BRG1 is indispensable for IFN-γ-induced TRIM22 expression, which is dependent on the recruitment of IRF-1

Biochemical strategies of E3 ubiquitin ligases target viruses in critical diseases

Viruses are known to cause various diseases in human and also infect other species such as animal plants, fungi, and bacteria. Replication of viruses depends upon their interaction with hosts. Human cells are prone to such unwanted viral infections. Disintegration and reconstitution require host machinery and various macromolecules like DNA, RNA, and proteins are invaded by viral particles. E3 ubiquitin ligases are known for their specific function, that is, recognition of their respective substrates for intracellular degradation. Still, we do not understand how ubiquitin proteasome system-based enzymes E3 ubiquitin ligases do their functional interaction with different viruses. Whether E3 ubiquitin ligases help in the elimination of viral components or viruses utilize their molecular capabilities in their intracellular propagation is not clear. The first time our current article comprehends fundamental concepts and new insights on the different viruses and their interaction with various E3 Ubiquitin Ligases. In this review, we highlight the molecular pathomechanism of viruses linked with E3 Ubiquitin Ligases dependent mechanisms. An enhanced understanding of E3 Ubiquitin Ligase-mediated removal of viral proteins may open new therapeutic strategies against viral infections.

Toxoplasma gondii Inhibits gamma interferon (IFN-γ)- and IFN-β-induced host cell STAT1 transcriptional activity by increasing the association of STAT1 with DNA

The gamma interferon (IFN-γ) response, mediated by the STAT1 transcription factor, is crucial for host defense against the intracellular pathogen Toxoplasma gondii, but prior infection with Toxoplasma can inhibit this response. Recently, it was reported that the Toxoplasma type II NTE strain prevents the recruitment of chromatin remodeling complexes containing Brahma-related gene 1 (BRG-1) to promoters of IFN-γ-induced secondary response genes such as Ciita and major histocompatibility complex class II genes in murine macrophages, thereby inhibiting their expression. We report here that a type I strain of Toxoplasma inhibits the expression of primary IFN-γ response genes such as IRF1 through a distinct mechanism not dependent on the activity of histone deacetylases. Instead, infection with a type I, II, or III strain of Toxoplasma inhibits the dissociation of STAT1 from DNA, preventing its recycling and further rounds of STAT1-mediated transcriptional activation. This leads to increased IFN-γ-induced binding of STAT1 at the IRF1 promoter in host cells and increased global IFN-γ-induced association of STAT1 with chromatin. Toxoplasma type I infection also inhibits IFN-β-induced interferon-stimulated gene factor 3-mediated gene expression, and this inhibition is also linked to increased association of STAT1 with chromatin. The secretion of proteins into the host cell by a type I strain of Toxoplasma without complete parasite invasion is not sufficient to block STAT1-mediated expression, suggesting that the effector protein responsible for this inhibition is not derived from the rhoptries.

p300, but not PCAF, collaborates with IRF-1 in stimulating TRIM22 expression independently of its histone acetyltransferase activity

Tripartite motif (TRIM) 22 plays an important role in IFN-mediated antiviral activity. We previously demonstrated that IFN regulatory factor-1 (IRF-1) was crucial for constitutive and IFN-induced TRIM22 expression via binding to a special cis-element named 5' extended IFN-stimulating response element. Here, we further investigate the molecular mechanisms of TRIM22 with a focus on the co-activators of IRF-1. Using an in vitro DNA affinity binding assay and an in vivo chromatin immunoprecipitation assay, we found that IFN-γ stimulation significantly enhanced the binding of p300 and p300/CBP-associated factor, but not other co-activators such as general control nondepressible 5, steroid receptor co-activator-1, and activator of thyroid and retinoic, to the 5' extended IFN-stimulating response element containing TRIM22 promoter region together with IRF-1. Overexpression and knockdown analysis demonstrated that it was p300, but not p300/CBP-associated factor, that functioned as a transcriptional co-activator of IRF-1 in IFN-γ induction of TRIM22. We further show that p300 contributed to both IFN-γ- and IRF-1-mediated TRIM22 transcription independent of its histone acetyltransferase activity, however, it was required for the recruitment of RNA polymerase II to TRIM22 promoter region. These data indicate that p300 plays a critical role in IFN-γ-induced TRIM22 expression via recruiting RNA polymerase II to the TRIM22 promoter, and might serve as a bridge between IRF-1 and the basal transcriptional apparatus in TRIM22 induction.

Induction of TRIM22 by IFN-γ Involves JAK and PC-PLC/PKC, but Not MAPKs and pI3K/Akt/mTOR Pathways

Tripartite motif (TRIM) 22 plays an important role in interferons (IFNs)-mediated antiviral activity. We previously demonstrated that interferon regulatory factor-1 (IRF-1) played a central role in IFN-γ-induced TRIM22 expression via binding to a special cis-element named 5' extended IFN-stimulating response element (5'eISRE). In this study, we sought to identify the signaling pathways involved in TRIM22 induction by IFN-γ. By using various pharmacological inhibitors, it was found that the activity of tyrosine kinase and phosphatidylcholine-phospholipase C (PC-PLC), but not phosphatidylinositol-phospholipase C (PI-PLC) and phospholipase D (PLD), was required for IFN-γ-induced TRIM22 expression in HepG2 cells. Tyrosine kinase Janus kinase (JAK), not SRC and PYK2, played an indispensable role in TRIM22 induction. Inhibition of protein kinase C (PKC) activity also significantly attenuated IFN-γ induction of TRIM22. Although treatment with IFN-γ resulted in the stimulation of mitogen-activated protein kinases (MAPKs) (p38, ERK, and JNK) and pI3K/Akt/mTOR pathways in HepG2 cells, the inhibition of their activity did not affect IFN-γ-stimulated TRIM22 expression. Further studies showed that overexpression of JAK1 and PKCα activated TRIM22 promoter activity in a 5'eISRE-dependent manner, and inhibition of not only JAK but also PC-PLC/PKC pathways significantly attenuated IFN-γ-induced IRF-1 expression in HepG2 cells. Taken together, these data indicated that IFN-γ induced TRIM22 expression via activation of JAK and PC-PLC/PKC signaling pathways, which involved the cis-element 5'eISRE and the transactivator IRF-1.

Toxoplasma gondii clonal strains all inhibit STAT1 transcriptional activity but polymorphic effectors differentially modulate IFNγ induced gene expression and STAT1 phosphorylation

Host defense against the parasite Toxoplasma gondii requires the cytokine interferon-gamma (IFNγ). However, Toxoplasma inhibits the host cell transcriptional response to IFNγ, which is thought to allow the parasite to establish a chronic infection. It is not known whether all strains of Toxoplasma block IFNγ-responsive transcription equally and whether this inhibition occurs solely through the modulation of STAT1 activity or whether other transcription factors are involved. We find that strains from three North American/European clonal lineages of Toxoplasma, types I, II, and III, can differentially modulate specific aspects of IFNγ signaling through the polymorphic effector proteins ROP16 and GRA15. STAT1 tyrosine phosphorylation is activated in the absence of IFNγ by the Toxoplasma kinase ROP16, but this ROP16-activated STAT1 is not transcriptionally active. Many genes induced by STAT1 can also be controlled by other transcription factors and therefore using these genes as specific readouts to determine Toxoplasma inhibition of STAT1 activity might be inappropriate. Indeed, GRA15 and ROP16 modulate the expression of subsets of IFNγ responsive genes through activation of the NF-κB/IRF1 and STAT3/6 transcription factors, respectively. However, using a stable STAT1-specific reporter cell line we show that strains from the type I, II, and III clonal lineages equally inhibit STAT1 transcriptional activity. Furthermore, all three of the clonal lineages significantly inhibit global IFNγ induced gene expression.

TRIM22: A Diverse and Dynamic Antiviral Protein

The tripartite motif (TRIM) family of proteins is an evolutionarily ancient group of proteins with homologues identified in both invertebrate and vertebrate species. Human TRIM22 is one such protein that has a dynamic evolutionary history that includes gene expansion, gene loss, and strong signatures of positive selection. To date, TRIM22 has been shown to restrict the replication of a number of viruses, including encephalomyocarditis virus (EMCV), hepatitis B virus (HBV), and human immunodeficiency virus type 1 (HIV-1). In addition, TRIM22 has also been implicated in cellular differentiation and proliferation and may play a role in certain cancers and autoimmune diseases. This comprehensive paper summarizes our current understanding of TRIM22 structure and function.