Abstract
In this issue of Cancer Research, Ranoa and colleagues report on the role of STING (stimulator of IFN genes, TMEM173) in regulating critical tumor cell–intrinsic functions including cell-cycle progression, chromosomal stability, and cellular response to therapeutic ionizing radiation. The authors used multiple methods including RNA expression profiling, molecular and biochemical techniques, cell biology, and reagents from genetically modified murine models to test their hypothesis that downregulating the STING pathway in cancer cells promotes cellular transformation through accumulation of chromosomal instability and premature progression of the cell cycle. Their findings demonstrate that STING is a tumor suppressor that inhibits cell proliferation by restricting entry to mitosis as well as protecting cells against aneuploidy. These findings significantly advance our understanding of the role of STING as a tumor gate keeper.
See related article by Ranoa et al., p. 1465
In this issue of Cancer Research, Ranoa and colleagues (1) address a fundamental theme in personalized cancer medicine. That is, how to identify subgroups of patients, based on molecular biomarkers and/or signatures, that will likely benefit from specific therapies, including therapeutic ionizing radiation (IR; ref. 1). In this regard, one potential molecular target may be STING (stimulator of IFN genes; also known as TMEM173, MITA, ERIS, or MPYS). STING is a cytoplasmic pattern recognition receptor that has been previously shown to regulate type I IFN production in response to the accumulation of double-stranded DNA (dsDNA) breaks in the cytoplasm. This is of significant biological and clinical importance because exposure to therapeutic IR induces dsDNA breaks that alter tumor cell cellular proliferation and survival. Previous studies suggest that genotoxic stress–induced DNA damage leads to the formation of micronuclei and/or chromosomal fragments in the cytosol that are recognized by cyclic GMP-AMP synthase (cGAS), triggering cellular senescence through STING-mediated production of type I IFN and other proinflammatory cytokines. In addition, it has been demonstrated that this cytosolic DNA-sensing pathway could also drive a STING-dependent adaptive immune response in response to IR-triggered antitumor responses (2).
In this study, Ranoa and colleagues suggest a different cellular homeostatic mechanism other than STING-dependent cytosolic DNA-sensing pathway. They found that STING could play a homeostatic role in cell proliferation by acting both as a mitotic checkpoint on cell division and as quality control of chromosomal stability. This mitotic checkpoint pathway requires the activation of NF-κB, p53, and cyclin-dependent kinase inhibitor CDKNA1 (p21, WAF1, CIP1). They showed that STING controls tumor growth in a cell-intrinsic mode and that loss of STING could lead to inactivation of CDKNA1 and abolish the mitotic checkpoint. This dysregulated cell cycle favored the survival of some tumor cells, especially after IR. Furthermore, early entry to the S-phase and mitosis caused by the loss of STING could further contribute to the genomic instability because of the increased aneuploidy and micronuclei in these tumor cells. Because STING is required for inducing inflammatory cytokines and chemokines that modulate the host immune response, the findings in this study provide a potential mechanism for how the progression of ontogenesis in certain tumor cells resulting from the accumulation of nuclear structure abnormalities avoids activating host-mediated immune responses.
Activation of the STING pathway could be beneficial for cancer therapy as activation of antigen-presenting cells in the tumor microenvironment leads to the production of IFNβ and the generation of CD8+ T cells, which can trigger an adaptive immune response against tumors and inhibit tumor growth (3). Several studies suggest STING agonists inhibit tumor growth through activation of dendritic cells and the innate immune response (4). Conversely, some studies have demonstrated that STING agonists can induce cancer cell death directly. Tang and colleagues observed that overexpression of STING in MCF-7 or T47D breast cancer cell lines with low basal expression of STING caused an increase in caspase-3 and/or -7 activity and apoptosis (5). These conflicting data suggest that additional STING-regulated pathways that control tumor cell growth may exist.
Ionizing radiation is a common treatment plan for patients with cancer. IR-induced cell killing is associated with dsDNA breaks. Increased DNA damage caused by IR or other genotoxic stress results in formation of micronuclei and/or chromosomal fragments. The balance between acute DNA damage and changes in growth factor and/or chemokine levels induced by IR can influence cell-cycle progression. In addition, some genomic stress induces chromosomal missegregation in subsequent cell mitosis, and the replicated chromosomes that fail to distribute equally into daughter cells will form micronuclei. When the nuclear envelope of micronuclei breaks apart, the DNA content is exposed to cGAS surveillance (6, 7). More importantly, cancer cells have been shown to have altered cell cycle and increased aneuploidy and micronuclei. This raises the possibility of a connection among the STING-dependent cytosolic DNA-sensing pathway, cell cycle, and tumor cell proliferation.
Ranoa and colleagues demonstrated that STING is directly involved in tumor cell-cycle progression and that STING-mediated regulation of p21 requires both NFκB and p53. Immortalized MEFs from STING−/− mice or cancer cells with downregulated STING expression had a faster growth rate and a shorter overall cell cycle compared with controls. Because several STING-dependent genes are components of the CDKN1A/RB/MDM2 network, including the stoichiometric cyclin-dependent kinase inhibitor p21, they also demonstrated that CDKN1A is a downstream target of STING and loss of STING could lead to the deactivation of p21. In addition, overexpression of STING led to higher activation of NFκB and p21, while p53 depletion in these cells abolished p21 activation, suggesting that NFκB and p53 are required for STING-CDKNA1A cell-cycle regulation. Progression from G2 to M phase is driven by activation of the CDK1/cyclin B1 complex. They observed that STING−/− MEFs exhibited lower levels of CDK1 and higher levels of the mitotic checkpoint proteins BUB1 and MAD2L1 that are always associated with aggressive tumor cell proliferation. The karyotypic analyses using STING−/− MEFs also suggested a higher degree of chromosomal aberrations and polyploidy, and these aberrations are enhanced by IR. Taken together, their findings suggest a putative tumor-suppressive role for STING, which is involved in cell-cycle regulation and chromosomal quality control.
The increased genomic instability could be favored by the tumor cells because it can promote transformation and create a tumor-permissive phenotype. However, excessive DNA damage with early mitotic entry could also lead to mitotic catastrophe and cell death. Ranoa and colleagues also provide a possible rationale for cancer therapy. They hypothesized that further impairing G2–M regulation in STING-deficient cells by blocking CDK1 Tyr15 phosphorylation by WEE1 might cause synergistic lethality and further sensitize the cells to IR. WEE1 inhibitors have been shown to trigger mitotic entry prior to completion of DNA synthesis and DNA damage repair. As expected, STING-depleted tumor cells treated with the WEE1 inhibitor MK1775 and, in combination with IR, exhibited greater growth inhibition and enhanced the cell-killing effects of IR.
In summary, this study identified a tumor-suppressive function of STING as a cell-cycle regulator in addition to a stimulator of antitumor immune response. More importantly, these findings suggest a connection in STING-depleted cells among dysregulated cell cycle, accumulated errors in their genome, and failure to activate host immune proinflammatory responses. This study could guide the design of future cancer treatment involving IR and cancer immunotherapies.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.