cGAS/STING signalling pathway in senescence and oncogenesis

Abstract

The cGAS/STING signaling pathway is a crucial component of the innate immune system, playing significant roles in sensing cytosolic DNA, regulating cellular senescence, and contributing to oncogenesis. Recent advances have shed new lights into the molecular mechanisms governing pathway activation in multiple pathophysiological settings, the indispensable roles of cGAS/STING signaling in cellular senescence, and its context-dependent roles in cancer development and suppression. This review summarizes current knowledge related to the biology of cGAS/STING signaling pathway and its participations into senescence and oncogenesis. We further explore the clinical implications and therapeutic potential for cGAS/STING targeted therapies, and faced challenges in the field. With a focus on molecular mechanisms and emerging pharmacological targets, this review underscores the importance of future studies to harness the therapeutic potential of the cGAS/STING pathway in treating senescence-related disorders and cancer. Advanced understanding of the regulatory mechanisms of cGAS/STING signaling, along with the associated deregulations in diseases, combined with the development of new classes of cGAS/STING modulators, holds great promise for creating novel and effective therapeutic strategies. These advancements could address current treatment challenges and unlock the full potential of cGAS/STING in treating senescence-related disorders and oncogenesis.

1. Introduction

Innate immunity is the first-line defense against foreign invasive materials to trigger adaptive immunity and clear infection in mammals by recognizing unique infectious materials [1–3]. Among all characterized innate immune signaling pathways, the cGAS/STING signaling pathway has drawn extensive attention due to its essential roles in DNA sensing and a plethora of distinct pathophysiological roles including viral infection, heart diseases, cancer and ageing [4]. cGAS (cGAMP synthase) is an evolutionarily conserved cytosolic DNA sensor that upon binding DNA, catalyzes the synthesis of a special second messenger 2’3’-cyclic GMP-AMP (cGAMP) [5, 6]. 2’3’-cGAMP subsequently binds and activates STING (stimulator of interferon genes), a transmembrane adaptor protein located in the endoplasmic reticulum (ER) [7]. Activation of STING leads to the recruitment and activation of downstream signaling molecules, such as TBK1 and IRF3, culminating in the production of type I interferons and other pro-inflammatory cytokines [8]. This cascade of events forms the backbone of the cGAS/STING pathway, essential for mounting an effective immune response against intracellular pathogens and damaged self-DNA. The cGAS/STING signaling pathway has emerged as a pivotal player in the innate immune response, responsible for detecting and responding to cytosolic DNA [9]. Beyond innate immunity, due to its unique role in detecting aberrant cytosolic DNA, which could be accumulated under pathological conditions with mitochondria dysfunction, DNA damage, ageing or upon cancer treatments, this pathway is also essential for maintaining cellular homeostasis and plays critical roles in various pathophysiological processes, including senescence and oncogenesis through distinct molecular mechanisms that we will focus on in this review [10].

Cellular senescence and oncogenesis are associated with fundamental biological processes with profound implications in human health and diseases [11, 12]. Cellular senescence is a state with stable and irreversible cell cycle arrest where cells enter in response to various cellular intrinsic or extrinsic stimuli, including DNA damage, oxidative stress, replicative stress, oncogenic signals (oncogene-induced senescence), mitochondria dysfunction and others [13–15]. This is largely caused by induced expression of cell cycle inhibitors (especially cyclin dependent kinase (CDK) inhibitors, including p14^ARF, p16^INK4 and p21^CIP1 [16]. Although senescence is characterized by a cell proliferation cease with enlarged and flattened cell morphology due to rupture of nuclear envelopes, they are metabolically active. Senescence-associated β-galactosidase (SA-β-gal) is a common senescence marker used for staining senescent cells. Another feature of cellular senescence is the senescence-associated secretory phenotype (SASP), which is characterized by the expression and secretion of pro-inflammatory cytokines, chemokines, and growth factors such as IL-6 and IL-8, to reinforce senescence-induced growth arrest and modulate tissue microenvironment [17]. Depending on the tissue context, SASP as a unique senescent cell-derived secretome could either facilitate or prevent the tumorigenic process by triggering inflammation. Moreover, although cancer treatments induce cancer cell senescence to kill cancer cells, these senescent cells could become resistant to apoptosis leading to evasion of cancer therapy [18]. In addition, senescence is also usually associated with observed DNA damage, epigenetic alternations and other molecular changes [19, 20].

Due to its ability to sense cytosolic DNA arising from genomic instability or mitochondrial damage, the cGAS/STING pathway has been implicated in the regulation of senescence [21, 22]. Activated cGAS/STING signaling in senescent cells could also induce SASP. In addition, the role of the cGAS/STING pathway in cancer is complex and context-dependent, exhibiting both tumor-suppressive and tumor-promoting effects. On one hand, activation of the tumor cell cGAS/STING pathway can enhance anti-tumor immunity by promoting the production of type I interferons for facilitating immune cell infiltrates into tumors and activating cytotoxic T cells [23]. This immune activation can lead to the recognition and destruction of tumor cells, thereby suppressing tumor growth. On the other hand, chronic activation of the cGAS/STING pathway, particularly within the tumor microenvironment and immune cells, could contribute to inflammation, immune cell destruction and immune suppression, facilitating tumor progression and metastasis [24]. In addition, tumors may exploit the SASP induced by senescent cells to create a pro-tumorigenic microenvironment. Moreover, some cancer cells develop mechanisms to evade cGAS/STING-mediated immune detection, further complicating the pathway’s role in cancer biology.

Understanding the mechanisms underlying cellular senescence and oncogenesis is crucial for developing therapeutic strategies to combat age-related diseases and cancer. The cGAS/STING signaling pathway has emerged as a key player in these processes, influencing both intrinsic cell behavior and extra-cellular immune responses in both senescence and oncogenesis. Thus, this review explores the roles of the cGAS/STING pathway in these contexts, highlighting its potential as a therapeutic target for combating human diseases.

2. The cGAS/STING Signaling Pathway

The cGAS/STING signaling pathway is a highly orchestrated molecular network that serves as a sentinel for cellular integrity (Figure 1). Central to this pathway is the cyclic GMP-AMP synthase (cGAS), a cytoplasmic DNA sensor discovered in 2013 poised to detect aberrantly presented cytoplasmic DNA within the cell [5, 6]. cGAS recognizes dsDNA in a DNA length-dependent, but DNA sequence-independent manner [25]. Upon binding cytosolic dsDNA, cGAS undergoes a conformational change and phase transition [26] necessary for its enzymatic activation to catalyze the synthesis of 2’3’-cyclic GMP-AMP (cGAMP) from ATP and GTP [27]. 2’3’-cGAMP functions as a secondary messenger, conveying the signal of DNA detection to distinct downstream effectors. The major well-characterized 2’3’-cGAMP effector is STING, an essential innate immune sensor discovered in 2008 as an ER-anchored protein [7]. 2’3’-cGAMP binding to STING induces a dramatic STING conformational change that facilitates its polymerization [28], trafficking from the ER to the Golgi apparatus [29] with a modification by palmitoylation [30] and subsequent recruitment and activation of downstream signaling components including TBK1 (TANK-binding kinase 1) [31] and IRF3 (interferon regulatory factor 3) (Figure 1)[32]. This leads to IRF3 phosphorylation, dimerization and translocation into nucleus to induce the transcription of type I interferons and other inflammatory cytokines, which play crucial roles in innate immune responses through autocrine and paracrine manners [32–34]. Moreover, the cGAS/STING pathway intersects with the NF-κB pathway, another central player in immune regulation. cGAS/STING-mediated activation of NF-κB contributes to the transcription of pro-inflammatory cytokines, strengthening the immune response against potential threats [35]. Activation of cGAS/STING also triggers death and clearance of infected cells via multiple cell death forms [36]. Some forms like apoptosis does not, while others further enhance immune activation such as necroptosis [37–39]. This intricate molecular cascade not only underscores the efficiency of the cGAS/STING pathway in sensing and responding to aberrant DNA but also highlights its role as a central hub in coordinating diverse immune signaling pathways. The precise regulation of these molecular events ensures a rapid and targeted response to cellular stresses, providing a robust defense mechanism against various challenges, including viral infections and cellular damage. As research in this field continues to grow rapidly, filling in knowledge gaps and uncovering regulatory mechanisms for the cGAS/STING signaling pathway will not only advance our understanding of innate immunity but also offer potential avenues for therapeutic interventions in human diseases where its dysregulation is implicated.

Figure 1. Overview of the cGAS/STING signaling.

cGAS is a cytosolic DNA sensor that, upon binding to DNA, catalyzes the synthesis of 2’3’-cyclic GMP-AMP (2’3’-cGAMP), a second messenger. cGAMP subsequently binds to and activates STING, a transmembrane adaptor protein located in the endoplasmic reticulum. Activation of STING leads to the recruitment and activation of downstream signaling molecules, such as TBK1 and IRF3, culminating in the production of type I interferons and other pro-inflammatory cytokines to activate innate immunity. In addition to the canonical innate immune regulatory function, multiple components within the cGAS/STING signaling, such as cGAS, 2’3’-cGAMP and STING also exert innate immunity-independent function. For example, cGAS binds PARP1 to suppress HR. 2’3’-cGAMP also binds EF1A1 to suppress protein translation. In RCC, STING binds VDAC2 on mitochondria to maintain calcium homeostasis.

Excessive activation of cytosolic DNA sensing and signaling leads to autoimmune diseases [40], whereas the suppression of cytosolic DNA sensing plays a role in evading immune destruction during tumorigenesis and contributes to resistance against immunotherapies [41]. Thus, the activities of cGAS/STING signaling must be tightly monitored and controlled. This balance is achieved via multi-layers of surveillance mechanisms, including but not limited to availability of cytosolic DNA (eg. TREX-1 degrades DNA to alleviate cGAS activation [42]; glucose binds and stimulates NSUN2/TREX-2 signaling to inactivate cGAS/STING), cellular localization of cGAS (eg. plasma membrane-located cGAS bypasses activation by self-DNA [43], and nuclear cGAS is inactivated by either tightly tethering to nucleosomes [44–50], binding to BAF [51] or undergoing extensive phosphorylation during mitosis [51, 52]) and STING (including nuclear STING and STING packed in extracellular vehicles [53, 54]), post-translational modifications of cGAS (see [8, 55, 56] for review) and STING [34, 57], binding proteins for cGAS (including both host proteins [58–61] and viral/bacterial proteins [62]) or STING [63], control by ions such as Mn²⁺ (Mn²⁺ has been shown to direct activate cGAS via a non-canonical 2’3’-cGAMP synthesis approach [64]), control of 2’3’-cGAMP cellular abundance (eg. by cGAMP transporters [65–69] and cGAMP hydrolyzing enzymes ENPP1 [70] and ENPP3 [71]) and others. Additionally, cGAS/STING interactions with other cellular components, such as DNA repair proteins and autophagy machinery, influence the activation and termination of the pathway [72].

In addition to canonical function of cGAS/STING in regulating innate immune responses, both cGAS and STING exert roles in governing other physiological function. For example, nuclear cGAS was reported to interact with PARP1 to suppress homologous recombination (HR) [73]. Upon cGAS activation, STING translocates to lysosomes/autophagosomes, where STING induces lysosome rupture leading to lysosomal cell death [74]. In addition, STING was also reported to modulate calcium homeostasis to prime T cell death [75]. Recently, we reported an unexpected STING function in maintaining renal cell mitochondrial calcium homeostasis and cell death [76].

Beyond cGAS and STING, 2’3’-cGAMP also exerts innate immunity-independent function. Effects of 2’3’-cGAMP in triggering anti-viral responses are not limited in mammals but also in lower organisms such as sea anemone Nematostella vectensis [77]. 2’3’-cGAMP can be transported in and out of cells by various uniporters and importers including SLC46A2 [65] LRRC8C [66, 67], SLC19A1 [68, 69] or via physical structures like gap junctions [78]. Beyond STING, additional 2’3’-cGAMP effectors have been reported including EF1A1 in protein translational control [79] to expand our knowledge of 2’3’-cGAMP biological function. Together, these innate immunity-dependent and -independent roles of cGAS/STING signaling are fundamental for roles of this signaling in senescence and oncogenesis.

3. Roles in Senescence

Cellular senescence is a state of irreversible cell proliferation and cell cycle arrest, which occurs during aging and in age-related diseases, and in response to internal or external stressors such as DNA damage, telomere shortening, UV, ionizing radiation, ER stress, oncogenic stress and distinct treatments [80]. Initially recognized as a mechanism to prevent the replication of damaged or potentially cancerous cells, senescence plays a crucial role in maintaining tissue homeostasis and a barrier for tumor development. Certain developed cancer treatments such as chemotherapy and immunotherapy aim to induce cancer cell senescence to stop tumor growth [81], while this also results in senescence of normal healthy cells due to the systemic nature of the treatments [82, 83]. In addition, senescent cells secrete SASPs including cytokines, chemokines, growth factors, and matrix metalloproteinases, to promote tissue repair and immune surveillance. A sustained DNA damage response is central to the induction of cellular senescence, leading to the expression of CDK inhibitors p16^INK4a and p21^WAF1/CIP1 [84, 85]. SASP changes dynamically during the life cycle of senescent cells. Mitochondrial dysfunction is associated with reduced expression of mitochondrial oxidative phosphorylation genes, which leads to cytoplasmic chromatin fragmentation and SASP by triggering the ROS-JNK retrograde signaling pathway [86]. The cGAS/STING signaling pathway contributes to cellular senescence through distinct paths that we will discuss in detail below.

3.1. Regulation of Senescence by cGAS/STING Signaling

Emerging evidence suggests that the cGAS/STING signaling pathway is integral to the induction and regulation of cellular senescence [87]. The clue for roles of cGAS/STING in regulating senescence comes from the observation that cGas^−/− primary MEFs become spontaneously immortalized more rapidly than their WT-counterparts in vitro [88, 89]. Mechanistic studies suggest that during this process, cGAS may sense oxidative DNA damage to facilitate expression of p16 and SA-β-Gal to induce senescence [90].

Due to the critical role of DNA damage signaling in senescence, DNA damage foci can serve as a biomarker for identifying senescent cells. Although a large number of factors contributing to senescence, persistent DNA damage has been proposed as a critical driver to cause and maintain senescence partially through activating both p53/p21 and p16/Rb pathways (see [91] for review). Chemotherapy, radiotherapy or other DNA damaging agents triggers the expression of type-I interferons [92–95] and this renders cell less responsive to viral infections [96, 97]. On the other hand, in vivo studies revealed that chemo- or radio-therapy-induced innate immune activation enhances anti-tumor immunity [97]. Emerging evidence suggests that the cGAS-STING pathway induces the SASPs through the accumulation of cytoplasmic DNA during senescence [88, 98, 99]. Upon DNA damage in cells received chemo- or radio-therapy, cGAS is recruited and enriched in micro-nuclei [88, 100, 101] resulted from nuclear envelop rupture [102], where cGAS could be activated by DNA fragments from chromosome. In cells under genotoxic stresses such as induced by hyperactivation of oncogenes, cGAS is activated evidenced by detectable 2’3’-cGAMP [103] within micronuclei to facilitate senescence. The likely molecular mechanism is that activated cGAS/STING signaling promotes SASP secretion especially IL-6 and IL-8 reported to feedback to cells to further augment and enhance the senescence phenotype (Figure 2) [104, 105]. However, SASP has also been indicated in recruiting immune cells to clear senescent cells to limit liver fibrosis [106] or tumor growth [103]. One possible explanation is that due to the large variety of SASPs, some SASPs such as IL-6 and IL-8 may enhance the senescence of SASP producing cells to augment SASP secretion, which could transform epithelial cells and induce metastasis [107], as well as matrix metalloproteases may increase growth factor concentrations by providing a less stiff microenvironment to facilitate tumor growth [108], while some other SASPs such as IFN-β and ISGs may primarily create immune friendly microenvironment through recruiting immune cells. Thus, SASPs from senescent cells function as a double-edged sword, which in part reflect a similar role of cGAS/STING signaling in senescence and oncogenesis.

Figure 2. Roles of cGAS/STING signaling in senescence.

Various stressors such as reactive oxygen species (ROS) or ultraviolet (UV) radiation can cause nuclear double-strand breaks (DSBs), leading to the accumulation of DNA fragments known as cytoplasmic chromatin fragments (CCFs). Damaged mitochondria can also release mitochondrial DNA (mtDNA) into the cytoplasm. These DNA fragments are recognized by cGAS, which generates 2′3′-cyclic GMP-AMP (2′3′-cGAMP). 2′3′-cGAMP activates STING and TANK-binding kinase 1 (TBK1), resulting in the phosphorylation of IRF3. 2′3′-cGAMP also activates IκBα. These transcription factors enter the nucleus and induce the expression of type I interferons (IFNs) and inflammatory cytokines. Both IRF3 and NF-κB induce the expression of senescence-associated secretory phenotype (SASP) factors such as IFN-β, IL-6, and IL-8 and sustain cellular senescence.

In addition to oncogene-induced senescence (OIS) and various stress-induced senescence models (such as oxidative stress, irradiation, and chemotherapy) cGAS/STING activation has also been observed in replicative senescence [109–113]. This implies that the cGAS/STING pathway is closely associated with detecting and responding to various biological process changes with chromosomes usually altered in human diseases and aging. Cell culture experiments using microglia suggest that the beneficial effects of NR are produced in part through the cGAS-STING-dependent pathway [114]. cGas knockout mice exhibit reduced induction of SASPs, affecting tissue homeostasis and cellular responses to DNA damage [103]. In addition, increased STING expression was observed in osteoarthritic articular (OA) cartilage to induce extracellular matrix (ECM) degradation by promoting NF-kB-mediated IL-1β production to induce chondrocyte senescence, where STING downregulation relieved OA development [115]. Similarly, STING blockade was also reported to reduce inflammatory cytokine production in various human organs to alleviate aging associated inflammation and neurodegeneration (see [4] for review).

The presence of cytoplasmic chromatin fragments is a hallmark of senescent cells capable to activate the cGAS/STING pathway, reinforcing the senescent state in paracrine manners [88, 116]. IL-6 maintains intracellular senescence by interacting with intracellular IL-6 receptors in forward traffic-specialized structures, facilitated by cGAS-STING activation [117]. Mechanotransduction of YAP/TAZ inhibits cGAS-STING signaling to the extent that inhibition of STING prevents tissue senescence following YAP/TAZ inactivation and tissue degeneration associated with premature aging. The decline of YAP/TAZ mechanotransduction drives senescence through the release of cGAS-STING signaling [118]. Mitochondrial outer membrane permeabilization (MOMP), which occurs in a subpopulation of mitochondria and is a feature of cellular senescence, has been termed minority mitochondrial membrane permeabilization (miMOMP) and requires BAX and BAK macropores to enable the release of mitochondrial DNA (mtDNA) into the cytoplasm, where the cytoplasmic mtDNA in turn activates the cGAS-STING pathway [119]. Cumulative evidence suggests that activation of the cGAS/STING signaling pathway may promote inflammation-associated senescence.

3.2. Dysregulation and Altered Cellular Processes

Dysregulation of the cGAS/STING signaling pathway in senescence can have profound implications for cellular processes. For example, when the pathway is hyperactivated or sustained over extended periods, it may contribute to chronic inflammation, a hallmark of the senescence state. The continuous secretion of inflammatory molecules by senescent cells, mediated by cGAS/STING, would presumably create a pro-inflammatory microenvironment that promotes further cellular damage and dysfunction. Furthermore, the persistent activation of cGAS/STING signaling in senescent cells may interfere with normal tissue repair and regeneration processes associated with senescence [120]. Instead of facilitating a controlled response to cellular stress, dysregulated cGAS/STING signaling may exacerbate the senescent phenotype, leading to tissue deterioration and dysfunction. One of such examples is immunosenescence that plays a critical role in aging and auto-immune diseases driven by senescent T cells (see for [121, 122] review).

3.3. Links to Age-Related Diseases

The dysregulation of the cGAS/STING pathway in senescence has been implicated in age-related diseases. Chronic activation of the pathway in senescent cells may contribute to the progression of diseases associated with aging, such as cancer and neurodegenerative disorders [123].

Understanding the associated changes and impact of cGAS/STING dysregulation on cellular processes during senescence is critical for uncovering potential therapeutic targets. Strategies aiming at modulating the cGAS/STING pathway may hold promises in mitigating the deleterious effects associated with senescence, providing avenues for interventions in age-related diseases, which has been observed in multiple disease models such as OA [115]. In addition, elevated cGAS-STING activation was observed in Alzheimer’s disease (AD) mouse models which could be partially restored by the nicotinamide riboside (NR) treatment [114]. Inhibition of cGAS and STING effectively reduced SASP expression triggered by its own DNA in A-T brain organoids, inhibited astrocyte senescence and neurodegeneration, and improved the neuropathology of A-T brain organoids [124, 125]. Notably, DNA damage primes the type I interferon system via activation of cGAS/STING, promoting inflammation and contributing to cardiovascular diseases ([126] see for [8] review). cGAS is essential for cellular senescence and related inflammatory responses that affect metabolic homeostasis, and chronic inflammation induced by the cGAS-STING pathway activation may influence the development of metabolic diseases such as type 2 diabetes [87, 127]. Whether and how modulating cGAS/STING activation in these human pathological settings could provide new therapeutic directions warrant further in-depth investigations.

4. Role in Oncogenesis

4.1. Connections of the cGAS/STING Pathway to Cancer

The cGAS/STING pathway, initially recognized for its role in innate immunity, has also emerged as a critical player in the intricate landscape of cancer development. Growing bodies of evidence support its involvement in the initiation, progression or suppression of various types of cancers, which underscores its multifaceted impact on oncogenesis. The primary connection between the cGAS/STING pathway and cancer lies within the recognition of cytosolic DNA anomalies within cancer cells presumably derived from unstable genome [128] through its canonical innate immune function. The cGAS/STING pathway serves as a sentinel, sensing these cytoplasmic DNA fragments and initiating downstream signaling events to mount an anti-tumor immune response. Interestingly, the cGAS/STING pathway exhibits a dual role in cancer development (Figure 3). While it can act as a barrier against tumorigenesis by promoting tumor immune surveillance and anti-tumor responses, its hyperactivation in immune cells such as T cells primes T cell death by inducing ER stress [75], thus dampening tumor destruction to facilitate tumor progression. In addition, various cGAS/STING signaling components also modulate tumorigenesis through innate immunity-independent paths that we will discuss in detail below.

Figure 3. Roles of cGAS/STING signaling in oncogenesis.

The cGAS/STING signaling exerts either tumor promoting or tumor suppressing effects in a context-dependent manner. Tumor Promotion: Double-stranded DNA (dsDNA) triggers the activation of the cGAS-STING signaling pathway, leading to a chronic inflammatory response that can ultimately promote tumorigenesis. Additionally, brain-metastatic tumor cells secreted 2’3’-cGAMP enters astrocytes through gap junctions. This activates the STING signaling in astrocytes to induce TNF-α and IFN-α production, which act in a paracrine manner to activate NF-κB and STAT1 signaling in tumor cells to enhance tumor growth. Tumor Suppression: In tumor cells, genomic DNA exposure due to genomic instability, mutations or deletions of oncogenes, oxidative stress, and metabolic disorders, as well as mitochondrial DNA leakage resulting from radiotherapy, chemotherapy, and oxidative stress, activate cGAS-STING signaling in the cytoplasm. The cGAS-STING pathway also promotes cellular senescence through SASP (senescence-associated secretory phenotype). Furthermore, the phagocytosis of tumor cell DNA by dendritic cells (DCs) and the transfer of cGAMP from tumor cells to DCs both activate STING-dependent antitumor immune responses.

4.2. Roles of the cGAS/STING Signaling Pathway in Suppressing Tumorigenesis

Notably, neither cGas^−/− nor Sting^gt/gt mice develop simultaneous tumors, suggesting cGAS or STING may not serve as a strong tumor suppressor in tumorigenesis.

Evading immune destruction is a hallmark of cancer [129] and STING loss has been observed in colon cancer [130], gastric cancer [131] and melanoma [132] to restrain IFN-mediated tissue repair and T cell priming, thus promoting tumorigenesis. Genetic changes of cGAS are not commonly observed in cancer. Pan-cancer analyses suggest that expression of both cGAS and STING mRNA is downregulated in colon cancer [130], gastric cancer [131] and melanoma [132] compared with normal adjacent tissues, which is largely due to promoter hypomethylation and other epigenetic regulations [133]. Both DNMT [134] and EZH2 inhibitors [135] restored STING expression by releasing suppression of STING transcription, contributing to tumor suppression and tumor responses to STING agonists [134, 136]. These reports suggest that cancer may hijack epigenetic inactivation of cGAS/STING signaling to evade immune surveillance in fueling tumorigenesis.

Pharmacological activation of cGAS/STING signaling is a promising adjuvant to anti-cancer therapies [137], and its activation has been shown to increase tumor immunogenicity and prime T cell responses [138]. This is largely because activation of the cGAS/STING signaling pathway in tumor cells triggers the release of type I interferons and pro-inflammatory cytokines, leading to increased immune cell infiltrates into tumors to enhance anti-tumor immune responses [139]. These cytokines also activate dendritic cells and promote the cross-priming of cytotoxic T cells, which target and destroy tumor cells [140]. For example, the absence of cGAS impaired the anti-tumor immune response in established colorectal cancer models. These mice exhibited reduced type I interferon production associated with increased tumor growth [141]. In addition, Sting knockout mice also showed impaired immune responses in established tumors, leading to enhanced tumor growth and metastasis [142, 143].

In addition, activation of cGAS/STING signaling in both tumor and immune cells seems critical in mediating the anti-tumor effect. Specifically, through a potent type I IFN response, cGAS/STING activation primed and activated dendritic cells (DCs) [139, 144, 145], T cells, and natural killer (NK) cells [139, 144, 146–148]. In tumor cells, the intact cGAS-STING signaling triggers the expression of NKG2D (natural killer cell group 2D) ligands on the surface of tumor cells, allowing for better binding of NKG2D receptors on the surface of NK cells to enhance NK-mediated tumor cell killing [149, 150]. Additionally, tumor derived NF-κB-dependent cytokine signaling plays a critical role in controlling tumor growth and synergistically enhances NK cell activity with the type I IFN response [151]. Beyond tumor and immune cells, the cytokine response induced by cGAS/STING activation in stromal cells further augments the inflammatory tumor microenvironment and causes tumor necrosis [152]. Thus, activation of cGAS/STING signaling functions through innate immune activation to suppress tumorigenesis in multiple types of tissues within tumors through distinct mechanisms.

In addition to type-I IFN responses, cGAS/STING signaling also modulates tumorigenesis independent of innate immunity. For example, IFNβ-independent STING function has also been observed in directly facilitating autophagy by binding LC3 [153] and promoting lysosome-mediated cell death in myeloid cells [74] establishing an anti-tumor immune environment.

4.3. Roles of the cGAS/STING Signaling Pathway in Promoting Tumorigenesis

Moreover, a strong association of certain viral infection with cancer has been observed. These viruses include human papillomaviruses (HPVs) as a cause for cancers of cervix, penis, anus, mouth and throat [154–156]; EBV (epstein-barr virus) with nasopharyngeal cancer and lymphomas [157–159]; HIV with Kaposi sarcoma, cervical cancer and non-Hodgkin lymphoma [160–162] and others. Viruses may largely alter genome or cellular programs such as triggering DNA damage response and chronic inflammation to facilitate tumorigenesis, where activation of cGAS/STING could be achieved directly by viral DNA/reversed transcribed RNA [163], or indirectly through damaging mitochondria to release mitochondrial DNA [164, 165] or causing ribosome collision to sensitize cGAS to DNA [166, 167]. This knowledge has been helping to develop viral vaccines for cancer prevention.

Echoing the dual roles of SASPs in tumorigenesis, cGAS/STING signaling has also been reported to play an oncogenic role in facilitating tumorigenesis in some cancer settings. This function depends on both innate immunity-dependent and - independent mechanisms.

Earlier studies found that mice with Sting knockdown were protected from carcinogen-induced skin cancer [168]. Moreover, STING was found to be indispensable for inflammation-driven carcinogenesis [168]. Surprisingly, although 2’3’-cGAMP is commonly thought as immune stimulatory to benefit cancer immune therapy [169], it was recently found to cause B-reg cell-mediated immune suppression [170], horning the alarm that cautions should be taken when these CDNs (cyclic dinucleotides) are used in clinical settings alone or as immune adjuvants. Additionally, the cGAS-STING pathway promotes tumor progression after tumor formation. In cancer cells with high levels of chromosomal instability (CIN), chronic activation of cGAS-STING signaling promoted tumor invasion and metastasis, attributed to the non-canonical NF-κB cascade [171]. Furthermore, atypical NF-κB activation or persistent type I IFN signaling attenuated the beneficial anti-tumor immune effects induced by radiotherapy by inhibiting DCs and mobilizing myeloid suppressor cells [172, 173]. Thus, cGAS/STING signaling in tumors promotes cancer progression by upsetting the balance of the immunosuppressive TME. Cancer cells may use tumor-secreted 2’3’-cGAMP to negatively influence T cell function in tumor transplantation [143, 174]. While acute activation of the turmeric cGAS-STING pathway may suppress tumor growth, chronic or persistent activation may exert the opposite effect to rather support tumorigenesis due to chronic inflammation [175]. Of note, brain metastatic breast or lung cancers were reported to transport 2’3’-cGAMP to astrocytes in promoting inflammatory molecules including IFNα and TNFα that functioned in a paracrine manner to feedback to activate STAT1/NF-kB signaling in metastatic cancer cells to support tumor establishment and growth at the metastasis site [176]. In addition to cGAS/STING signaling in tumor cells, activating of host cGAS/STING signaling, especially in T cells, has been shown to either positively [137] or negatively [75] affect immunotherapy efficacy in cancer treatments. These reports reveal multiple layers of tumor promoting ability from distinct cGAS/STING signaling pathway members.

In addition to functioning through 2’3’-cGAMP or IFN/ISGs to regulate tumorigenesis, innate immunity-independent mechanisms for cGAS/STING signaling in tumorigenesis have also been reported. For example, cGAS was reported to bind PARP1 to suppress HR (homologous recombination) leading to genome instability, where cGAS overexpression facilitated lung cancer growth [73]. Profiling cGAS/STING expression in pan-cancer indicated reduced STING expression in most cancer types; however, increased STING expression was observed in renal cell carcinoma (RCC) [177]. Further mechanistic studies revealed that STING interacted with mitochondrial calcium transporter VDAC2 to maintain calcium homeostasis and growth in RCC and suggested STING/VDAC2 interaction as a novel target in treating RCC. Together, these studies cumulatively suggest that roles of cGAS/STING signaling in tumorigenesis is cancer type-dependent, cell type-specific and also probably activation threshold and duration-related.

5. Clinical Implications and Therapeutic Potential

Considering the above-described roles of cGAS/STING signaling in senescence and oncogenesis, and the interplays between senescence and oncogenesis, it is plausible to targetedly manipulate the activity of cGAS/STING signaling to promote senescence and suppress tumorigenesis as therapeutic directions. Given multiple cancer treatments aim to induce cellular senescence such as DNA damaging agents [178], where cGAS/STING activation plays indispensable roles, whether activating cGAS/STING synergizes with these senescence inducers to augment senescence-induction in cells and animals warrants further investigations. Notably, the senescence enhancement effect (such as promoting p16 and SA-β-Gal expression [90]) is mediated by cell intrinsic cGAS/STING activation, while the ability of stimulating cGAS/STING-mediated secreted factors including various type I IFNs and ISGs exert function in modulating immune environment and death of infected cells. Activating cGAS/STING pathway has been implicated in influencing the tumor response to immunotherapies, such as immune checkpoint blockade [137]. Thus, understanding and targeting the intricate crosstalks between the cGAS/STING pathway and other signaling pathways offer therapeutic opportunities for senescence-related disorders and cancer.

It seems acute and moderate STING signaling facilitates tumor suppression, whereas sustained or excessive STING signaling leads to immunosuppression. Moreover, preferential targeting of STING in DCs while avoiding T cell exposure is critical for inducing durable tumor-specific T cell responses [179]. Small molecule inhibitors to antagonize cGAS or STING were initially investigated to reduce the deleterious inflammatory phenotype of autoinflammatory diseases (Figure 4). Subsequently, it was found that blocking cGAS or STING may also help prevent inflammation-induced tumor development especially in tumors with high CIN (chromosome instability) [180]. Moreover, Cdc2-like kinase 1 (CLK1) is critical for residual tumor cell survival after treatment with cGAS inhibitors, and CLK1 suppression enhances sensitivity to cGAS inhibitors [181]. In addition, single-cell dynamic transcriptomic analysis indicated reduced proliferation-associated nascent RNAs as the underlying mechanism. In first-line therapy, chemotherapy-triggered DNA damage activates the cGAS-STING pathway, and cGAS inhibitors can synergize with chemotherapeutic agents to kill tumors [181].

Figure 4. Overview of agents targeting the cGAS/STING signaling.

Reagents in clinical trials are labeled in red and species specific-targeting agents are also clearly labeled.

5.1. Therapeutic Strategies:

a. Activating the cGAS/STING Pathway:

One therapeutic strategy involves activating the cGAS/STING pathway to enhance anti-tumor immune responses. Small molecule agonists targeting cGAS or STING have shown promises in preclinical models. Direct activation of the pathway in tumors could potentially overcome immune evasion strategies employed by cancer cells. Evading immune surveillance and destruction is a cancer hallmark [182], and therapies restoring or activating the cGAS/STING pathway can be explored to overcome immune suppression. Indeed, inhibitors targeting DNMT [134] or EZH2 [135] efficiently restored STING expression suppressed by epigenetic silencing and showed a combination effects with immune checkpoint blockade in reducing tumor burden [134, 136]. Similarly, activating cGAS/STING signaling by small molecules (discussed in later section), nanoagonists ([183–185]) improved anti-tumor immune environment [138] either directly from inflammatory cytokines induced by cGAS/STING, or indirectly enhanced by cGAS/STING activation-induced cell death that further enhances immunogenic effects [184]. with combinatory effects with immunotherapy and other therapies [186]. Thus, combining cGAS/STING-targeted therapies with existing modalities, such as chemotherapy or immunotherapies, represents a promising approach. The cGAS/STING pathway’s ability to modulate the immune microenvironment suggests that combining therapies could have synergistic effects, addressing the complexity of the tumor landscape and improving treatment outcomes.

In addition to targeting cGAS or STING, manipulating 2’3’-cGAMP abundance could be another viable approach to tune cGAS/STING activation. ENPP1 [70] and ENPP3 [71]are the two characterized extracellular 2’3’-cGAMP hydrolyzing enzymes and increased ENPP1 expression has been reported in cancer [187] possible serving as an immune evasion approach by reducing 2’3’-cGAMP levels. A selective ENPP1 inhibitor RSB2481 is currently evaluated in phase I clinical trials in treating patients with advanced, metastatic solid tumors (NCT05270213). In addition, multiple 2’3’-cGAMP transporters and uniporters including SLC46A2 [65] LRRC8C [66, 67], SLC19A1 [68, 69] or gap junctions [78] have been reported to mediate the 2’3’-cGAMP transportation. Whether targeting these 2’3’-cGAMP transportation machinery to enrich endogenous 2’3’-cGAMP concentrations achieves anti-tumor effects warrants further investigations.

b. Challenges and Considerations:

Despite the therapeutic potential for targeting the cGAS/STING signaling in various human disease treatments, challenges do exist. Ideally, activating tumor cell cGAS/STING signaling without disturbing cGAS/STING signaling in non-tumor cells would be an idea approach. However, whether this specificity can be achieved remains challenging. Tumor-specific delivery of 2’3’-cGAMP might be a way; however, the transfer of 2’3’-cGAMP among cells with the assistance of various uniporters or transporters could theoretically reduce its specificity. Due to the critical roles of cGAS/STING in innate immunity to fight against viral infection and maintaining anti-tumor immune environment, a delicate balance to fine tune activation of this signaling to avoid excessive immune activation, potential off-target effects, and the need for a thorough understanding of the context-dependent roles of the pathway are critical considerations in therapeutic development.

5.2. Current Status in Preclinical and Clinical Settings:

The status of active testing of various agents targeting the cGAS/STING pathway in preclinical and clinical settings also reflects its emerging importance in senescence and oncogenesis. Next, we provide an overview of the developments and ongoing efforts towards this direction.

a. Preclinical Advances:

Based on mechanisms of action, a large set of agents have been developed to target various steps of the activation of cGAS/STING signaling. Briefly, there are multiple types of developed cGAS inhibitors including (1) disrupting cGAS binding to DNA [188]; (2) inhibiting the cGAS catalytic pockets [189]; (3) targeting cGAS regulations (for example, aspirin induces cGAS acetylation to block its activation [190]). Different types of STING inhibitors have also been reported such as (1) CDN (cyclic dinucleotide) competitors/STING LBD binding compounds such as sulfonamide derivatives including SN11 [191]; (2) STING palmitoylation inhibitors such as indole ureas [192], nitro fatty acids [193]; acrylamides an others; (3) STING oligomerization inhibitors [194] and others. In addition, degraders such as PROTACs for cGAS or STING have also been developed and tested. STING-PROTACs depending on CRBN [195], VHL [196, 197], HERC4 (AK59, [198]) or RNF126 [199] E3 ligases have been synthesized with efficacy towards degrading active STING or disease-related STING mutants (such as AK59).

Preclinical studies have demonstrated the feasibility and efficacy of targeting the cGAS/STING pathway in treating various diseases in cell and animal models. Small molecule agonists and inhibitors targeting different components of the cGAS/STING pathway have shown promising results in preclinical models of cancer, autoimmune disorders, and neurodegenerative diseases (summarized in Table 1).

Table 1.

cGAS-STING targeted reagents in preclinical studies

Role	Type	Name	Disease model	References
cGAS Inhibitors	Disrupting cGAS binding	X6	AGS	[239]
		Hydroxychloroquine		[240]
		Suramin		[241]
		A151		[200]
		Quinacrine		[242]
	Inhibiting catalytic activity	RU.521	NDs	[201]
		RU.365	NDs	[201]
		RU.332	NDs	[201]
		PF-06928215		[243]
		G150		[189]
		Compound S3		[244]
	Targeting inhibition	Aspirin	AGS	[245]
	unknown	CU-76		[246]
		CU-32		[246]
STING agonists and antagonists	CDN competitors	Tetrahydroisoquinolines		[247]
		Astin C		[248]
		SN-011		[249]
		2’3’-cGAMP		[27]
		3’3’-cGAMP		[6]
		3’5’-c-di-GMP		[250]
		diABZI	ARDS, glioblastoma,	[203]
		MSA-2	Cervical cancer, HCC	[251]
		SR-717	Melanoma	[252]
		IMSA172		[253]
		STINGVAX	Melanoma, colon carcinoma	[254]
	Palmitoylation inhibitors	H-151 (Indole ureas)	Psoriasis	[255]
		C-178		[192]
		Acrylamides		[256]
		Nitro fatty acid		[257]
		C-176	Osteoclast-related osteolytic diseases	[258]
	Unknown	Butenolide heterodimer		[259]
TBK1 inhibitors	ATP competitor	BX795		[260]
	Targeted inhibition	AZ13102909	Melanoma	[261]
		Azabenzimidazole HIT 1a		[262]
		Amlexanox	Liver fibrosis and biliary fibrosis	[263]
	Selective inhibition	MRT67307		[264]
	Phosphorylation inhibitor	Idronoxil		[265]

Inhibitory oligodeoxynucleotides (ODNs) containing repetitive TTAGGG motifs are utilized in the treatment of autoimmune diseases, including systemic lupus erythematosus (SLE). Recently, ODN A151 as a novel cGAS inhibitor has been reported [200]. A151 blocks the activation of cGAS by competitively binding DNA, thereby inhibiting the production of type I IFNs in human monocytes. This mechanism positions A151 as a promising therapeutic agent for SLE [200]. In addition, the inhibition of cGAS catalytic activity by RU.521 reduces the constitutive expression of interferon in macrophages in a mouse model of Aicardi-Goutières syndrome. RU.521 may serve as a molecular scaffold for developing future anti-autoimmune therapies [201].

Single B-LNPs (bridging-lipid nanoparticle) suppressed CD47 and PD-L1 and enhanced tumor-associated macrophage (TAMC) phagocytosis. B-LNPs encapsulated with diABZI, a non-nucleotide agonist that stimulates interferon genes, converted immune-suppressive cells into anti-tumor effector cells, inducing T-cell infiltration and activation in brain tumors. In a preclinical mouse model, B-LNP/diABZI synergized with radiotherapy to promote brain tumor regression and induced immune memory against gliomas [202]. The diABZI-induced antigen presentation in cancer cells also increases the affinity of TCR-T cells for tumor cells. diABZI enhanced the immunotherapeutic efficacy of TCR-T cells by activating STING and TCR signaling, improving interferon-γ expression, and increasing antigen presentation in tumor cells [203]. The STING agonist MSA-2 inhibited tumor progression and prolongs survival in a mouse model of clear cell renal cell carcinoma (ccRCC) by promoting cytokine secretion [204]. In vitro and in vivo studies have shown that active D2.0R cancer stem cells (CSC) were resistant to natural killer (NK) cell cytotoxicity, whereas proliferating D2.0R CSC were sensitive to NK cell cytotoxicity. MSA-2 enhanced NK cell killing in breast cancer cells and efficiently suppressed the growth of subcutaneously implanted cervical tumors [205, 206]. A STING inhibitor H-151 effectively inhibited the activation of cGAS-STING pathway evidenced by the reduced levels of p-TBK1, p-p65 and p-IRF3 and reduced Alzheimer’s disease pathogenesis in 5×FAD mice [207].

b. Clinical Trials:

While preclinical studies are encouraging, the translation of cGAS/STING-targeted therapies into clinical applications is still at its early stage. Clinical trials are underway to evaluate the safety and efficacy of cGAS/STING modulators in human patients with no FDA-approved therapies to date. In the clinical setting, ongoing efforts balance the antitumor and pro-tumor effects of cGAS-STING signaling (Table 2).

Table 2.

cGAS-STING targeted reagents in clinical trial

Role	Name	Type	Phase	NCT code	Disease	References
STING agonists	ADU-S100 (MIW815)	synthetic CDN	I, II	NCT02675439 NCT03172936 NCT03937141	Advanced/Meta static solid tumors and lymphomas	[212, 213]
	DMXAA (Vadimezan)	non-CDN	I, II, III	NCT01285453 NCT00738387 NCT00111618	Advanced or chemotherapy-refractory tumors	[209, 210]
	MK-1454	synthetic CDN	I, II	NCT03010176 NCT04220866	HNSCC, advanced or metastatic solid tumors or lymphomas	[214, 215]
	E7766	synthetic CDN	I	NCT04144140 NCT04109092	Advanced solid tumors or lymphomas, non-muscle invasive bladder cancer	[216, 217]
	TAK-676	synthetic CDN	I	NCT04420884 NCT04879849	Advanced or metastatic solid tumors	[266]
	BMS-986301	synthetic CDN	I	NCT03956680	Advanced solid cancers	[267]
	SB11285	synthetic CDN	I	NCT04096638	Advanced solid tumors	[218]
	DN015089	synthetic CDN	I	CTR20212462	Advanced or metastatic solid tumors	[268]
	IMSA101	synthetic CDN	I, II	NCT05846646 NCT04020185	Oligometastatic NSCLC and RCC, advanced or metastatic solid tumors	[269]
	BI1703880	synthetic CDN	I	NCT05471856	Advanced, unresectable and/or metastatic or relapsed/refract ory solid tumor	[270]
	BI1387446	synthetic CDN	I	NCT04147234		[271]
	MK-2118	synthetic CDN	I	NCT03249792	Advanced or metastatic solid tumors or lymphomas	[219]
	GSK3745417	non-CDN	I	NCT03843359 NCT05424380	Advanced cancer (solid tumors)	[272]
	HG-381	non-CDN	I	NCT04998422	Advanced solid tumors	[268]
	KL340399	non-CDN	I	NCT05549804 NCT05387928	Advanced solid tumors	[273]
	ONM-501	non-CDN	I	NCT06022029	Advanced solid tumors and lymphoma	[274, 275]
	SNX281	non-CDN	I	NCT04609579	Advanced solid tumors and lymphoma	[276, 277]
	CRD3874-SI		I	NCT06021626	Advanced or metastatic cancer	[278]
	TAK-500	ADC	I	NCT05070247	Selected locally advanced or metastatic solid tumors	[279]
	XMT-2056	ADC	I	NCT05514717	Advanced/recurrent solid tumors	[280]
	CDK-002	engineered exosome	I, II	NCT04592484	Advanced/metastatic, recurrent, injectable solid tumors	[281]
	SYNB1891	engineered bacteria vector	I	NCT04167137	Advanced/meta static solid tumors or lymphoma	[282]

DMXAA, as one of the most well-characterized STING agonists primarily for its ability to induce apoptosis in endothelial cells, was originally developed as a vascular disrupting agent unrelated to STING [208]. DMXAA has been preferentially tested in patients with solid tumors, primarily in patient populations with advanced or chemotherapy-refractory tumors. A Japanese Phase I clinical study (NCT01285453) tested the tolerability, safety, efficacy, and pharmacokinetics of DMXAA in combination with the microtubule toxicant docetaxel in patients with advanced or recurrent solid tumors [209]. The results demonstrated that DMXAA with docetaxel was generally well tolerated, with only grade 1–2 adverse events, including constipation, decreased appetite, alopecia, fatigue, and neutropenia. These findings led to a subsequent Phase III clinical study (ATTRACT-2) utilizing DMXAA with docetaxel as a second-line therapy for advanced non-small cell lung cancer (NSCLC), though the ATTRACT-2 trial ended prematurely, likely due to poor initial efficacy (NCT00738387). A similar Phase II study in 70 patients with hormone-refractory metastatic prostate cancer (NCT00111618) found a favorable safety profile and some degree of clinical activity [210]. The failure of DMXAA in clinical trials led to the discovery that DMXAA is specific to mouse but not human STING [211]. Overviews of the failure experiences horn the alarm for later research to consider compound specificity to targets in distinct species [211] given the pre-clinical studies are primarily performed with human cell lines in mouse models, while the clinical trials are on human patients. Unfortunately, although the function of cGAS/STING signaling is conserved between mouse and human, both proteins do not share a high sequence homology. Species-specific agents or agents targeting both mouse and human cGAS/STING have been developed to date.

For example, ADU-S100 (also known as MIW815) is currently being treated with the cytotoxic T-lymphocyte-associated protein 4 (CTLA4) blocker ipilimumab (NCT02675439) in advanced solid tumors in combination with the PD-1 blocker pembrolizumab in CD274 (PD-L1)-positive recurrent or metastatic HNSCC patients (NCT03937141) and concurrently receiving a PD-1 blocker other than pembrolizumab (NCT03172936) [212, 213]. Preliminary results from NCT03172936 indicate that the combination of ADU-S100 with spartalizumab is well tolerated with some clinical activity in patients with solid tumors, particularly against PD-1-naïve triple-negative breast cancer and PD-1 relapsed/refractory melanoma [213]. MK-1454 is a potent (STING) activator and is currently being investigated as an immuno-oncology therapeutic agent. MK-1454 is being used to treat patients with metastatic or unresectable recurrent head and neck squamous cell carcinoma (HNSCC) (NCT04220866) [214, 215]. Additionally, patients with advanced or metastatic solid tumors or lymphomas are being tested in a clinical trial of MK-1454 alone or in combination with Pembrolizumab (NCT03010176) [214, 215]. E7766 belongs to a novel class of macrocycle-bridged STING agonists (MBSAs) and is currently the only STING agonist tested as a stand-alone intravenous therapy in cancer patients. The tolerability, safety, and preliminary activity of this molecule have been studied in patients with advanced solid tumors or lymphomas (NCT04144140) and in patients with non-muscle invasive bladder cancer (NCT04109092) [216, 217]. The combination of the PD-1 blocker nivolumab with the STING agonist SB11285 is also being tested in subjects with advanced solid tumors (NCT04096638) [218]. Another compound, MK-2118, is being tested in combination with Pembrolizumab in patients with advanced or metastatic solid tumors or lymphomas (NCT03249792) [219]. With active ongoing clinical evaluations, we hope effective cures targeting the cGAS/STING signaling in treating human diseases would be approved by FDA as a single agent or as an adjuvant for combination therapies.

6. Challenges and Future Directions:

6.1. Current Challenges

a. Technological Limitations:

One of the major challenges in studying the cGAS/STING signaling lies within the technological limitations associated with approaches to acutely and faithfully monitor the dynamic cellular processes. Visualizing real-time cGAS activation, STING trafficking/activation, and downstream signaling events within living cells in a single-cell resolution remains an action item on the to-do list. Improving imaging techniques and developing innovative spatial and temporal methodologies to monitor these processes in cells or in vivo would offer invaluable insights into the spatiotemporal dynamics of the cGAS/STING pathway. In addition, although 2’3’-cGAMP sensors have been reported [220, 221], due to the high mobility nature of this special molecule in vivo, it remains challenging to determine the original location of 2’3’-cGAMP synthesis and the migratory path once it is administrated into patients as an adjuvant therapy. In addition, it remains unclear how 2’3’-cGAMP recognizes distinct downstream targets to exert different biological function.

b. Context-Dependent Functions:

Understanding the context-dependent functions of the cGAS/STING pathway presents a significant challenge. The pathway exhibits diverse roles in different cellular contexts, including senescence, oncogenesis, and immune responses. Deciphering the factors that dictate the pathway’s outcomes in specific situations is crucial for designing targeted interventions. Integrating systems biology approaches, such as single-cell omics and computational modeling, may help unravel the intricate network of interactions influencing cGAS/STING functions.

c. Immune Tolerance and Autoimmunity:

Balancing the immune response is essential, thus the cGAS/STING pathway’s dual roles in promoting anti-tumor immunity and contributing to autoimmune disorders poses a challenge. Achieving precise fine tunes without triggering excessive inflammation or compromising immune surveillance remains a delicate balance. Identifying specific regulators that fine-tune the cGAS/STING pathway in a context-dependent manner could be key to addressing this challenge.

d. Biomarker Development:

Developing reliable biomarkers to assess the activity and status of the cGAS/STING pathway in clinical settings also remains challenging. Reliable markers are necessary for patient stratification, treatment monitoring, and predicting responses to therapeutic interventions. Advances in liquid biopsy techniques and the identification of specific molecular signatures associated with cGAS/STING activation could facilitate the development of robust biomarkers.

6.2. Future Research Avenues

a. Advanced Imaging Technologies:

Investments in the development of advanced imaging technologies can revolutionize our understanding of the cGAS/STING pathway. Techniques such as super-resolution microscopy, live-cell imaging, and proximity-based labeling methods could enable studies to visualize real-time interactions and dynamics of cGAS, STING, and associated molecules within given cellular compartments. This would provide essential insights into the spatiotemporal regulation of the pathway. Single-cell resolution techniques will also help to address the heterogeneous responses to the same stimuli within a population of cells/tissues to allow the analyses for cell response variations.

b. Inhibiting the cGAS/STING Pathway:

In certain contexts, inhibiting the cGAS/STING pathway might be beneficial for human health and suitable for disease treatments. For example, aberrant cGAS/STING activation by genomic or mitochondrial DNA has been reported in autoinflammatory, autoimmune and neurodegenerative diseases [222], where cGAS/STING inhibition demonstrated a promise in treating these diseases. For example, SN-011 a STING inhibitor inhibited SAVI (stimulator of IFN genes associated vasculopathy) associated STING activation and partially alleviated SAVI symptom in Trex1^−/− mice to prolong the animal survival [191]. Improved highly selective STING inhibitors C-176, C178 and H-151 were developed to significantly reduce the animal autoimmune disease phenotypes by inhibiting innate immune activation [192]. Similarly, X6 a compound to interfere with cGAS binding to DNA, significantly reduced spleen inflammation signatures in a AGS (Aicardi-Goutieres syndrome) mouse model [188]. To date, multiple cGAS/STING antagonists have been tested with a promise in reducing the inflammatory signatures in human cell lines or auto-immune disease animal models (see [223] for review).

In addition, cardiovascular disorders could also be induced by sterile inflammation (persistent chronic inflammation) [224], where either cGAS inhibition [225] has been shown to reduce the contractile dysfunction in cardiomyocytes [226] or doxorubicin-induced cardiotoxicity [227], or STING inhibition has been observed with protective effects against multiple cardiovascular diseases [228–230]. Recent SARS-CoV-2 infection also induced STING activation (by damaging and releasing mitochondrial DNA) in patient lung and skin manifestations, where applying the STING inhibitor H-151 alleviated SARS-CoV-2 infection induced lung inflammation and disease outcome in mice [164]. In addition, due to the critical role of damaged mitochondrial DNA in activating cGAS/STING signaling, and mitochondria deregulation in neurogenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS) and others, activation of cGAS/STING signaling and subsequent chronic inflammation have been tightly linked to neurogenerative diseases [231, 232]. STING is largely expressed in microglia [233], upon viral infection activation of which promotes type I IFN secretion to fight against infection [234]. However, when the host immune system mistakenly attacks myelin in nerve fibers, MS occurs with inflammation and nerve damages [235]. Interestingly, deleting cGas in tauopathy mice could alleviate tauopathy-induced microglial inflammation and partially restore synapse integrity and plasticity, as well as cognitive deficits [236]. Further, cGAS inhibition indeed reduced IFN-I signaling in microglia cells [236], while whether inhibiting cGAS/STING in relieving AD symptom in patients would require further clinical evaluations. Thus, small molecule inhibitors targeting either cGAS or STING could be explored to dampen the excessive immune response in treating these inflammatory diseases.

c. Biomarker Development:

Understanding the clinical relevance of the cGAS/STING pathway in cancer could lead to the development of biomarkers for predicting treatment responses. For example, a recent study revealed that STING expression and cGAS/STING activation status correlated with genome instability and immune cell infiltrates in breast cancer [237]. This suggests these “cGAS/STING-high” cancer with an “immune hot” signature may benefit from immunotherapy. In addition, activation of the cGAS/STING signaling has also been reported to predicted response to other cancer treatments like chemotherapy in breast cancer [238].Thus, assessing the status of the cGAS/STING pathway in tumors may guide personalized treatment strategies, allowing clinicians to tailor interventions based on the immune landscape of the tumor microenvironment.

d. Systems Biology Approaches:

The complex nature of the cGAS/STING pathway calls for an integrative approach to decipher its context-dependent functions. Systems biology approaches, including single-cell omics, network analysis, and mathematical modeling, can aid in unraveling the intricate regulatory networks governing the cGAS/STING pathway. These methods can help identify key nodes, predict pathway behavior, and uncover novel interactions, providing a holistic understanding of its role in various cellular processes.

e. Immunomodulation in Cancer Therapeutics:

Future research could delve into innovative immunomodulatory approaches that leverage the cGAS/STING pathway for cancer therapeutics. Combining cGAS/STING-targeted therapies with other immunomodulators, such as checkpoint inhibitors, might enhance the overall anti-tumor immune response. Understanding the interplay between the cGAS/STING pathway and other immune regulatory pathways will be crucial for designing effective combination strategies. 2’3’-cGAMP has been shown to synergize with immune checkpoint blockade (ICB) in animal models [169]. However, cGAMP administration to animals induces B-reg activation, leading to a dampened immune response [170]. This could be due to different levels of 2’3’-cGAMP in tumor sites or different major downstream effectors engaged. To mitigate these unwanted side effects, future strategies may include: (1) Using a targeted delivery system to ensure that 2’3’-cGAMP reaches specific immune cells or tissues, thereby minimizing off-target effects on B-reg cells; (2) Optimizing the dose of 2’3’-cGAMP to achieve the desired therapeutic effect while minimizing activation of B-reg cells; (3) Preventing suppression of the immune response by combining 2’3’-cGAMP with drugs that specifically inhibit B-reg activation or function through combination therapy; (4) In addition, considering timing the administration of cGAMP to coincide with periods of low B-reg activity to minimize B-reg activation. Genetic approaches or pharmacological agents can also be utilized to selectively modulate pathways of B-reg activation; (5) Further modifying the chemical moiety of 2’3’-cGAMP to allow it exerting specific function towards tumor STING but not B-reg activation.

7. Conclusion:

This review summarizes critical roles of the cGAS/STING signaling pathway in senescence and oncogenesis. We dissect the key components, molecular mechanisms, and regulatory elements of the pathway to provide a comprehensive overview for its participation into senescence and oncogenesis. The cGAS/STING pathway acts as a sentinel, responding to cytoplasmic DNA and shaping immune responses, ultimately influencing senescence-related disorders and cancer.

This review reveals the context-dependent dual function of cGAS/STING signaling in cancer that acts as both a protector to enhance anti-tumor immune responses, and a facilitator that allows immune evasion depending on cellular-contexts. We also review the cGAS/STING pathway’s impact on senescence, from its role in regulating cellular processes to its involvements in age-related diseases, further highlighting its significance in maintaining cellular homeostasis. Ongoing research in the cGAS/STING pathway is of paramount importance for several compelling reasons. First, increasing numbers of human diseases have been connected to deregulation of the cGAS/STING signaling, ranging from inflammatory diseases, cancer, cardiovascular diseases to neurodegenerative diseases due to the importance of this signaling in anti-infection and tissue homeostasis control. Second, the clinical implications of the cGAS/STING pathway targeting therapies underscore the importance of continued research. Senescence-related disorders and cancer present significant challenges in healthcare and unraveling the intricacies of the cGAS/STING pathway offers promising avenues for therapeutic interventions. The pathway’s potential as a diagnostic tool and a target for personalized treatments necessitates further investigation.

Ongoing research are warranted to provide insights into the crosstalks between the cGAS/STING pathway and other signaling networks. Understanding these interactions with precision is pivotal for designing effective combination therapies and exploiting crosstalks for therapeutic purposes. The challenges faced in targeting the cGAS/STING pathway, including maintaining therapeutic specificity, highlight the need for future research to refine and optimize therapeutic strategies. Moreover, the clinical relevance of the cGAS/STING pathway in neurodegenerative disorders, cardiovascular diseases, and chronic inflammatory conditions offers avenues for translational research. Developing therapeutic strategies that modulate the pathway’s activity in a targeted manner could revolutionize the treatment landscape for these diverse diseases.

In conclusion, the cGAS/STING pathway stands as a fascinating and versatile subject of study with profound implications for both basic science and clinical applications. Summarizing the key findings underscores the pathway’s dual roles, its impact on cellular processes, and its relevance to age-related diseases and cancer. As research in this field advances, the cGAS/STING pathway holds the promise of transformative discoveries that could reshape our understanding of diseases and pave the way for innovative therapeutic interventions. The importance of continued research cannot be overstated, as it is the key to unlocking the full potential of the cGAS/STING pathway and translating these discoveries into tangible clinical benefits.