The role of cGAS in pathologic liver injury and viral hepatitis is still in debate. Although studies had reported cGAS deletion may attenuate alcohol and radiation induced innate-immunity-driven pathology and hepatic injury[20, 21], others observed a contrary protective effect of cGAS deletion on ischemia-reperfusion liver injury[22]. In terms of viral hepatitis, cGAS-STING mediated innate immune responses are responsible to inhibit the replication of HBV and HCV[23–26]. The difference in disease models might be a potential contributing factor to these discrepancies, this also suggests a complex role of cGAS in liver immunopathology, and more work is required to refine the role of cGAS in mediating pathologic liver injury.
In this study, we provided evidence that cGAS may have a protective effect on ConA-induced acute hepatitis. Therefore, deletion of cGAS exacerbates ConA-induced deterioration of liver function and promotes hepatocyte apoptosis. It has been established that following crosslinking T cell receptors and mannose-rich glycoproteins on liver sinusoidal endothelial cells and Kupffer cells, ConA activated T cells, leading to the secretion of large amounts of pro-inflammatory cytokines[3]. Inflammatory cells such as macrophages and neutrophils recruited to the injury site, amplified innate immune signals, and further contributed to liver necrosis and impairment of liver function [3, 17]. Our results showed that, upon ConA induction, the hepatic infiltration of macrophages, neutrophils, and T cells were all significantly increased in the cGAS deficient livers, suggesting that the amplified inflammatory responses might be a major contributor to the aggravated liver damage in the KO mice. Moreover, the in vitro data proved that devoid of cGAS may enhance the migration potential of macrophages and elevate the expression pro-inflammatory genes. Therefore, cGAS may prevent ConA-induced acute hepatic injury by restricting leukocyte migration and suppressing inflammatory response. However, a limitation is that here we only used the global cGAS knockout mice in which cGAS was deleted both in immune cells and in hepatic parenchymal cells, thus it is reasonable to speculate that the expression of cGAS in hepatocytes may have similar or even greater contribution to the observed phenotypes. Evidence from experiments using the tissue-specific knockout mice would shed light on this important issue. Moreover, how cGAS influences the interplay between hepatocytes and immune cells during ConA challenge is still elusive.
Interestingly, our data implied that the protective role of cGAS upon ConA challenge is independent of its usual downstream signaling partner STING. This finding is kind of consistent with previous reports that cGAS protected the liver from ischemia-reperfusion injury independent of STING[22]. Although we observed up-regulation of STING and its mediated interferon genes after ConA treatment, inhibition of STING failed to replicate the protective effect of cGAS deletion on ConA-induced acute liver injury. Honestly, here we only used a pharmaceutical approach to lower STING expression which represents a limitation that STING activity might be not completely blocked. Genetically deletion of STING gene in mice would be required to confirm the conclusion in the future. Notably, in addition to the initially described cytoplasmic localization, cGAS also localized on inner leaflet of the plasma membrane[27] and in the nucleus[28]. A recent work indicates that cGAS constitutively resides in the nucleus[29, 30] and tightly sequestered to chromatin[31]. The nuclear cGAS needs to export to the cytoplasm and perform DNA-sensing function, which depends on a nuclear export signal [32]. Though some studies suggest that the nuclear cGAS inhibits the repair of DNA double-strand breaks and promotes tumorigenesis[29, 33], other findings demonstrated that cGAS protects cells from DNA damage and safeguards genome stability during mitosis[34, 35]. This may partly explain the results what we have observed that deletion of cGAS suppressed the surveillance of DNA damage upon ConA challenge. Recently, increasing studies noticed that cGAS not only promotes innate immune response through STING but also plays an important role in other biological process in a STING-independent manner. For example, some studies demonstrated that primary hepatocytes express cGAS [22] but not STING [24, 36]. However, disruption of cGAS in hepatocytes resulted in higher levels of cell death and apoptosis and reduced protective autophagy, suggesting that cGAS participates in the regulation of liver function in a STING-independent manner[22]. In addition, in CCl4-induced liver fibrosis and nonalcoholic steatohepatitis, whether STING-mediated liver injury depends on the DNA sensing of cGAS also remains unclear[37, 38]. Therefore, clarifying the function and mechanisms of cGAS, STING and the related signaling molecules in liver diseases warrants further in-depth investigation.
In conclusion, our results demonstrated that cGAS deficiency accelerated ConA-induced liver injury by promoting leukocyte chemotaxis and accelerating liver inflammatory response. Although much more work is still needed to clarify the molecular mechanisms underlying the role of the cGAS in liver pathology, these studies may provide a novel therapeutic approach for acute immune-mediated hepatitis.