Cooperation between STAT3 and c-Jun Suppresses Fas Transcription

Decreased Fas expression during tumor progression often results in a loss of Fas-ligand (FasL)-mediated apoptosis. Human and mouse melanoma exhibit an inverse correlation between the degree of Fas cell surface expression, tumorigenicity, and metastatic capacity. The expression of dominant negative Stat3 or c-Jun in melanoma cells efficiently increased Fas expression and sensitized cells to FasL-induced apoptosis. Stat3+/− as well as c-Jun−/− cells exhibited increased Fas cell surface expression and higher sensitivity to FasL-mediated apoptosis. Suppression of Fas expression by Stat3 and c-Jun is uncoupled from Stat3-mediated transcriptional activation. Our findings indicate that Stat3 oncogenic activities could also be mediated through its cooperation with c-Jun, resulting in downregulation of Fas surface expression, which is implicated in the tumor's ability to resist therapy and metastasize

Impaired Cellular Responses to Cytosolic DNA or Infection with Listeria monocytogenes and Vaccinia Virus in the Absence of the Murine LGP2 Protein

Innate immune signaling is crucial for detection of and the initial response to microbial pathogens. Evidence is provided indicating that LGP2, a DEXH box domain protein related to the RNA recognition receptors RIG-I and MDA5, participates in the cellular response to cytosolic double-stranded DNA (dsDNA). Analysis of embryonic fibroblasts and macrophages from mice harboring targeted disruption in the LGP2 gene reveals that LGP2 can act as a positive regulator of type I IFN and anti-microbial gene expression in response to transfected dsDNA. Results indicate that infection of LGP2-deficient mice with an intracellular bacterial pathogen, Listeria monocytogenes, leads to reduced levels of type I IFN and IL12, and allows increased bacterial growth in infected animals, resulting in greater colonization of both spleen and liver. Responses to infection with vaccinia virus, a dsDNA virus, are also suppressed in cells lacking LGP2, reinforcing the ability of LGP2 to act as a positive regulator of antiviral signaling. In vitro mechanistic studies indicate that purified LGP2 protein does not bind DNA but instead mediates these responses indirectly. Data suggest that LGP2 may be acting downstream of the intracellular RNA polymerase III pathway to activate anti-microbial signaling. Together, these findings demonstrate a regulatory role for LGP2 in the response to cytosolic DNA, an intracellular bacterial pathogen, and a DNA virus, and provide a plausible mechanistic hypothesis as the basis for this activity

Positive and Negative Regulation of the Innate Antiviral Response and Beta Interferon Gene Expression by Deacetylation

Beta interferon (IFN-β) gene expression in response to virus infection relies on the dynamic assembly of a multiprotein enhanceosome complex that is initiated by the activation of two inducible transcription factors, interferon regulatory factor 3 (IRF3) and NF-κB. Virus or double-stranded RNA-induced activation of IFN-β gene expression is prevented by the addition of protein deacetylase inhibitors. The isolated IRF-responsive positive regulatory domain was found to require deacetylation for its activity, but IRF3 protein activation leading to its nuclear translocation and DNA binding was not impaired by deacetylase inhibition. In contrast, NF-κB activity was not affected by deacetylase inhibitors. RNA interference indicated that several deacetylase enzymes, including histone deacetylase 1 (HDAC1), HDAC8, and HDAC6, influence IFN-β gene expression with opposing effects. While HDAC1 and HDAC8 repress IFN-β expression, HDAC6 acts as a coactivator essential for enhancer activity. Virus replication is enhanced in HDAC6-depleted cells, demonstrating HDAC6 is an essential component of innate antiviral immunity

RNA- and Virus-Independent Inhibition of Antiviral Signaling by RNA Helicase LGP2

Antiviral innate immune responses can be triggered by accumulation of intracellular nucleic acids resulting from virus infections. Double-stranded RNA (dsRNA) can be detected by the cytoplasmic RNA helicase proteins RIG-I and MDA5, two proteins that share sequence similarities within a caspase recruitment domain (CARD) and a DExD/H box RNA helicase domain. These proteins are considered dsRNA sensors and are thought to transmit the signal to the mitochondrial adapter, IPS-1 (also known as MAVS, VISA, or CARDIF) via CARD interactions. IPS-1 coordinates the activity of protein kinases that activate transcription factors needed to induce beta interferon (IFN-β) gene transcription. Another helicase protein, LGP2, lacks the CARD region and does not activate IFN-β gene expression. LGP2 mRNA is induced by interferon, dsRNA treatments, or Sendai virus infection and acts as a feedback inhibitor for antiviral signaling. Results indicate that LGP2 can inhibit antiviral signaling independently of dsRNA or virus infection intermediates by engaging in a protein complex with IPS-1. Experiments suggest that LGP2 can compete with the kinase IKKi (also known as IKKɛ) for a common interaction site on IPS-1. These results provide the first demonstration of protein interaction as an element of negative-feedback regulation of intracellular antiviral signaling by LGP2

STAT2 Is a Primary Target for Measles Virus V Protein-Mediated Alpha/Beta Interferon Signaling Inhibition▿

Measles virus, a member of the Morbillivirus family, infects millions of people each year despite the availability of effective vaccines. The V protein of measles virus is an important virulence factor that can interfere with host innate immunity by inactivating alpha/beta interferon (IFN-α/β) and IFN-γ signaling through protein interactions with signal transducer and activator of transcription proteins STAT1 and STAT2. Here we demonstrate that although STAT1 interference results from protein interactions within a V protein N-terminal region encompassed by amino acids 110 to 130, detection of STAT1 interaction and IFN-γ signaling inhibition requires the presence of cellular STAT2. Cell-specific variability in STAT1 interference was observed to correlate with V protein expression level. A more direct target for measles virus V protein-mediated IFN-α/β evasion is STAT2. Results indicate that the widely conserved C-terminal zinc finger domain of measles virus V protein is both necessary and sufficient to bind STAT2 and disrupt IFN-α/β signal transduction. Mutagenesis and molecular modeling define a contact surface for STAT2 association that includes aspartic acid residue 248 as critical for STAT2 interference and IFN antiviral immune suppression. These findings clearly define the molecular determinants for measles virus IFN evasion and validate specific targets as candidates for therapeutic intervention

Nipah Virus V Protein Evades Alpha and Gamma Interferons by Preventing STAT1 and STAT2 Activation and Nuclear Accumulation

Characterization of recent outbreaks of fatal encephalitis in southeast Asia identified the causative agent to be a previously unrecognized enveloped negative-strand RNA virus of the Paramyxoviridae family, Nipah virus. One feature linking Nipah virus to this family is a conserved cysteine-rich domain that is the hallmark of paramyxovirus V proteins. The V proteins of other paramyxovirus species have been linked with evasion of host cell interferon (IFN) signal transduction and subsequent antiviral responses by inducing proteasomal degradation of the IFN-responsive transcription factors, STAT1 or STAT2. Here we demonstrate that Nipah virus V protein escapes IFN by a distinct mechanism involving direct inhibition of STAT protein function. Nipah virus V protein differs from other paramyxovirus V proteins in its subcellular distribution but not in its ability to inhibit cellular IFN responses. Nipah virus V protein does not induce STAT degradation but instead inhibits IFN responses by forming high-molecular-weight complexes with both STAT1 and STAT2. We demonstrate that Nipah virus V protein accumulates in the cytoplasm by a Crm1-dependent mechanism, alters the STAT protein subcellular distribution in the steady state, and prevents IFN-stimulated STAT redistribution. Consistent with the formation of complexes, STAT protein tyrosine phosphorylation is inhibited in cells expressing the Nipah virus V protein. As a result, Nipah virus V protein efficiently prevents STAT1 and STAT2 nuclear translocation in response to IFN, inhibiting cellular responses to both IFN-α and IFN-γ