Unphosphorylated ISGF3 drives constitutive expression of interferon-stimulated genes to protect against viral infections, Wang et al. (2017)

[jnmaciuch]

Unphosphorylated ISGF3 drives constitutive expression of interferon-stimulated genes to protect against viral infections​

Wenshi Wang, Yuebang Yin, Lei Xu, Junhong Su, Fen Huang, Yijin Wang, Patrick P. C. Boor, Kan Chen, Wenhui Wang, Wanlu Cao, Xinying Zhou, Pengyu Liu, Luc J. W. van der Laan, Jaap Kwekkeboom, Maikel P. Peppelenbosch, and Qiuwei Pan

An interferon-independent antiviral defense​

Virally infected cells produce type I interferons (IFNs), which stimulate the phosphorylation and activation of the STAT1 and STAT2 transcription factors. When combined with the transcriptional regulator IRF9, phosphorylated STAT1 and STAT2 form the ISGF3 complex, which drives the expression of IFN-stimulated genes (ISGs) that are important for antiviral immunity. Wang et al. report the formation of an alternative form of ISGF3, U-ISGF3, which drove the expression of ISGs and protected cells from viral infection in the absence of detection of detectable IFN or IFN signaling. In addition to IRF9, U-ISGF3 contained unphosphorylated STAT1 and STAT2. Together, these data suggest that U-ISGF3 drives constitutive expression of ISGs as part of an IFN-independent, antiviral immune response.

Abstract
Interferon (IFN)–stimulated genes (ISGs) are antiviral effectors that are induced by IFNs through the formation of a tripartite transcription factor ISGF3, which is composed of IRF9 and phosphorylated forms of STAT1 and STAT2. However, we found that IFN-independent ISG expression was detectable in immortalized cell lines, primary intestinal and liver organoids, and liver tissues. The constitutive expression of ISGs was mediated by the unphosphorylated ISGF3 (U-ISGF3) complex, consisting of IRF9 together with unphosphorylated STAT1 and STAT2. Under homeostatic conditions, STAT1, STAT2, and IRF9 were found in the nucleus. Analysis of a chromatin immunoprecipitation sequencing data set revealed that STAT1 specifically bound to the promoters of ISGs even in the absence of IFNs. Knockdown of STAT1, STAT2, or IRF9 by RNA interference led to the decreased expression of various ISGs in Huh7.5 human liver cells, which was confirmed in mouse embryonic fibroblasts (MEFs) from STAT1−/−, STAT2−/−, or IRF9−/− mice. Furthermore, decreased ISG expression was accompanied by increased replication of hepatitis C virus and hepatitis E virus. Conversely, simultaneous overexpression of all ISGF3 components, but not any single factor, induced the expression of ISGs and inhibited viral replication; however, no phosphorylated STAT1 and STAT2 were detected. A phosphorylation-deficient STAT1 mutant was comparable to the wild-type protein in mediating the IFN-independent expression of ISGs and antiviral activity, suggesting that ISGF3 works in a phosphorylation-independent manner. These data suggest that the U-ISGF3 complex is both necessary and sufficient for constitutive ISG expression and antiviral immunity under homeostatic conditions.

Link (Paywalled) | Sci-hub

 

[jnmaciuch]

Following discussion in another thread about the possibility of interferon signaling in tissue, just wanted to post a potentially interesting paper exploring a mechanism by which and interferon response can be perpetuated long term without active interferon binding to receptor.

Explain like I'm brain-foggy:
The interferon response is often thought of as being mediated by interferon binding to the interferon receptor, which triggers a cascade (JAK-STAT) that ultimately results in the transcription of interferon stimulated genes (ISGs). This study confirms findings from other studies in cultured cells showing that it is actually possible for interferon-stimulated genes to get transcribed in the absence of active interferon signaling.

This occurs through a protein complex known as ISGF3, which normally gets "activated" (phosphorylated) when interferon signaling is induced by interferon receptor binding or detection of a virus. It turns out that the "unactivated" form of this complex is still capable of inducing gene expression, just to a lesser degree than its activated form.

 

[jnmaciuch]

Something to note is that some of the findings here, and in the studies it references, are in cell lines which are usually derived from cancer cells or cells that are otherwise weird in a way that renders them immortal. However, it seems likely that this is also something that occurs in organisms, since expression of interferon-stimulated genes in the absence of detectable interferon has also been found in certain animal models and might have to do with differential efficacy of vaccination. It's just not certain whether it occurs by this same mechanism in organisms vs. cell lines.

Interestingly, some of these findings were in organoids taken from primary tissue and cultured. There remains a possibility that the conditions of culturing may induce this signature, but per the findings in the Newton lab studies, it would not be expected to do so differentially in ME/CFS vs. healthy control unless there was already something different related to interferon and ISG regulation.

 

[Utsikt]

It turns out that the "unactivated" form of this complex is still capable of inducing gene expression, just to a lesser degree than its activated form.

What does «inducing gene expression» mean in this context? Does it mean that that gene has been used to make something?

 

[jnmaciuch]

What does «inducing gene expression» mean in this context? Does it mean that that gene has been used to make something?

It means that transcription of interferon-stimulated genes has been induced by the protein complex serving as a transcription factor. Which then get translated and turned into an active protein (presumably, barring other regulatory interference at these later steps)