FURIN regulates cytotoxic T-lymphocyte effector function and memory cell transition in mice.

The proprotein convertase subtilisin/kexins (PCSKs) regulate biological actions by cleaving immature substrate proteins. The archetype PCSK, FURIN, promotes the pathogenicity of viruses by proteolytically processing viral proteins. FURIN has also important regulatory functions in both innate and adaptive immune responses but its role in the CD8+ CTLs remains enigmatic. We used a T-cell-specific FURIN deletion in vivo to demonstrate that FURIN promotes host response against the CTL-dependent lymphocytic choriomeningitis virus by virtue of restricting viral burden and augmenting interferon gamma (IFNG) production. We also characterized Furin KO CD8+ T cells ex vivo, including after their activation with FURIN regulating cytokines IL12 or TGFB1. Furin KO CD8+ T cells show an inherently activated phenotype characterized by the upregulation of effector genes and increased frequencies of CD44+ , TNF+ , and IFNG+ cells. In the activated CTLs, FURIN regulates the productions of IL2, TNF, and GZMB and the genes associated with the TGFBR-signaling pathway. FURIN also controls the expression of Eomes, Foxo1, and Bcl6 and the levels of ITGAE and CD62L, which implies a role in the development of CTL memory. Collectively, our data suggest that the T-cell expressed FURIN is important for host responses in viral infections, CTL homeostasis/activation, and memory development.

IL-7Rα Expression Regulates Murine Dendritic Cell Sensitivity to Thymic Stromal Lymphopoietin.

Thymic stromal lymphopoietin (TSLP) and IL-7 are related cytokines that mediate growth and differentiation events in the immune system. They signal through IL-7Rα-containing receptors. Target cells of TSLP in Th2 responses include CD4 T cells and dendritic cells (DCs). Although it has been reported that expression of TSLP receptor (TSLPR) on CD4 T cells is required for OVA-induced lung inflammation, DCs have also been shown to be target cells of TSLP. In this study, we show that murine ex vivo splenic DCs are unresponsive to TSLP, as they fail to phosphorylate STAT5, but in vitro overnight culture, especially in presence of IL-4, renders DCs responsive to both TSLP and IL-7. This induced responsiveness is accompanied by dramatic upregulation of IL-7Rα on DCs with little change in expression of TSLPR or of γc In splenic DCs, the induction of IL-7Rα occurs mainly in CD8- DCs. In vivo, we found that IL-4 has a differential regulatory role on expression of IL-7Rα depending on the cell type; IL-4 decreases IL-7Rα expression on CD4 T cells whereas it upregulates the expression on DCs. Our results indicate that the induction of IL-7Rα expression on DCs is critical for TSLP responsiveness and that IL-4 can upregulate IL-7Rα on DCs.

Furin deficiency in myeloid cells leads to attenuated revascularization in a mouse-model of oxygen-induced retinopathy.

Ischemic retinopathy is a vision-threatening disease associated with chronic retinal inflammation and hypoxia leading to abnormal angiogenesis. Furin, a member of the proprotein convertase family of proteins, has been implicated in the regulation of angiogenesis due to its essential role in the activation of several angiogenic growth factors, including vascular endothelial growth factor-C (VEGF-C), VEGF-D and transforming growth factor - ß (TGF- ß). In the present study, we evaluated expression of furin in the retina and its role in retinal angiogenesis. As both inflammation and hypoxia contribute to angiogenesis, the role of furin was evaluated using myeloid-cell specific furin knockout (KO) mice (designated LysMCre-fur(fl/fl)) both in developmental retinal angiogenesis as well as in hypoxia-driven angiogenesis using the oxygen-induced retinopathy (OIR) model. In the retina, furin expression was detected in endothelial cells, macrophages and, to some extent, in neurons. The rate of angiogenesis was not different in LysMCre-fur(fl/fl) mice when compared to their wild-type littermates during development. In the OIR model, the revascularization of retina was significantly delayed in LysMCre-fur(fl/fl) mice compared to their wild-type littermates, while there was no compensatory increase in the preretinal neovascularization in LysMCre-fur(fl/fl) mice. These results demonstrate that furin expression in myeloid cells plays a significant role in hypoxia-induced angiogenesis in retina.

Proprotein convertase Furin1 expression in the Drosophila fat body is essential for a normal antimicrobial peptide response and bacterial host defense.

Invading pathogens provoke robust innate immune responses in Dipteran insects, such as Drosophila melanogaster In a systemic bacterial infection, a humoral response is induced in the fat body. Gram-positive bacteria trigger the Toll signaling pathway, whereas gram-negative bacterial infections are signaled via the immune deficiency (IMD) pathway. We show here that the RNA interference-mediated silencing of Furin1-a member of the proprotein convertase enzyme family-specifically in the fat body, results in a reduction in the expression of antimicrobial peptides. This, in turn, compromises the survival of adult fruit flies in systemic infections that are caused by both gram-positive and -negative bacteria. Furin1 plays a nonredundant role in the regulation of immune responses, as silencing of Furin2, the other member of the enzyme family, had no effect on survival or the expression of antimicrobial peptides upon a systemic infection. Furin1 does not directly affect the Toll or IMD signaling pathways, but the reduced expression of Furin1 up-regulates stress response factors in the fat body. We also demonstrate that Furin1 is a negative regulator of the Janus kinase/signal transducer and activator of transcription signaling pathway, which is implicated in stress responses in the fly. In summary, our data identify Furin1 as a novel regulator of humoral immunity and cellular stress responses in Drosophila-Aittomäki, S., Valanne, S., Lehtinen, T., Matikainen, S., Nyman, T. A., Rämet, M., Pesu, M. Proprotein convertase Furin1 expression in the Drosophila fat body is essential for a normal antimicrobial peptide response and bacterial host defense.

Proprotein convertase FURIN constrains Th2 differentiation and is critical for host resistance against Toxoplasma gondii.

The proprotein convertase subtilisin/kexin enzymes proteolytically convert immature proproteins into bioactive molecules, and thereby they serve as key regulators of cellular homeostasis. The archetype proprotein convertase subtilisin/kexin, FURIN, is a direct target gene of the IL-12/STAT4 pathway and it is upregulated in Th1 cells. We have previously demonstrated that FURIN expression in T cells critically regulates the maintenance of peripheral immune tolerance and the functional maturation of pro-TGF-ß1 in vivo, but FURIN's role in cell-mediated immunity and Th polarization has remained elusive. In this article, we show that T cell-expressed FURIN is essential for host resistance against a prototypic Th1 pathogen, Toxoplasma gondii, and for the generation of pathogen-specific Th1 lymphocytes, including Th1-IL-10 cells. FURIN-deficient Th cells instead show elevated expression of IL-4R subunit α on cell surface, sensitized IL-4/STAT6 signaling, and a propensity to polarize toward the Th2 phenotype. By exploring FURIN-interacting proteins in Jurkat T cells with Strep-Tag purification and mass spectrometry, we further identify an association with a cytoskeleton modifying Ras-related C3 botulinum toxin substrate/dedicator of cytokinesis 2 protein complex and unravel that FURIN promotes F-actin polymerization, which has previously been shown to downregulate IL-4R subunit α cell surface expression and promote Th1 responses. In conclusion, our results demonstrate that in addition to peripheral immune tolerance, T cell-expressed FURIN is also a central regulator of cell-mediated immunity and Th1/2 cell balance.

Patients with acute pancreatitis complicated by organ failure show highly aberrant monocyte signaling profiles assessed by phospho-specific flow cytometry.

OBJECTIVES: To outline signaling profiles and transmigration capacity of monocytes of patients with severe acute pancreatitis. DESIGN: Prospective study. SETTING: University hospital intensive care unit. PATIENTS: Thirteen patients with severe acute pancreatitis. All patients had organ dysfunction (acute respiratory distress syndrome in 12, renal dysfunction in eight). Healthy volunteers served as reference subjects. INTERVENTIONS: Blood samples were collected after admission to the intensive care unit. MEASUREMENTS AND MAIN RESULTS: Phosphorylation of nuclear factor-kappaB and p38, signal transducers and activators of transcription (STATs) 1, 3, 5, and extracellular signal-regulated kinases 1/2 in appropriately stimulated and nonstimulated samples were studied using phospho-specific whole-blood flow cytometry. Monocyte chemotactic protein-1-induced transmigration of monocytes among mononuclear cells obtained by density gradient centrifugation was studied using Transwell cell culture inserts covered with confluent layer of endothelial EA-HY cells. Phosphorylation levels of nuclear factor-kappaB induced by tumor necrosis factor, bacterial lipopolysaccharide, muramyl dipeptide, Escherichia coli, Staphylococcus aureus, and Staphylococcus epidermidis were significantly lower in patients' monocytes than monocytes of healthy reference subjects, whereas mitogen-activated protein kinase p38 phosphorylation levels were normal. Phosphorylation levels induced by interleukin-6 in STAT1 and STAT3 and by combination of phorbol 12-myristate 13-acetate and calcium ionophore A23187 in extracellular signal-regulated kinases 1/2, members of a mitogen-activated protein kinase family, were depressed in patients' monocytes, whereas phosphorylation levels induced by granulocyte-macrophage colony-stimulating factor in STAT5 was normal. In nonstimulated samples, phosphorylation levels were normal. The transmigration percentage of patients' monocytes was significantly lower than that of reference monocytes. CONCLUSIONS: In severe acute pancreatitis, monocytes show impaired nuclear factor kappaB and STAT1 activation, which may increase susceptibility to secondary infections. p38 activation is normal and STAT3 activation is depressed, which may contribute to maintenance of systemic inflammation. Extracellular signal-regulated kinases 1/2 activation is impaired, which may depress monocytes' transmigration and may consequently increase risk of infection. Monitoring of monocyte signaling profiles may aid in finding new therapeutic approaches and predictors of outcome of severe acute pancreatitis.

T-cell-expressed proprotein convertase FURIN inhibits DMBA/TPA-induced skin cancer development.

Proprotein convertases (PCSK) have a critical role in the body homeostasis as enzymes responsible for processing precursor proteins into their mature forms. FURIN, the first characterized member of the mammalian PCSK family, is overexpressed in multiple malignancies and the inhibition of its activity has been considered potential cancer treatment. FURIN has also an important function in the adaptive immunity, since its deficiency in T cells causes an impaired peripheral immune tolerance and accelerates immune responses. We addressed whether deleting FURIN from the immune cells would strengthen anticancer responses by subjecting mouse strains lacking FURIN from either T cells or macrophages and granulocytes to the DMBA/TPA two-stage skin carcinogenesis protocol. Unexpectedly, deficiency of FURIN in T cells resulted in enhanced and accelerated development of tumors, whereas FURIN deletion in macrophages and granulocytes had no effect. The epidermises of T-cell-specific FURIN deficient mice were significantly thicker with more proliferating Ki67+ cells. In contrast, there were no differences in the numbers of the T cells. The flow cytometric analyses of T-cell populations in skin draining lymph nodes showed that FURIN T-cell KO mice have an inherent upregulation of early activation marker CD69 as well as more CD4+CD25+Foxp3+ positive T regulatory cells. In the early phase of tumor promotion, T cells from the T-cell-specific FURIN knockout animals produced more interferon gamma, whereas at later stage the production of Th2- and Th17-type cytokines was more prominent than in wild-type controls. In conclusion, while PCSK inhibitors are promising therapeutics in cancer treatment, our results show that inhibiting FURIN specifically in T cells may promote squamous skin cancer development.

Myeloid cell expressed proprotein convertase FURIN attenuates inflammation.

The proprotein convertase enzyme FURIN processes immature pro-proteins into functional end- products. FURIN is upregulated in activated immune cells and it regulates T-cell dependent peripheral tolerance and the Th1/Th2 balance. FURIN also promotes the infectivity of pathogens by activating bacterial toxins and by processing viral proteins. Here, we evaluated the role of FURIN in LysM+ myeloid cells in vivo. Mice with a conditional deletion of FURIN in their myeloid cells (LysMCre-fur(fl/fl)) were healthy and showed unchanged proportions of neutrophils and macrophages. Instead, LysMCre-fur(fl/fl) mice had elevated serum IL-1ß levels and reduced numbers of splenocytes. An LPS injection resulted in accelerated mortality, elevated serum pro-inflammatory cytokines and upregulated numbers of pro-inflammatory macrophages. A genome-wide gene expression analysis revealed the overexpression of several pro-inflammatory genes in resting FURIN-deficient macrophages. Moreover, FURIN inhibited Nos2 and promoted the expression of Arg1, which implies that FURIN regulates the M1/M2-type macrophage balance. FURIN was required for the normal production of the bioactive TGF-ß1 cytokine, but it inhibited the maturation of the inflammation-provoking TACE and Caspase-1 enzymes. In conclusion, FURIN has an anti-inflammatory function in LysM+ myeloid cells in vivo.

Proprotein convertase FURIN regulates T cell receptor-induced transactivation.

Antigen emergence rapidly stimulates T cells, which leads to changes in cytokine production, cell proliferation, and differentiation. Some of the key molecules involved in these events, such as TGF-ß1 and NOTCH1, are synthesized initially as inactive precursors and are proteolytically activated during T cell activation. PCSKs regulate proprotein maturation by catalyzing the proteolytic cleavage of their substrates. The prototype PCSK FURIN is induced upon TCR activation, and its expression in T cells is critical for the maintenance of peripheral immune tolerance. In this study, we tested the hypothesis that FURIN regulates T cell activation. Our data demonstrate that IL-2 is increased initially in FURIN-deficient mouse CD4(+) T cells, but the TCR-induced IL-2 mRNA expression is not sustained in the absence of FURIN. Accordingly, the inhibition of FURIN in human Jurkat T cell lines also results in a decrease in IL-2 production, whereas the overexpression of WT FURIN is associated with elevated IL-2 levels. In Jurkat cells, FURIN is dispensable for immediate TCR signaling steps, such as ERK, ZAP70, or LAT phosphorylation. However, with the use of gene reporter assays, we demonstrate that FURIN regulates the AP-1, NFAT, and NF-κB transcription factors. Finally, by performing a transcription factor-binding site enrichment analysis on FURIN-dependent transcriptomes, we identify the FURIN-regulated transcription factors in mouse CD4(+) T cell subsets. Collectively, our work confirms the hypothesis that the TCR-regulated protease FURIN plays an important role in T cell activation and that it can specifically modulate TCR-activated transactivation.

Therapeutic targeting of the Jak/STAT pathway.

Antibodies that block cytokine function provide a powerful therapeutic tool especially for the treatment of autoimmune diseases. Cytokines are a group of small hydrophilic glycoproteins that bind their receptors on the cell surface and subsequently activate intracellular signalling cascades, such as the JAK/STAT pathway. A bulk of evidence has demonstrated that genetic mutations in signalling molecules can cause immunodeficiencies and malignant cell growth. As a result, several drug companies have begun to develop therapeutics that inhibit the function of JAK tyrosine kinases. Currently, two JAK inhibitors, tofacitinib and ruxolitinib, are used in the clinic for treating rheumatoid arthritis and myeloproliferative diseases, respectively. Inhibiting JAK function has been shown to efficiently prevent the uncontrolled growth of cancerous cells and to harness overly active immune cells. In the future, other small molecule compounds are likely to come into clinical use, and intense work is ongoing to develop inhibitors that specifically target the constitutively active mutant JAKs. This MiniReview will summarize the basic features of the JAK/STAT pathway, its role in human disease and the therapeutic potential of JAK/STAT inhibitors.

Monocyte anergy is present in patients with severe acute pancreatitis and is significantly alleviated by granulocyte-macrophage colony-stimulating factor and interferon-gamma in vitro.

OBJECTIVES: Severe acute pancreatitis (AP) is frequently associated with immune suppression, which increases the risk of infections, organ failure, and death. Our aims were to measure monocyte function (ie, HLA-DR expression and tumor necrosis factor-alpha [TNF-alpha] production as markers of immune suppression) in patients with severe AP and to determine whether treatment of blood samples with granulocyte-macrophage colony-stimulating factor (GM-CSF) and/or interferon-gamma (IFN-gamma) corrected the functional defects of monocytes in vitro. METHODS: The study consisted of 28 patients with severe AP who were treated at intensive care unit and in whom the proportion of HLA-DR-positive monocytes in the circulation was less than 70%, and 28 matched control subjects who were selected from healthy laboratory personnel. HLA-DR density was determined by whole blood flow cytometry. Monocyte TNF-alpha production in response to bacterial lipopolysaccharides (LPSs) was studied in a whole blood assay. Aliquots of blood were supplemented with IFN-gamma (all 28 patients), GM-CSF (the last 24 patients), or both (the last 12 patients). RESULTS: The median proportion of HLA-DR-positive monocytes was 45% in patients (range, 18%-73%) and was 98% in controls (range, 86%-100%; P < 0.001). TNF-alpha levels in response to LPSs were lower in patients (545 pg/mL; range, 84-1990 pg/mL) than in controls (1415 pg/mL; range, 660-5490 pg/mL; P < 0.001). The proportion of HLA-DR-positive cells correlated positively with TNF-alpha levels (r = 0.56; P < 0.01). Both GM-CSF and IFN-gamma increased HLA-DR expression of monocytes in patients (98%; range, 74%-100% for GM-CSF; 99%; range, 86%-100% for IFN-gamma; both P < 0.001). The combination restored monocyte HLA-DR expression (99%; range, 96%-100%; P = 0.002). Compared with basal levels, GM-CSF increased TNF-alpha production of monocytes both in blood samples from patients (median, 1320 pg/mL; range, 35-8015 pg/mL) and controls (median, 3450 pg/mL; range, 1040-9835 pg/mL; both P < 0.001). IFN-gamma increased TNF-alpha production by monocytes in patients (683 pg/mL; range, 186-2705 pg/mL; P < 0.05) but not in controls (1658 pg/mL; range, 765-4755 pg/mL; P = 0.31). With the combination of GM-CSF and IFN-gamma, the TNF-alpha levels of monocytes in patients (3185 pg/mL; range, 545-8280 pg/mL) and in controls (2800 pg/mL; range, 1080-6860 pg/mL) were comparable. CONCLUSIONS: The proportion of HLA-DR-positive monocytes correlates with TNF-alpha production, and they both reflect the degree of immune suppression. The low proportion of HLA-DR-positive monocytes in AP can be reversed in vitro by GM-CSF and/or IFN-gamma. The GM-CSF and IFN-gamma treatments also increase LPS-induced TNF-alpha production. By the combination of GM-CSF and IFN-gamma, but not by either agent alone, LPS-induced TNF-alpha production of monocytes was equally high in patients and in controls.

PU.1 is required for transcriptional activation of the Stat6 response element in the Igepsilon promoter.

Signal transducer and activator of transcription 6 (Stat6) has a crucial role in regulation of IL-4-induced gene responses. Stat6-binding sites are present in the promoters of both ubiquitously and cell-type-specifically expressed genes. The promoter regions of IL-4-inducible genes contain cis-acting elements for several transcription factors that act in concert with Stat6 and are also likely to modulate lineage-specific gene expression. We have observed that the Stat6 response element from the B-cell-specific Igepsilon promoter is readily activated upon IL-4 stimulation in B cells but not in non-hematopoietic cells. A minimal low-affinity PU.1-core-binding sequence (5'-AGAA-3') was identified within the Stat6 DNA-binding site in the Igepsilon promoter. Ectopic expression of the myeloid- and B-cell-specific transcription factor PU.1 restored the IL-4-inducibility of the Igepsilon-Stat6 response element in HepG2 cells, and the induction required an intact PU.1-binding sequence. Both the transactivation and the DNA-binding domains of PU.1 were required for induction of Stat6-mediated transcription. The co-operation between PU.1 and Stat6 in transactivation of the Igepsilon gene represents a molecular mechanism for the fine-tuning of cell-type-restricted expression of IL-4-induced gene responses.

Molecular basis of Stat1 and PU.1 cooperation in cytokine-induced Fcgamma receptor I promoter activation.

The high-affinity receptor for IgG (FcgammaRI) is a myeloid cell-specific and IFN-gamma-induced gene, and thereby serves as a paradigm for cytokine-induced cell type-specific gene responses. The expression of FcgammaRI is regulated by PU.1 and Stat1 transcription factors. We established an experimental model to analyze the individual functions of Stat1 and PU.1 in cytokine-induced transcription of the natural FcgammaRI promoter in U3A cells lacking both factors. PU.1 was required for both the basal activity and for the IFN-gamma-induced FcgammaRI promoter activation, while Stat1 alone could not initiate transcription. In contrast, in the context of a heterologous promoter, PU.1 inhibited the Stat1-mediated transcription. Systematic analysis of Stat1 and PU.1 mutants and FcgammaRI promoter elements revealed that activation of the promoter required the DNA binding, and the transactivation functions of both Stat1 and PU.1. PU.1 and Stat1 bound the promoter elements independently, and no physical interaction between the proteins was observed. The requirement of PU.1 for FcgammaRI promoter activity was supported by demonstration of in vitro interaction between PU.1 and components of the basal transcription machinery TBP and RNA polymerase II. Deletion of the acidic transactivation domain of PU.1 greatly diminished both the FcgammaRI promoter activity as well as the interaction with RNA polymerase II. In contrast, Stat1 did not interact with TBP or RNA polymerase II. These results define functional cooperativity between PU.1 and Stat1 in FcgammaRI promoter activation where PU.1 serves as an amplifier and bridging factor with the basal transcription machinery.

Distinct functions for signal transducer and activator of transcription 1 and PU.1 in transcriptional activation of Fc gamma receptor I promoter.

The myeloid cell-specific expression and interferon-gamma (IFN-gamma) induction of Fc gamma receptor I (FcgammaRI) requires cooperation between PU.1 and signal transducer and activator of transcription 1 (Stat1) by means of mechanisms that are unknown. We found that PU.1 and Stat1 mediated distinct functions in the activation of FcgammaRI promoter. The basal activity of the natural FcgammaRI promoter was strictly dependent on PU.1, and IFN-gamma induction required both PU.1 and Stat1. Recruitment of TATA-binding protein (TBP) to the FcgammaRI promoter did not replace PU.1 in promoter activation, suggesting that TBP is not sufficient for FcgammaRI activation and that PU.1 mediates additional contacts with basal transcription machinery. In contrast, Stat1 did not interact with basal transcription machinery, but the Stat1-mediated activation of FcgammaRI promoter critically required CREB-binding protein (CBP)/p300. These results define functional cooperativity between PU.1 and Stat1 in FcgammaRI promoter activation, in which PU.1 appears to serve as a bridging factor with the basal transcription machinery and IFN-gamma-mediated induction of transcription occurs through recruitment of CBP/p300 by Stat1.

p38 Mitogen-activated protein kinase regulates interleukin-4-induced gene expression by stimulating STAT6-mediated transcription.

STAT6 functions as a critical mediator of IL-4-stimulated gene activation, and the function of STAT6 is regulated by both tyrosine and serine kinase activities. Here we analyzed the role of serine phosphorylation in regulation of STAT6-mediated transcription. Optimal transcriptional response of IL-4-inducible promoters requires costimulatory signals through CD40-stimulated intracellular kinases such as p38 MAPK. We found that the p38 MAPK inhibitor SB202190 as well as the dominant negative p38 MAPK inhibited interleukin (IL)-4 regulated expression of CD23 in Ramos B cells. IL-4 stimulation did not stimulate p38 MAPK activity, but inhibition of p38 MAPK activity directly correlated with inhibition of IL-4-induced gene activation. Dissection of individual response elements on IL-4-regulated promoter showed that C/EBP beta-mediated transcription was insensitive to SB202190 treatment in B cells whereas STAT6-mediated transcription was regulated by p38 MAPK. The IL-4-induced immediate activation events of STAT6 were not affected by p38 MAPK activity. Furthermore, phosphoamino acid analysis and phosphopeptide mapping indicated that STAT6 is not a direct substrate for p38 MAPK. Instead, p38 MAPK was found to directly regulate the activity of the transactivation domain of STAT6. These results show that, in addition to the well established proinflammatory effects, p38 MAPK also provides a costimulatory signal for IL-4-induced gene responses by directly stimulating the transcriptional activation of STAT6.

PIAS proteins promote SUMO-1 conjugation to STAT1.

Signal transducer and activator of transcription 1 (STAT1) is a critical mediator of interferon-gamma (IFN-gamma)-induced transcription that is regulated through posttranslational modifications and through transacting proteins such as protein inhibitor of activated STAT1 (PIAS1). PIAS proteins have been shown to function as E3-type small ubiquitin-like modifier (SUMO) ligases, and sumoylation has been identified as a modulatory mechanism for several transcription factors. Here we show that STAT1 is subject to SUMO-1 modification, and sumoylation occurs in vivo and in vitro at a single, evolutionary conserved amino acid residue Lys703. Members of the PIAS family of proteins were found to strongly stimulate sumoylation of STAT1. Furthermore, activation of STAT1 by IFN-gamma or pervanadate induced SUMO-1 conjugation. Mutation of Lys703 in STAT1 resulted in increased IFN-gamma-mediated transactivation, suggesting a negative regulatory function for sumoylation. These results indicate that STAT1 is covalently modified by SUMO-1 in cytokine signaling and that PIAS proteins promote SUMO-1 conjugation to STAT1.

Identification of p100 as a coactivator for STAT6 that bridges STAT6 with RNA polymerase II.

STAT6 is a central mediator of IL-4-induced gene responses. STAT6-mediated transcription is depend ent on the C-terminal transcription activation domain (TAD), but the mechanisms by which STAT6 activates transcription are poorly understood. Here, we have identified the staphylococcal nuclease (SN)-like domain and tudor domain containing protein p100 as a STAT6 TAD interacting protein. p100 was originally characterized as a transcriptional coactivator for Epstein-Barr virus nuclear antigen 2. STAT6 interacted with p100 in vitro and in vivo. The interaction was mediated by the TAD domain of STAT6 and the SN-like domain of p100. p100 did not affect the immediate activation events of STAT6, but enhanced STAT6-mediated transcriptional activation and the IL-4-induced Igepsilon gene transcription in human B-cell line. Finally, p100 associated with the large subunit of RNA polymerase II and was mediating interaction between STAT6 and RNA polymerase II. These findings identify p100 as a novel coactivator for STAT6 and suggest that p100 functions as a bridging factor between STAT6 and the basal transcription machinery.