STAT6 and Ets-1 Form a Stable Complex That Modulates Socs-1 Expression by Interleukin-4 in Keratinocytes

ETS2 repressor factor (ERF) is involved in T lymphocyte maturation acting as regulator of thymocyte lineage commitment

Thymocyte differentiation and lineage commitment is regulated by an extensive network of transcription factors and signaling molecules among which Erk plays a central role. However, Erk effectors as well as the molecular mechanisms underlying this network are not well understood. Erf is a ubiquitously expressed transcriptional repressor regulated by Erk-dependent phosphorylation. Here, we investigated the role of Erf in T cell maturation and lineage commitment, using a double-fluorescent Erf-floxed mouse to produce thymus-specific Erf knockouts. We observed significant accumulation of thymocytes in the CD4/CD8 DP stage, followed by a significant reduction in CD4SP cells, a trend for lower CD8SP cell frequency, and an elevated percentage of γδ expressing thymocytes in Erf-deficient mice. Also, an elevated number of CD69⁺ TCRβ⁺ cells indicates that thymocytes undergoing positive selection accumulate at this stage. The expression of transcription factors Gata3, ThPOK, and Socs1 that promote CD4⁺ cell commitment was significantly decreased in Erf-deficient mice. These findings suggest that Erf is involved in T cell maturation, acting as a positive regulator during CD4 and eventually CD8 lineage commitment, while negatively regulates the production of γδ T cells. In addition, Erf-deficient mice displayed decreased percentages of CD4⁺ and CD8⁺ splenocytes and elevated levels of IL-4 indicating that Erf may have an additional role in the homeostasis, differentiation, and immunologic response of helper and cytotoxic T cells in the periphery. Overall, our results show, for the first time, Erf's involvement in T cell biology suggesting that Erf acts as a potential regulator during thymocyte maturation and thymocyte lineage commitment, in γδ T cell generation, as well as in Th cell differentiation.

Hierarchical control of interleukin 13 (IL-13) signals in lung fibroblasts by STAT6 and SOX11

Interleukin (IL)-13 is a signature cytokine of type 2 inflammation important for the pathogenesis of various diseases, including allergic diseases. Signal transducer and activator of transcription (STAT) 6 is a critical transcriptional factor for the IL-13 signals; however, it remains unknown how expression of the IL-13-induced genes is differentiated by the transcriptional machineries. In this study, we identified IL-13-induced transcriptional factors in lung fibroblasts using DNA microarrays in which SOX11 was included. Knockdown of SOX11 down-regulated expression of periostin and CCL26, both of which are known to be downstream molecules of IL-13, whereas enforced expression of SOX11 together with IL-13 stimulation enhanced expression of periostin. Moreover, we found that in DNA microarrays combining IL-13 induction and SOX11 knockdown there exist both SOX11-dependent and -independent molecules in IL-13-inducible molecules. In the former, many inflammation-related and fibrosis-related molecules, including periostin and CCL26, are involved. These results suggest that SOX11 acts as a trans-acting transcriptional factor downstream of STAT6 and that in lung fibroblasts the IL-13 signals are hierarchically controlled by STAT6 and SOX11.

Interferon-γ Protects from Staphylococcal Alpha Toxin-Induced Keratinocyte Death through Apolipoprotein L1

Staphylococcus aureus is a bacterial pathogen that frequently infects the skin, causing lesions and cell destruction through its primary virulence factor, alpha-toxin. Here we show that interferon gamma (IFN-γ) protects human keratinocytes from cell death induced by staphylococcal alpha toxin. We find that IFN-γ prevents alpha toxin binding and reduces expression of the alpha toxin receptor, ADAM10. We determine that the mechanism for IFN-γ mediated resistance to alpha toxin involves the induction of autophagy, a process of cellular adaptation to sublethal damage. We find that IFN-γ potently stimulates activation of the primary autophagy effector, LC3. This process is dependent upon up-regulation of Apolipoprotein L1 (ApoL1). Depletion of ApoL1 by siRNA significantly increases alpha toxin induced lethality and inhibits activation of LC3. We conclude that IFN-γ plays a significant role to protect human keratinocytes from the lethal effects of staphylococcal alpha toxin through ApoL1 induced autophagy.

Interleukin-4 up-regulation of epidermal interleukin-19 expression in keratinocytes involves the binding of signal transducer and activator of transcription 6 (Stat6) to the imperfect Stat6 sites

Interleukin-19 (IL-19) plays an important role in asthma by stimulating T helper type 2 (Th2) cytokine production. Interestingly, IL-4, a key Th2 cytokine, in turn up-regulates IL-19 expression in bronchial epithelial cells, so forming a positive feedback loop. In atopic dermatitis (AD), another Th2 disease closely related to asthma, IL-19 is up-regulated in the skin. We propose to use IL-4 transgenic (Tg) mice and human keratinocyte culture to delineate the molecular mechanisms involved in the up-regulation of IL-19 in AD. IL-19 is similarly up-regulated in the skin of IL-4 Tg mice as in human AD. Next we show that IL-4 up-regulates IL-19 expression in keratinocytes. Interestingly, the up-regulation was suppressed by a pan-Janus kinase (Jak) inhibitor, suggesting that the Jak-signal transducer and activator of transcription (Jak-STAT) pathway may be involved. Dominant negative studies further indicate that STAT6, but not other STATs, mediates the up-regulation. Serial 5' deletion of the IL-19 promoter and mutagenesis studies demonstrate that IL-4 up-regulation of IL-19 in keratinocytes involves two imperfect STAT6 response elements. Finally, chromatin immunoprecipitation assay studies indicate that IL-4 increases the binding of STAT6 to its response elements in the IL-19 promoter. Taken together, we delineate the detailed molecular pathway for IL-4 up-regulation of IL-19 in keratinocytes, which may play an important role in AD pathogenesis.

Identification and characterization of a novel promoter for the human ANO1 gene regulated by the transcription factor signal transducer and activator of transcription 6 (STAT6)

Anoctamin-1 (Ano1) is a widely expressed protein responsible for endogenous Ca(2+)-activated Cl(-) currents. Ano1 is overexpressed in cancer. Differential expression of transcriptional variants is also found in other diseases. However, the mechanisms underlying regulation of Ano1 are unknown. This study identifies the Ano1 promoter and defines a mechanism for regulating its expression. Next-generation RNA sequencing (RNA-seq) analysis in human gastric muscle found a new exon upstream of the reported exon 1 and identified a promoter proximal to this new exon. Reporter assays in human embryonic kidney 293 cells showed a 6.7 ± 2.1-fold increase in activity over empty vector. Treatment with a known regulator of Ano1 expression, IL-4, increased promoter activity by 1.6 ± 0.02-fold over untreated cells. The promoter region contained putative binding sites for multiple transcription factors including signal transducer and activator of transcription 6 (STAT6), a downstream effector of IL-4. Chromatin immunoprecipitation (ChIP) experiments on T84 cells, which endogenously express Ano1, showed a 2.1 ± 0.12-fold increase in binding of STAT6 to P0 after IL-4 treatment. These results were confirmed by mutagenesis, expression, and RNA interference techniques. This work allows deeper understanding of the regulation of Ano1 in physiology and as a potential therapeutic target in a variety of diseases.

Suppressor of cytokine signaling 1 modulates invasion and metastatic potential of colorectal cancer cells

Suppressor of cytokine signaling (SOCS) 1 is an inducible negative regulator of cytokine signaling but its role in human cancer is not completely established. Here we report that, while SOCS1 is expressed in normal colonic epithelium and colon adenocarcinomas, its level decreases during progression of colon adenocarcinomas, the lowest level being found in the most aggressive stage and least differentiated carcinomas. Forced expression of SOCS1 in metastatic colorectal SW620 cells reverses many characteristics of Epithelial-Mesenchymal Transition (EMT), as highlighted by the disappearance of the transcription factor ZEB1 and the mesenchymal form of p120ctn and the re-expression of E-cadherin. Furthermore, miRNA profiling indicated that SOCS1 also up-regulates the expression of the mir-200 family of miRNAs, which can promote the mesenchymal-epithelial transition and reduce tumor cell migration. Accordingly, overexpression of SOCS1 induced cell morphology changes and dramatically reduced tumor cell invasion in vitro. When injected in nude mice, SOCS1-expressing SW620 cells induced metastases in a smaller number of animals than parental SW620 cells, and did not generate any adrenal gland or bone metastasis. Overall, our results suggest that SOCS1 controls metastatic progression of colorectal tumors by preventing the mesenchymal-epithelial transition (MET), including E-cadherin expression. This pathway may be associated with survival to colorectal cancer by reducing the capacity of generating metastases.

Th2 cytokines increase Staphylococcus aureus alpha toxin-induced keratinocyte death through the signal transducer and activator of transcription 6 (STAT6)

Atopic dermatitis (AD) is an inflammatory skin disease characterized by increased Th2 cytokine expression. AD skin lesions are often exacerbated by Staphylococcus aureus mediated secretion of the lytic virulence factor, alpha toxin. In the current study, we report that alpha toxin induced cell death is greater in the skin from patients with AD compared to controls. Furthermore, we find that keratinocyte differentiation and Th2 cytokine exposure influence sensitivity to S. aureus alpha toxin induced cell death. Differentiated keratinocytes are protected from cell death, while cells treated with Th2 cytokines have increased sensitivity to alpha toxin induced lethality. Our data demonstrates that downstream effects mediated by Th2 cytokines are dependent upon host expression of STAT6. We determine that Th2 cytokines induce biochemical changes that decrease levels of acid sphingomyelinase, an enzyme that cleaves sphingomyelin, an alpha toxin receptor. Furthermore, Th2 cytokines inhibit production of lamellar bodies, organelles critical for epidermal barrier formation. Finally, we determine that sphingomyelinase and its enzymatic product, phosphocholine, prevent Th2 mediated increases in alpha toxin induced cell death. Therefore, our studies may help explain the increased propensity for Th2 cytokines to exacerbate S. aureus induced skin disease, and provide a potential therapeutic target for treatment of AD.

Gfi-1 is the transcriptional repressor ofSOCS1in acute myeloid leukemia cells

Silencing of SOCS1, a TSG, has been detected in various malignancies, including AML. However, the underlying mechanism of SOCS1 inactivation remains elusive. In this study, we explored the role of histone methylation in SOCS1 expression in AML cells. By ChIP assay, we demonstrated that G9a and SUV39H1, two enzymes catalyzing H3K9 methylation, were physically associated with the SOCS1 promoter, and treatment with chaetocin, a histone methyltransferase inhibitor, suppressed H3K9 methylation on the SOCS1 promoter and enhanced SOCS1 expression. Furthermore, knockdown of G9a and SUV39H1 by siRNA could also induce SOCS1 expression. On the other hand, SOCS1 knockdown by shRNA eliminated chaetocin-induced cell apoptosis. To investigate further whether any transcription factor was involved in H3K9 methylation-related SOCS1 repression, we scanned the sequences of the SOCS1 gene promoter and found two binding sites for Gfi-1, a transcription repressor. By DNA pull-down and ChIP assays, we showed that Gfi-1 directly bound the SOCS1 promoter, and ectopic Gfi-1 expression suppressed STAT5-induced SOCS1 promoter activation. In contrast, Gfi-1 knockdown by shRNA enhanced SOCS1 expression and inhibited STAT5 expression. Moreover, the knockdown of G9a completely rescued the repressive effect of Gfi-1 on STAT5A-induced SOCS1 promoter activation. Collectively, our study indicates that the expression of Gfi-1 contributes to SOCS1 silencing in AML cells through epigenetic modification, and suppression of histone methyltransferase can provide new insight in AML therapy.

Downregulation of inflammatory microRNAs by Ig-like transcript 3 is essential for the differentiation of human CD8(+) T suppressor cells

We have investigated the mechanism underlying the immunoregulatory function of membrane Ig-like transcript 3 (ILT3) and soluble ILT3Fc. microRNA (miRNA) expression profile identified genes that were downregulated in ILT3-induced human CD8(+) T suppressor cells (Ts) while upregulated in T cells primed in the absence of ILT3. We found that miR-21, miR-30b, and miR-155 target the 3'-untranslated region of genes whose expression was strongly increased in ILT3Fc-induced Ts, such as dual specificity phosphatase 10, B cell CLL/lymphoma 6, and suppressor of cytokine signaling 1, respectively. Transfection of miRNA mimics or inhibitors and site-specific mutagenesis of their 3'-untranslated region binding sites indicated that B cell CLL/lymphoma 6, dual specificity phosphatase 10, and suppressor of cytokine signaling 1 are direct targets of miR-30b, miR-21, and miR-155. Primed CD8(+) T cells transfected with miR-21&30b, miR-21&155, or miR-21&30b&155 inhibitors displayed suppressor activity when added to autologous CD3-triggered CD4 T cells. Luciferase reporter assays of miR-21 and miR-155 indicated that their transcription is highly dependent on AP-1. Analysis of activated T cells showed that ILT3Fc inhibited the translocation to the nucleus of the AP-1 subunits, FOSB and c-FOS, and the phosphorylation of ZAP70 and phospholipase C-γ 1. In conclusion, ILT3Fc inhibits T cell activation and induces the generation of Ts targeting multiple inflammatory miRNA pathways.

The IFN-gamma-dependent suppressor of cytokine signaling 1 promoter activity is positively regulated by IFN regulatory factor-1 and Sp1 but repressed by growth factor independence-1b and Krüppel-like factor-4, and it is dysregulated in psoriatic keratinocytes

Epidermal keratinocytes can counteract the detrimental effects of IFN-gamma by inducing the expression of suppressor of cytokine signaling (SOCS)1, which plays an important anti-inflammatory and self-protective role. To date, limited information exists on its expression and regulation in human diseased keratinocytes. In this study, we compared the expression levels of SOCS1 in keratinocytes isolated from skin affected by psoriasis with cells obtained from healthy donors, unveiling that keratinocytes are more prone than healthy cells to upregulate SOCS1 mRNA expression in response to IFN-gamma. We explored the regulatory mechanisms involved in socs1 gene transcription, and found that Sp1 and IFN regulatory factor-1 transcription factors are, respectively, responsible for the basal and IFN-gamma-induced activity of human socs1 promoter. In parallel, we demonstrated that socs1 promoter is negatively regulated by two transcriptional repressors, namely, growth factor independence-1b and Krüppel-like factor 4, which tightly control SOCS1 transcription on IFN-gamma stimulation. Interestingly, although the expression of Sp1 and IFN regulatory factor-1 activators of socs1 promoter is unaltered, growth factor independence-1b and Krüppel-like factor 4 are significantly reduced in psoriatic compared with healthy keratinocytes. This reduction and the consequent unbalanced binding of transcriptional activators and repressors to socs1 promoter after IFN-gamma stimulation might be responsible for the enhanced expression of SOCS1 in psoriatic cells. We suggest that SOCS1 exaggerated upregulation in psoriatic keratinocytes could represent a mechanism through which these cells attempt to protect themselves from IFN-gamma effects. However, the SOCS1 increased levels in psoriatic keratinocytes are not sufficient to completely inhibit the expression of proinflammatory genes.

Functional cooperation between Stat-1 and ets-1 to optimize icam-1 gene transcription

Intercellular adhesion molecule-1 (ICAM-1) plays an important role in the immune system, enabling the interactions between effector cells and target cells. It is also known to be involved in tumor growth and metastasis. Its expression is transcriptionally regulated by several proinflammatory cytokines including IFN-gamma, which induces ICAM-1 transcription via the JAK-STAT signaling pathway in a Stat1-dependent fashion. The ICAM-1 promoter contains several cis-active regulatory elements including 2 Ets binding sites (EBSs) located at positions -158 and -138 relatively to the AUG, which were previously shown to play a role in the constitutive activity of the ICAM-1 promoter. In the present study, we have determined whether the EBSs are also involved in the regulation of ICAM-1 gene transcription by pro-inflammatory cytokines. Transient transfection assays were performed with reporter genes containing ICAM-1 promoter constructions cloned upstream from the firefly luciferase gene. Site-specific mutations of the EBS diminished the promoter activity stimulated by IFN-gamma, although the IFN-gamma responsive element (pIgammaRE), which binds Stat1, was intact. Stimulation of the transcriptional activity following IFN-gamma treatment was significantly reduced when both EBSs were inactivated. Co-immunoprecipitation experiments provided evidence of a physical interaction involving Ets1 and Stat1. In COS-1 and HEK 293 cells cotransfected with CFP-Stat1 and YFP-Ets fusion protein, fluorescence resonance energy transfer experiments confirmed the close proximity of these 2 proteins in living cells following treatment with IFN-gamma.

Sp2 regulates interferon-gamma-mediated socs1 gene expression

Suppressor of cytokine signalling (SOCS) proteins are inducible feedback inhibitors of Janus kinase (JAK) and signal transducers and activators of transcription signalling (STAT) pathways. Interferon (IFN)-gamma induces the expression of the socs1 gene in several cell types through several cis elements present in its promoter and their binding proteins. Socs1 expression is induced in the human keratinocytes HaCaT cell line through sequential activation of STAT1 and IRF-1. Comparison of the 5'-upstream sequences of the mouse and human socs1 genes identified conserved binding sites for IRF-1 regulatory elements. Although this response element is able to bind IRF-1 in human cells, no IFN-gamma responsiveness was observed with human socs1 promoter reporter constructs containing this element. In contrast the mouse socs1 promoter was fully responsive. The mouse promoter contains two cis-acting elements which modulate its expression and are recognized by IRF-1 and Sp2. Despite the absence of Sp2 in the 5'-upstream sequence of the human promoter, silencing of Sp2 by RNA interference clearly demonstrated that Sp2 is required for IFN-gamma-induced regulation of socs1 mRNA both in human and mouse.

IL-4 independent nuclear translocalization of STAT6 in HeLa cells by entry of Toxoplasma gondii

Toxoplasma gondii provokes rapid and sustained nuclear translocation of the signal transducer and activator of transcription 6 (STAT6) in HeLa cells. We observed activation of STAT6 as early as 2 hr after infection with T. gondii by the nuclear translocation of fluorescence expressed from exogenously transfected pDsRed2-STAT6 plasmid and by the detection of phosphotyrosine-STAT6 in Western blot. STAT6 activation occurred only by infection with live tachyzoites but not by co-culture with killed tachyzoites or soluble T. gondii extracts. STAT6 phosphorylation was inhibited by small interfering RNA of STAT6 (siSTAT6). In view of the fact that STAT6 is a central mediator of IL-4 induced gene expression, activation of STAT6 by T. gondii infection resembles that infected host cells has been stimulated by IL-4 treatment. STAT1 was affected to increase the transcription and expression by the treatment of siSTAT6. STAT6 activation was not affected by any excess SOCS's whereas that with IL-4 was inhibited by SOCS-1 and SOCS-3. T. gondii infection induced Eotaxin-3 gene expression which was reduced by IFN-gamma. These results demonstrate that T. gondii exploits host STAT6 to take away various harmful reactions by IFN-gamma. This shows, for the first time, IL-4-like action by T. gondii infection modulates microbicidal action by IFN-gamma in infected cells.

Involvement of SOCS3 in regulation of CD11c+ dendritic cell-derived osteoclastogenesis and severe alveolar bone loss

To investigate the role of suppressor of cytokine signaling (SOCS) molecules in periodontal immunity and RANKL-mediated dendritic cell (DC)-associated osteoclastogenesis, we analyzed SOCS expression profiles in CD4(+) T cells and the effect of SOCS3 expression in CD11c(+) DCs during periodontal inflammation-induced osteoclastogenesis and bone loss in nonobese diabetic (NOD) versus humanized NOD/SCID mice. Our results of ex vivo and in vitro analyses showed that (i) there is significantly higher SOCS3 expression associated with RANKL(+) T-cell-mediated bone loss in correlation with increased CD11c(+) DC-mediated osteoclastogenesis; (ii) the transfection of CD11c(+) DC using an adenoviral vector carrying a dominant negative SOCS3 gene significantly abrogates TRAP and bone-resorptive activity; and (iii) inflammation-induced TRAP expression, bone resorption, and SOCS3 activity are not associated with any detectable change in the expression levels of TRAF6 and mitogen-activated protein kinase signaling adaptors (i.e., Erk, Jnk, p38, and Akt) in RANKL(+) T cells. We conclude that SOCS3 plays a critical role in modulating cytokine signaling involved in RANKL-mediated DC-derived osteoclastogenesis during immune interactions with T cells and diabetes-associated severe inflammation-induced alveolar bone loss. Therefore, the development of SOCS3 inhibitors may have therapeutic potential as the target to halt inflammation-induced bone loss under pathological conditions in vivo.

Interferon alpha increases metalloproteinase-13 gene expression through a polyomavirus enhancer activator 3-dependent pathway in hepatic stellate cells

BACKGROUND/AIMS

To determine the effects of IFNalpha on MMP-13 gene expression in primary culture of hepatic stellate cells.

METHODS

We measured MMP-13 mRNA, MMP-13 protein, MMP-13 luciferase activity, binding of AP1 and PEA3 to DNA, and binding of PEA3 to Jak1 and Stat1.

RESULTS

IFNalpha increased MMP-13 mRNA, MMP-13 protein, and luciferase activity in cells transfected either with a luciferase plasmid driven by the MMP-13 promoter or with the same plasmid in which the AP1 binding site has been mutated. IFNalpha induced the binding of nuclear proteins to a radiolabeled PEA3 probe, but not to a AP1 probe. Supershift assays demonstrated that PEA3 and Stat1 are implicated in the formation of this complex. Immunoprecipitation assays showed that PEA3 interacts physically with Stat1 and that IFNalpha treatment increases this interaction. Downregulation of PEA3 or JAK1 with appropriated siRNAs or mutation of the PEA3 binding site in the MMP-13 promoter abrogated the effects of IFNalpha on MMP-13 gene expression. Finally, IFNalpha induced the binding of PEA3 to JAK1, as well as PEA3 tyrosine and serine phosphorylation.

CONCLUSIONS

IFNalpha determines the binding of PEA3 to JAK1 and its tyrosine phosphorylation. Activated PEA3 binds to MMP-13 promoter and activates its expression.

IL-4 and IL-13 negatively regulate TNF-alpha- and IFN-gamma-induced beta-defensin expression through STAT-6, suppressor of cytokine signaling (SOCS)-1, and SOCS-3

Human beta-defensins (HBDs) are a major class of antimicrobial peptides that play an important role in the innate immune response, however, the induction and regulation of these antimicrobial peptides is not well understood. We demonstrate here that stimulation of keratinocytes with TNF-alpha/IFN-gamma induces HBD-2 and HBD-3 by activating STAT-1 and NF-kappaB signaling. We further demonstrate that IL-4 and IL-13 activate STAT-6 and induce the suppressors of cytokine signaling (SOCS)-1 and -3. This interferes with STAT-1 and NF-kappaB signaling, thereby inhibiting TNF-alpha/IFN-gamma-mediated induction of HBD-2 and HBD-3. These data suggest that targeting the STAT-1-signaling pathway or suppressor of cytokine signaling expression enhances beta-defensin expression and represents a new therapeutic strategy for reduction of infection in human diseases associated with beta-defensin deficiency.

LCAT-null mice develop improved hepatic insulin sensitivity through altered regulation of transcription factors and suppressors of cytokine signaling

We previously reported that LCAT-deficient mice develop not only low HDL-cholesterol but also hypertriglyceridemia, hepatic triglyceride (TG) overproduction, and, unexpectedly, improved hepatic insulin sensitivity and reduced hepatic TG content. Here, we examined the mechanistic links underlying this apparent paradox. The LDL receptor-deficient (Ldlr)(-/-)xLcat(-/-) mouse model and age- and sex-matched Ldlr(-/-)xLcat(+/+) littermates, both in C57Bl/6 background, were employed. Studies of hepatic insulin signal transduction showed an upregulation of hepatic Irs2 mRNA level (5.3-fold, P = 0.02), IRS-2 protein mass level (1.5-fold, P = 0.009) and pIRS-2 (1.8-fold. P = 0.02) in the Ldlr(-/-)xLcat(-/-) mice. There was a 1.2-fold increase in pAkt (P = 0.03) with a nonsignificant change in total Akt. We observed a significant shift in its downstream transcription factor FoxO-1 to the cytosolic compartment (2.3-fold increase in cytosolic/nuclear ratio, P = 0.04). We also observed a significant 3.1-fold increase in nuclear abundance of FoxA-2 mass (P = 0.017) and a 1.5-fold upregulation of its coactivator PGC-1beta (P = 0.002), the coordinated actions of which promotes hepatic TG production and beta-oxidation. Increased hepatic insulin signaling in the Ldlr(-/-)xLcat(-/-) mice was associated with an upregulation of the Tcfe3 gene (1.7-fold, P = 0.024), a selective downregulation of the Socs-1 gene by 60% (P = 0.01), and no change in PTP-1B protein mass. These data suggest that LCAT deficiency induces complex alterations in hepatic signal transduction cascades, which explain, at least in part, the observed enhanced insulin signaling in association with hepatic TG overproduction and reduced hepatic TG content.

IL-4-induced AID expression and its relevance to IgA class switch recombination

Activation-induced cytidine deaminase (AID) is an inducible gene that plays a critical role in Ig class switch recombination and somatic hypermutation in B cells. We explored the mechanisms by which IL-4 induces AID expression in mouse B cells. IL-4 increased AID expression and over-expression of Stat6 further augmented IL-4-induced promoter activity. The involvement of Stat6 in the promoter activity was confirmed using ChIP assays and site-directed mutagenesis. Treatment with H89, a PKA inhibitor, markedly decreased IL-4-induced AID expression, and over-expression of CREB enhanced it. These results indicate that Stat6 and PKA/CREB are involved in IL-4-induced AID expression. The relevance of these signal transducing molecules was verified using the TGFbeta1-induced IgA isotype switching model. Our results indicate that IL-4, through Stat6 and PKA/CREB, induces AID expression leading to Ig isotype switching event.

Promoter profiling and coexpression data analysis identifies 24 novel genes that are coregulated with AMPA receptor genes, GRIAs

We identified a set of transcriptional elements that are conserved and overrepresented within the promoters of human, mouse, and rat GRIAs by comparing these promoters against a collection of 10,741 gene promoters. Cells regulate functional groups of genes by coordinating the transcriptional and/or posttranscriptional mRNA levels of interacting genes. As such, it is expected that functional groups of genes share the same transcriptional features within their promoters. We found 47 genes whose promoters contain the same combination of transcriptional elements that are overrepresented within the promoters of the GRIA gene family. Coexpressed genes may be transcriptionally coregulated, which in turn suggests that these genes may play complementary roles within a particular functional context. Using microarray expression data, we found 24 (of the 47) genes that share not only a similar promoter profile with GRIAs but also a well-correlated gene expression profile and, thus, we believe these to be coregulated with GRIAs.

Suppressor of cytokine signaling-1 is an IL-4-inducible gene in macrophages and feedback inhibits IL-4 signaling

Interferon-gamma and interleukin-4 (IL-4) induce distinct gene expression profiles in macrophages by differentially activating signal transducers and activators of transcription (STAT)1 and STAT6, respectively. The role of suppressor of cytokine signaling (SOCS)-1 as a negative regulator of IFN-gamma signaling is well established. However, its potential role as a negative regulator of IL-4 signaling has not been explored. We found that IL-4, like IFN-gamma, induces rapid de novo expression of SOCS-1 in primary macrophages. Induction of SOCS-1 gene expression by IL-4 is STAT6-dependent, whereas induction of SOCS-1 by IFN-gamma is STAT1-dependent. Unlike their common ability to induce expression of SOCS-1, IL-4 also induced expression of SOCS-2 but not SOCS-3 in macrophages, whereas IFN-gamma induced expression of SOCS-3 but not SOCS-2. Forced expression of SOCS-1 or SOCS-3, but not SOCS-2, inhibited activation of STAT6 by IL-4. Moreover, SOCS-1 appears to serve as an endogenous regulator of IL-4 signaling in macrophages because the magnitude and duration of STAT6 activation as well as IL-4-mediated gene expression were much greater in SOCS-1-deficient (SOCS-1(-/-)) macrophages than in wild-type macrophages. Our findings demonstrate that, like IFN-gamma, IL-4 also induces expression of SOCS-1 in macrophages, and SOCS-1 feedback inhibits expression of STAT6-responsive genes.

IL-4 induces expression of TARC/CCL17 via two STAT6 binding sites

A cardinal feature of allergic disorders and immune responses is enhanced leukocyte trafficking. This is largely orchestrated by chemokines. The CC chemokine thymus- and activation-regulated chemokine (TARC/CCL17) selectively attracts Th2 cells via the G protein-coupled chemokine receptor CCR4. We show here that TARC/CCL17 is expressed by human T cells upon stimulation with IL-4. Mapping of the transcriptional start site revealed the presence of two putative STAT6 binding motifs in proximity to the start position. EMSA and chromatin immunoprecipitation experiments demonstrated that STAT6 was able to bind to both motifs. A fragment of the TARC/CCL17 promoter containing both sites was tested in reporter gene assays for IL-4 inducibility. The promoter was inducible in a STAT6-deficient cell line only after introduction of functional STAT6. When mutations were inserted into one of the STAT6 motifs, IL-4-induced promoter activation was reduced. With both sites mutated, inducibility was completely abrogated. These data demonstrate collectively that T cells serve as a source of TARC/CCL17 when stimulated with IL-4 and that STAT6 is essential for this.

IFN‐β‐induced SOCS‐1 negatively regulates CD40 gene expression in macrophages and microglia

Costimulation between T cells and antigen-presenting cells is required for adaptive immune responses. CD40, a costimulatory molecule, is expressed in macrophages and microglia. The aberrant expression of CD40 is involved in human diseases including multiple sclerosis, rheumatoid arthritis, and Alzheimer's disease. CD40 expression is induced by a variety of stimuli, including IFN-gamma and lipopolysaccharide (LPS). In this study, we describe the molecular basis by which IFN-beta, a cytokine with immunomodulatory properties, regulates CD40 gene expression. IFN-beta induces CD40 expression in macrophages and microglia at the transcriptional level, and GAS elements in the CD40 promoter are required for IFN-beta-induced CD40 promoter activity. The critical role of signal transducers and activators of transcription-1alpha (STAT-1alpha) in this response was confirmed by utilizing primary microglia from STAT-1alpha deficient mice. IFN-beta induces suppressor of cytokine signaling-1 (SOCS-1) gene expression, which inhibits cytokine signaling by inhibiting activation of STAT proteins. The ectopic expression of SOCS-1 abrogates IFN-beta-mediated STAT-1alpha activation and inhibits IFN-beta-induced CD40 expression. IFN-beta-induced recruitment of STAT-1alpha and RNA Pol II and permissive histone modifications on the CD40 promoter are also inhibited by SOCS-1 overexpression. These novel results indicate that IFN-beta-induced SOCS-1 plays an important role in the negative regulation of IFN-beta-induced CD40 gene expression.

Chemical Genetic Identification of the IGF-Linked Pathway that Is Mediated by STAT6 and MFP2

Insulin-like growth factor 2 (IGF2) is a potent mitogen whose deregulation plays a role in developing liver, breast, and prostate cancers. Here, we take a small-molecule approach to investigate molecular pathways that modulate IGF2 signaling, by using chromeceptin, a synthetic molecule that selectively impairs the viability and growth of IGF2-overexpressing hepatocellular carcinoma cells. Affinity purification revealed that chromeceptin binds to multifunctional protein 2 (MFP-2), a seemingly multifunctional enzyme implicated in peroxisomal beta-oxidation. The small molecule-protein interaction stimulates the expression of IGF binding protein 1 (IGFBP-1) and suppressor of cytokine signaling-3 (SOCS-3), two cellular attenuators of the IGF signals, through activation of signal transducers and activators of transcription 6 (STAT6). The results underline the importance of STATs in IGF/insulin regulation, and they implicate a new pathway for STAT6 activation that is amenable to small-molecule intervention.

JAK/STAT signal transduction: regulators and implication in hematological malignancies

Signal transducers and activators of transcription (STATs) comprise a family of several transcription factors that are activated by a variety of cytokines, hormones and growth factors. STATs are activated through tyrosine phosphorylation, mainly by JAK kinases, which lead to their dimerization, nuclear translocation and regulation of target genes expression. Stringent mechanisms of signal attenuation are essential for insuring appropriate, controlled cellular responses. Among them phosphotyrosine phosphatases (SHPs, CD45, PTP1B/TC-PTP), protein inhibitors of activated STATs (PIAS) and suppressors of cytokine signaling (SOCS) inhibit specific and distinct aspects of cytokine signal transduction. SOCS proteins bind through their SH2 domain to phosphotyrosine residues in either cytokine receptors or JAK and thus can suppress cytokine signaling. Many recent findings indicate that SOCS proteins act, in addition, as adaptors that regulate the turnover of certain substrates by interacting with and activating an E3 ubiquitin ligase. Thus, SOCS proteins act as negative regulators of JAK/STAT pathways and may represent tumour suppressor genes. The discovery of oncogenic partner in this signaling pathway, more especially in diverse hematologic malignancies support a prominent role of deregulated pathways in the pathogenesis of diseases. Fusion proteins implicating the JH1 domain of JAK2 (TEL-JAK2, BCR-JAK2), leading to deregulated activity of JAK2, have been described as the result of translocation. Somatic point mutation in JH2 domain of JAK2 (JAK2V617F), leading also to constitutive tyrosine phosphorylation of JAK2 and its downstream effectors was reported in myeloproliferative disorders. Furthermore, silencing of socs-1 and shp-1 expression by gene methylation is observed in some cancer cells.

Convergence of protein kinase C and JAK-STAT signaling on transcription factor GATA-4

Angiotensin II (AII), a potent vasoactive hormone, acts on numerous organs via G-protein-coupled receptors and elicits cell-specific responses. At the level of the heart, AII stimulation alters gene transcription and leads to cardiomyocyte hypertrophy. Numerous intracellular signaling pathways are activated in this process; however, which of these directly link receptor activation to transcriptional regulation remains undefined. We used the atrial natriuretic factor (ANF) gene (NPPA) as a marker to elucidate the signaling cascades involved in AII transcriptional responses. We show that ANF transcription is activated directly by the AII type 1 receptor and precedes the development of myocyte hypertrophy. This response maps to STAT and GATA binding sites, and the two elements transcriptionally cooperate to mediate signaling through the JAK-STAT and protein kinase C (PKC)-GATA-4 pathways. PKC phosphorylation enhances GATA-4 DNA binding activity, and STAT-1 functionally and physically interacts with GATA-4 to synergistically activate AII and other growth factor-inducible promoters. Moreover, GATA factors are able to recruit STAT proteins to target promoters via GATA binding sites, which are sufficient to support synergy. Thus, STAT proteins can act as growth factor-inducible coactivators of tissue-specific transcription factors. Interactions between STAT and GATA proteins may provide a general paradigm for understanding cell specificity of cytokine and growth factor signaling.

Interleukin-4 downregulates TNFalpha-induced IL-8 production in keratinocytes

Interleukin (IL)-8 is a CXC chemokine induced by pro-inflammatory cytokines such as TNFalpha, IL-1beta and IL-6 in different cell types including keratinocytes. IL-4 regulation of TNFalpha-induced IL-8 expression is cell-type specific. In this study, we show that in the keratinocyte cell line HaCaT, IL-4 decreases TNFalpha-induced IL-8 mRNA expression. We then investigated the mechanism of IL-4 effect and showed that IL-4 downregulates TNFalpha-induced IL-8 promoter activity in luciferase reporter assays. Moreover, overexpression of either the endogenous JAK inhibitor SOCS-1 or a dominant negative form of the STAT6 transcription factor (STAT6DeltaC) interferes with the IL-4 inhibitory effect on IL-8 promoter. Finally we demonstrate, using a NF-kappaB-dependent promoter luciferase construct that IL-4 interferes, at least in part, with NF-kappaB transcriptional activity. Overall our results suggest that IL-4 regulates TNFalpha-induced IL-8 expression at a transcriptional level and this mechanism involves STAT6 and NF-kappaB transcription factors.

STAT1 and Nmi are downstream targets of Ets-1 transcription factor in MCF-7 human breast cancer cell

Ets-1 is a cellular homologue of the product of the viral ets oncogene of the E26 virus, and it functions as a tissue-specific transcription factor. It plays an important role in cell proliferation, differentiation, lymphoid cell development, transformation, angiogenesis, and apoptosis. Ets-1 controls the expression of critical genes involved in these processes by binding to ets binding sites present in the transcriptional regulatory regions. Here, we transiently overexpressed Ets-1 in MCF-7 and comprehensively searched for potential downstream targets of Ets-1 by cDNA microarray analysis. The expressions of several interferon-related genes including STAT1 and Nmi were augmented by the overexpression of Ets-1. RT-PCR and Western blotting confirmed the increase in the levels of STAT1 and Nmi mRNA and protein. In contrast, Ets-1 siRNA decreased the expression of STAT1 and Nmi proteins. As in our transient transfection experiments, stable overexpression of Ets-1, also increased the protein expression of STAT1 and Nmi in MCF-7 cells. Taken together, our results indicate that STAT1 and Nmi are downstream targets of Ets-1 in MCF-7 human breast cancer cells.