IFNs, ISGylation and cancer: Cui prodest?

Bacteria conjugate ubiquitin-like proteins to interfere with phage assembly

Several immune pathways in humans conjugate ubiquitin-like proteins to virus and host molecules as a means of antiviral defence1-5. Here we studied an antiphage defence system in bacteria, comprising a ubiquitin-like protein, ubiquitin-conjugating enzymes E1 and E2, and a deubiquitinase. We show that during phage infection, this system specifically conjugates the ubiquitin-like protein to the phage central tail fibre, a protein at the tip of the tail that is essential for tail assembly as well as for recognition of the target host receptor. Following infection, cells encoding this defence system release a mixture of partially assembled, tailless phage particles and fully assembled phages in which the central tail fibre is obstructed by the covalently attached ubiquitin-like protein. These phages show severely impaired infectivity, explaining how the defence system protects the bacterial population from the spread of phage infection. Our findings demonstrate that conjugation of ubiquitin-like proteins is an antiviral strategy conserved across the tree of life.

The mRNA stability of NCAPG2, a novel contributor to breast invasive carcinoma, is enhanced by the RNA-binding protein PCBP2

Non-SMC condensin II complex subunit G2 (NCAPG2) is one of the three non-SMC subunits in condensin II, which plays a vital role in regulating chromosome condensation and segregation. Although the tumor-promoting role of NCAPG2 has been reported in several solid malignancies, its function in breast invasive carcinoma (BRCA) remains unknown. Data both from GEPIA and GSE36295 indicated that NCAPG2 mRNA expression was abnormally upregulated in cancer tissues, which was further verified in 40 paired BRCA and para-carcinoma samples. Kaplan-Meier Plotter further illustrated that BRCA patients with higher NCAPG2 expression have a poorer prognosis. Functional experiments carried out in two BRCA cell lines (MCF-7 and T-47D) showed that NCAPG2-silenced BRCA cells acquired less aggressive behavior - weakened growth and metastasis both in vitro and in vivo. Label-free proteomics quantified the protein expression patterns in MCF-7 cells, and the results revealed 684 differentially expressed proteins (|log2FC| > 1 and P < 0.05) downstream to NCAPG2. Interestingly, poly(C)-binding protein 2 (PCBP2), an RNA binding protein previously known to increase RNA stability of its target genes, was found to directly bind to and protect NCAPG2 mRNA from degradation-PCBP2 knockdown accelerated the degradation half-life time of NCAPG2 mRNA from approximately 8 h to 5 h. Taken together, our study indicates that NCAPG2 acts as a novel contributor to BRCA growth and metastasis under the regulation of PCBP2, providing insights into BRCA treatment.

ISG15 and ISGylation in Human Diseases

Type I Interferons (IFNs) induce the expression of >500 genes, which are collectively called ISGs (IFN-stimulated genes). One of the earliest ISGs induced by IFNs is ISG15 (Interferon-Stimulated Gene 15). Free ISG15 protein synthesized from the ISG15 gene is post-translationally conjugated to cellular proteins and is also secreted by cells into the extracellular milieu. ISG15 comprises two ubiquitin-like domains (UBL1 and UBL2), each of which bears a striking similarity to ubiquitin, accounting for its earlier name ubiquitin cross-reactive protein (UCRP). Like ubiquitin, ISG15 harbors a characteristic β-grasp fold in both UBL domains. UBL2 domain has a conserved C-terminal Gly-Gly motif through which cellular proteins are appended via an enzymatic cascade similar to ubiquitylation called ISGylation. ISG15 protein is minimally expressed under physiological conditions. However, its IFN-dependent expression is aberrantly elevated or compromised in various human diseases, including multiple types of cancer, neurodegenerative disorders (Ataxia Telangiectasia and Amyotrophic Lateral Sclerosis), inflammatory diseases (Mendelian Susceptibility to Mycobacterial Disease (MSMD), bacteriopathy and viropathy), and in the lumbar spinal cords of veterans exposed to Traumatic Brain Injury (TBI). ISG15 and ISGylation have both inhibitory and/or stimulatory roles in the etiology and pathogenesis of human diseases. Thus, ISG15 is considered a “double-edged sword” for human diseases in which its expression is elevated. Because of the roles of ISG15 and ISGylation in cancer cell proliferation, migration, and metastasis, conferring anti-cancer drug sensitivity to tumor cells, and its elevated expression in cancer, neurodegenerative disorders, and veterans exposed to TBI, both ISG15 and ISGylation are now considered diagnostic/prognostic biomarkers and therapeutic targets for these ailments. In the current review, we shall cover the exciting journey of ISG15, spanning three decades from the bench to the bedside.

Inhibition of UBE2L6 attenuates ISGylation and impedes ATRA-induced differentiation of leukemic cells

Ubiquitin/ISG15‐conjugating enzyme E2L6 (UBE2L6) is a critical enzyme in ISGylation, a post‐translational protein modification that conjugates the ubiquitin‐like modifier, interferon‐stimulated gene 15 (ISG15), to target substrates. Previous gene expression studies in acute promyelocytic leukemia (APL) cells showed that all‐trans‐retinoic acid (ATRA) altered the expression of many genes, including UBE2L6 (200‐fold) and other members of the ISGylation pathway. Through gene expression analyses in a cohort of 98 acute myeloid leukemia (AML) patient samples and in primary neutrophils from healthy donors, we found that UBE2L6 gene expression is reduced in primary AML cells compared with normal mature granulocytes. To assess whether UBE2L6 expression is important for leukemic cell differentiation—two cell line models were employed: the human APL cell line NB4 and its ATRA‐resistant NB4R counterpart, as well as the ATRA‐sensitive human AML HL60 cells along with their ATRA‐resistant subclone—HL60R. ATRA strongly induced UBE2L6 in NB4 APL cells and in ATRA‐sensitive HL60 AML cells, but not in the ATRA‐resistant NB4R and HL60R cells. Furthermore, short hairpin (sh)RNA‐mediated UBE2L6 depletion in NB4 cells impeded ATRA‐mediated differentiation, suggesting a functional role for UBE2L6 in leukemic cell differentiation. In addition, ATRA induced ISG15 gene expression in NB4 APL cells, leading to increased levels of both free ISG15 protein and ISG15 conjugates. UBE2L6 depletion attenuated ATRA‐induced ISG15 conjugation. Knockdown of ISG15 in NB4 APL cells inhibited ISGylation and also attenuated ATRA‐induced differentiation. In summary, we demonstrate the functional importance of UBE2L6 in ATRA‐induced neutrophil differentiation of APL cells and propose that this may be mediated by its catalytic role in ISGylation.

ISG15 suppresses translation of ABCC2 via ISGylation of hnRNPA2B1 and enhances drug sensitivity in cisplatin resistant ovarian cancer cells

Cisplatin-based chemotherapies have long been considered as a standard chemotherapy in ovarian cancer. However, cisplatin resistance restricts beneficial therapy for patients with ovarian cancer. The ubiquitin-like protein interferon-stimulated gene 15 (ISG15) encodes a 15-kDa protein, that is implicated in the post-translational modification of diverse proteins. In this work, we found that ISG15 was downregulated in cisplatin resistant tissues and cell lines of ovarian cancer. Functional studies demonstrated that overexpression of wild type (WT) ISG15, but not nonISGylatable (Mut) ISG15 increased cell responses to cisplatin in resistant ovarian cancer cells. Furthermore, we found that WT ISG15 decreased ABCC2 expression at the protein level. Importantly, overexpression of ABCC2 blocked sensitizing effect of ISG15 on cisplatin. In addition, we identified that hnRNPA2B1 was recruited to 5'UTR of ABCC2 mRNA and promoted its translation, which was blocked by ISG15. We further demonstrated that hnRNPA2B1 could be ISGylated, and ISGylation blocked its recruitment to ABCC2 mRNA, thereby suppressed translation of ABCC2. Altogether, our data support targeting ISG15 might be a potential therapeutic strategy for patients with cisplatin-resistant ovarian cancer.

ISG15 induction is required during L1-mediated colon cancer progression and metastasis

Hyperactivation of Wnt/β-catenin target gene expression is a hallmark of colorectal cancer (CRC) development. We identified L1-CAM (L1) and Nr-CAM, members of the immunoglobulin family of nerve cell adhesion receptors, as target genes of the Wnt/β-catenin pathway in CRC cells. L1 overexpression in CRC cells enhances their motile and tumorigenic capacity and promotes liver metastasis. L1 is often localized at the invasive edge of CRC tissue. Using gene arrays and proteomic analyses we identified downstream signaling pathways and targets of L1-mediated signaling. Here, we found that the expression of interferon-stimulated gene 15 (ISG15) that operates much like ubiquitin (is conjugated to proteins by ISGylation), is elevated in the conditioned medium and in CRC cells overexpressing L1. Suppression of endogenous ISG15 levels in L1-expressing cells blocked the increased proliferative, motile, tumorigenic and liver metastatic capacities of CRC cells. ISG15 overexpression, on its own, could enhance these properties in CRC cells, but only to a much lower extent compared to L1. We show that NF-κB signaling is involved in the L1-mediated increase in ISG15, since blocking the NF-κB pathway abolished the induction of ISG15 by L1. Point mutations in the L1 ectodomain that interfere with its binding to L1 ligands, also inhibited the increase in ISG15. We detected high levels of ISG15 in human CRC tissue cells and in the adjacent stroma, but not in the normal mucosa. The results suggest that ISG15 is involved in L1-mediated CRC development and is a potential target for CRC therapy.

An Approach for the Identification of Proteins Modified with ISG15

Interferon-stimulated gene 15 (ISG15) encodes a protein that is most upregulated by type I interferon stimulation and, upon activation, is conjugated to various target proteins in a process known as ISGylation. ISGylation has been shown to have roles in various biological phenomena such as viral infection and cancer. To gain further insight into the function of ISGylation, it would be useful to be able to identify ISGylated proteins. Here, we describe a method for the identification of proteins modified with ISG15. This method involves the generation of stable ISG15-transfectant cells, followed by affinity purification, and then identification of the ISGylated proteins by mass spectrometry.

Type I interferons and dendritic cells in cancer immunotherapy

Type I interferons (IFNs) facilitate cancer immunosurveillance, antitumor immunity and antitumor efficacy of conventional cell death-inducing therapies (chemotherapy/radiotherapy) as well as immunotherapy. Moreover, it is clear that dendritic cells (DCs) play a significant role in aiding type I IFN-driven immunity. Owing to these antitumor properties several immunotherapies involving, or inducing, type I IFNs have received considerable clinical attention, e.g., recombinant IFNα2 or agonists targeting pattern recognition receptor (PRR) pathways like Toll-like receptors (TLRs), cGAS-STING or RIG-I/MDA5/MAVS. A series of preclinical and clinical evidence concurs that the success of anticancer therapy hinges on responsiveness of both cancer cells and DCs to type I IFNs. In this article, we discuss this link between type I IFNs and DCs in the context of cancer biology, with particular attention to mechanisms behind type I IFN production, their impact on DC driven anticancer immunity, and the implications of this for cancer immunotherapy, including DC-based vaccines.

USP18 and ISG15 coordinately impact on SKP2 and cell cycle progression

USP18 is an isopeptidase that cleaves the ubiquitin-like ISG15 from conjugates and is also an essential negative feedback regulator of type I interferon signaling. We and others reported that USP18 protein is stabilized by ISG15 and targeted for degradation by SKP2 (S-phase kinase associated protein 2), the substrate-recognition subunit of the SCF^SKP2 ubiquitin E3 ligase complex, which operates in cell cycle progression. Here, we have analyzed how, under non stimulated conditions, USP18, ISG15 and SKP2 communicate with each other, by enforcing or silencing their expression. We found that USP18 and SKP2 interact and that free ISG15 abrogates the complex, liberating USP18 from degradation and concomitantly driving SKP2 to degradation and/or ISGylation. These data reveal a dynamic interplay where the substrate USP18 stabilizes SKP2, both exogenous and endogenous. Consistent with this we show that silencing of baseline USP18 slows down progression of HeLa S3 cells towards S phase. Our findings point to USP18 and ISG15 as unexpected new SKP2 regulators, which aid in cell cycle progression at homeostasis.

Interferon-stimulated gene 15 modulates cell migration by interacting with Rac1 and contributes to lymph node metastasis of oral squamous cell carcinoma cells

In an effort to understand the underlying mechanisms of lymph node metastasis in oral squamous cell carcinoma (OSCC), through in vivo selection, LN1-1 cells were previously established from OEC-M1 cells and showed enhanced lymphangiogenesis and lymphatic metastasis capabilities. In the current study, we use a stable isotope labeling with amino acids in cell culture (SILAC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based proteomic platform to compare LN1-1 to OEC-M1 cells. Interferon-stimulated gene 15 (ISG15) was found highly expressed in LN1-1 cells. Immunohistochemical analysis and meta-analysis of publicly available microarray datasets revealed that the ISG15 level was increased in human OSCC tissues and associated with poor disease outcome. Knockdown of ISG15 had minimal effects on tumor growth but did decrease tumor lymphangiogenesis and lymphatic metastasis of LN1-1 cells. Consistent with the in vivo assay, ISG15 knockdown did not impair cell growth but diminished cell migration, invasion, and transendothelial migration in vitro. ISG15-induced cell migration was independent of ISGylation and associated with membrane protrusion. Ectopic expression of ISG15 increased Rac1 activity and knockdown of Rac1 impaired ISG15-enhanced migration. Furthermore, Rac1 colocalized with ISG15 to a region of membrane protrusion and ISG15 coimmunoprecipitated with Rac1, especially with the Rac1-GDP form. Importantly, as shown by proximity ligation assays, ISG15 and Rac1 physically interacted with each other. Our results indicated that ISG15 affects cell migration by interacting with Rac1 and regulating Rac1 activity and contributes to lymphatic metastasis in OSCC.

Cells with stemness features are generated from in vitro transformed human fibroblasts

Cancer stem cells (CSCs) have been involved in the maintenance, progression and relapse of several tumors, but their origin is still elusive. Here, in vitro transformed human fibroblasts (cen3tel cells) and the tumorsphere assay were used to search for and possibly characterize CSCs in transformed somatic cells. Cen3tel cells formed spheres showing self-renewal capacity and Sox2 overexpression, suggesting that they contained a subset of cells with CSC-like features. Sphere cells displayed deregulation of a c-MYC/miR-34a circuitry, likely associated with cell protection from apoptosis. Gene expression profiles of sphere cells revealed an extensive transcriptional reprogramming. Genes up-regulated in tumorspheres identified processes related to tumorigenesis and stemness, as cholesterol biosynthesis, apoptosis suppression, interferon and cytokine mediated signalling pathways. Sphere cells engrafted into NSG mice more rapidly than adherent cells, but both cell populations were tumorigenic. These results indicate that, during transformation, human somatic cells can acquire CSC properties, confirming the high plasticity of tumor cells. However, CSC-like cells are not the only tumorigenic population in transformed cells, indicating that the CSC phenotype and tumorigenicity can be uncoupled.

Promyelocytic Leukemia Protein (PML) Requirement for Interferon-induced Global Cellular SUMOylation

We report that interferon (IFN) α treatment at short and long periods increases the global cellular SUMOylation and requires the presence of the SUMO E3 ligase promyelocytic leukemia protein (PML), the organizer of PML nuclear bodies (NBs). Several PML isoforms (PMLI-PMLVII) derived from a single PML gene by alternative splicing, share the same N-terminal region but differ in their C-terminal sequences. Introducing each of the human PML isoform in PML-negative cells revealed that enhanced SUMOylation in response to IFN is orchestrated by PMLIII and PMLIV. Large-scale proteomics experiments enabled the identification of 558 SUMO sites on 389 proteins, of which 172 sites showed differential regulation upon IFNα stimulation, including K49 from UBC9, the sole SUMO E2 protein. Furthermore, IFNα induces PML-dependent UBC9 transfer to the nuclear matrix where it colocalizes with PML within the NBs and enhances cellular SUMOylation levels. Our results demonstrate that SUMOylated UBC9 and PML are key players for IFN-increased cellular SUMOylation.

LIPG signaling promotes tumor initiation and metastasis of human basal-like triple-negative breast cancer

Current understanding of aggressive human basal-like triple-negative breast cancer (TNBC) remains incomplete. In this study, we show endothelial lipase (LIPG) is aberrantly overexpressed in basal-like TNBCs. We demonstrate that LIPG is required for in vivo tumorigenicity and metastasis of TNBC cells. LIPG possesses a lipase-dependent function that supports cancer cell proliferation and a lipase-independent function that promotes invasiveness, stemness and basal/epithelial-mesenchymal transition features of TNBC. Mechanistically, LIPG executes its oncogenic function through its involvement in interferon-related DTX3L-ISG15 signaling, which regulates protein function and stability by ISGylation. We show that DTX3L, an E3-ubiquitin ligase, is required for maintaining LIPG protein levels in TNBC cells by inhibiting proteasome-mediated LIPG degradation. Inactivation of LIPG impairs DTX3L-ISG15 signaling, indicating the existence of DTX3L-LIPG-ISG15 signaling. We further reveal LIPG-ISG15 signaling is lipase-independent. We demonstrate that DTX3L-LIPG-ISG15 signaling is essential for malignancies of TNBC cells. Targeting this pathway provides a novel strategy for basal-like TNBC therapy.

UBE2L6/UBCH8 and ISG15 attenuate autophagy in esophageal cancer cells

Esophageal cancer remains a poor prognosis cancer due to advanced stage of presentation and drug resistant disease. To understand the molecular mechanisms influencing response to chemotherapy, we examined genes that are differentially expressed between drug sensitive, apoptosis competent esophageal cancer cells (OE21, OE33, FLO-1) and those which are more resistant and do not exhibit apoptosis (KYSE450 and OE19). Members of the ISG15 (ubiquitin-like) protein modification pathway, including UBE2L6 and ISG15, were found to be more highly expressed in the drug sensitive cell lines. In this study, we evaluated the contribution of these proteins to the response of drug sensitive cells. Depletion of UBE2L6 or ISG15 with siRNA did not influence caspase-3 activation or nuclear fragmentation following treatment with 5-fluorouracil (5-FU). We assessed autophagy by analysis of LC3II expression and Cyto-ID staining. Depletion of either ISG15 or UBE2L6 resulted in enhanced endogenous autophagic flux. An increase in autophagic flux was also observed following treatment with cytotoxic drugs (5-FU, rapamycin). In ISG15 depleted cells, this increase in autophagy was associated with improved recovery of drug treated cells. In contrast, UBE2L6 depleted cells, did not show enhanced recovery. UBE2L6 may therefore influence additional targets that limit the pro-survival effect of ISG15 depletion. These data identify UBE2L6 and ISG15 as novel inhibitors of autophagy, with the potential to influence chemosensitivity in esophageal cancer cells.

Dual regulation of Stat1 and Stat3 by the tumor suppressor protein PML contributes to interferon α-mediated inhibition of angiogenesis

IFNs are effective in inhibiting angiogenesis in preclinical models and in treating several angioproliferative disorders. However, the detailed mechanisms of IFNα-mediated anti-angiogenesis are not completely understood. Stat1/2/3 and PML are IFNα downstream effectors and are pivotal regulators of angiogenesis. Here, we investigated PML's role in the regulation of Stat1/2/3 activity. In Pml knock-out (KO) mice, ablation of Pml largely reduces IFNα angiostatic ability in Matrigel plug assays. This suggested an essential role for PML in IFNα's anti-angiogenic function. We also demonstrated that PML shared a large cohort of regulatory genes with Stat1 and Stat3, indicating an important role of PML in regulating Stat1 and Stat3 activity. Using molecular tools and primary endothelial cells, we demonstrated that PML positively regulates Stat1 and Stat2 isgylation, a ubiquitination-like protein modification. Accordingly, manipulation of the isgylation system by knocking down USP18 altered IFNα-PML axis-mediated inhibition of endothelial cell migration and network formation. Furthermore, PML promotes turnover of nuclear Stat3, and knockdown of PML mitigates the effect of LLL12, a selective Stat3 inhibitor, on IFNα-mediated anti-angiogenic activity. Taken together, we elucidated an unappreciated mechanism in which PML, an IFNα-inducible effector, possess potent angiostatic activity, doing so in part by forming a positive feedforward loop with Stat1/2 and a negative feedback loop with Stat3. The interplay between PML, Stat1/Stat2, and Stat3 contributes to IFNα-mediated inhibition of angiogenesis, and disruption of this network results in aberrant IFNα signaling and altered angiostatic activity.

Structural basis of the specificity of USP18 toward ISG15

Protein modification by ubiquitin and ubiquitin-like modifiers (Ubls) is counteracted by ubiquitin proteases and Ubl proteases, collectively termed DUBs. In contrast to other proteases of the ubiquitin-specific protease (USP) family, USP18 shows no reactivity toward ubiquitin but specifically deconjugates the interferon-induced Ubl ISG15. To identify the molecular determinants of this specificity, we solved the crystal structures of mouse USP18 alone and in complex with mouse ISG15. USP18 was crystallized in an open and a closed conformation, thus revealing high flexibility of the enzyme. Structural data, biochemical and mutational analysis showed that only the C-terminal ubiquitin-like domain of ISG15 is recognized and essential for USP18 activity. A critical hydrophobic patch in USP18 interacts with a hydrophobic region unique to ISG15, thus providing evidence that USP18's ISG15 specificity is mediated by a small interaction interface. Our results may provide a structural basis for the development of new drugs modulating ISG15 linkage.

Free ISG15 triggers an antitumor immune response against breast cancer: a new perspective

Interferon-Stimulated Gene 15 (ISG15), an antagonist of the canonical ubiquitin pathway, is frequently overexpressed in various cancers. In cancer cells, ISG15 is detected as free (intracellular) and conjugated to cellular proteins (ISGylation). Free ISG15 is also secreted into the extracellular milieu. ISGylation has protumor functions and extracellular free ISG15 has immunomodulatory properties in vitro. Therefore, whether ISG15 is a tumor suppressor or tumor promoter in vivo remains controversial. The current study aimed to clarify the role of free ISG15 in tumorigenesis. Breast cancer cells stably expressing control, ISG15, and UbcH8 (ISG15-specific E2 ligase) shRNAs were used to assess the immunoregulatory and antitumor function of free ISG15 in cell culture (in vitro) and in nude mice (in vivo). We show that extracellular free ISG15 suppresses breast tumor growth and increases NK cell infiltration into xenografted breast tumors in nude mice, and intracellular free ISG15 enhances major histocompatibility complex (MHC) class I surface expression in breast cancer cells. We conclude that free ISG15 may have antitumor and immunoregulatory function in vivo. These findings provides the basis for developing strategies to increase systemic levels of free ISG15 to treat cancer patients overexpressing the ISG15 pathway.

STAT3-activated GM-CSFRα translocates to the nucleus and protects CLL cells from apoptosis

Here, it was determined that chronic lymphocytic leukemia (CLL) cells express the α subunit, but not the β subunit, of the granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR/CSF2R). GM-CSFRα was detected on the surface, in the cytosol, and in the nucleus of CLL cells via confocal microscopy, cell fractionation, and GM-CSFRα antibody epitope mapping. Because STAT3 is frequently activated in CLL and the GM-CSFRα promoter harbors putative STAT3 consensus binding sites, MM1 cells were transfected with truncated forms of the GM-CSFRα promoter, then stimulated with IL6 to activate STAT3 and to identify STAT3-binding sites. Chromatin immunoprecipitation (ChIP) and an electoromobility shift assay (EMSA) confirmed STAT3 occupancy to those promoter regions in both IL6-stimulated MM1 and CLL cells. Transfection of MM1 cells with STAT3-siRNA or CLL cells with STAT3-shRNA significantly downregulated GM-CSFRα mRNA and protein levels. RNA transcripts, involved in regulating cell survival pathways, and the proteins KAP1 (TRIM28) and ISG15 coimmunoprecipitated with GM-CSFRα. GM-CSFRα-bound KAP1 enhanced the transcriptional activity of STAT3, whereas GM-CSFRα-bound ISG15 inhibited the NF-κB pathway. Nevertheless, overexpression of GM-CSFRα protected MM1 cells from dexamethasone-induced apoptosis, and GM-CSFRα knockdown induced apoptosis in CLL cells, suggesting that GM-CSFRα provides a ligand-independent survival advantage.

IMPLICATIONS

Constitutively, activation of STAT3 induces the expression of GM-CSFRα that protects CLL cells from apoptosis, suggesting that inhibition of STAT3 or GM-CSFRα may benefit patients with CLL.

Interferon-stimulated genes: a complex web of host defenses

Interferon-stimulated gene (ISG) products take on a number of diverse roles. Collectively, they are highly effective at resisting and controlling pathogens. In this review, we begin by introducing interferon (IFN) and the JAK-STAT signaling pathway to highlight features that impact ISG production. Next, we describe ways in which ISGs both enhance innate pathogen-sensing capabilities and negatively regulate signaling through the JAK-STAT pathway. Several ISGs that directly inhibit virus infection are described with an emphasis on those that impact early and late stages of the virus life cycle. Finally, we describe ongoing efforts to identify and characterize antiviral ISGs, and we provide a forward-looking perspective on the ISG landscape.

The genetics of innate immunity sensors and human disease

Since their discovery, innate immunity microbial sensors have been increasingly studied and shown to play a critical role in innate responses to microbes in several experimental in vitro, ex vivo, and animal models. However, their role in the human response to infection in natural conditions has just started to be deciphered, by means of clinical studies of primary immunodeficiencies and epidemiological genetic studies. Here, we summarize the major findings concerning the genetic diversity of the various families of microbial sensors in humans, and of other molecules involved in the signaling pathways they trigger. Specifically, we review the genetic associations, revealed by both clinical and epidemiological genetics studies, of microbial sensors from five different families: Toll-like receptors, C-type lectin receptors, NOD-like receptors, RIG-I-like receptors, and cytosolic DNA sensors. In particular, we consider the relationships between variation at the genes encoding these molecules and susceptibility to and the severity of infectious diseases and other clinical conditions associated with immune dysfunction, including autoimmunity, inflammation, allergy, and cancer. Despite the fact that the genetic links between innate immunity sensors and human disorders remain still limited, human genetics studies are increasingly improving our understanding of the genuine functions of microbial sensors and downstream signaling molecules in the natural setting.

A negative feedback of the HIF-1α pathway via interferon-stimulated gene 15 and ISGylation

PURPOSE

The IFN-stimulated gene 15 (ISG15)- and ubiquitin-conjugation pathways play roles in mediating hypoxic and inflammatory responses. To identify interaction(s) between these two tumor microenvironments, we investigated the effect of ISG15 on the activity of the master hypoxic transcription factor HIF-1α.

EXPERIMENTAL DESIGN

IFN and desferoxamine treatments were used to induce the expression of ISGs and HIF-1α, respectively. Interactions between HIF-1α and the ISG15 and ISGylation system were studied using knockdown of mRNA expression, immunoblotting, coimmunoprecipitation, and pull-down analyses. Effects of the ISG15 and ISGylation system on the HIF-1α-directed processes were examined using reporter, reverse transcription polymerase chain reaction (RT-PCR), and tumorigenic growth assays.

RESULTS

We found that the level of the free form of HIF-1α is differentially regulated by IFN treatment, and that the free ISG15 level is lower under hypoxia. Mechanism-directed studies have shown that HIF-1α not only interacts physically with ISG15, but is also ISGylated in multiple domains. ISG15 expression disrupts the functional dimerization of HIF-1α and -1β. Subsequently, expression of the ISG15 and/or ISGylation system attenuates HIF-1α-mediated gene expression and tumorigenic growth.

CONCLUSION

In summary, our results revealed cross-talk between inflammatory and hypoxic pathways through the ISGylation of HIF-1α. On the basis of these results, we propose a novel negative feedback loop for the HIF-1α-mediated pathway involving the regulation of HIF-1α via IFN-induced ISGylation.

The DeISGylase USP18 limits TRAIL-induced apoptosis through the regulation of TRAIL levels: Cellular levels of TRAIL influences responsiveness to TRAIL-induced apoptosis

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a promising molecule for anti-cancer therapies. Unfortunately, cancer cells frequently acquire resistance to rhTRAIL. Various co-treatments have been proposed to overcome apoptosis resistance to TRAIL. Here we show that downregulation of the deISGylase USP18 sensitizes cancer cells to rhTRAIL, whereas, elevate levels of USP18 inhibit TRAIL-induced apoptosis, in a deISGylase-independent manner. USP18 influences TRAIL signaling through the control of the IFN autocrine loop. In fact, cells with downregulated USP18 expression augment the expression of cellular TRAIL. Downregulation of cellular TRAIL abrogates the synergism between TRAIL and USP18 siRNA and also limits cell death induced by rhTRAIL. By comparing the apoptotic responsiveness to TRAIL in a panel of cancer cell lines, we have discovered a correlation between TRAIL levels and the apoptotic susceptibility to rhTRAIL, In cells expressing high levels of TRAIL-R2 susceptibility to rhTRAIL correlates with TRAIL expression. In conclusion, we propose that cellular TRAIL is an additional factor that can influence the apoptotic response to rhTRAIL.