 10.1126/science.aaa2630 CrossRefPubMedPubMedCentralGoogle Scholar
  10. 10.
    Dey B, Dey RJ, Cheung LS, Pokkali S, Guo H, Lee JH, Bishai WR (2015) A bacterial cyclic dinucleotide activates the cytosolic surveillance pathway and mediates innate resistance to tuberculosis. Nat Med 21(4):401–406. doi: 10.1038/nm.3813 CrossRefPubMedPubMedCentralGoogle Scholar
  11. 11.
    Woodward JJ, Iavarone AT, Portnoy DA (2010) c-di-AMP secreted by intracellular Listeria monocytogenes activates a host type I interferon response. Science 328(5986):1703–1705. doi: 10.1126/science.1189801 CrossRefPubMedPubMedCentralGoogle Scholar
  12. 12.
    Ishikawa H, Ma Z, Barber GN (2009) STING regulates intracellular DNA-mediated, type I interferon-dependent innate immunity. Nature 461(7265):788–792. doi: 10.1038/nature08476 CrossRefPubMedPubMedCentralGoogle Scholar
  13. 13.
    Bhat N, Fitzgerald KA (2014) Recognition of cytosolic DNA by cGAS and other STING-dependent sensors. Eur J Immunol 44(3):634–640. doi: 10.1002/eji.201344127 CrossRefPubMedPubMedCentralGoogle Scholar
  14. 14.
    Sun L, Wu J, Du F, Chen X, Chen ZJ (2013) Cyclic GMP-AMP synthase is a cytosolic DNA sensor that activates the type I interferon pathway. Science 339(6121):786–791. doi: 10.1126/science.1232458 CrossRefPubMedGoogle Scholar
  15. 15.
    Wu J, Sun L, Chen X, Du F, Shi H, Chen C, Chen ZJ (2013) Cyclic GMP-AMP is an endogenous second messenger in innate immune signaling by cytosolic DNA. Science 339(6121):826–830. doi: 10.1126/science.1229963 CrossRefPubMedGoogle Scholar
  16. 16.
    Gao P, Ascano M, Wu Y, Barchet W, Gaffney BL, Zillinger T, Serganov AA, Liu Y, Jones RA, Hartmann G, Tuschl T, Patel DJ (2013) Cyclic [G(2′,5′)pA(3′,5′)p] is the metazoan second messenger produced by DNA-activated cyclic GMP-AMP synthase. Cell 153(5):1094–1107. doi: 10.1016/j.cell.2013.04.046 CrossRefPubMedPubMedCentralGoogle Scholar
  17. 17.
    Ablasser A, Goldeck M, Cavlar T, Deimling T, Witte G, Rohl I, Hopfner KP, Ludwig J, Hornung V (2013) cGAS produces a 2′-5′-linked cyclic dinucleotide second messenger that activates STING. Nature 498(7454):380–384. doi: 10.1038/nature12306 CrossRefPubMedPubMedCentralGoogle Scholar
  18. 18.
    Diner EJ, Burdette DL, Wilson SC, Monroe KM, Kellenberger CA, Hyodo M, Hayakawa Y, Hammond MC, Vance RE (2013) The innate immune DNA sensor cGAS produces a noncanonical cyclic dinucleotide that activates human STING. Cell Rep 3(5):1355–1361. doi: 10.1016/j.celrep.2013.05.009 CrossRefPubMedPubMedCentralGoogle Scholar
  19. 19.
    Zhang X, Shi H, Wu J, Zhang X, Sun L, Chen C, Chen ZJ (2013) Cyclic GMP-AMP containing mixed phosphodiester linkages is an endogenous high-affinity ligand for STING. Mol Cell 51(2):226–235. doi: 10.1016/j.molcel.2013.05.022 CrossRefPubMedGoogle Scholar
  20. 20.
    Mankan AK, Schmidt T, Chauhan D, Goldeck M, Honing K, Gaidt M, Kubarenko AV, Andreeva L, Hopfner KP, Hornung V (2014) Cytosolic RNA:DNA hybrids activate the cGAS-STING axis. EMBO J 33(24):2937–2946. doi: 10.15252/embj.201488726 CrossRefPubMedPubMedCentralGoogle Scholar
  21. 21.
    Pepin G, Nejad C, Thomas BJ, Ferrand J, McArthur K, Bardin PG, Williams BR, Gantier MP (2017) Activation of cGAS-dependent antiviral responses by DNA intercalating agents. Nucleic Acids Res 45(1):198–205. doi: 10.1093/nar/gkw878 CrossRefPubMedGoogle Scholar
  22. 22.
    White MJ, McArthur K, Metcalf D, Lane RM, Cambier JC, Herold MJ, van Delft MF, Bedoui S, Lessene G, Ritchie ME, Huang DC, Kile BT (2014) Apoptotic caspases suppress mtDNA-induced STING-mediated type I IFN production. Cell 159(7):1549–1562. doi: 10.1016/j.cell.2014.11.036 CrossRefPubMedPubMedCentralGoogle Scholar
  23. 23.
    Konig N, Fiehn C, Wolf C, Schuster M, Cura Costa E, Tungler V, Alvarez HA, Chara O, Engel K, Goldbach-Mansky R, Gunther C, Lee-Kirsch MA (2017) Familial chilblain lupus due to a gain-of-function mutation in STING. Ann Rheum Dis 76(2):468–472. doi: 10.1136/annrheumdis-2016-209841 CrossRefPubMedGoogle Scholar
  24. 24.
    Mackenzie KJ, Carroll P, Lettice L, Tarnauskaite Z, Reddy K, Dix F, Revuelta A, Abbondati E, Rigby RE, Rabe B, Kilanowski F, Grimes G, Fluteau A, Devenney PS, Hill RE, Reijns MA, Jackson AP (2016) Ribonuclease H2 mutations induce a cGAS/STING-dependent innate immune response. EMBO J 35(8):831–844. doi: 10.15252/embj.201593339 CrossRefPubMedPubMedCentralGoogle Scholar
  25. 25.
    Gray EE, Treuting PM, Woodward JJ, Stetson DB (2015) Cutting edge: cGAS Is required for lethal autoimmune disease in the trex1-deficient mouse model of aicardi-goutieres syndrome. J Immunol 195(5):1939–1943. doi: 10.4049/jimmunol.1500969 CrossRefPubMedPubMedCentralGoogle Scholar
  26. 26.
    Ahn J, Ruiz P, Barber GN (2014) Intrinsic self-DNA triggers inflammatory disease dependent on STING. J Immunol 193(9):4634–4642. doi: 10.4049/jimmunol.1401337 CrossRefPubMedPubMedCentralGoogle Scholar
  27. 27.
    Dunn GP, Bruce AT, Sheehan KC, Shankaran V, Uppaluri R, Bui JD, Diamond MS, Koebel CM, Arthur C, White JM, Schreiber RD (2005) A critical function for type I interferons in cancer immunoediting. Nat Immunol 6(7):722–729. doi: 10.1038/ni1213 CrossRefPubMedGoogle Scholar
  28. 28.
    Swann JB, Hayakawa Y, Zerafa N, Sheehan KC, Scott B, Schreiber RD, Hertzog P, Smyth MJ (2007) Type I IFN contributes to NK cell homeostasis, activation, and antitumor function. J Immunol 178(12):7540–7549CrossRefPubMedGoogle Scholar
  29. 29.
    Diamond MS, Kinder M, Matsushita H, Mashayekhi M, Dunn GP, Archambault JM, Lee H, Arthur CD, White JM, Kalinke U, Murphy KM, Schreiber RD (2011) Type I interferon is selectively required by dendritic cells for immune rejection of tumors. J Exp Med 208(10):1989–2003. doi: 10.1084/jem.20101158 CrossRefPubMedPubMedCentralGoogle Scholar
  30. 30.
    Fuertes MB, Kacha AK, Kline J, Woo SR, Kranz DM, Murphy KM, Gajewski TF (2011) Host type I IFN signals are required for antitumor CD8+ T cell responses through CD8{alpha}+ dendritic cells. J Exp Med 208(10):2005–2016. doi: 10.1084/jem.20101159 CrossRefPubMedPubMedCentralGoogle Scholar
  31. 31.
    Klarquist J, Hennies CM, Lehn MA, Reboulet RA, Feau S, Janssen EM (2014) STING-mediated DNA sensing promotes antitumor and autoimmune responses to dying cells. J Immunol 193(12):6124–6134. doi: 10.4049/jimmunol.1401869 CrossRefPubMedPubMedCentralGoogle Scholar
  32. 32.
    Woo SR, Fuertes MB, Corrales L, Spranger S, Furdyna MJ, Leung MY, Duggan R, Wang Y, Barber GN, Fitzgerald KA, Alegre ML, Gajewski TF (2014) STING-dependent cytosolic DNA sensing mediates innate immune recognition of immunogenic tumors. Immunity 41(5):830–842. doi: 10.1016/j.immuni.2014.10.017 CrossRefPubMedPubMedCentralGoogle Scholar
  33. 33.
    Demaria O, De Gassart A, Coso S, Gestermann N, Di Domizio J, Flatz L, Gaide O, Michielin O, Hwu P, Petrova TV, Martinon F, Modlin RL, Speiser DE, Gilliet M (2015) STING activation of tumor endothelial cells initiates spontaneous and therapeutic antitumor immunity. Proc Natl Acad Sci U S A 112(50):15408–15413. doi: 10.1073/pnas.1512832112 CrossRefPubMedPubMedCentralGoogle Scholar
  34. 34.
    Kang J, Demaria S, Formenti S (2016) Current clinical trials testing the combination of immunotherapy with radiotherapy. J Immunother Cancer 4:51. doi: 10.1186/s40425-016-0156-7 CrossRefPubMedPubMedCentralGoogle Scholar
  35. 35.
    Ohkuri T, Ghosh A, Kosaka A, Zhu J, Ikeura M, David M, Watkins SC, Sarkar SN, Okada H (2014) STING contributes to antiglioma immunity via triggering type I IFN signals in the tumor microenvironment. Cancer Immunol Res 2(12):1199–1208. doi: 10.1158/2326-6066.CIR-14-0099 CrossRefPubMedPubMedCentralGoogle Scholar
  36. 36.
    Tang CH, Zundell JA, Ranatunga S, Lin C, Nefedova Y, Del Valle JR, Hu CC (2016) Agonist-mediated activation of STING induces apoptosis in malignant B cells. Cancer Res 76(8):2137–2152. doi: 10.1158/0008-5472.CAN-15-1885 CrossRefPubMedPubMedCentralGoogle Scholar
  37. 37.
    Corrales L, Glickman LH, McWhirter SM, Kanne DB, Sivick KE, Katibah GE, Woo SR, Lemmens E, Banda T, Leong JJ, Metchette K, Dubensky TW Jr, Gajewski TF (2015) Direct activation of STING in the tumor microenvironment leads to potent and systemic tumor regression and immunity. Cell Rep 11(7):1018–1030. doi: 10.1016/j.celrep.2015.04.031 CrossRefPubMedPubMedCentralGoogle Scholar
  38. 38.
    Lara PN Jr, Douillard JY, Nakagawa K, von Pawel J, McKeage MJ, Albert I, Losonczy G, Reck M, Heo DS, Fan X, Fandi A, Scagliotti G (2011) Randomized phase III placebo-controlled trial of carboplatin and paclitaxel with or without the vascular disrupting agent vadimezan (ASA404) in advanced non-small-cell lung cancer. J Clin Oncol 29(22):2965–2971. doi: 10.1200/JCO.2011.35.0660. Epub 2011 Jun 27.CrossRefPubMedGoogle Scholar
  39. 39.
    Corrales L, Gajewski TF (2016) Endogenous and pharmacologic targeting of the STING pathway in cancer immunotherapy. Cytokine 77:245–247. doi: 10.1016/j.cyto.2015.08.258 CrossRefPubMedGoogle Scholar
  40. 40.
    Baird JR, Friedman D, Cottam B, Dubensky TW Jr, Kanne DB, Bambina S, Bahjat K, Crittenden MR, Gough MJ (2016) Radiotherapy combined with novel STING-targeting oligonucleotides results in regression of established tumors. Cancer Res 76(1):50–61  https://doi.org/10.1158/0008-5472.CAN-14-3619 CrossRefPubMedGoogle Scholar
  41. 41.
    Li T, Cheng H, Yuan H, Xu Q, Shu C, Zhang Y, Xu P, Tan J, Rui Y, Li P, Tan X (2016) Antitumor activity of cGAMP via stimulation of cGAS-cGAMP-STING-IRF3 mediated innate immune response. Sci Rep 6:19049  https://doi.org/10.1038/srep19049 CrossRefPubMedPubMedCentralGoogle Scholar
  42. 42.
    Nakamura T, Miyabe H, Hyodo M, Sato Y, Hayakawa Y, Harashima H (2015) Liposomes loaded with a STING pathway ligand, cyclic di-GMP, enhance cancer immunotherapy against metastatic melanoma. J Control Release 216:149–157. doi: 10.1016/j.jconrel.2015.08.026 CrossRefPubMedGoogle Scholar
  43. 43.
    Topalian SL, Drake CG, Pardoll DM (2015) Immune checkpoint blockade: a common denominator approach to cancer therapy. Cancer Cell 27(4):450–461. doi: 10.1016/j.ccell.2015.03.001 CrossRefPubMedPubMedCentralGoogle Scholar
  44. 44.
    Wang H, Hu S, Chen X, Shi H, Chen C, Sun L, Chen ZJ (2017) cGAS is essential for the antitumor effect of immune checkpoint blockade. Proc Natl Acad Sci U S A. doi: 10.1073/pnas.1621363114 CrossRefGoogle Scholar
  45. 45.
    Moore E, Clavijo PE, Davis R, Cash H, Van Waes C, Kim Y, Allen C (2016) Established T cell-inflamed tumors rejected after adaptive resistance was reversed by combination STING activation and PD-1 pathway blockade. Cancer Immunol Res 4(12):1061–1071. doi: 10.1158/2326-6066.CIR-16-0104 CrossRefPubMedPubMedCentralGoogle Scholar
  46. 46.
    Fu J, Kanne DB, Leong M, Glickman LH, McWhirter SM, Lemmens E, Mechette K, Leong JJ, Lauer P, Liu W, Sivick KE, Zeng Q, Soares KC, Zheng L, Portnoy DA, Woodward JJ, Pardoll DM, Dubensky TW Jr, Kim Y (2015) STING agonist formulated cancer vaccines can cure established tumors resistant to PD-1 blockade. Sci Transl Med 7(283):283ra252. doi: 10.1126/scitranslmed.aaa4306 CrossRefGoogle Scholar
  47. 47.
    Lemos H, Mohamed E, Huang L, Ou R, Pacholczyk G, Arbab AS, Munn D, Mellor AL (2016) STING promotes the growth of tumors characterized by low antigenicity via IDO activation. Cancer Res 76(8):2076–2081. doi: 10.1158/0008-5472.CAN-15-1456 CrossRefPubMedPubMedCentralGoogle Scholar
  48. 48.
    Huang L, Li L, Lemos H, Chandler PR, Pacholczyk G, Baban B, Barber GN, Hayakawa Y, McGaha TL, Ravishankar B, Munn DH, Mellor AL (2013) Cutting edge: DNA sensing via the STING adaptor in myeloid dendritic cells induces potent tolerogenic responses. J Immunol 191(7):3509–3513. doi: 10.4049/jimmunol.1301419 CrossRefPubMedPubMedCentralGoogle Scholar
  49. 49.
    Lemos H, Huang L, Chandler PR, Mohamed E, Souza GR, Li L, Pacholczyk G, Barber GN, Hayakawa Y, Munn DH, Mellor AL (2014) Activation of the STING adaptor attenuates experimental autoimmune encephalitis. J Immunol 192(12):5571–5578. doi: 10.4049/jimmunol.1303258 CrossRefPubMedPubMedCentralGoogle Scholar
  50. 50.
    Andzinski L, Spanier J, Kasnitz N, Kroger A, Jin L, Brinkmann MM, Kalinke U, Weiss S, Jablonska J, Lienenklaus S (2016) Growing tumors induce a local STING dependent Type I IFN response in dendritic cells. Int J Cancer 139(6):1350–1357. doi: 10.1002/ijc.30159 CrossRefPubMedGoogle Scholar
  51. 51.
    Stetson DB, Medzhitov R (2006) Recognition of cytosolic DNA activates an IRF3-dependent innate immune response. Immunity 24(1):93–103. doi: 10.1016/j.immuni.2005.12.003 CrossRefPubMedGoogle Scholar
  52. 52.
    Ablasser A, Schmid-Burgk JL, Hemmerling I, Horvath GL, Schmidt T, Latz E, Hornung V (2013) Cell intrinsic immunity spreads to bystander cells via the intercellular transfer of cGAMP. Nature 503(7477):530–534. doi: 10.1038/nature12640 CrossRefPubMedPubMedCentralGoogle Scholar
  53. 53.
    Stetson DB, Ko JS, Heidmann T, Medzhitov R (2008) Trex1 prevents cell-intrinsic initiation of autoimmunity. Cell 134(4):587–598. doi: 10.1016/j.cell.2008.06.032 CrossRefPubMedPubMedCentralGoogle Scholar
  54. 54.
    Lan YY, Londono D, Bouley R, Rooney MS, Hacohen N (2014) Dnase2a deficiency uncovers lysosomal clearance of damaged nuclear DNA via autophagy. Cell Rep 9(1):180–192. doi: 10.1016/j.celrep.2014.08.074 CrossRefPubMedPubMedCentralGoogle Scholar
  55. 55.
    Kretschmer S, Wolf C, Konig N, Staroske W, Guck J, Hausler M, Luksch H, Nguyen LA, Kim B, Alexopoulou D, Dahl A, Rapp A, Cardoso MC, Shevchenko A, Lee-Kirsch MA (2015) SAMHD1 prevents autoimmunity by maintaining genome stability. Ann Rheum Dis 74(3):e17. doi: 10.1136/annrheumdis-2013-204845 CrossRefPubMedGoogle Scholar
  56. 56.
    Gehrke N, Mertens C, Zillinger T, Wenzel J, Bald T, Zahn S, Tuting T, Hartmann G, Barchet W (2013) Oxidative damage of DNA confers resistance to cytosolic nuclease TREX1 degradation and potentiates STING-dependent immune sensing. Immunity 39(3):482–495. doi: 10.1016/j.immuni.2013.08.004 CrossRefPubMedGoogle Scholar
  57. 57.
    Hartlova A, Erttmann SF, Raffi FA, Schmalz AM, Resch U, Anugula S, Lienenklaus S, Nilsson LM, Kroger A, Nilsson JA, Ek T, Weiss S, Gekara NO (2015) DNA damage primes the type I interferon system via the cytosolic DNA sensor STING to promote anti-microbial innate immunity. Immunity 42(2):332–343. doi: 10.1016/j.immuni.2015.01.012 CrossRefPubMedGoogle Scholar
  58. 58.
    Wolf C, Rapp A, Berndt N, Staroske W, Schuster M, Dobrick-Mattheuer M, Kretschmer S, Konig N, Kurth T, Wieczorek D, Kast K, Cardoso MC, Gunther C, Lee-Kirsch MA (2016) RPA and Rad51 constitute a cell intrinsic mechanism to protect the cytosol from self DNA. Nat Commun 7:11752. doi: 10.1038/ncomms11752 CrossRefPubMedPubMedCentralGoogle Scholar
  59. 59.
    Hanahan D, Weinberg RA (2011) Hallmarks of cancer: the next generation. Cell 144(5):646–674. doi: 10.1016/j.cell.2011.02.013 CrossRefPubMedPubMedCentralGoogle Scholar
  60. 60.
    Lam AR, Le Bert N, Ho SS, Shen YJ, Tang ML, Xiong GM, Croxford JL, Koo CX, Ishii KJ, Akira S, Raulet DH, Gasser S (2014) RAE1 ligands for the NKG2D receptor are regulated by STING-dependent DNA sensor pathways in lymphoma. Cancer Res 74(8):2193–2203. doi: 10.1158/0008-5472.CAN-13-1703 CrossRefPubMedPubMedCentralGoogle Scholar
  61. 61.
    Parkes EE, Walker SM, Taggart LE, McCabe N, Knight LA, Wilkinson R, McCloskey KD, Buckley NE, Savage KI, Salto-Tellez M, McQuaid S, Harte MT, Mullan PB, Harkin DP, Kennedy RD (2017) Activation of STING-dependent innate immune signaling By S-phase-specific DNA damage in breast cancer. J Natl Cancer Inst 109(1). doi: 10.1093/jnci/djw199 CrossRefPubMedPubMedCentralGoogle Scholar
  62. 62.
    Shen YJ, Le Bert N, Chitre AA, Koo CX, Nga XH, Ho SS, Khatoo M, Tan NY, Ishii KJ, Gasser S (2015) Genome-derived cytosolic DNA mediates type I interferon-dependent rejection of B cell lymphoma cells. Cell Rep 11(3):460–473. doi: 10.1016/j.celrep.2015.03.041 CrossRefPubMedGoogle Scholar
  63. 63.
    Ho SS, Zhang WY, Tan NY, Khatoo M, Suter MA, Tripathi S, Cheung FS, Lim WK, Tan PH, Ngeow J, Gasser S (2016) The DNA structure-specific endonuclease MUS81 mediates DNA sensor STING-dependent host rejection of prostate cancer cells. Immunity 44(5):1177–1189. doi: 10.1016/j.immuni.2016.04.010 CrossRefPubMedGoogle Scholar
  64. 64.
    Xia T, Konno H, Ahn J, Barber GN (2016) Deregulation of STING signaling in colorectal carcinoma constrains DNA damage responses and correlates with tumorigenesis. Cell Rep 14(2):282–297. doi: 10.1016/j.celrep.2015.12.029 CrossRefPubMedGoogle Scholar
  65. 65.
    Xia T, Konno H, Barber GN (2016) Recurrent loss of STING signaling in melanoma correlates with susceptibility to viral oncolysis. Cancer Res. doi: 10.1158/0008-5472.CAN-16-1404 CrossRefPubMedGoogle Scholar
  66. 66.
    Gall A, Treuting P, Elkon KB, Loo YM, Gale M Jr, Barber GN, Stetson DB (2012) Autoimmunity initiates in nonhematopoietic cells and progresses via lymphocytes in an interferon-dependent autoimmune disease. Immunity 36(1):120–131. doi: 10.1016/j.immuni.2011.11.018 CrossRefPubMedPubMedCentralGoogle Scholar
  67. 67.
    Gajewski TF (2006) Identifying and overcoming immune resistance mechanisms in the melanoma tumor microenvironment. Clin Cancer Res 12(7 Pt 2):2326s–2330s. doi: 10.1158/1078-0432.CCR-05-2517 CrossRefPubMedGoogle Scholar
  68. 68.
    Hwang WT, Adams SF, Tahirovic E, Hagemann IS, Coukos G (2012) Prognostic significance of tumor-infiltrating T cells in ovarian cancer: a meta-analysis. Gynecol Oncol 124(2):192–198. doi: 10.1016/j.ygyno.2011.09.039 CrossRefPubMedGoogle Scholar
  69. 69.
    Mahmoud SM, Paish EC, Powe DG, Macmillan RD, Grainge MJ, Lee AH, Ellis IO, Green AR (2011) Tumor-infiltrating CD8+ lymphocytes predict clinical outcome in breast cancer. J Clin Oncol 29(15):1949–1955. doi: 10.1200/JCO.2010.30.5037 CrossRefPubMedGoogle Scholar
  70. 70.
    Galon J, Costes A, Sanchez-Cabo F, Kirilovsky A, Mlecnik B, Lagorce-Pages C, Tosolini M, Camus M, Berger A, Wind P, Zinzindohoue F, Bruneval P, Cugnenc PH, Trajanoski Z, Fridman WH, Pages F (2006) Type, density, and location of immune cells within human colorectal tumors predict clinical outcome. Science 313(5795):1960–1964. doi: 10.1126/science.1129139 CrossRefPubMedGoogle Scholar
  71. 71.
    Pages F, Kirilovsky A, Mlecnik B, Asslaber M, Tosolini M, Bindea G, Lagorce C, Wind P, Marliot F, Bruneval P, Zatloukal K, Trajanoski Z, Berger A, Fridman WH, Galon J (2009) In situ cytotoxic and memory T cells predict outcome in patients with early-stage colorectal cancer. J Clin Oncol 27(35):5944–5951. doi: 10.1200/JCO.2008.19.6147 CrossRefPubMedGoogle Scholar
  72. 72.
    Yang CA, Huang HY, Chang YS, Lin CL, Lai IL, Chang JG (2017) DNA-sensing and nuclease gene expressions as markers for colorectal cancer progression. Oncology 92(2):115–124. doi: 10.1159/000452281 CrossRefPubMedGoogle Scholar
  73. 73.
    Kodigepalli KM, Nanjundan M (2015) Induction of PLSCR1 in a STING/IRF3-dependent manner upon vector transfection in ovarian epithelial cells. PLoS One 10(2):e0117464. doi: 10.1371/journal.pone.0117464 CrossRefPubMedPubMedCentralGoogle Scholar
  74. 74.
    Small W Jr, Bacon MA, Bajaj A, Chuang LT, Fisher BJ, Harkenrider MM, Jhingran A, Kitchener HC, Mileshkin LR, Viswanathan AN, Gaffney DK (2017) Cervical cancer: a global health crisis. Cancer. doi: 10.1002/cncr.30667 CrossRefPubMedGoogle Scholar
  75. 75.
    Xiao D, Huang W, Ou M, Guo C, Ye X, Liu Y, Wang M, Zhang B, Zhang N, Huang S, Zang J, Zhou Z, Wen Z, Zeng C, Wu C, Huang C, Wei X, Yang G, Jing C (2016) Interaction between susceptibility loci in cGAS-STING pathway, MHC gene and HPV infection on the risk of cervical precancerous lesions in Chinese population. Oncotarget. doi: 10.18632/oncotarget.12399
  76. 76.
    Sunthamala N, Thierry F, Teissier S, Pientong C, Kongyingyoes B, Tangsiriwatthana T, Sangkomkamhang U, Ekalaksananan T (2014) E2 proteins of high risk human papillomaviruses down-modulate STING and IFN-kappa transcription in keratinocytes. PLoS One 9(3):e91473. doi: 10.1371/journal.pone.0091473 CrossRefPubMedPubMedCentralGoogle Scholar
  77. 77.
    Lau L, Gray EE, Brunette RL, Stetson DB (2015) DNA tumor virus oncogenes antagonize the cGAS-STING DNA-sensing pathway. Science 350(6260):568–571. doi: 10.1126/science.aab3291 CrossRefPubMedGoogle Scholar
  78. 78.
    Song S, Peng P, Tang Z, Zhao J, Wu W, Li H, Shao M, Li L, Yang C, Duan F, Zhang M, Zhang J, Wu H, Li C, Wang X, Wang H, Ruan Y, Gu J (2017) Decreased expression of STING predicts poor prognosis in patients with gastric cancer. Sci Rep 7:39858. doi: 10.1038/srep39858 CrossRefPubMedPubMedCentralGoogle Scholar
  79. 79.
    Loewenstein WR, Kanno Y (1966) Intercellular communication and the control of tissue growth: lack of communication between cancer cells. Nature 209(5029):1248–1249CrossRefPubMedGoogle Scholar
  80. 80.
    Aasen T, Mesnil M, Naus CC, Lampe PD, Laird DW (2016) Gap junctions and cancer: communicating for 50 years. Nat Rev Cancer 16(12):775–788. doi: 10.1038/nrc.2016.105 CrossRefPubMedPubMedCentralGoogle Scholar
  81. 81.
    Xu N, Chen HJ, Chen SH, Xue XY, Chen H, Zheng QS, Wei Y, Li XD, Huang JB, Cai H, Sun XL (2016) Reduced Connexin 43 expression is associated with tumor malignant behaviors and biochemical recurrence-free survival of prostate cancer. Oncotarget 7(41):67476–67484. doi: 10.18632/oncotarget.11231 CrossRefPubMedPubMedCentralGoogle Scholar
  82. 82.
    Danos K, Brauswetter D, Birtalan E, Pato A, Bencsik G, Krenacs T, Petak I, Tamas L (2016) The potential prognostic value of Connexin 43 expression in head and neck squamous cell carcinomas. Appl Immunohistochem Mol Morphol 24(7):476–481. doi: 10.1097/PAI.0000000000000212 CrossRefPubMedGoogle Scholar
  83. 83.
    Ostrow RS, Manias DA, Fong WJ, Zachow KR, Faras AJ (1987) A survey of human cancers for human papillomavirus DNA by filter hybridization. Cancer 59(3):429–434CrossRefPubMedGoogle Scholar
  84. 84.
    Sun P, Dong L, MacDonald AI, Akbari S, Edward M, Hodgins MB, Johnstone SR, Graham SV (2015) HPV16 E6 controls the gap junction protein Cx43 in cervical tumour cells. Virus 7(10):5243–5256. doi: 10.3390/v7102871 CrossRefGoogle Scholar
  85. 85.
    Tittarelli A, Guerrero I, Tempio F, Gleisner MA, Avalos I, Sabanegh S, Ortiz C, Michea L, Lopez MN, Mendoza-Naranjo A, Salazar-Onfray F (2016) Overexpression of Connexin 43 reduces melanoma proliferative and metastatic capacity. Br J Cancer 115(9):e14. doi: 10.1038/bjc.2016.296 CrossRefPubMedPubMedCentralGoogle Scholar
  86. 86.
    Li X, Pan JH, Song B, Xiong EQ, Chen ZW, Zhou ZS, Su YP (2012) Suppression of CX43 expression by miR-20a in the progression of human prostate cancer. Cancer Biol Ther 13(10):890–898. doi: 10.4161/cbt.20841 CrossRefPubMedGoogle Scholar
  87. 87.
    Qin H, Shao Q, Curtis H, Galipeau J, Belliveau DJ, Wang T, Alaoui-Jamali MA, Laird DW (2002) Retroviral delivery of Connexin genes to human breast tumor cells inhibits in vivo tumor growth by a mechanism that is independent of significant gap junctional intercellular communication. J Biol Chem 277(32):29132–29138. doi: 10.1074/jbc.M200797200 CrossRefPubMedGoogle Scholar
  88. 88.
    Shi H, Shi D, Wu Y, Shen Q, Li J (2016) Qigesan inhibits migration and invasion of esophageal cancer cells via inducing connexin expression and enhancing gap junction function. Cancer Lett 380(1):184–190. doi: 10.1016/j.canlet.2016.06.015 CrossRefPubMedGoogle Scholar
  89. 89.
    Stewart MK, Bechberger JF, Welch I, Naus CC, Laird DW (2015) Cx26 knockout predisposes the mammary gland to primary mammary tumors in a DMBA-induced mouse model of breast cancer. Oncotarget 6(35):37185–37199. doi: 10.18632/oncotarget.5953 CrossRefPubMedPubMedCentralGoogle Scholar
  90. 90.
    Pepin G, Ferrand J, Honing K, Jayasekara WS, Cain JE, Behlke MA, Gough DJ, Williams BRG, Hornung V, Gantier MP (2016) Cre-dependent DNA recombination activates a STING-dependent innate immune response. Nucleic Acids Res 44(11):5356–5364. doi: 10.1093/nar/gkw405 CrossRefPubMedPubMedCentralGoogle Scholar
  91. 91.
    Ito A, Katoh F, Kataoka TR, Okada M, Tsubota N, Asada H, Yoshikawa K, Maeda S, Kitamura Y, Yamasaki H, Nojima H (2000) A role for heterologous gap junctions between melanoma and endothelial cells in metastasis. J Clin Invest 105(9):1189–1197. doi: 10.1172/JCI8257 CrossRefPubMedPubMedCentralGoogle Scholar
  92. 92.
    Zhang A, Hitomi M, Bar-Shain N, Dalimov Z, Ellis L, Velpula KK, Fraizer GC, Gourdie RG, Lathia JD (2015) Connexin 43 expression is associated with increased malignancy in prostate cancer cell lines and functions to promote migration. Oncotarget 6(13):11640–11651. doi: 10.18632/oncotarget.3449 CrossRefPubMedPubMedCentralGoogle Scholar
  93. 93.
    Chen Q, Boire A, Jin X, Valiente M, Er EE, Lopez-Soto A, Jacob LS, Patwa R, Shah H, Xu K, Cross JR, Massague J (2016) Carcinoma-astrocyte gap junctions promote brain metastasis by cGAMP transfer. Nature 533(7604):493–498. doi: 10.1038/nature18268 CrossRefPubMedPubMedCentralGoogle Scholar
  94. 94.
    Zou ZW, Chen HJ, Yu JL, Huang ZH, Fang S, Lin XH (2016) Gap junction composed of connexin43 modulates 5fluorouracil, oxaliplatin and irinotecan resistance on colorectal cancers. Mol Med Rep 14(5):4893–4900. doi: 10.3892/mmr.2016.5812 CrossRefPubMedGoogle Scholar
  95. 95.
    Yang Y, Zhu J, Zhang N, Zhao Y, Li WY, Zhao FY, Ou YR, Qin SK, Wu Q (2016) Impaired gap junctions in human hepatocellular carcinoma limit intrinsic oxaliplatin chemosensitivity: a key role of connexin 26. Int J Oncol 48(2):703–713. doi: 10.3892/ijo.2015.3266 CrossRefPubMedGoogle Scholar
  96. 96.
    Murphy SF, Varghese RT, Lamouille S, Guo S, Pridham KJ, Kanabur P, Osimani AM, Sharma S, Jourdan J, Rodgers CM, Simonds GR, Gourdie RG, Sheng Z (2016) Connexin 43 inhibition sensitizes chemoresistant glioblastoma cells to temozolomide. Cancer Res 76(1):139–149. doi: 10.1158/0008-5472.CAN-15-1286 CrossRefPubMedGoogle Scholar
  97. 97.
    Minn AJ (2015) Interferons and the immunogenic effects of cancer therapy. Trends Immunol 36(11):725–737. doi: 10.1016/j.it.2015.09.007 CrossRefPubMedPubMedCentralGoogle Scholar
  98. 98.
    Ahn J, Xia T, Konno H, Konno K, Ruiz P, Barber GN (2014) Inflammation-driven carcinogenesis is mediated through STING. Nat Commun 5:5166. doi: 10.1038/ncomms6166 CrossRefPubMedPubMedCentralGoogle Scholar
  99. 99.
    Kitai Y, Kawasaki T, Sueyoshi T, Kobiyama K, Ishii KJ, Zou J, Akira S, Matsuda T, Kawai T (2017) DNA-containing exosomes derived from cancer cells treated with topotecan activate a STING-dependent pathway and reinforce antitumor immunity. J Immunol 198(4):1649–1659. doi: 10.4049/jimmunol.1601694 CrossRefPubMedGoogle Scholar
  100. 100.
    Deng L, Liang H, Xu M, Yang X, Burnette B, Arina A, Li XD, Mauceri H, Beckett M, Darga T, Huang X, Gajewski TF, Chen ZJ, Fu YX, Weichselbaum RR (2014) STING-dependent cytosolic DNA sensing promotes radiation-induced type I interferon-dependent antitumor immunity in immunogenic tumors. Immunity 41(5):843–852. doi: 10.1016/j.immuni.2014.10.019 CrossRefPubMedPubMedCentralGoogle Scholar
  101. 101.
    Erdal E, Haider S, Rehwinkel J, Harris AL, McHugh PJ (2017) A prosurvival DNA damage-induced cytoplasmic interferon response is mediated by end resection factors and is limited by Trex1. Genes Dev 31(4):353–369. doi: 10.1101/gad.289769.116 CrossRefPubMedPubMedCentralGoogle Scholar
  102. 102.
    Gaston J, Cheradame L, Yvonnet V, Deas O, Poupon MF, Judde JG, Cairo S, Goffin V (2016) Intracellular STING inactivation sensitizes breast cancer cells to genotoxic agents. Oncotarget. doi: 10.18632/oncotarget.12858

Copyright information

© Springer Nature Singapore Pte Ltd. 2017

Authors and Affiliations

  1. 1.Centre for Innate Immunity and Infectious DiseasesHudson Institute of Medical ResearchClaytonAustralia
  2. 2.Department of Molecular and Translational ScienceMonash UniversityClaytonAustralia

Personalised recommendations