Differential activation of IFN regulatory factor (IRF)-3 and IRF-5 transcription factors during viral infection

Members of the IFN regulatory factor (IRF) family regulate gene expression critical to immune response, hemopoiesis, and proliferation. Although related by homology at their N-terminal DNA-binding domain, they display individual functional properties. The distinct properties result from differences in regulated expression, response to activating signals, and interaction with DNA regulatory elements. IRF-3 is expressed ubiquitously and is activated by serine phosphorylation in response to viral infection or TLR signaling. Evidence indicates that the kinases TANK-binding kinase 1 and inhibitor of NF-kappaB kinase-epsilon specifically phosphorylate and thereby activate IRF-3. We evaluated the contribution of another member of the IRF family, IRF-5, during viral infection since prior studies provided varied results. Analysis of phosphorylation, nuclear translocation, dimerization, binding to CREB-binding protein, recognition of DNA, and induction of gene expression were used comparatively with IRF-3 as a measure of IRF-5 activation. IRF-5 was not activated by viral infection; however, expression of TANK-binding kinase 1 or inhibitor of NF-kappaB kinase-epsilon did provide clear activation of IRF-5. IRF-5 is therefore distinct in its activation profile from IRF-3. However, similar to the biological effects of IRF-3 activation, a constitutively active mutation of IRF-5 promoted apoptosis. The apoptosis was inhibited by expression of Bcl-x(L) but not a dominant-negative mutation of the Fas-associated death domain. These studies support the distinct activation profiles of IRF-3 in comparison to IRF-5, but reveal a potential shared biological effect.     

Human cytomegalovirus IE86 attenuates virus- and tumor necrosis factor alpha-induced NFkappaB-dependent gene expression

Human cytomegalovirus (HCMV) infection regulates a number of genes involved in the host antiviral response. We have previously reported that HCMV attenuates the expression of beta interferon (IFN-beta) and a number of proinflammatory chemokines, and this attenuation is mediated by the HCMV immediate-early protein IE86. The present study seeks to identify the mechanism by which IE86 blocks IFN-beta expression. We demonstrate that the induction of IFN-beta during HCMV infection requires the activation of both the IRF-3 and the NFkappaB pathways. Therefore, IE86 may target either pathway to block IFN-beta expression. Our results show that IE86 does not block IRF-3 phosphorylation, dimerization, nuclear translocation, or target gene expression. However, using gel shift analysis, we demonstrate that IE86 efficiently inhibits virus-induced binding of NFkappaB to the IFN-beta promoter, resulting in attenuation of IFN-beta and NFkappaB-dependent gene expression. Furthermore, IE86 expression inhibits tumor necrosis factor alpha-induced NFkappaB DNA binding and target gene expression. Together, these results identify IE86 as a NFkappaB antagonist, which results in the suppression of NFkappaB-dependent cytokine and chemokine gene expression.     

Lipopolysaccharide-binding protein critically regulates lipopolysaccharide-induced IFN-beta signaling pathway in human monocytes

LPS binding to Toll-like receptor 4 induces a large number of genes through activation of NF-kappaB and IFN-regulatory factor-3 (IRF-3). However, no previous reports have tested the role of serum proteins in LPS-induced gene expression profiles. To investigate how serum proteins affect LPS-induced signaling, we investigated LPS-inducible genes in PBMC using an oligonucleotide probe-array system. Approximately 120 genes up-regulated by LPS were hierarchically divided into two clusters. Induction of one cluster, containing only IFN-inducible genes, was serum dependent. Real-time PCR analysis confirmed that IFN-inducible genes were induced only in the presence of serum, whereas inflammatory genes were induced both in the presence and absence of serum. Further analysis demonstrated that addition of LPS-binding protein (LBP), but not of soluble CD14 to the serum-free medium enabled the induction of IFN-inducible genes and IFN-beta itself by LPS in human monocytes. The mRNAs for IFN-beta and IFN-inducible genes were induced by LPS only in the presence of serum from LBP(+/+) mice, and not in the presence of serum from LBP(-/-) mice. Blocking experiments also confirmed the involvement of LBP in this phenomenon. Immunoblotting analysis showed that phosphorylation of c-Jun N-terminal kinase, p38, IRF-3, tyrosine kinase 2, and STAT1 by LPS, but not of NF-kappaB and extracellular signal-regulated kinase was abrogated in the absence of LBP. This critical role for LBP implies the presence of possible mechanisms linking LBP to the intracellular signaling between Toll-like receptor 4 and IRF-3, leading to the induction of IFN-beta by LPS.