ISG15 counteracts Listeria monocytogenes infection.
Bottom Line: ISG15 is an interferon-stimulated, linear di-ubiquitin-like protein, with anti-viral activity.We made use of stable isotope labeling in tissue culture (SILAC) to identify ISGylated proteins that could be responsible for the protective effect.Strikingly, infection or overexpression of ISG15 leads to ISGylation of ER and Golgi proteins, which correlates with increased secretion of cytokines known to counteract infection.
Affiliation: Unité, Institut Pasteur, Paris, France.
ISG15 is an interferon-stimulated, linear di-ubiquitin-like protein, with anti-viral activity. The role of ISG15 during bacterial infection remains elusive. We show that ISG15 expression in nonphagocytic cells is dramatically induced upon Listeria infection. Surprisingly this induction can be type I interferon independent and depends on the cytosolic surveillance pathway, which senses bacterial DNA and signals through STING, TBK1, IRF3 and IRF7. Most importantly, we observed that ISG15 expression restricts Listeria infection in vitro and in vivo. We made use of stable isotope labeling in tissue culture (SILAC) to identify ISGylated proteins that could be responsible for the protective effect. Strikingly, infection or overexpression of ISG15 leads to ISGylation of ER and Golgi proteins, which correlates with increased secretion of cytokines known to counteract infection. Together, our data reveal a previously uncharacterized ISG15-dependent restriction of Listeria infection, reinforcing the view that ISG15 is a key component of the innate immune response.
No MeSH data available.
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Mentions: Certain UBL proteins, such as SUMO and ISG15, modify 1–5% of a given substrate; nevertheless, this partial modification is sufficient to lead to a phenotypic effect on the substrate. We thus investigated whether the enrichment of ER and Golgi proteins modified by ISG15 could have an effect on the primary function of these organelles. Canonical secretion of cytokines and growth factors involves translation and import of the proteins into the ER, folding and modification (e.g., glycosylation), within the ER, followed by sorting and trafficking within the Golgi culminating in targeting to the plasma membrane. Since we found proteins implicated in many of these processes to be ISGylated (Figure 4C), we thus hypothesized that canonical secretion could be altered in the ISG15-overexpressing cells. As TNF-α is known to lead to canonical secretion of many cytokines, via activation of NF-κB and MAPK pathways, we assessed secreted cytokine levels in ISG15-overexpressing cells compared to control cells following TNF-α treatment. Among a panel of 31 human cytokines, we detected increased secretion of IL-8 and IL-6 in ISG15-expressing cells relative to control cells (Figure 5—figure supplement 2). We validated these findings with quantitative ELISAs for IL-6 and IL-8 (Figure 5D). ISG15-expressing cells secreted significantly more IL-6 and IL-8 following TNF-α treatment than control cells (Figure 5D). As ISG15 itself has been reported to be able to act as a secreted cytokine, we also assessed ISG15 in the supernatant. However, we did not observe secreted ISG15 from either cell type (Figure 5—figure supplement 3).
No MeSH data available.