Regulation of Socs gene expression by the proto-oncoprotein GFI-1B: two routes for STAT5 target gene induction by erythropoietin

Histone Deacetylase Inhibitor Trichostatin A Reduces Endothelial Cell Proliferation by Suppressing STAT5A-Related Gene Transcription

Inhibitors of histone deacetylases (HDACi) have shown promising effects in preclinical applications for the treatment of many diseases. Confusedly though, the effects of the HDACi trichostatin A (TSA) on angiogenesis are variable among different diseases. This study investigated the direct effects of TSA on endothelial cells, which plays essential roles in angiogenesis and the underlying molecular events. TSA reduced the viability of human umbilical vein endothelial cells (HUVECs), in which proliferation-related genes including BIRC5, CKS1B, and NDC80 were found to be involved. Furthermore, signal transducer and activator of transcription 5 A (STAT5A) was demonstrated to be reduced by TSA and to mediate TSA-induced downregulation of BIRC5, CKS1B, and NDC80 and HUVEC proliferation. Mechanistically, data showed that STAT5A directly bound to the promoters of BIRC5, CKS1B, and NDC80 and activated their transcription through special DNA sequence sites. Finally, the TSA–STAT5A–BIRC5, CKS1B, and NDC80 axis also worked in a cancerous endothelial cell angiogenesis model. The results of this study revealed novel mechanisms underlying the effects of TSA on endothelial cells and provided insights for angiogenesis-associated diseases.

SOCS Proteins in Immunity, Inflammatory Diseases, and Immune-Related Cancer

Cytokine signaling represents one of the cornerstones of the immune system, mediating the complex responses required to facilitate appropriate immune cell development and function that supports robust immunity. It is crucial that these signals be tightly regulated, with dysregulation underpinning immune defects, including excessive inflammation, as well as contributing to various immune-related malignancies. A specialized family of proteins called suppressors of cytokine signaling (SOCS) participate in negative feedback regulation of cytokine signaling, ensuring it is appropriately restrained. The eight SOCS proteins identified regulate cytokine and other signaling pathways in unique ways. SOCS1–3 and CISH are most closely involved in the regulation of immune-related signaling, influencing processes such polarization of lymphocytes and the activation of myeloid cells by controlling signaling downstream of essential cytokines such as IL-4, IL-6, and IFN-γ. SOCS protein perturbation disrupts these processes resulting in the development of inflammatory and autoimmune conditions as well as malignancies. As a consequence, SOCS proteins are garnering increased interest as a unique avenue to treat these disorders.

Growth Factor Independence 1B-Mediated Transcriptional Repression and Lineage Allocation Require Lysine-Specific Demethylase 1-Dependent Recruitment of the BHC Complex

Growth factor independence 1B (GFI1B) coordinates assembly of transcriptional repressor complexes comprised of corepressors and histone-modifying enzymes to control gene expression programs governing lineage allocation in hematopoiesis. Enforced expression of GFI1B in K562 erythroleukemia cells favors erythroid over megakaryocytic differentiation, providing a platform to define molecular determinants of binary fate decisions triggered by GFI1B. We deployed proteome-wide proximity labeling to identify factors whose inclusion in GFI1B complexes depends upon GFI1B's obligate effector, lysine-specific demethylase 1 (LSD1). We show that GFI1B preferentially recruits core and putative elements of the BRAF-histone deacetylase (HDAC) (BHC) chromatin-remodeling complex (LSD1, RCOR1, HMG20A, HMG20B, HDAC1, HDAC2, PHF21A, GSE1, ZMYM2, and ZNF217) in an LSD1-dependent manner to control acquisition of erythroid traits by K562 cells. Among these elements, depletion of both HMG20A and HMG20B or of GSE1 blocks GFI1B-mediated erythroid differentiation, phenocopying impaired differentiation brought on by LSD1 depletion or disruption of GFI1B-LSD1 binding. These findings demonstrate the central role of the GFI1B-LSD1 interaction as a determinant of BHC complex recruitment to enable cell fate decisions driven by GFI1B.

Transcription Factor GFI1B in Health and Disease

Many human diseases arise through dysregulation of genes that control key cell fate pathways. Transcription factors (TFs) are major cell fate regulators frequently involved in cancer, particularly in leukemia. The GFI1B gene, coding a TF, was identified by sequence homology with the oncogene growth factor independence 1 (GFI1). Both GFI1 and GFI1B have six C-terminal C2H2 zinc fingers and an N-terminal SNAG (SNAIL/GFI1) transcriptional repression domain. Gfi1 is essential for neutrophil differentiation in mice. In humans, GFI1 mutations are associated with severe congenital neutropenia. Gfi1 is also required for B and T lymphopoiesis. However, knockout mice have demonstrated that Gfi1b is required for development of both erythroid and megakaryocytic lineages. Consistent with this, human mutations of GFI1B produce bleeding disorders with low platelet count and abnormal function. Loss of Gfi1b in adult mice increases the absolute numbers of hematopoietic stem cells (HSCs) that are less quiescent than wild-type HSCs. In keeping with this key role in cell fate, GFI1B is emerging as a gene involved in cancer, which also includes solid tumors. In fact, abnormal activation of GFI1B and GFI1 has been related to human medulloblastoma and is also likely to be relevant in blood malignancies. Several pieces of evidence supporting this statement will be detailed in this mini review.

From cytopenia to leukemia: the role of Gfi1 and Gfi1b in blood formation

The DNA-binding zinc finger transcription factors Gfi1 and Gfi1b were discovered more than 20 years ago and are recognized today as major regulators of both early hematopoiesis and hematopoietic stem cells. Both proteins function as transcriptional repressors by recruiting histone-modifying enzymes to promoters and enhancers of target genes. The establishment of Gfi1 and Gfi1b reporter mice made it possible to visualize their cell type-specific expression and to understand their function in hematopoietic lineages. We now know that Gfi1 is primarily important in myeloid and lymphoid differentiation, whereas Gfi1b is crucial for the generation of red blood cells and platelets. Several rare hematologic diseases are associated with acquired or inheritable mutations in the GFI1 and GFI1B genes. Certain patients with severe congenital neutropenia carry mutations in the GFI1 gene that lead to the disruption of the C-terminal zinc finger domains. Other mutations have been found in the GFI1B gene in families with inherited bleeding disorders. In addition, the Gfi1 locus is frequently found to be a proviral integration site in retrovirus-induced lymphomagenesis, and new, emerging data suggest a role of Gfi1 in human leukemia and lymphoma, underlining the role of both factors not only in normal hematopoiesis, but also in a wide spectrum of human blood diseases.

Kit transduced signals counteract erythroid maturation by MAPK-dependent modulation of erythropoietin signaling and apoptosis induction in mouse fetal liver

Signaling by the stem cell factor receptor Kit in hematopoietic stem and progenitor cells is functionally associated with the regulation of cellular proliferation, differentiation and survival. Expression of the receptor is downregulated upon terminal differentiation in most lineages, including red blood cell terminal maturation, suggesting that omission of Kit transduced signals is a prerequisite for the differentiation process to occur. However, the molecular mechanisms by which Kit signaling preserves the undifferentiated state of progenitor cells are not yet characterized in detail. In this study, we generated a mouse model for inducible expression of a Kit receptor carrying an activating mutation and studied its effects on fetal liver hematopoiesis. We found that sustained Kit signaling leads to expansion of erythroid precursors and interferes with terminal maturation beyond the erythroblast stage. Primary KIT(D816V) erythroblasts stimulated to differentiate fail to exit cell cycle and show elevated rates of apoptosis because of insufficient induction of survival factors. They further retain expression of progenitor cell associated factors c-Myc, c-Myb and GATA-2 and inefficiently upregulate erythroid transcription factors GATA-1, Klf1 and Tal1. In KIT(D816V) erythroblasts we found constitutive activation of the mitogen-activated protein kinase (MAPK) pathway, elevated expression of the src kinase family member Lyn and impaired Akt activation in response to erythropoietin. We demonstrate that the block in differentiation is partially rescued by MAPK inhibition, and completely rescued by the multikinase inhibitor Dasatinib. These results show that a crosstalk between Kit and erythropoietin receptor signaling cascades exists and that continuous Kit signaling, partly mediated by the MAPK pathway, interferes with this crosstalk.

Growth factor independence 1b (gfi1b) is important for the maturation of erythroid cells and the regulation of embryonic globin expression

Growth factor independence 1b (GFI1B) is a DNA binding repressor of transcription with vital functions in hematopoiesis. Gfi1b-null embryos die at midgestation very likely due to defects in erythro- and megakaryopoiesis. To analyze the full functionality of Gfi1b, we used conditionally deficient mice that harbor floxed Gfi1b alleles and inducible (Mx-Cre, Cre-ERT) or erythroid specific (EpoR-Cre) Cre expressing transgenes. In contrast to the germline knockout, EpoR-Cre mediated erythroid specific ablation of Gfi1b allows full gestation, but causes perinatal lethality with very few mice surviving to adulthood. Both the embryonic deletion of Gfi1b by EpoR-Cre and the deletion in adult mice by Mx-Cre or Cre-ERT leads to reduced numbers of erythroid precursors, perturbed and delayed erythroid maturation, anemia and extramedullary erythropoiesis. Global expression analyses showed that the Hba-x, Hbb-bh1 and Hbb-y embryonic globin genes were upregulated in Gfi1b deficient TER119⁺ fetal liver cells over the gestation period from day 12.5–17.5 p.c. and an increased level of Hbb-bh1 and Hbb-y embryonic globin gene expression was even maintained in adult Gfi1b deficient mice. While the expression of Bcl11a, a regulator of embryonic globin expression was not affected by Gfi1b deficiency, the expression of Gata1 was reduced and the expression of Sox6, also involved in globin switch, was almost entirely lost when Gfi1b was absent. These findings establish Gfi1b as a regulator of embryonic globin expression and embryonic and adult erythroid maturation.

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.

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.

SOCS3: An essential physiological inhibitor of signaling by interleukin-6 and G-CSF family cytokines

SOCS3 is an inducible negative feedback inhibitor of cytokine signaling. Conditional deletion of SOCS3 in mice using the Cre-lox system has now been applied to a range of cell types in the steady-state and under inflammatory, pathogenic, or tumorigenic stress, with the resulting phenotypes demonstrating the effects of SOCS3 in physiological and disease contexts. Together with recent structural and biochemical studies on the mechanisms of SOCS3 binding to cytokine receptors and associated kinases, we now have a better understanding of the non-redundant roles of SOCS3 in the inhibition of cytokine signaling via the receptors gp130, G-CSFR, leptinR, and IL-12Rβ. This review discusses the known functional activities of SOCS3 in fertility and development, inflammation, innate and adaptive immunity, and malignancy as determined by genetic studies in mice.

Snail/Gfi-1 (SNAG) family zinc finger proteins in transcription regulation, chromatin dynamics, cell signaling, development, and disease

The Snail/Gfi-1 (SNAG) family of zinc finger proteins is a group of transcriptional repressors that have been intensively studied in mammals. SNAG family members are similarly structured with an N-terminal SNAG repression domain and a C-terminal zinc finger DNA binding domain, however, the spectrum of target genes they regulate and the ranges of biological functions they govern vary widely between them. They play active roles in transcriptional regulation, formation of repressive chromatin structure, cellular signaling and developmental processes. They can also result in disease states due to deregulation. We have performed a thorough investigation of the relevant literature and present a comprehensive mini-review. Based on the available information, we also propose a mechanism by which SNAG family members may function.

Integrating novel signaling pathways involved in erythropoiesis

Many extrinsic and intrinsic factors control the development of red blood cells from committed progenitors, with the Erythropoietin-receptor (Epo-R) signaling network being the primary controlling molecular hub. Although much is understood about erythroid signaling pathways, new and intriguing factors that influence different aspects of erythroid cell development are still being uncovered. New extrinsic effectors include hypoxia and polymeric IgA1 (pIgA1), and new Epo-R signaling pathway components include Lyn/Cbp and Lyn/Liar. Hypoxia directly activates committed erythroid progenitors to expand, whereas pIgA1 activates the Akt and MAP-Kinase (MAPK) pathways through transferrin receptors on more mature erythroid cells. The Lyn/Cbp pathway controls the activity and protein levels of Lyn through recruitment of Csk and SOCS1, as well as feeding into the control of other pathways mediated by recruitment of ras-GAP, PI3-kinase, PLCγ, Fes, and EBP50. Nuclear/cytoplasmic shuttling of Lyn and other signaling molecules is influenced by Liar and results in regulation of their intersecting signaling pathways. The challenge of future research is to flesh out the details of these new signaling regulators/networks and integrate their influences during the different stages of erythropoiesis.

Inhibition of DNA methyltransferase induces G2 cell cycle arrest and apoptosis in human colorectal cancer cells via inhibition of JAK2/STAT3/STAT5 signalling

DNA methyltransferase inhibitors (MTIs) have recently emerged as promising chemotherapeutic or preventive agents for cancer, despite their poorly characterized mechanisms of action. The present study shows that DNA methylation is integral to the regulation of SH2-containing protein tyrosine phosphatase 1 (SHP1) expression, but not for regulation of suppressors of cytokine signalling (SOCS)1 or SOCS3 in colorectal cancer (CRC) cells. SHP1 expression correlates with down-regulation of Janus kinase/signal transducers and activators of transcription (JAK2/STAT3/STAT5) signalling, which is mediated in part by tyrosine dephosphorylation events and modulation of the proteasome pathway. Up-regulation of SHP1 expression was achieved using a DNA MTI, 5-aza-2'-deoxycytidine (5-aza-dc), which also generated significant down-regulation of JAK2/STAT3/STAT5 signalling. We demonstrate that 5-aza-dc suppresses growth of CRC cells, and induces G2 cell cycle arrest and apoptosis through regulation of downstream targets of JAK2/STAT3/STAT5 signalling including Bcl-2, p16(ink4a), p21(waf1/cip1) and p27(kip1). Although 5-aza-dc did not significantly inhibit cell invasion, 5-aza-dc did down-regulate expression of focal adhesion kinase and vascular endothelial growth factor in CRC cells. Our results demonstrate that 5-aza-dc can induce SHP1 expression and inhibit JAK2/STAT3/STAT5 signalling. This study represents the first evidence towards establishing a mechanistic link between inhibition of JAK2/STAT3/STAT5 signalling and the anticancer action of 5-aza-dc in CRC cells that may lead to the use of MTIs as a therapeutic intervention for human colorectal cancer.

Trichostatin A, a histone deacetylase inhibitor, suppresses JAK2/STAT3 signaling via inducing the promoter-associated histone acetylation of SOCS1 and SOCS3 in human colorectal cancer cells

Aberrant janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling is involved in the oncogenesis of several cancers. Suppressors of cytokine signaling (SOCS) genes and SH2-containing protein tyrosine phosphatase 1 (SHP1) proteins, which are negative regulators of JAK/STAT signaling, have been reported to have tumor suppressor functions. However, in colorectal cancer (CRC) cells, the mechanisms that regulate SOCS and SHP1 genes, and the cause of abnormalities in the JAK/STAT signaling pathway, remain largely unknown. The present study shows that trichostatin A (TSA), a histone deacetylase (HDAC) inhibitor, leads to the hyperacetylation of histones associated with the SOCS1 and SOCS3 promoters, but not the SHP1 promoter in CRC cells. This indicates that histone modifications are involved in the regulation of SOCS1 and SOCS3. Moreover, upregulation of SOCS1 and SOCS3 expression was achieved using TSA, which also significantly downregulated JAK2/STAT3 signaling in CRC cells. We also demonstrate that TSA suppresses the growth of CRC cells, and induces G1 cell cycle arrest and apoptosis through the regulation of downstream targets of JAK2/STAT3 signaling, including Bcl-2, survivin and p16(ink4a) . Therefore, our data demonstrate that TSA may induce SOCS1 and SOCS3 expression by inducing histone modifications and consequently inhibits JAK2/STAT3 signaling in CRC cells. These results also establish a mechanistic link between the inhibition of JAK2/STAT3 signaling and the anticancer action of TSA in CRC cells.

Human growth hormone receptor gene expression is regulated by Gfi-1/1b and GAGA cis-elements

Human growth hormone receptor (hGHR) gene regulation is complex: mRNAs are transcribed from multiple variant (V) 5'UTR exons, several ubiquitously while others only in the postnatal hepatocyte. The liver-specific V1 exon promoter contains Gfi-1/1b repressor sites adjacent to a GAGA box, a GH response element (GHRE) in several mammalian genes. GAGA boxes are also present in the ubiquitously expressing V3 exon promoter. Heterologous sites in bovine, ovine and murine GHR genes suggest conserved roles. GAGA factor stimulated V1 and V3 promoters while Gfi-1/1b repressed basal and GAF-stimulated V1 transcription. HGH treatment of HepG2 cells resulted in a new complex forming with V3 GAGA elements, suggesting a functional GHRE. Data suggest liver-specific V1 transcription is regulated by inhibitory Gfi-1/1b and stimulatory GAGA cis-elements and Gfi-1/1b may control the lack of V1 expression in fetal liver, hepatic tumours and non-hepatic tissues. In addition, hGH may regulate hGHR expression through V3 GAGA boxes.

Quantitative analysis of interferon alpha receptor subunit 1 and suppressor of cytokine signaling 1 gene transcription in blood cells of patients with chronic hepatitis C

Background

Interferon (IFN)-α receptor 1 (ifnar1) and suppressor of cytokine signaling 1 (socs1) transcription levels were quantified in peripheral blood mononuclear cells (PBMC) of 59 patients infected with hepatitis C virus (HCV) and 17 non-infected individuals. Samples were obtained from patients infected with HCV that were either untreated or treated with IFN-α2 plus ribavirin for 1 year and divided into responders and non-responders based on viral load reduction 6 months after treatment. Ifnar1 and socs1 transcription was quantified by real-time RT-PCR, and the fold difference (2^-ΔΔCT) with respect to hprt housekeeping gene was calculated.

Results

Ifnar1 transcription increased significantly in HCV-infected patients either untreated (3.26 ± 0.31), responders (3.1 ± 0.23) and non-responders (2.18 ± 0.23) with respect to non-infected individuals (1 ± 0.34; P = 0.005). Ifnar1 transcription increased significantly (P = 0.003) in patients infected with HCV genotypes 1a (4.74 ± 0.25) and 1b (2.81 ± 0.25) but not in 1a1b (1.58 ± 0.21). No association was found of Ifnar1 transcription with disease progress, initial viral load or other clinical factors. With respect to socs1 transcription, values were similar for non-infected individuals (1 ± 0.28) and untreated patients (0.99 ± 0.41) but increased in responders (2.81 ± 0.17) and non-responder patients (1.67 ± 0.41). Difference between responder and non-responder patients was not statistically significant. Socs1 transcription increased in patients infected with HCV genotypes 1a and 1b (2.87 ± 0.45 and 2.22 ± 0.17, respectively) but not in 1a1b (1.28 ± 0.40). Socs1 transcript was absent in three patients infected with HCV genotype 1b. A weak correlation between ifnar1 and socs1 transcription was found, when Spearman's correlation coefficient was calculated.

Conclusion

Our results suggest that HCV infection may up-regulate ifnar1 transcription. HCV genotypes differ in their capacity to affect ifnar1 and socs1 transcription, as well as in the ability to evade the antiviral response.

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.

Early growth response 2 regulates the survival of thymocytes during positive selection

The early growth response (Egr) transcription factor family regulates multiple steps during T-cell development. We examine here the role played by Egr2 in positive selection. In double-positive cells, Egr2 is upregulated immediately following TCR ligation, and its expression requires both the MAPK and calcineurin signaling pathways. Inducible transgenic and knockout mice were generated to cause gain- or loss-of-function of Egr2 in double-positive cells, and had reciprocal effects; more mature single-positive cells were made when Egr2 was overexpressed, and fewer when Egr2 was absent. These defects were associated with changes in the survival of positively selected cells rather than perturbation of positive selection or immediate post-selection signaling. The survival function of Egr2 at least partly depends upon its ability to activate the cytokine-mediated survival pathway, likely through negative regulation of both the IL-7R and suppressor of cytokine signaling 1 (Socs1), the molecular switch whose downregulation normally results in restored responsiveness to cytokine signaling following selection. While gain of Egr2 caused a decrease in Socs1 mRNA, loss of Egr2 resulted in downregulation of IL-7R, upregulation of Socs1, and inhibition of Stat5 phosphorylation and IL-7-mediated survival post-selection. Therefore, expression of Egr2 following positive selection links the initial TCR signaling event to subsequent survival of signaled cells.

Regulation of multiple cytokine signalling pathways by SOCS3 is independent of SOCS2

Suppressor of cytokine signalling (SOCS) 3 is an essential regulator of cytokine signalling, and in turn its expression is tightly regulated. Data from overexpression studies in cell lines suggest that SOCS2 regulates SOCS3 protein degradation, by forming a molecular bridge to an E3 ubiquitin-ligase complex. Whether this regulation is relevant in primary cells is unknown. In this study, we utilized Socs2( - / - ) mice to examine the role of SOCS2 in modulating SOCS3 expression and degradation, and its impact on interleukin-2 (IL-2) and IL-6 signalling in primary haemopoietic cells. Both biochemical and biological analyses demonstrated unperturbed SOCS3 expression and cytokine signalling in the absence of SOCS2. Our results suggest that SOCS2 is not a physiological regulator of SOCS3 expression and action in primary haemopoietic cells.

Erythropoiesis: model systems, molecular regulators, and developmental programs

Human erythropoiesis is a complex multistep developmental process that begins at the level of pluripotent hematopoietic stem cells (HSCs) at bone marrow microenvironment (HSCs niche) and terminates with the production of erythrocytes (RBCs). This review covers the basic and contemporary aspects of erythropoiesis. These include the: (a) cell-lineage restricted pathways of differentiation originated from HSCs and going downward toward the blood cell development; (b) model systems employed to study erythropoiesis in culture (erythroleukemia cell lines and embryonic stem cells) and in vivo (knockout animals: avian, mice, zebrafish, and xenopus); (c) key regulators of erythropoiesis (iron, hypoxia, stress, and growth factors); (d) signaling pathways operating at hematopoietic stem cell niche for homeostatic regulation of self renewal (SCF/c-kit receptor, Wnt, Notch, and Hox) and for erythroid differentiation (HIF and EpoR). Furthermore, this review presents the mechanisms through which transcriptional factors (GATA-1, FOG-1, TAL-1/SCL/MO2/Ldb1/E2A, EKLF, Gfi-1b, and BCL11A) and miRNAs regulate gene pattern expression during erythroid differentiation. New insights regarding the transcriptional regulation of alpha- and beta-globin gene clusters were also presented. Emphasis was also given on (i) the developmental program of erythropoiesis, which consists of commitment to terminal erythroid maturation and hemoglobin production, (two closely coordinated events of erythropoieis) and (ii) the capacity of human embryonic and umbilical cord blood (UCB) stem cells to differentiate and produce RBCs in culture with highly selective media. These most recent developments will eventually permit customized red blood cell production needed for transfusion.

GFI1B controls its own expression binding to multiple sites

BACKGROUND

Transcription factors play essential roles in both normal and malignant hematopoiesis. This is the case for the growth factor independent 1b (GFI1B) transcription factor, which is required for erythroid and megakaryocytic differentiation and over-expressed in leukemic patients and cell lines.

DESIGN AND METHODS

To investigate GFI1B regulation, we searched for multispecies conserved non-coding elements between GFI1B and neighboring genes. We used a formaldehyde-assisted isolation of regulatory elements (FAIRE) assay and DNase1 hypersensitivity to assess the chromatin conformation of these sites. Next, we analyzed transcription factor binding and histone modifications at the GFI1B locus including the conserved non-coding elements by a chromatin immunoprecipitation assay. Finally, we studied the interaction of the GFI1B promoter and the conserved non-coding elements with the chromatin conformation capture technique and used immunofluorescence to evaluate GFI1B levels in individual cells.

RESULTS

We localized several conserved non-coding elements containing multiple erythroid specific transcription factor binding sites at the GFI1B locus. In GFI1B-expressing cells a subset of these conserved non-coding elements and the promoter adopt a close spatial conformation, localize with open chromatin sites, harbor chromatin modifications associated with gene activation and bind multiple transcription factors and co-repressors. Conclusions Our findings indicate that GFI1B regulatory elements behave as activators and repressors. Different protein levels within a cell population suggest that cells must activate and repress GFI1B continuously to control its final level. These data are consistent with a model of GFI1B regulation in which GFI1B binds to its own promoter and to the conserved non-coding elements as its levels rise. This would attract repressor complexes that progressively down-regulate the gene. GFI1B expression would decrease until a stage at which the activating complexes predominate and expression increases.

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.

Expression and functional significance of SOCS-1 and SOCS-3 in astrocytes

Astrocytes play a number of important physiological roles in CNS homeostasis. Inflammation stimulates astrocytes to secrete cytokines and chemokines that guide macrophages/microglia and T cells to sites of injury/inflammation. Herein, we describe how these processes are controlled by the suppressor of cytokine signaling (SOCS) proteins, a family of proteins that negatively regulate adaptive and innate immune responses. In this study, we describe that the immunomodulatory cytokine IFN-beta induces SOCS-1 and SOCS-3 expression in primary astrocytes at the transcriptional level. SOCS-1 and SOCS-3 transcriptional activity is induced by IFN-beta through IFN-gamma activation site (GAS) elements within their promoters. Studies in STAT-1alpha-deficient astrocytes indicate that STAT-1alpha is required for IFN-beta-induced SOCS-1 expression, while STAT-3 small interfering RNA studies demonstrate that IFN-beta-induced SOCS-3 expression relies on STAT-3 activation. Specific small interfering RNA inhibition of IFN-beta-inducible SOCS-1 and SOCS-3 in astrocytes enhances their proinflammatory responses to IFN-beta stimulation, such as heightened expression of the chemokines CCL2 (MCP-1), CCL3 (MIP-1alpha), CCL4 (MIP-1beta), CCL5 (RANTES), and CXCL10 (IP-10), and promoting chemotaxis of macrophages and CD4(+) T cells. These results indicate that IFN-beta induces SOCS-1 and SOCS-3 in primary astrocytes to attenuate its own chemokine-related inflammation in the CNS.

Deletion of Mtg16, a target of t(16;21), alters hematopoietic progenitor cell proliferation and lineage allocation

While a number of DNA binding transcription factors have been identified that control hematopoietic cell fate decisions, only a limited number of transcriptional corepressors (e.g., the retinoblastoma protein [pRB] and the nuclear hormone corepressor [N-CoR]) have been linked to these functions. Here, we show that the transcriptional corepressor Mtg16 (myeloid translocation gene on chromosome 16), which is targeted by t(16;21) in acute myeloid leukemia, is required for hematopoietic progenitor cell fate decisions and for early progenitor cell proliferation. Inactivation of Mtg16 skewed early myeloid progenitor cells toward the granulocytic/macrophage lineage while reducing the numbers of megakaryocyte-erythroid progenitor cells. In addition, inactivation of Mtg16 impaired the rapid expansion of short-term stem cells, multipotent progenitor cells, and megakaryocyte-erythroid progenitor cells that is required under hematopoietic stress/emergency. This impairment appears to be a failure to proliferate rather than an induction of cell death, as expression of c-Myc, but not Bcl2, complemented the Mtg16(-/-) defect.

TLR Ligand-Induced Type I IFNs Affect Thymopoiesis

The interactions between TLRs and their ligands have profound immune modulation properties. Attention has focused mostly on the impact of TLR ligands on peripheral innate and adaptive immunity during viral infections, whereas little impact of TLR activation has been shown on thymic development. Here we show that treatment of murine fetal thymic organ cultures (FTOCs) with TLR3 or TLR7 ligands induced rapid expression of IFN-alpha and -beta mRNA, hallmarks of acute and chronic viral infections. This resulted in an early developmental blockade, increased frequencies of apoptotic cells, and decreased proliferation of thymocytes, which led to an immediate decrease in cellularity. FTOCs infected with vesicular stomatitis virus, known to act through TLR7, were similarly affected. Down-regulation of IL-7R alpha-chain expression, together with an increased expression of suppressor of cytokine signaling-1 and a concomitant decreased expression of the transcriptional regulator growth factor independence 1 were observed in TLR ligands or IFN-treated FTOCs. This indicates a role for these pathways in the observed changes in thymocyte development. Taken together, our data demonstrate that TLR activation and ensuing type I IFN production exert a deleterious effect on T cell development. Because TLR ligands are widely used as vaccine adjuvants, their immunomodulatory actions mediated mainly by IFN-alpha suggested by our results should be taken in consideration.

Age-associated changes in immune and inflammatory responses: impact of vitamin E intervention

Aging is associated with dysregulated immune and inflammatory responses. Declining T cell function is the most significant and best-characterized feature of immunosenescence. Intrinsic changes within T cells and extrinsic factors contribute to the age-associated decline in T cell function. T cell defect seen in aging involves multiple stages from early receptor activation events to clonal expansion. Among extrinsic factors, increased production of T cell-suppressive factor PGE(2) by macrophages (Mphi) is most recognized. Vitamin E reverses an age-associated defect in T cells, particularly naïve T cells. This effect of vitamin E is also reflected in a reduced rate of upper respiratory tract infection in the elderly and enhanced clearance of influenza infection in a rodent model. The T cell-enhancing effect of vitamin E is accomplished via its direct effect on T cells and indirectly by inhibiting PGE(2) production in Mphi. Up-regulated inflammation with aging has attracted increasing attention as a result of its implications in the pathogenesis of diseases. Increased PGE(2) production in old Mphi is a result of increased cyclooxygenase 2 (COX-2) expression, leading to higher COX enzyme activity, which in turn, is associated with the ceramide-induced up-regulation of NF-kappaB. Similar to Mphi, adipocytes from old mice have a higher expression of COX-2 as well as inflammatory cytokines IL-1beta, IL-6, and TNF-alpha, which might also be related to elevated levels of ceramide and NF-kappaB activation. This review will discuss the above age-related immune and inflammatory changes and the effect of vitamin E as nutritional intervention with a focus on the work conducted in our laboratory.

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.

Characterization of the SOCS3 promoter response to prostaglandin E2 in T47D cells

Suppressor of cytokine signaling 3 (SOCS3), a negative regulator of cytokine signaling, is expressed in breast cancer cells where it can modify sensitivity and responsiveness to cytokine signaling through the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathways. Although it is widely accepted that SOCS3 expression is in itself regulated by STATs, we and others have shown that prostaglandins can also up-regulate SOCS3 expression. Here we used T47D breast cancer cells treated with prostaglandin E2 (PGE2) to examine this pathway. T47D cells responded to PGE2 stimulation with a significant increase in SOCS3 mRNA that was independent of de novo protein synthesis. PGE2 stimulation resulted in STAT3 serine and tyrosine phosphorylation, although mutation of either of the two previously characterized STAT response elements on the SOCS3 promoter did not affect SOCS3 promoter activation by PGE2. In addition, overexpression of STAT3 wild-type, constitutively active or dominant-negative constructs did not affect PGE2-induced SOCS3 promoter activation, indicating that STATs are unlikely mediators of this pathway in these cells. PGE2 is a known activator of the cAMP/protein kinase A (PKA) pathway, and in T47D cells, up-regulation of SOCS3 mRNA by PGE2 was abolished by pretreatment with H89, a PKA inhibitor and increased by cAMP and forskolin treatment. Consistent with this, PGE2 treatment increased cAMP response element (CRE)-binding protein serine phosphorylation. However, mutation of the activator protein 1/CRE on the promoter did not affect basal or PGE2-stimulated activation, suggesting a role for cAMP/PKA that is independent of CRE-binding protein binding. Mutation of the GC-rich region of the SOCS3 promoter, a putative Sp1/Sp3 binding site, abolished both basal and PGE2-stimulated activation. Gel-shift assays showed increased complex formation after treatment, and this was inhibited by the addition of an Sp1 antibody or pretreatment with PKA inhibitor. Chromatin immunoprecipitation assay verified Sp1 binding to the promoter in response to PGE2. Sp1 overexpression increased SOCS3 promoter activation, and both basal and PGE2-induced SOCS3 mRNA expression was prevented by mithramycin, an inhibitor of Sp1 DNA binding. Finally, a physiological role for PGE2 was demonstrated with PGE2 pretreatment reducing lipopolysaccharide-induced STAT3 activation. Collectively, this study details a novel mechanism of SOCS3 up-regulation by PGE2 in breast cancer cells that appears to be STAT independent and involve Sp1 binding to the promoter. This process has possible implications for cytokine responsiveness and tumor progression.

GATA-1 and Gfi-1B interplay to regulate Bcl-xL transcription

The induction of Bcl-x(L) is critical for the survival of late proerythroblasts. The erythroid-specific transcriptional network that regulates Bcl-x(L) expression in erythropoiesis remains unclear. The activation of the central erythropoietic transcriptional factor, GATA-1, leads to the early, transient induction of a transcription repressor, Gfi-1B, followed by the late induction of Bcl-x(L) during erythroid maturation in G1ER cells. Chromatin immunoprecipitation assays demonstrated that a constant level of GATA-1 binds to the Bcl-x promoter throughout the entire induction period, while Gfi-1B is transiently associated with the promoter in the early phase. The sustained expression of Gfi-1B abolished GATA-1-induced Bcl-x(L) expression. Here, we present evidence that GATA-1 binds to the noncanonical GATT motif of the Bcl-x promoter for trans-activation. Gfi-1B expressed at increased levels is recruited to the Bcl-x promoter through its association with GATA-1, suppressing Bcl-x(L) transcription. Therefore, the down-regulation of Gfi-1B in the late phase of erythroid maturation is necessary for Bcl-x(L) induction. Furthermore, we show that the inhibition of Bcr-Abl kinase by treatment with imatinib caused the up-regulation of Gfi-1B in K562 cells, where Gfi-1B also cooperated with GATA-1 to repress Bcl-x(L) transcription. Gfi-1B knockdown by RNA interference diminished imatinib-induced apoptosis, while the overexpression of Gfi-1B sensitized K562 cells to arsenic-induced death. These findings illuminate the role of Gfi-1B in GATA-1-mediated transcription in the survival aspect of erythroid cells.

Transcriptional repressor Gfi1 integrates cytokine-receptor signals controlling B-cell differentiation

Hematopoietic stem cell differentiation is specified by cytokines and transcription factors, but the mechanisms controlling instructive and permissive signalling networks are poorly understood. We provide evidence that CLP1-dependent IL7-receptor mediated B cell differentiation is critically controlled by the transcriptional repressor Gfi1. Gfi1-deficient progenitor B cells show global defects in IL7Rα-dependent signal cascades. Consequently, IL7-dependent trophic, proliferative and differentiation-inducing responses of progenitor B cells are perturbed. Gfi1 directly regulates expression levels of IL7Rα and indirectly controls STAT5 signalling via expression of SOCS3. Thus, Gfi1 selectively specifies IL7-dependent development of B cells from CLP1 progenitors, providing clues to the transcriptional networks integrating cytokine signals and lymphoid differentiation.

Gfi-1 plays an important role in IL-2-mediated Th2 cell expansion

Enforced expression of growth factor independent 1 (Gfi-1), a transcription repressor induced by T cell activation and IL-4/Stat6 signaling, strikingly enhances Th2 cell expansion. Using conditionally Gfi1-deficient mice prepared for this study, we show that in vitro or in vivo deletion of this factor dramatically reduces Th2, but not Th1, cell expansion in response to IL-2. Both increased cell apoptosis and reduced cell proliferation resulted from Gfi1 deletion. IL-2-Stat5 signaling was partially reduced in Gfi1-deficient Th2 cells, but overexpression of Stat5 failed to restore normal Th2 expansion in these cells, suggesting that Gfi-1 also functioned downstream of, or in parallel with, Stat5 signaling. Reduced Th2 cell expansion in the absence of Gfi-1 was confirmed by the diminished frequency of IL-4-producing cells when these mice were infected with Schistosoma mansoni.

Gfi1b:green fluorescent protein knock-in mice reveal a dynamic expression pattern of Gfi1b during hematopoiesis that is largely complementary to Gfi1

Gfi1b and Gfi1 are 37- and 55-kDa transcriptional repressors that share common features such as a 20-amino acid (aa) N-terminal SNAG domain, a nonconserved intermediary domain, and 6 highly conserved C-terminal zinc fingers. Both gene loci are under autoregulatory and cross-regulatory feedback control. We have generated a reporter mouse strain by inserting the cDNA for green fluorescent protein (GFP) into the Gfi1b gene locus which allowed us to follow Gfi1b expression during hematopoiesis and lymphopoiesis by measuring green fluorescence. We found highly dynamic expression patterns of Gfi1b in erythroid cells, megakaryocytes, and their progenitor cells (MEPS) where Gfi1 is not detected. Vice versa, Gfi1b could not be found in granulocytes, activated macrophages, or their granulomonocytic precursors (GMPs) or in mature naive or activated lymphocytes where Gfi1 is expressed, suggesting a complementary regulation of both loci during hematopoiesis. However, Gfi1b was found to be up-regulated in early stages of B-cell and in a subset of early T-cell development, where Gfi1 is also present, suggesting that cross-regulation of both loci exists but is cell-type specific.

Age and Vitamin E-Induced Changes in Gene Expression Profiles of T Cells

T cells are vulnerable to age-associated changes. Vitamin E has been shown to improve T cell functions in the old. We studied gene expression profiles of T cells to better understand the underlying mechanisms of age and vitamin E-induced changes in T cell function. Young and old C57BL mice were fed diets containing 30 (control) or 500 (supplemented) ppm of vitamin E for 4 wks. Gene expression profiles of T cells were assessed using microarray analysis with/without anti-CD3/anti-CD28 stimulation. Genes associated with cytokines/chemokines, transcriptional regulation, signal transduction, cell cycle, and apoptosis were significantly up-regulated upon stimulation. Higher SOCS3 and lower growth factor independent 1 (Gfi-1) expression in old T cells may contribute to age-associated decline in proliferation. Higher Gadd45 and lower Bcl2 expression may contribute to increased apoptosis in old T cells. Vitamin E supplementation resulted in higher expression of genes involved in cell cycle regulation (Ccnb2, Cdc2, Cdc6) in old T cells. Vitamin E supplementation resulted in higher up-regulation of IL-2 expression in young and old T cells and lower up-regulation of IL-4 expression in old T cells following stimulation. These findings suggest that aging has significant effects on the expression of genes associated with signal transduction, transcriptional regulation, and apoptosis pathways in T cells, and vitamin E has a significant impact on the expression of genes associated with cell cycle and Th1/Th2 balance in old T cells. Further studies are needed to determine whether these changes are due to the effects of aging at a single-cell level or to the shift in the ratio of naïve:memory T cells with age.

Senseless physically interacts with proneural proteins and functions as a transcriptional co-activator

The zinc-finger transcription factor Senseless is co-expressed with basic helix-loop-helix (bHLH) proneural proteins in Drosophila sensory organ precursors and is required for their normal development. High levels of Senseless synergize with bHLH proteins and upregulate target gene expression,whereas low levels of Senseless act as a repressor in vivo. However, the molecular mechanism for this dual role is unknown. Here, we show that Senseless binds bHLH proneural proteins via its core zinc fingers and is recruited by proneural proteins to their target enhancers to function as a co-activator. Some point mutations in the Senseless zinc-finger region abolish its DNA-binding ability but partially spare the ability of Senseless to synergize with proneural proteins and to induce sensory organ formation in vivo. Therefore, we propose that the structural basis for the switch between the repressor and co-activator functions of Senseless is the ability of its core zinc fingers to interact physically with both DNA and bHLH proneural proteins. As Senseless zinc fingers are ∼90% identical to the corresponding zinc fingers of its vertebrate homologue Gfi1, which is thought to cooperate with bHLH proteins in several contexts, the Senseless/bHLH interaction might be evolutionarily conserved.

Gfi1b alters histone methylation at target gene promoters and sites of gamma-satellite containing heterochromatin

Gfi1b is a 37 kDa nuclear protein with six C2H2 zinc-finger domains that can silence transcription upon binding to specific target gene promoters. Here we show by using a chromatin immunoprecipitation and cloning protocol that Gfi1b also binds to gamma-satellite sequences that mainly occur in pericentric heterochromatin. Immuno-FISH experiments demonstrated that Gfi1b is localized at foci of pericentric heterochromatin identified by DAPI staining. Elevated levels of Gfi1b correlated with increased histone H3 lysine 9 dimethylation at sites neighboring gamma-satellite sequences but also at Gfi1b target gene promoters. In Gfi1b-deficient cells, however, a decrease of histone H3 lysine 9 trimethylation and a loss of heterochromatic structures was observed. Strikingly, we found that Gfi1b binds to both SUV39H1 and G9A histone methyl transferases, which provides a direct link between histone methylation and Gfi1b at heterochromatic and euchromatic sites. We propose that Gfi1b functions in heterochromatin formation and silencing of euchromatic transcription by recruiting histone methyl transferases to either gamma-satellite sequences or specific target gene promoters.

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.

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.

Estrogen receptor-alpha regulates SOCS-3 expression in human breast cancer cells

The suppressor of cytokine signalling (SOCS) protein family negatively regulates cytokine action. In this study, we investigated the effects of estrogen (E2) on SOCS-3 expression in T47D and MCF-7 human breast cancer cells. Real-time PCR analysis of E2-treated T47D cells revealed a ligand and time-dependent increase in of SOCS-3 mRNA levels. Cloning of a 1.7 kb fragment of the human SOCS-3 5' flanking sequence, and subsequent analysis of potential transcription factor-binding sites identified an incomplete ERE motif located -1493 to -1489 upstream of the start site. Transient transfection of the cloned fragment in MCF-7 cells showed that both E2 and genistein treatment caused an increase in reporter gene activity, which was inhibited by co-treatment with ICI 182,780. Chromatin immunoprecipitation analysis revealed an E2 and time-dependent recruitment of ERalpha to the E2 responsive region of the human SOCS-3 promoter. In summary, this study shows that ERalpha directly regulates human SOCS-3 promoter activity in human breast cancer cells, thus modulating cytokine activity.

GATA-1 mediates auto-regulation of Gfi-1B transcription in K562 cells

Gfi-1B (growth factor independence-1B) gene is an erythroid-specific transcription factor, whose expression plays an essential role in erythropoiesis. Our laboratory has previously defined the human Gfi-1B promoter region and shown that GATA-1 mediates erythroid-specific Gfi-1B transcription. By further investigating the regulation of the Gfi-1B promoter, here we report that (i) Gfi-1B transcription is negatively regulated by its own gene product, (ii) GATA-1, instead of Gfi-1B, binds directly to the Gfi-1-like sites in the Gfi-1B promoter and (iii) Gfi-1B suppresses GATA-1-mediated stimulation of Gfi-1B promoter through their protein interaction. These results not only demonstrate that interaction of GATA-1 and Gfi-1B participates in a feedback regulatory pathway in controlling the expression of the Gfi-1B gene, but also provide the first evidence that Gfi-1B can exert its repression function by acting on GATA-1-mediated transcription without direct binding to the Gfi-1 site of the target genes. Based on these data, we propose that this negative auto-regulatory feedback loop is important in restricting the expression level of Gfi-1B, thus optimizing its function in erythroid cells.

The transcriptional repressor Gfi1 controls STAT3-dependent dendritic cell development and function

The mechanisms controlling the differentiation of dendritic cells (DCs) remain largely unknown. Using a transcriptional profiling approach, we identified Gfi1 as a novel critical transcription factor in DC differentiation. Gfi1-/- mice showed a global reduction of myeloid and lymphoid DCs in all lymphoid organs whereas epidermal Langerhans cells were enhanced in number. In vivo, Gfi1-/- DCs showed striking phenotypic and functional alterations such as defective maturation and increased cytokine production. In vitro, Gfi1-/- hematopoietic progenitor cells were unable to develop into DCs. Instead, they differentiated into macrophages, suggesting that Gfi1 is a critical modulator of DC versus macrophage development. Analysis of hematopoietic chimeras and retrovirus-reconstituted hematopoietic progenitor cells established a cell autonomous and nonredundant role for Gfi1 in DC development. The developmental defect of Gfi1-/- progenitor cells was associated with decreased STAT3 activation. In conclusion, we have identified Gfi1 as a critical transcription factor that controls DC versus macrophage development and dissociates DC maturation and activation.

Differential STAT5 signaling by ligand-dependent and constitutively active cytokine receptors

Many leukemia and cancer cells exhibit constitutive activation of STAT5, which was suggested to provide an anti-apoptotic advantage. Transformation of cytokine-dependent hematopoietic cells, such as Ba/F3 cells to autonomous growth and tumorigenicity equally results in selection for constitutive activation of STAT5. We compared STAT5 signaling between erythropoietin(Epo)-dependent cells and cells that were transformed by oncogenic activation of the erythropoietin receptor (EpoR) by coexpression of the gp55-P envelope protein of the spleen focus forming virus or by expression of the R129C constitutively active EpoR mutant. In transformed cells it was mainly STAT5B that was constitutively activated. In contrast, Epo stimulation activated both STAT5A and STAT5B. In transformed cells, chromatin immunoprecipitation (ChIP) showed STAT5 to be physically bound to promoters of STAT5 target genes, such as Bcl(XL), and to be able to promote transactivation of the Bcl(XL) promoter in a constitutive fashion. Sequencing of native sequences after ChIP with anti-STAT5 antibodies in Epo-dependent and -transformed cells indicated that in gp55-transformed cells, STAT5B bound in the chromatin not only to N3 high affinity, but also to low affinity N4 GAS sites. Transactivation for N3 GAS sites in luciferase reporters was specific to gp55 transformation. Because we also found preferential constitutive STAT5B activation after transformation of cells by a truncated form of the G-CSF-R that produces severe neutropenia (Kostmann syndrome) and favors leukemia in humans, we discuss the potential role of STAT5B in oncogenic transformation of hematopoietic cells.

Regulation of the immune system by SOCS family adaptor proteins

Signal transduction via cytokine receptors is regulated by several mechanisms that control initiation, magnitude and duration of the signaling pathways. Cytokine-induced SOCS family adaptors function as feedback inhibitors of cytokine receptor signaling by inhibiting the JAK-STAT signal transduction pathway. Specific gene-targeted mice have unveiled critical, non-overlapping functions for SOCS1 and SOCS3 in lymphocyte development and homeostasis, and in the regulation of macrophage and dendritic cell functions. In this review, we will discuss the structure of SOCS proteins, mechanisms by which they control the JAK-STAT pathway and their role in immune regulation.