STAT5 represses BCL6 expression by binding to a regulatory region frequently mutated in lymphomas

STAT3 induction of miR-146b forms a feedback loop to inhibit the NF-κB to IL-6 signaling axis and STAT3-driven cancer phenotypes

Interleukin-6 (IL-6)-mediated activation of signal transducer and activator of transcription 3 (STAT3) is a mechanism by which chronic inflammation can contribute to cancer and is a common oncogenic event. We discovered a pathway, the loss of which is associated with persistent STAT3 activation in human cancer. We found that the gene encoding the tumor suppressor microRNA miR-146b is a direct STAT3 target gene, and its expression was increased in normal breast epithelial cells but decreased in tumor cells. Methylation of the miR-146b promoter, which inhibited STAT3-mediated induction of expression, was increased in primary breast cancers. Moreover, we found that miR-146b inhibited nuclear factor κB (NF-κB)-dependent production of IL-6, subsequent STAT3 activation, and IL-6/STAT3-driven migration and invasion in breast cancer cells, thereby establishing a negative feedback loop. In addition, higher expression of miR-146b was positively correlated with patient survival in breast cancer subtypes with increased IL6 expression and STAT3 phosphorylation. Our results identify an epigenetic mechanism of crosstalk between STAT3 and NF-κB relevant to constitutive STAT3 activation in malignancy and the role of inflammation in oncogenesis.

Distinct roles of STAT3 and STAT5 in the pathogenesis and targeted therapy of breast cancer

The transcription factors STAT3 and STAT5 play important roles in the regulation of mammary gland function during pregnancy, lactation, and involution. Given that STAT3 and STAT5 regulate genes involved in proliferation and survival, it is not surprising that inappropriate activation of STAT3 and STAT5 occurs commonly in breast cancer. Although these proteins are structurally similar, they have divergent and opposing effects on gene expression and cellular phenotype. Notably, when STAT5 and STAT3 are activated simultaneously, STAT5 has a dominant effect, and leads to decreased proliferation and increased sensitivity to cell death. Similarly, in breast cancer, activation of both STAT5 and STAT3 is associated with longer patient survival than activation of STAT3 alone. Pharmacological inhibitors of STAT3 and STAT5 are being developed for cancer therapy, though understanding the activation state and functional interaction of STAT3 and STAT5 in a patient's tumor may be critical for the optimal use of this strategy.

Targeting STAT5 in hematologic malignancies through inhibition of the bromodomain and extra-terminal (BET) bromodomain protein BRD2

The transcription factor signal STAT5 is constitutively activated in a wide range of leukemias and lymphomas, and drives the expression of genes necessary for proliferation, survival, and self-renewal. Thus, targeting STAT5 is an appealing therapeutic strategy for hematologic malignancies. Given the importance of bromodomain-containing proteins in transcriptional regulation, we considered the hypothesis that a pharmacologic bromodomain inhibitor could inhibit STAT5-dependent gene expression. We found that the small-molecule bromodomain and extra-terminal (BET) bromodomain inhibitor JQ1 decreases STAT5-dependent (but not STAT3-dependent) transcription of both heterologous reporter genes and endogenous STAT5 target genes. JQ1 reduces STAT5 function in leukemia and lymphoma cells with constitutive STAT5 activation, or inducibly activated by cytokine stimulation. Among the BET bromodomain subfamily of proteins, it seems that BRD2 is the critical mediator for STAT5 activity. In experimental models of acute T-cell lymphoblastic leukemias, where activated STAT5 contributes to leukemia cell survival, Brd2 knockdown or JQ1 treatment shows strong synergy with tyrosine kinase inhibitors (TKI) in inducing apoptosis in leukemia cells. In contrast, mononuclear cells isolated form umbilical cord blood, which is enriched in normal hematopoietic precursor cells, were unaffected by these combinations. These findings indicate a unique functional association between BRD2 and STAT5, and suggest that combinations of JQ1 and TKIs may be an important rational strategy for treating leukemias and lymphomas driven by constitutive STAT5 activation.

Active STAT5 regulates T-bet and eomesodermin expression in CD8 T cells and imprints a T-bet-dependent Tc1 program with repressed IL-6/TGF-β1 signaling

In adoptive therapy, CD8 T cells expressing active STAT5 (STAT5CA) transcription factors were found to be superior to unmanipulated counterparts in long-term persistence, capacity to infiltrate autochthonous mouse melanomas, thrive in their microenvironment, and induce their regression. However, the molecular mechanisms sustaining these properties were undefined. In this study, we report that STAT5CA induced sustained expression of genes controlling tissue homing, cytolytic granule composition, type 1 CD8 cytotoxic T cell-associated effector molecules granzyme B(+), IFN-γ(+), TNF-α(+), and CCL3(+), but not IL-2, and transcription factors T-bet and eomesodermin (Eomes). Chromatin immunoprecipitation sequencing analyses identified the genes possessing regulatory regions to which STAT5 bound in long-term in vivo maintained STAT5CA-expressing CD8 T cells. This analysis identified 34% of the genes differentially expressed between STAT5CA-expressing and nonexpressing effector T cells as direct STAT5CA target genes, including those encoding T-bet, Eomes, and granzyme B. Additionally, genes encoding the IL-6R and TGFbRII subunits were stably repressed, resulting in dampened IL-17-producing CD8 T cell polarization in response to IL-6 and TGF-β1. The absence of T-bet did not affect STAT5CA-driven accumulation of the T cells in tissue or their granzyme B expression but restored IL-2 secretion and IL-6R and TGFbRII expression and signaling, as illustrated by IL-17 induction. Therefore, concerted STAT5/T-bet/Eomes regulation controls homing, long-term maintenance, recall responses, and resistance to polarization towards IL-17-producing CD8 T cells while maintaining expression of an efficient type 1 CD8 cytotoxic T cell program (granzyme B(+), IFN-γ(+)).

Effect of STAT5 silenced by siRNA on proliferation apoptosis and invasion of esophageal carcinoma cell line Eca-109

Background

STAT is the backward position of cytokine and growth factor receptors in the nucleus, STAT dimers could bind to DNA and induce transcription of specific target genes. Several lines of evidence support the important roles of STAT, especially STAT5, in carcinogenesis. The overexpression of STAT 5 is related to the differentiation and apoptosis of tumor cells. However, the role of STAT5 in esophageal squamous cell carcinoma remains unclear.

Methods

The siRNA vectors aiming to STAT5 gene were constructed. STAT5 siRNA was transfected into Eca-109 cells by Lipofectamine™2000. Expression of STAT5、Bcl-2 and Cyclin D1 were analyzed by Western blot and RT-PCR. Eca-109 cells proliferation was determined by MTT. Eca-109 cell cycle and apoptosis were detected by the flow cytometry. Boyden chamber was used to evaluate the invasion and metastasis capabilities of Eca-109 cells.

Results

The double strands oligonucleotide of siRNA aiming to STAT5 was successfully cloned into the pRNAT-U6.1 vector, and the target sequence coincided with the design. RT-PCR and Western blotting detection demonstrated that the expression levels of STAT5、Bcl-2 and Cyclin D1 gene were obviously decreased in Eca-109 cells transfected with STAT5 siRNA. STAT5 siRNA could suppress the proliferation of Eca-109 cells. The proportion of S and G2/M period frequency was significantly decreased (p < 0.05). The proportion of G0/G1 period frequency was significantly increased (p < 0.05). The average amount of cells penetrating Matrigel was significantly decreased (p < 0.05).

Conclusions

STAT5 silenced by siRNA could induce the apoptosis and suppress the proliferation、invasion and metastasis of esophageal carcinoma cell line Eca-109, which indicated STAT5 might be a novel therapeutic strategy for the human ESCC.

Virtual slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1351913072103000

New effector functions and regulatory mechanisms of BCL6 in normal and malignant lymphocytes

The BCL6 oncogenic repressor is a master regulator of humoral immunity and B-cell lymphoma survival. Whereas much research has focused on its regulation and function in germinal center B-cells, its role in other mature lymphoid cell compartments is less clear. A novel role for BCL6 in follicular T helper cell development was recently uncovered. The latest discoveries reveal that BCL6 is also an important regulator of other specialized helper T-cell subsets within germinal centers, pre-germinal center events, and peripheral T-cell effector functions. Here, we review newly discovered roles for BCL6 in lymphocyte subsets residing within and outside of germinal centers, and discuss their implications with respect to the molecular mechanisms of BCL6 regulation and potential links to B and T-cell lymphomas.

STAT5 outcompetes STAT3 to regulate the expression of the oncogenic transcriptional modulator BCL6

Inappropriate activation of the transcription factors STAT3 and STAT5 has been shown to drive cancer pathogenesis through dysregulation of genes involved in cell survival, growth, and differentiation. Although STAT3 and STAT5 are structurally related, they can have opposite effects on key genes, including BCL6. BCL6, a transcriptional repressor, has been shown to be oncogenic in diffuse large B cell lymphoma. BCL6 also plays an important role in breast cancer pathogenesis, a disease in which STAT3 and STAT5 can be activated individually or concomitantly. To determine the mechanism by which these oncogenic transcription factors regulate BCL6 transcription, we analyzed their effects at the levels of chromatin and gene expression. We found that STAT3 increases expression of BCL6 and enhances recruitment of RNA polymerase II phosphorylated at a site associated with transcriptional initiation. STAT5, in contrast, represses BCL6 expression below basal levels and decreases the association of RNA polymerase II at the gene. Furthermore, the repression mediated by STAT5 is dominant over STAT3-mediated induction. STAT5 exerts this effect by displacing STAT3 from one of the two regulatory regions to which it binds. These findings may underlie the divergent biology of breast cancers containing activated STAT3 alone or in conjunction with activated STAT5.

Impact of CUX2 on the female mouse liver transcriptome: activation of female-biased genes and repression of male-biased genes

The growth hormone-regulated transcription factors STAT5 and BCL6 coordinately regulate sex differences in mouse liver, primarily through effects in male liver, where male-biased genes are upregulated and many female-biased genes are actively repressed. Here we investigated whether CUX2, a highly female-specific liver transcription factor, contributes to an analogous regulatory network in female liver. Adenoviral overexpression of CUX2 in male liver induced 36% of female-biased genes and repressed 35% of male-biased genes. In female liver, CUX2 small interfering RNA (siRNA) preferentially induced genes repressed by adenovirus expressing CUX2 (adeno-CUX2) in male liver, and it preferentially repressed genes induced by adeno-CUX2 in male liver. CUX2 binding in female liver chromatin was enriched at sites of male-biased DNase hypersensitivity and at genomic regions showing male-enriched STAT5 binding. CUX2 binding was also enriched near genes repressed by adeno-CUX2 in male liver or induced by CUX2 siRNA in female liver but not at genes induced by adeno-CUX2, indicating that CUX2 binding is preferentially associated with gene repression. Nevertheless, direct CUX2 binding was seen at several highly female-specific genes that were positively regulated by CUX2, including A1bg, Cyp2b9, Cyp3a44, Tox, and Trim24. CUX2 expression and chromatin binding were high in immature male liver, where repression of adult male-biased genes and expression of adult female-biased genes are common, suggesting that the downregulation of CUX2 in male liver at puberty contributes to the developmental changes establishing adult patterns of sex-specific gene expression.

The origins, function, and regulation of T follicular helper cells

The generation of high-affinity antibodies (Abs) plays a critical role in the neutralization and clearance of pathogens and subsequent host survival after natural infection with a variety of microorganisms. Most currently available vaccines rely on the induction of long-lived protective humoral immune responses by memory B cells and plasma cells, underscoring the importance of Abs in host protection. Ab responses against most antigens (Ags) require interactions between B cells and CD4(+) T helper cells, and it is now well recognized that T follicular helper cells (Tfh) specialize in providing cognate help to B cells and are fundamentally required for the generation of T cell-dependent B cell responses. Perturbations in the development and/or function of Tfh cells can manifest as immunopathologies, such as immunodeficiency, autoimmunity, and malignancy. Unraveling the cellular and molecular requirements underlying Tfh cell formation and maintenance will help to identify molecules that could be targeted for the treatment of immunological diseases that are characterized by insufficient or excessive Ab responses.

Rac1 signalling modulates a STAT5/BCL-6 transcriptional switch on cell-cycle-associated target gene promoters

Gene expression depends on binding of transcriptional regulators to gene promoters, a process controlled by signalling pathways. The transcriptional repressor B-cell lymphoma (BCL)-6 downregulates genes involved in cell-cycle progression and becomes inactivated following phosphorylation by the Rac1 GTPase-activated protein kinase PAK1. Interestingly, the DNA motifs recognized by BCL-6 and signal transducers and activators of transcription 5 (STAT5) are similar. Because STAT5 stimulation in epithelial cells can also be triggered by Rac1 signalling, we asked whether both factors have opposing roles in transcriptional regulation and whether Rac1 signalling may coordinate a transcription factor switch. We used chromatin immunoprecipitation to show that active Rac1 promotes release of the repressor BCL-6 while increasing binding of STAT5A to a BCL-6-regulated reporter gene. We further show in colorectal cell lines that the endogenous activation status of the Rac1/PAK1 pathway correlated with the phosphorylation status of BCL-6 and STAT5A. Three cellular genes (cyclin D2, p15^INK4B, small ubiquitin-like modifier 1) were identified to be inversely regulated by BCL-6 and STAT5A and responded to Rac1 signalling with increased expression and corresponding changes in promoter occupancy. Together, our data show that Rac1 signalling controls a group of target genes that are repressed by BCL-6 and activated by STAT5A, providing novel insights into the modulation of gene transcription by GTPase signalling.

The Drosophila BCL6 homolog Ken and Barbie promotes somatic stem cell self-renewal in the testis niche

Stem cells sustain tissue regeneration by their remarkable ability to replenish the stem cell pool and to generate differentiating progeny. Signals from local microenvironments, or niches, control stem cell behavior. In the Drosophila testis, a group of somatic support cells called the hub creates a stem cell niche by locally activating the Janus Kinase-Signal Transducer and Activator of Transcription (JAK-STAT) pathway in two adjacent types of stem cells: germline stem cells (GSCs) and somatic cyst stem cells (CySCs). Here, we find that ken and barbie (ken) is autonomously required for the self-renewal of CySCs but not GSCs. Furthermore, Ken misexpression in the CySC lineage induces the cell-autonomous self-renewal of somatic cells as well as the nonautonomous self-renewal of germ cells outside the niche. Thus, Ken, like Stat92E and its targets ZFH1 (Leatherman and Dinardo, 2008) and Chinmo (Flaherty et al., 2010), is necessary and sufficient for CySC renewal. However, ken is not a JAK-STAT target in the testis, but instead acts in parallel to Stat92E to ensure CySC self-renewal. Ken represses a subset of Stat92E targets in the embryo (Arbouzova et al., 2006) suggesting that Ken maintains CySCs by repressing differentiation factors. In support of this hypothesis, we find that the global JAK-STAT inhibitor Protein tyrosine phosphatase 61F (Ptp61F) is a JAK-STAT target in the testis that is repressed by Ken. Together, our work demonstrates that Ken has an important role in the inhibition of CySC differentiation. Studies of ken may inform our understanding of its vertebrate orthologue B-Cell Lymphoma 6 (BCL6) and how misregulation of this oncogene leads to human lymphomas.

The role of STAT5 in lymphocyte development and transformation

STAT5 plays a crucial role in B and T lymphocyte development. However, whether STAT5 primarily plays a role as a permissive factor, involved in lymphocyte survival, or an instructive factor, involved in lymphocyte differentiation, has been unclear. In addition, while STAT5 has been suggested to act as a transcriptional repressor, the mechanism by which it represses transcription was undefined. Recent reports have begun to shed new light on these roles for STAT5 in lymphocyte development, transcriptional repression, and leukemic transformation.

STAT5 is a potent negative regulator of TFH cell differentiation

Interleukin 2, STAT5, and Blimp-1 work together to suppress differentiation of follicular helper T cells in mice.

Follicular helper T cells (T_FH cells) constitute the CD4⁺ T cell subset that is specialized to provide help to germinal center (GC) B cells and, consequently, mediate the development of long-lived humoral immunity. T_FH cell differentiation is driven by the transcription factor Bcl6, and recent studies have identified cytokine and cell–cell signals that drive Bcl6 expression. However, although T_FH dysregulation is associated with several major autoimmune diseases, the mechanisms underlying the negative regulation of T_FH cell differentiation are poorly understood. In this study, we show that STAT5 inhibits T_FH cell differentiation and function. Constitutive STAT5 signaling in activated CD4⁺ T cells selectively blocked T_FH cell differentiation and GCs, and IL-2 signaling was a primary inducer of this pathway. Conversely, STAT5-deficient CD4⁺ T cells (mature STAT5^fl/fl CD4⁺ T cells transduced with a Cre-expressing vector) rapidly up-regulated Bcl6 expression and preferentially differentiated into T_FH cells during T cell priming in vivo. STAT5 signaling failed to inhibit T_FH cell differentiation in the absence of the transcription factor Blimp-1, a direct repressor of Bcl6 expression and T_FH cell differentiation. These results demonstrate that IL-2, STAT5, and Blimp-1 collaborate to negatively regulate T_FH cell differentiation.

Transcriptional and epigenetic control of T helper cell specification: molecular mechanisms underlying commitment and plasticity

T helper cell differentiation occurs in the context of the extracellular cytokine milieu evoked by diverse microbes and other pathogenic stimuli along with T cell receptor stimulation. The culmination of these signals results in specification of T helper lineages, which occurs through the combinatorial action of multiple transcription factors that establish distinctive transcriptomes. In this manner, inducible, but constitutively active, master regulators work in conjunction with factors such as the signal transducer and activator of transcriptions (STATs) that sense the extracellular environment. The acquisition of a distinctive transcriptome also depends on chromatin modifications that impact key cis elements as well as the changes in global genomic organization. Thus, signal transduction and epigenetics are linked in these processes of differentiation. In this review, recent advances in understanding T helper lineage specification and deciphering the action of transcription factors are summarized with emphasis on comprehensive views of the dynamic T cell epigenome.

Emerging cellular networks for regulation of T follicular helper cells

The cellular networks that regulate humoral immune responses have been a focus of research over the past three decades. Studies have shown that inhibition of immune responses can be attributed to both suppressor T cells and B cells. More recently, T follicular helper (Tfh) cells have been identified as a target of immune regulation. Tfh cells are a subset of highly activated T helper cells specialized for providing cognate help to B cells during germinal center reactions. In this review, we describe emerging evidence for cellular networks that alter Tfh cell phenotype and function and regulate antibody production during the germinal center reaction. We discuss how these new findings influence our understanding of Tfh cells.

Type I interferons directly down-regulate BCL-6 in primary and transformed germinal center B cells: differential regulation in B cell lines derived from endemic or sporadic Burkitt's lymphoma

Type I interferons (IFN) exert multiple effects on both the innate and adaptive immune system in addition to their antiviral and antiproliferative activities. Little is known, however about the direct effects of type I IFNs on germinal center (GC) B cells, the central components of adaptive B cell responses. We used Burkitt's lymphoma (BL) lines, as a model system of normal human GC B cells, to examine the effect of type I IFNs on the expression of BCL-6, the major regulator of the GC reaction. We show that type I IFNs, but not IFNγ, IL-2 and TNFα rapidly down-regulate BCL-6 protein and mRNA expression, in cell lines derived from endemic, but not from sporadic BL. IFNα-induced down-regulation is specific for BCL-6, independent of Epstein-Barr virus and is not accompanied by IRF-4 up-regulation. IFNα-induced BCL-6 mRNA down-regulation does not require de novo protein synthesis and is specifically inhibited by piceatannol. The proteasome inhibitor MG132 non-specifically prevents, while inhibitors of alternate type I IFN signaling pathways do not inhibit IFNα-induced BCL-6 protein downregulation. We validate our results with showing that IFNα rapidly down-regulates BCL-6 mRNA in purified mouse normal GC B cells. Our results identify type I IFNs as the first group of cytokines that can down-regulate BCL-6 expression directly in GC B cells.

BCL6 enables Ph+ acute lymphoblastic leukaemia cells to survive BCR-ABL1 kinase inhibition

Tyrosine kinase inhibitors (TKI) are widely used to treat patients with leukemia driven by BCR-ABL11 and other oncogenic tyrosine kinases2,3. Recent efforts focused on the development of more potent TKI that also inhibit mutant tyrosine kinases4,5. However, even effective TKI typically fail to eradicate leukemia-initiating cells6–8, which often cause recurrence of leukemia after initially successful treatment. Here we report on the discovery of a novel mechanism of drug-resistance, which is based on protective feedback signaling of leukemia cells in response to TKI-treatment. We identified BCL6 as a central component of this drug-resistance pathway and demonstrate that targeted inhibition of BCL6 leads to eradication of drug-resistant and leukemia-initiating subclones.

BCL6 is a known proto-oncogene that is often translocated in diffuse large B cell lymphoma (DLBCL)9. In response to TKI-treatment, BCR-ABL1 acute lymphoblastic leukemia (ALL) cells upregulate BCL6 protein levels by ~90-fold, i.e. to similar levels as in DLBCL (Fig. 1a). Upregulation of BCL6 in response to TKI-treatment represents a novel defense mechanism, which enables leukemia cells to survive TKI-treatment: Previous work suggested that TKI-mediated cell death is largely p53-independent. Here we demonstrate that BCL6 upregulation upon TKI-treatment leads to transcriptional inactivation of the p53 pathway. BCL6-deficient leukemia cells fail to inactivate p53 and are particularly sensitive to TKI-treatment. BCL6^−/− leukemia cells are poised to undergo cellular senescence and fail to initiate leukemia in serial transplant recipients. A combination of TKI-treatment and a novel BCL6 peptide inhibitor markedly increased survival of NOD/SCID mice xenografted with patient-derived BCR-ABL1 ALL cells. We propose that dual targeting of oncogenic tyrosine kinases and BCL6-dependent feedback (Supplementary Fig. 1) represents a novel strategy to eradicate drug-resistant and leukemia-initiating subclones in tyrosine kinase-driven leukemia.

Genomic views of STAT function in CD4+ T helper cell differentiation

Signal transducer and activator of transcription (STAT) proteins are well known for their essential roles in transmitting cytokine-mediated signals and specifying T helper (T(H)) cell differentiation. Recent technological advances have revealed that STAT proteins have broad and complex roles in gene regulation and epigenetic control, including important roles as functional repressors. However, the challenge of how to link signal transduction, nucleosome biology and gene regulation remains. The relevance of tackling this problem is highlighted by genome-wide association studies that link cytokine signalling and STATs to various autoimmune or immune deficiency disorders. Defining exactly how extrinsic signals control the specification and plasticity of T(H) cells will provide important insights and perhaps therapeutic opportunities in these diseases.

The transcription factor B-cell lymphoma (BCL)-6 modulates pancreatic {beta}-cell inflammatory responses

Type 1 diabetes is a chronic autoimmune disease with a strong inflammatory component. We have previously shown that expression of the transcriptional repressor B-cell lymphoma (BCL)-6 is very low in pancreatic β-cells, which may favor prolonged proinflammatory responses after exposure to the cytokines IL-1β and interferon γ. Here we investigated whether cytokine-induced inflammation and apoptosis can be prevented in β-cells by BCL-6 expression using plasmid, prolactin, and adenoviral approaches. The induction of mild or abundant BCL-6 expression in β-cells by prolactin or an adenoviral BCL-6 expression construct, respectively, reduced cytokine-induced inflammatory responses in a dose-dependent manner through inhibition of nuclear factor-κB activation. BCL-6 decreased Fas and inducible nitric oxide synthase expression and nitric oxide production, but it inhibited the expression of the antiapoptotic proteins Bcl-2 and JunB while increasing the expression of the proapoptotic death protein 5. The net result of these opposite effects was an augmentation of β-cell apoptosis. In conclusion, BCL-6 expression tones down the unrestrained cytokine-induced proinflammatory response of β-cells but it also favors gene networks leading to apoptosis. This suggests that cytokine-induced proinflammatory and proapoptotic signals can be dissociated in β-cells. Further understanding of these pathways may open new possibilities to improve β-cell survival in early type 1 diabetes or after transplantation.

The STAT5 inhibitor pimozide decreases survival of chronic myelogenous leukemia cells resistant to kinase inhibitors

The transcription factor STAT5 is an essential mediator of the pathogenesis of chronic myelogenous leukemia (CML). In CML, the BCR/ABL fusion kinase causes the constitutive activation of STAT5, thereby driving the expression of genes promoting survival. BCR/ABL kinase inhibitors have become the mainstay of therapy for CML, although CML cells can develop resistance through mutations in BCR/ABL. To overcome this problem, we used a cell-based screen to identify drugs that inhibit STAT-dependent gene expression. Using this approach, we identified the psychotropic drug pimozide as a STAT5 inhibitor. Pimozide decreases STAT5 tyrosine phosphorylation, although it does not inhibit BCR/ABL or other tyrosine kinases. Furthermore, pimozide decreases the expression of STAT5 target genes and induces cell cycle arrest and apoptosis in CML cell lines. Pimozide also selectively inhibits colony formation of CD34(+) bone marrow cells from CML patients. Importantly, pimozide induces similar effects in the presence of the T315I BCR/ABL mutation that renders the kinase resistant to presently available inhibitors. Simultaneously inhibiting STAT5 with pimozide and the kinase inhibitors imatinib or nilotinib shows enhanced effects in inhibiting STAT5 phosphorylation and in inducing apoptosis. Thus, targeting STAT5 may be an effective strategy for the treatment of CML and other myeloproliferative diseases.

T cells and follicular dendritic cells in germinal center B-cell formation and selection

Germinal centers (GCs) are specialized microenvironments formed after infection where activated B cells can mutate their B-cell receptors to undergo affinity maturation. A stringent process of selection allows high affinity, non-self-reactive B cells to become long-lived memory B cells and plasma cells. While the precise mechanism of selection is still poorly understood, the last decade has advanced our understanding of the role of T cells and follicular dendritic cells (FDCs) in GC B-cell formation and selection. T cells and non-T-cell-derived CD40 ligands on FDCs are essential for T-dependent (TD) and T-independent GC formation, respectively. TD-GC formation requires Bcl-6-expressing T cells capable of signaling through SAP, which promotes formation of stable T:B conjugates. By contrast, differentiation of B blasts along the extrafollicular pathway is less dependent on SAP. T-follicular helper (Tfh) cell-derived CD40L, interleukin-21, and interleukin-4 play important roles in GC B-cell proliferation, survival, and affinity maturation. A role for FDC-derived integrin signals has also emerged: GC B cells capable of forming an immune synapse with FDCs have a survival advantage. This emerges as a powerful mechanism to ensure death of B cells that bind self-reactive antigen, which would not normally be presented on FDCs.

BCL6 repression of EP300 in human diffuse large B cell lymphoma cells provides a basis for rational combinatorial therapy

B cell lymphoma 6 (BCL6), which encodes a transcriptional repressor, is a critical oncogene in diffuse large B cell lymphomas (DLBCLs). Although a retro-inverted BCL6 peptide inhibitor (RI-BPI) was recently shown to potently kill DLBCL cells, the underlying mechanisms remain unclear. Here, we show that RI-BPI induces a particular gene expression signature in human DLBCL cell lines that included genes associated with the actions of histone deacetylase (HDAC) and Hsp90 inhibitors. BCL6 directly repressed the expression of p300 lysine acetyltransferase (EP300) and its cofactor HLA-B-associated transcript 3 (BAT3). RI-BPI induced expression of p300 and BAT3, resulting in acetylation of p300 targets including p53 and Hsp90. Induction of p300 and BAT3 was required for the antilymphoma effects of RI-BPI, since specific blockade of either protein rescued human DLBCL cell lines from the BCL6 inhibitor. Consistent with this, combination of RI-BPI with either an HDAC inhibitor (HDI) or an Hsp90 inhibitor potently suppressed or even eradicated established human DLBCL xenografts in mice. Furthermore, HDAC and Hsp90 inhibitors independently enhanced RI-BPI killing of primary human DLBCL cells in vitro. We also show that p300-inactivating mutations occur naturally in human DLBCL patients and may confer resistance to BCL6 inhibitors. Thus, BCL6 repression of EP300 provides a basis for rational targeted combinatorial therapy for patients with DLBCL.

The role of Stat5 transcription factors as tumor suppressors or oncogenes

Stat5 is constitutively activated in many human cancers affecting the expression of cell proliferation and cell survival controlling genes. These oncogenic functions of Stat5 have been elegantly reproduced in mouse models. Aberrant Stat5 activity induces also mitochondrial dysfunction and reactive oxygen species leading to DNA damage. Although DNA damage can stimulate tumorigenesis, it can also prevent it. Stat5 can inhibit tumor progression like in the liver and it is a tumor suppressor in fibroblasts. Stat5 proteins are able to regulate cell differentiation and senescence activating the tumor suppressors SOCS1, p53 and PML. Understanding the context dependent regulation of tumorigenesis through Stat5 function will be central to understand proliferation, survival, differentiation or senescence of cancer cells.

Microtubule-targeted chemotherapeutic agents inhibit signal transducer and activator of transcription 3 (STAT3) signaling

The transcription factor signal transducer and activator of transcription 3 (STAT3) is inappropriately activated in the majority of breast tumors, especially in aggressive and invasive ones. In addition to driving the expression of genes promoting malignancy, STAT3 associates with tubulin and can promote cell migration. Because microtubule-targeted drugs are among the most active agents used in the treatment of breast cancer, we examined whether microtubule-based chemotherapy modulates STAT3 activity. When treated with paclitaxel or vinorelbine, breast cancer cells with constitutive activation of STAT3 display a loss of STAT3 phosphorylation, and paclitaxel disrupts the interaction of STAT3 with tubulin. Paclitaxel also inhibits cytokine-induced STAT3 activation. This effect is specific for microtubule-targeted agents, because other chemotherapeutic drugs, such as doxorubicin, have no effect on STAT3. The loss of STAT3 tyrosine phosphorylation is also reflected in an inhibition of expression of STAT3 target genes. This effect is not restricted to breast cancer, because similar effects are also seen in ovarian cancer and prostate cancer cells. Thus, in addition to their role in disrupting microtubule function, microtubule-targeted agents also suppress STAT3 signaling. This may be an important component of their activity, raising the possibility that microtubule targeted therapy may be particularly effective in tumors characterized by STAT3 activation.

Erythropoietin receptor response circuits

PURPOSE OF REVIEW

In 1985-1989, erythropoietin (EPO), its receptor (EPOR), and janus kinase 2 were cloned; established to be essential for definitive erythropoiesis; and initially intensely studied. Recently, new impetus, tools, and model systems have emerged to re-examine EPO/EPOR actions, and are addressed in this review. Impetus includes indications that EPO affects significantly more than standard erythroblast survival pathways, the development of novel erythropoiesis-stimulating agents, increasing evidence for EPO/EPOR cytoprotection of ischemically injured tissues, and potential EPO-mediated worsening of tumorigenesis.

RECENT FINDINGS

New findings are reviewed in four functional contexts: (pro)erythroblast survival mechanisms, new candidate EPO/EPOR effects on erythroid cell development and new EPOR responses, EPOR downmodulation and trafficking, and novel erythropoiesis-stimulating agents.

SUMMARY

As Current Opinion, this monograph seeks to summarize, and provoke, new EPO/EPOR action concepts. Specific problems addressed include: beyond (and before) BCL-XL, what key survival factors are deployed in early-stage proerythroblasts? Are distinct EPO/EPOR signals transduced in stage-selective fashions? Is erythroblast proliferation also modulated by EPO/EPOR signals? What functions are subserved by new noncanonical EPO/EPOR response factors (e.g. podocalyxin like-1, tribbles 3, reactive oxygen species, and nuclear factor kappa B)? What key regulators mediate EPOR inhibition and trafficking? And for emerging erythropoiesis-stimulating agents, to what extent do activities parallel EPOs (or differ in advantageous, potentially complicating ways, or both)?

Cyclophilin B as a co-regulator of prolactin-induced gene expression and function in breast cancer cells

The effects of prolactin (PRL) during the pathogenesis of breast cancer are mediated in part though Stat5 activity enhanced by its interaction with its transcriptional inducer, the prolyl isomerase cyclophilin B (CypB). We have demonstrated that knockdown of CypB decreases cell growth, proliferation, and migration, and CypB expression is associated with malignant progression of breast cancer. In this study, we examined the effect of CypB knockdown on PRL signaling in breast cancer cells. CypB knockdown with two independent siRNAs was shown to impair PRL-induced reporter expression in breast cancer cell line. cDNA microarray analysis was performed on these cells to assess the effect of CypB reduction, and revealed a significant decrease in PRL-induced endogenous gene expression in two breast cancer cell lines. Parallel functional assays revealed corresponding alterations of both anchorage-independent cell growth and cell motility of breast cancer cells. Our results demonstrate that CypB expression levels significantly modulate PRL-induced function in breast cancer cells ultimately resulting in enhanced levels of PRL-responsive gene expression, cell growth, and migration. Given the increasingly appreciated role of PRL in the pathogenesis of breast cancer, the actions of CypB detailed here are of biological significance.

BCL6 is critical for the development of a diverse primary B cell repertoire

BCL6 protects germinal center (GC) B cells against DNA damage-induced apoptosis during somatic hypermutation and class-switch recombination. Although expression of BCL6 was not found in early IL-7-dependent B cell precursors, we report that IL-7Ralpha-Stat5 signaling negatively regulates BCL6. Upon productive VH-DJH gene rearrangement and expression of a mu heavy chain, however, activation of pre-B cell receptor signaling strongly induces BCL6 expression, whereas IL-7Ralpha-Stat5 signaling is attenuated. At the transition from IL-7-dependent to -independent stages of B cell development, BCL6 is activated, reaches expression levels resembling those in GC B cells, and protects pre-B cells from DNA damage-induced apoptosis during immunoglobulin (Ig) light chain gene recombination. In the absence of BCL6, DNA breaks during Ig light chain gene rearrangement lead to excessive up-regulation of Arf and p53. As a consequence, the pool of new bone marrow immature B cells is markedly reduced in size and clonal diversity. We conclude that negative regulation of Arf by BCL6 is required for pre-B cell self-renewal and the formation of a diverse polyclonal B cell repertoire.

Stochastic modeling of B lymphocyte terminal differentiation and its suppression by dioxin

Background

Upon antigen encounter, naïve B lymphocytes differentiate into antibody-secreting plasma cells. This humoral immune response is suppressed by the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other dioxin-like compounds, which belong to the family of aryl hydrocarbon receptor (AhR) agonists.

Results

To achieve a better understanding of the immunotoxicity of AhR agonists and their associated health risks, we have used computer simulations to study the behavior of the gene regulatory network underlying B cell terminal differentiation. The core of this network consists of two coupled double-negative feedback loops involving transcriptional repressors Bcl-6, Blimp-1, and Pax5. Bifurcation analysis indicates that the feedback network can constitute a bistable system with two mutually exclusive transcriptional profiles corresponding to naïve B cells and plasma cells. Although individual B cells switch to the plasma cell state in an all-or-none fashion when stimulated by the polyclonal activator lipopolysaccharide (LPS), stochastic fluctuations in gene expression make the switching event probabilistic, leading to heterogeneous differentiation response among individual B cells. Moreover, stochastic gene expression renders the dose-response behavior of a population of B cells substantially graded, a result that is consistent with experimental observations. The steepness of the dose response curve for the number of plasma cells formed vs. LPS dose, as evaluated by the apparent Hill coefficient, is found to be inversely correlated to the noise level in Blimp-1 gene expression. Simulations illustrate how, through AhR-mediated repression of the AP-1 protein, TCDD reduces the probability of LPS-stimulated B cell differentiation. Interestingly, stochastic simulations predict that TCDD may destabilize the plasma cell state, possibly leading to a reversal to the B cell phenotype.

Conclusion

Our results suggest that stochasticity in gene expression, which renders a graded response at the cell population level, may have been exploited by the immune system to launch humoral immune response of a magnitude appropriately tuned to the antigen dose. In addition to suppressing the initiation of the humoral immune response, dioxin-like compounds may also disrupt the maintenance of the acquired immunity.

Prolactin inhibits BCL6 expression in breast cancer through a Stat5a-dependent mechanism

BCL6 is a transcriptional repressor that recognizes DNA target sequences similar to those recognized by signal transducer and activator of transcriptions 5 (Stat5). BCL6 disrupts differentiation of breast epithelia, is downregulated during lactation, and is upregulated in poorly differentiated breast cancer. In contrast, Stat5a mediates prolactin-induced differentiation of mammary epithelia, and loss of Stat5 signaling in human breast cancer is associated with undifferentiated histology and poor prognosis. Here, we identify the mammary cell growth factor prolactin as a potent suppressor of BCL6 protein expression in human breast cancer through a mechanism that requires Stat5a, but not prolactin-activated Stat5b, MEK-ERK, or PI3K-AKT pathways. Prolactin rapidly suppressed BCL6 mRNA in T47D, MCF7, ZR75.1, and SKBr3 breast cancer cell lines, followed by prolonged reduction of BCL6 protein levels within 3 hours. Prolactin suppression of BCL6 was enhanced by overexpression of Stat5a but not Stat5b, was mimicked by constitutively active Stat5a, but did not require the transactivation domain of Stat5a. Stat5 chromatin immunoprecipitation demonstrated physical interaction with a BCL6 gene regulatory region, and BCL6 transcript repression required histone deacetylase activity based on sensitivity to trichostatin A. Functionally, BCL6 overexpression disrupted prolactin induction of Stat5 reporter genes. Prolactin suppression of BCL6 was extended to xenotransplant tumors in nude mice in vivo and to freshly isolated human breast cancer explants ex vivo. Quantitative immunohistochemistry revealed elevated BCL6 in high-grade and metastatic breast cancer compared with ductal carcinoma in situ and nonmalignant breast, and cellular BCL6 protein levels correlated negatively with nuclear Stat5a (r = -0.52; P < 0.001) but not with Stat5b. Loss of prolactin-Stat5a signaling and concomitant upregulation of BCL6 may represent a regulatory switch facilitating undifferentiated histology and poor prognosis of breast cancer.

Integrated biochemical and computational approach identifies BCL6 direct target genes controlling multiple pathways in normal germinal center B cells

BCL6 is a transcriptional repressor required for mature B-cell germinal center (GC) formation and implicated in lymphomagenesis. BCL6's physiologic function is only partially known because the complete set of its targets in GC B cells has not been identified. To address this issue, we used an integrated biochemical-computational-functional approach to identify BCL6 direct targets in normal GC B cells. This approach includes (1) identification of BCL6-bound promoters by genome-wide chromatin immunoprecipitation, (2) inference of transcriptional relationships by the use of a regulatory network reverse engineering approach (ARACNe), and (3) validation of physiologic relevance of the candidate targets down-regulated in GC B cells. Our approach demonstrated that a large set of promoters (> 4000) is physically bound by BCL6 but that only a fraction of them is repressed in GC B cells. This set of 1207 targets identifies several cellular functions directly controlled by BCL6 during GC development, including activation, survival, DNA-damage response, cell cycle arrest, cytokine signaling, Toll-like receptor signaling, and differentiation. These results define a broad role of BCL6 in preventing centroblasts from responding to signals leading to exit from the GC before they complete the phase of proliferative expansion and of antibody affinity maturation.

Male-specific hepatic Bcl6: growth hormone-induced block of transcription elongation in females and binding to target genes inversely coordinated with STAT5

The transcriptional repressor Bcl6 is a male-specific rat liver gene product and one of 24 early GH-response genes encoding DNA-binding proteins. Presently, the sex specificity of Bcl6 was shown to emerge at puberty, when hepatic Bcl6 mRNA was induced in males and repressed in females by the female plasma GH profile. Hepatic Bcl6 mRNA was increased to near-normal male levels in hypophysectomized females and was extinguished in intact males given a continuous GH infusion (female-like GH pattern). Bcl6 was also repressed in adult male somatostatin-deficient mice, where plasma GH profiles are female like. Hepatic Bcl6 RNA was rapidly down-regulated by GH pulse treatment, both in hypophysectomized male rats and in primary rat hepatocytes. Bcl6 was substantially induced in female mice deficient in hepatic signal transducer and activator of transcription (STAT)5a/STAT5b, suggesting that these STAT transcriptional mediators of GH signaling repress Bcl6. Indeed, STAT5 was bound to Bcl6 STAT5-binding region-B, previously associated with Bcl6 repression, in both male and female liver chromatin. STAT5 also bound to Bcl6 region-A in male chromatin but only during a plasma GH pulse. Analysis of primary transcripts (heterogeneous nuclear RNA) across the Bcl6 gene revealed a novel mechanism of GH-dependent sex specificity, with two apparent blocks in Bcl6 transcription elongation seen in female liver and in continuous GH-treated male liver, one early in intron 4 and one in exon 5, which together reduced transcription beyond exon 5 more than 300-fold. Finally, Bcl6 was bound to a subset of STAT5-binding sites in male liver chromatin, including a Socs2 STAT5-binding site where Bcl6 binding increased substantially between plasma GH pulses, i.e. when STAT5 binding was low. Bcl6 and STAT5 binding are thus inversely coordinated by the endogenous pulses of pituitary GH release, suggesting this male-specific transcriptional repressor modulates hepatic GH signaling to select STAT5 target genes.

Ras orchestrates exit from the cell cycle and light-chain recombination during early B cell development

Signals through the pre-B cell antigen receptor (pre-BCR) and interleukin 7 receptor (IL-7R) coordinate pre-B cell population expansion with subsequent recombination of the locus encoding immunoglobulin kappa-chain (Igk). Although many 'downstream' effectors of each receptor are known, how they integrate to mediate development has remained unclear. Here we report that pre-BCR-mediated activation of the Ras-MEK-Erk signaling pathway silenced transcription of Ccnd3 (encoding cyclin D3) and coordinated exit from the cell cycle with induction of the transcription factor E2A and the initiation of Igk recombination. IL-7R-mediated activation of the transcription factor STAT5 opposed this pathway by promoting Ccnd3 expression and concomitantly inhibiting Igk transcription by binding to the Igk intronic enhancer and preventing E2A recruitment. Our data show how pre-BCR signaling poises pre-B cells to undergo differentiation after escape from IL-7R signaling.

Computational and functional analysis of growth hormone (GH)-regulated genes identifies the transcriptional repressor B-cell lymphoma 6 (Bc16) as a participant in GH-regulated transcription

For insight into transcriptional mechanisms mediating physiological responses to GH, data mining was performed on a profile of GH-regulated genes induced or inhibited at different times in highly responsive 3T3-F442A adipocytes. Gene set enrichment analysis indicated that GH-regulated genes are enriched in pathways including phosphoinositide and insulin signaling and suggested that suppressor of cytokine signaling 2 (SOCS2) and phosphoinositide 3' kinase regulatory subunit p85alpha (Pik3r1) are important targets. Model-based Chinese restaurant clustering identified a group of genes highly regulated by GH at times consistent with its key physiological actions. This cluster included IGF-I, phosphoinositide 3' kinase p85alpha, SOCS2, and cytokine-inducible SH2-containing protein. It also contains the most strongly repressed gene in the profile, B cell lymphoma 6 (Bcl6), a transcriptional repressor. Quantitative real-time PCR verified the strong decrease in Bcl6 mRNA after GH treatment and induction of the other genes in the cluster. Transcriptional network analysis of the genes implicated signal transducer and activator of transcription (Stat) 5 as hub regulating the most responsive genes, Igf1, Socs2, Cish, and Bcl6. Transcriptional activation analysis demonstrated that Bcl6 inhibits SOCS2-luciferase and blunts its stimulation by GH. Occupancy of endogenous Bcl6 on SOCS2 DNA decreased after GH treatment, whereas occupancy of Stat5 increased concomitantly. Thus, GH-mediated inhibition of Bcl6 expression may reverse the repression of SOCS2 and facilitate SOCS2 activation by GH. Together these analyses identify Bcl6 as a participant in GH-regulated gene expression and suggest an interplay between the repressor Bcl6 and the activator Stat5 in regulating genes, which contribute to GH responses.

Reciprocal effects of STAT5 and STAT3 in breast cancer

Breast cancer is often associated with inappropriate activation of transcription factors involved in normal mammary development. Two related transcription factors, signal transducer and activator of transcription (STAT) 5 and STAT3, play important and distinct roles in mammary development and both can be activated in breast cancer. However, the relative contribution of these STATs to mammary tumorigenesis is unknown. We have found that primary human breast tumors displaying activation of both STATs are more differentiated than those with STAT3 activation alone and display more favorable prognostic characteristics. To understand this difference, we have analyzed the effect of these STATs on gene regulation and phenotype of mammary carcinoma cells. STAT5 and STAT3 mediate opposing effects on several key target genes, with STAT5 exerting a dominant role. Using a model system of paired breast cancer cell lines, we found that coactivation of STAT5 and STAT3 leads to decreased proliferation and increased sensitivity to the chemotherapeutic drugs paclitaxel and vinorelbine compared with cells that have only STAT3 activation. Thus, STAT5 can modify the effects of STAT3 from the level of gene expression to cellular phenotype and analysis of the activation state of both STAT5 and STAT3 may provide important diagnostic and prognostic information in breast cancer.

Inhibition of STAT5 induces G1 cell cycle arrest and reduces tumor cell invasion in human colorectal cancer cells

Abnormalities in the signal transducer and activator of transcription (STAT) pathway are involved in the oncogenesis of several cancers. However, the mechanism by which dysregulated STAT5 signaling contributes to the progression of human colorectal cancer (CRC) has not been elucidated. To investigate the role of STAT5 in CRC progression, we depleted STAT5 with a small interfering RNA (siRNA). Our results demonstrate that STAT5 is involved in CRC cell growth, cell cycle progression, invasion and migration through regulation of gene expression, such as Bcl-2, p16(ink4a), p21(waf1/cip1), p27(kip1), E-cadherin, the focal adhesion kinase (FAK), vascular endothelial growth factor (VEGF) and matrix metalloproteinases. In addition, immunohistochemical staining reveals upregulation of STAT5 during CRC tumorigenesis. Moreover, phospho-STAT5 (pSTAT5) is predominantly localized in the cytoplasm of adenomas cells and colon adenocarcinoma cells, but primarily presented in the nucleus of normal colonic epithelium cells. Thus, pSTAT5 protein is shuttled from the nucleus to the cytoplasm in the oncogenesis of CRC, suggesting that activated STAT5 may also have cytoplasmic functions. In support of this hypothesis, we found that STAT5 formed a complex with p44/42 MAPK and SAPK/JNK in CRC cells, suggesting cross talk between STAT5 signaling and the MAPK pathway in the development of human CRC. Our findings illustrate the biological significance of STAT5 signaling in CRC progression, and provide novel evidence that intervention in STAT5 signaling may have potential therapeutic value in the prevention of human colorectal cancer.

CD40 and B-cell receptor signalling induce MAPK family members that can either induce or repress Bcl-6 expression

Bcl-6 is essential for germinal centre development and normal antibody responses, and has major roles in controlling B-cell proliferation and differentiation. Bcl-6 expression is tightly controlled, but neither the nature of all the regulatory signals nor their interactions are known. Bcl-6 expression is induced in Bcr-Abl expressing lymphoid cell lines by the tyrosine kinase inhibitor, imatinib. We show that p38 MAPK mediates induction of Bcl-6 following inhibition of Bcr-Abl by imatinib. Next we analyze p38 function in a germinal centre B-cell line, Ramos. p38 is phosphorylated under basal conditions, and studies with p38 inhibitors show that it induces Bcl-6 expression. Membrane bound CD40 ligand activates p38 but also other MAPK pathways that strongly repress Bcl-6 and the overall effect is reduction in Bcl-6 expression. Surprisingly soluble CD40 ligand induces Bcl-6 by activating p38 without activating the repressive pathways. Hence different types of CD40 signalling are associated with varying effects on Bcl-6 expression. Transcription reporter assays demonstrate p38 responsive sequences at about 4.5 kb from the transcription start site. Immunocytochemistry of tonsil sections show phosphorylated p38 in a minor population of germinal centre B-cells. We demonstrate for the first time that p38 induces Bcl-6 transcription, but increased protein expression occurs only when the strong pathways repressing Bcl-6 are not activated.

Nifuroxazide inhibits survival of multiple myeloma cells by directly inhibiting STAT3

Constitutive activation of the transcription factor STAT3 contributes to the pathogenesis of many cancers, including multiple myeloma (MM). Since STAT3 is dispensable in most normal tissue, targeted inhibition of STAT3 is an attractive therapy for patients with these cancers. To identify STAT3 inhibitors, we developed a transcriptionally based assay and screened a library of compounds known to be safe in humans. We found the drug nifuroxazide to be an effective inhibitor of STAT3 function. Nifuroxazide inhibits the constitutive phosphorylation of STAT3 in MM cells by reducing Jak kinase autophosphorylation, and leads to down-regulation of the STAT3 target gene Mcl-1. Nifuroxazide causes a decrease in viability of primary myeloma cells and myeloma cell lines containing STAT3 activation, but not normal peripheral blood mononuclear cells. Although bone marrow stromal cells provide survival signals to myeloma cells, nifuroxazide can overcome this survival advantage. Reflecting the interaction of STAT3 with other cellular pathways, nifuroxazide shows enhanced cytotoxicity when combined with either the histone deacetylase inhibitor depsipeptide or the MEK inhibitor UO126. Therefore, using a mechanistic-based screen, we identified the clinically relevant drug nifuroxazide as a potent inhibitor of STAT signaling that shows cytotoxicity against myeloma cells that depend on STAT3 for survival.

Therapeutic targeting of the BCL6 oncogene for diffuse large B-cell lymphomas

BCL6 is a transcriptional repressor often expressed constitutively in diffuse large B-cell lymphomas (DLBCL) due to mutations of its genomic locus. BCL6 mediates aberrant survival, proliferation, genomic instability and differentiation blockade in DLBCL cells. The biochemical study of BCL6 mediated gene repression has provided the basis for design of agents that inhibit BCL6 and kill lymphoma cells. The repressor activity of the BCL6 BTB domain is particularly well defined from the structural standpoint. Design of inhibitors targeting BCL6 BTB domain protein interaction surfaces appears to be an effective approach, which reactivates important BCL6 target genes and readily kills DLBCL cells. Targeting other domains of BCL6 or using histone deacetylase inhibitors to overcome BCL6 mediated repression may also be useful. Recent studies in DLBCL transcriptional signatures have revealed a subset of DLBCLs that are particularly dependent on BCL6 to maintain their survival and these patients could be candidates for clinical trials of BCL6 inhibitors.

B-cell lymphoma 6 and the molecular pathogenesis of diffuse large B-cell lymphoma

PURPOSE OF REVIEW

The B-cell lymphoma 6 transcriptional repressor is the most commonly involved oncogene in B-cell lymphomas. Sustained expression of B-cell lymphoma 6 causes malignant transformation of germinal center B cells. Understanding the mechanism of action of B-cell lymphoma 6 is crucial for the study of how aberrant transcriptional programming leads to lymphomagenesis and development of targeted antilymphoma therapy.

RECENT FINDINGS

Identification of B-cell lymphoma 6 target genes indicates a critical role for B-cell lymphoma 6 in facilitating a state of physiological genomic instability required for germinal center B cells to undergo affinity maturation, and suggests its contribution to several additional cellular functions. The discovery of several layers of counterregulatory mechanisms reveals how B cells can control and fine-tune the potentially lymphomagenic actions of B-cell lymphoma 6. From the biochemical standpoint, B-cell lymphoma 6 can regulate distinct biological pathways through different cofactors. This observation explains how the biological actions of B-cell lymphoma 6 can be physiologically controlled through separate mechanisms and affords the means for improved therapeutic targeting. The fact that patients with B-cell lymphoma 6-dependent lymphoma can be identified on the basis of gene signatures suggests that therapeutic trials of B-cell lymphoma 6 inhibitors could be personalized to these individuals.

SUMMARY

B-cell lymphoma 6 plays a fundamental role in lymphomagenesis and is an excellent therapeutic target for development of improved antilymphoma therapeutic regimens.

STAT5 activity in pancreatic β-cells

Signal transducer and activator of transcription (STAT)5A and -5B are latent transcription factors activated by cytokines and hormones of the cytokine family. In pancreatic insulin-secreting β-cells, STAT5A and -5B are activated primarily by prolactin and growth hormone stimulation and are important mediators of the potent stimulation of proliferation and insulin production caused by these hormones. STAT5A and -5B are both expressed in β-cells and control the expression of a number of mRNAs implicated in cell replication control, insulin biosynthesis and secretion. In addition to STAT5A and -5B being transcriptional activators, they may also repress gene transcription. By these means, STAT5 proteins increase the levels of anti-apoptotic transcripts in β-cells and repress expression of pro-apoptotic genes. This review focuses on the anti-apoptotic role of STAT5 signaling, providing a mechanism for β-cell resistance to pro-apoptotic cytokines, Type 1 diabetes mellitus and obesity-associated β-cell stress. It is clear from studies of STAT5 signaling in pancreatic β-cells that STAT5 is important for postnatal β-cell compensatory growth (as in pregnancy or obesity) and in the defense against β-cell stress factors.

Activation-dependent induction of Blimp-1

B lymphocyte induced maturation protein-1 (Blimp-1) mRNA is induced upon antigen-dependent activation of both T and B lymphocytes, in spite of the fact that it plays very different roles in the two lineages. B cells have at least four different mechanisms to repress Blimp-1 and repression is relieved before induction. Only one repressor, Bcl-6, is known in T cells. Activators must also be present to induce Blimp-1 in both T and B cells. Cytokines IL-21, IL-10, and IL-6, activating STAT3, are crucial in B cells along with toll-like receptor (TLR) signals, whereas IL-2 is crucial in T cells. AP-1, NF-kappaB, and IRF4 also activate Blimp-1.

STAT3-mediated up-regulation of BLIMP1 Is coordinated with BCL6 down-regulation to control human plasma cell differentiation

STAT family members have been implicated in regulating the balance between B cell lymphoma (BCL)6 and B lymphocyte induced maturation protein (BLIMP)1 to control plasma cell differentiation. We previously showed that STAT5 induces BCL6 to block plasma cell differentiation and extend the life span of human B cells. The heterogeneity in STAT activation by cytokines and their effects on B cell differentiation prompted us to investigate the effect of STAT3 activation in plasma cell differentiation. First stimulation with IL-21, which promotes plasma cell differentiation, induced robust and prolonged STAT3 activation in primary human B cells. We then investigated effects of direct STAT3 activation on regulation of plasma cell genes, cellular phenotype, and Ig production. Activation of a tamoxifen-regulated STAT3-estrogen receptor fusion protein triggered BLIMP1 mRNA and protein up-regulation, plasma cell phenotypic features, and Ig secretion. When STAT3 was activated by IL-21 in B cells ectopically expressing BCL6, BLIMP1 was up-regulated, but only partial plasma cell differentiation was achieved. Lastly, through coexpression of BCL6 and STAT3-ER, we verified that STAT3 activation functionally mimicked IL-21 treatment and that STAT3-mediated BLIMP1 up-regulation occurred despite high BCL6 expression levels indicating that BCL6 is not the dominant repressor of BLIMP1. Thus, up-regulation of BLIMP1 alone is not sufficient for differentiation of primary human B cells into plasma cells; concomitant down-regulation of BCL6 is absolutely required for completion of the plasma cell differentiation program.

Inhibition of JAK1, 2/STAT3 signaling induces apoptosis, cell cycle arrest, and reduces tumor cell invasion in colorectal cancer cells

Abnormalities in the STAT3 pathway are involved in the oncogenesis of several cancers. However, the mechanism by which dysregulated STAT3 signaling contributes to the progression of human colorectal cancer (CRC) has not been elucidated, nor has the role of JAK, the physiological activator of STAT3, been evaluated. To investigate the role of both JAK and STAT3 in CRC progression, we inhibited JAK with AG490 and depleted STAT3 with a SiRNA. Our results demonstrate that STAT3 and both JAK1 and 2 are involved in CRC cell growth, survival, invasion, and migration through regulation of gene expression, such as Bcl-2, p1(6ink4a), p21(waf1/cip1), p27(kip1), E-cadherin, VEGF, and MMPs. Importantly, the FAK is not required for STAT3-mediated regulation, but does function downstream of JAK. In addition, our data show that proteasome-mediated proteolysis promotes dephosphorylation of the JAK2, and consequently, negatively regulates STAT3 signaling in CRC. Moreover, immunohistochemical staining reveals that nuclear staining of phospho-STAT3 mostly presents in adenomas and adenocarcinomas, and a positive correlation is found between phospho-JAK2 immunoreactivity and the differentiation of colorectal adenocarcinomas. Therefore, our findings illustrate the biologic significance of JAK1, 2/STAT3 signaling in CRC progression and provide novel evidence that the JAK/STAT3 pathway may be a new potential target for therapy of CRC.

Plasma cell differentiation and survival

Humoral immunity depends on the regulated production and maintenance of antibody secreting cells during the course of an immune response. Recent insights into the transcriptional regulation of the initiation of plasma cell differentiation have clarified aspects of this process, particularly with respect to the choice between the memory B cell and plasma cell differentiation pathways. It is now possible to specify the conditions favouring these outcomes and to predict where they might occur within the germinal center. Once formed, plasma cell survival is critically dependent on accessing niches that are formed by stomal elements in both normal and inflamed tissues. The apparent homeostasis of plasma cell numbers means that new specificities can persist only at the expense of existing ones, raising questions on how immunological memory is maintained in the face of new immune responses. The answer appears to be through the reduction of the process to a single cell level, thereby introducing an element of stochasticity.

Poly-(ADP-ribose) polymerase-1 (Parp-1) binds in a sequence-specific manner at the Bcl-6 locus and contributes to the regulation of Bcl-6 transcription

Bcl-6 is a transcription factor that is normally expressed in germinal centre B cells. It is essential for the formation of germinal centres and the production of high-affinity antibodies. Transcriptional downregulation of Bcl-6 occurs on terminal differentiation to plasma cells. Bcl-6 is highly expressed in B-cell non-Hodgkin's lymphoma and, in a subset of cases of diffuse large cell lymphoma, the mechanism of Bcl-6 overexpression involves interruption of normal transcriptional controls. Transcriptional control of Bcl-6 is, therefore, important for normal antibody responses and lymphomagenesis, but little is known of the cis-acting control elements. This report focuses on a region of mouse/human sequence homology in the first intron of Bcl-6, which is a candidate site for such a control element. We demonstrate that poly-(ADP-ribose) polymerase-1 (Parp-1) binds in vitro and in vivo to specific sequences in this region. We further show that PARP inhibitors, and Parp-1 knockdown by siRNA induce Bcl-6 mRNA expression in Bcl-6 expressing cell lines. We speculate that Parp-1 activation plays a role in switching off Bcl-6 transcription and subsequent B-cell exit from the germinal centre.