STING (stimulator of IFN genes; also known as MITA/ERIS/MPYS/TMEM173) is an endoplasmic reticulum (ER) resident, which is required for effective type I IFN production in response to nucleic acids. Indeed, select pathogen-derived DNA or RNA were shown to activate STING in human and mouse cells (Ishikawa H and Barber GN 2008; Ishikawa H et al. 2009; Sun W et al. 2009; Prantner D et al. 2010). Importantly, in vitro studies have shown that STING is essential for Mycobacterium tuberculosis (Manzanillo PS et al. 2012), Plasmodium falciparum (Sharma S et al. 2011) and human immunodeficiency virus (HIV) induced type I IFN production [Yan N et al 2010]. Mycobacterium tuberculosis, plasmodium falciparum and HIV are three deadliest pathogens, which kill millions of people each year worldwide.
STING has been also implicated in type I IFN response which was stimulated by fusion of viral and target-cell membrane in a manner independent of DNA, RNA and viral capsid [Holm CK et al 2012].
Under steady state conditions, STING is positioned at the translocon complex within the ER membrane. However upon stimulation with intracellular DNA it translocates from ER to perinuclear vesicles via the Golgi by mechanisms that remain unclear (Ishikawa H and Barber GN 2008; Sun W et al. 2009; Ishikawa H et al. 2009; Saitoh T et al. 2009). Mouse Sting trafficking in dsDNA-stimulated mouse embryonic fibroblasts (MEF) cells was found to depend on autophagy-related gene 9a (Atg9a) (Saitoh T et al. 2009).
STING was reported to function as a signaling adaptor or coreceptor in response to cytosolic dsDNA (Unterholzner L et al. 2010; Zhang Z et al. 2011). STING was also shown to function as a direct DNA sensor to induce the innate immune response in human telomerase fibroblasts (hTERT-BJ1) and murine embryonic fibroblasts (MEFs) (Abe T et al. 2013). Additionally, STING is thought to function as a direct sensor of cyclic dinucleotides. STING was shown to interact directly with c-di-GMP in human embryonic kidney HEK293T cell lysates (Burdette DL et al. 2011). Once STING is stimulated, its C-terminus serves as a signaling scaffold to recruit IRF3 and TBK1, which leads to TBK1-dependent phosphorylation of IRF3 (Tanaka Y and Chen ZJ 2012).
Mouse, but not human STING, can also bind vascular disrupting agents 5,6-dimethylxanthenone-4-acetic acid (DMXAA) and the antiviral small molecule 10-carboxymethyl-9-acridanone (CMA) to induce type I IFN production, suggesting a species-specific drug effect on the STING-mediated host response (Conlon J et al. 2013; Cavlar T et al. 2013).