STAT1 acts as a tumor promoter for leukemia development

STAT5 regulation of BCL10 parallels constitutive NFkappaB activation in lymphoid tumor cells

Background

Signal Transducer and Activator of Transcription 5 A and B (STAT5) are key survival factors in cells of the lymphoid lineage. Identification of novel, tissue-specific STAT5 regulated genes would advance the ability to combat diseases due to aberrant STAT5 signaling. In the present work a library of human STAT5 bound genomic elements was created and validated.

Results

Of several STAT5 responsive genomic regulatory elements identified, one was located within the first intron of the human BCL10 gene. Chromatin immuno-precipitation reactions confirmed constitutive in vivo STAT5 binding to this intronic fragment in various human lymphoid tumor cell lines. Interestingly, non-phosphorylated STAT5 was found in the nuclei of Kit225 and YT cells in the absence of cytokine stimulation that paralleled constitutive NFκB activation. Inhibition of the hyperactive JAK3/STAT5 pathway in MT-2 cells via the Mannich-base, NC1153, diminished the constitutive in vivo occupancy of BCL10-SBR by STAT5, reduced NFκB activity and BCL10 protein expression in a dose dependent manner. Moreover, depletion of STAT5 via selective antisense oligonucleotide treatment similarly resulted in decreased BCL10 mRNA and protein expression, cellular viability and impaired NFκB activity independent of IL-2.

Conclusion

These results suggest that the NFκB regulator BCL10 is an IL-2-independent STAT5 target gene. These findings proffer a model in which un-activated STAT5 can regulate pathways critical for lymphoid cell survival and inhibitors that disrupt STAT5 function independent of tyrosine phosphorylation may be therapeutically effective in treating certain leukemias/lymphomas.

Dendritic cells require STAT-1 phosphorylated at its transactivating domain for the induction of peptide-specific CTL

Phosphorylation of transcription factor STAT-1 on Y701 regulates subcellular localization whereas phosphorylation of the transactivating domain at S727 enhances transcriptional activity. In this study, we investigate the impact of STAT-1 and the importance of transactivating domain phosphorylation on the induction of peptide-specific CTL in presence of the TLR9-dependent immune adjuvant IC31. STAT-1 deficiency completely abolished CTL induction upon immunization, which was strongly reduced in animals carrying the mutation of the S727 phospho-acceptor site. A comparable reduction of CTL was found in mice lacking the type I IFN (IFN-I) receptor, whereas IFN-gamma-deficient mice behaved like wild-type controls. This finding suggests that S727-phosphorylated STAT-1 supports IFN-I-dependent induction of CTL. In adoptive transfer experiments, IFN-I- and S727-phosphorylated STAT-1 were critical for the activation and function of dendritic cells. Mice with a T cell-specific IFN-I receptor ablation did not show impaired CTL responses. Unlike the situation observed for CTL development S727-phosphorylated STAT-1 restrained proliferation of naive CD8(+) T cells both in vitro and following transfer into Rag-deficient mice. In summary, our data reveal a dual role of S727-phosphorylated STAT-1 for dendritic cell maturation as a prerequisite for the induction of CTL activity and for T cell autonomous control of activation-induced or homeostatic proliferation.

T-cell protein tyrosine phosphatase is a key regulator in immune cell signaling: lessons from the knockout mouse model and implications in human disease

The immune system requires for its proper ontogeny, differentiation, and maintenance the function of several tyrosine kinases and adapters that create and modify tyrosine phosphorylation sites. Tyrosine phosphorylation is a crucial protein modification in immune cell signaling and can be reversed by protein tyrosine phosphatases (PTPs). Much progress has been made in identifying and understanding PTP function in the immune system. In this review, we present one of these proteins, named T-cell PTPs (TC-PTP) (gene name PTPN2), a classical, non-receptor PTP that is ubiquitously expressed with particularly high expression in hematopoietic tissues. TC-PTP is remarkable not only by the fact that it appears to influence most, if not all, cells involved in the development of the immune system, from stem cells to differentiated lineages, but also recent findings have positioned it at the core of several human diseases from autoimmune disease to cancer.

Leukemic challenge unmasks a requirement for PI3Kδ in NK cell–mediated tumor surveillance

Specific inhibitors of PI3K isoforms are currently evaluated for their therapeutic potential in leukemia. We found that BCR/ABL+ human leukemic cells express PI3Kδ and therefore explored its impact on leukemia development. Using PI3Kδ-deficient mice, we define a dual role of PI3Kδ in leukemia. We observed a growth-promoting effect in tumor cells and an essential function in natural killer (NK) cell–mediated tumor surveillance: Abelson-transformed PI3Kδ-deficient cells induced leukemia in RAG2-deficient mice with an increased latency, indicating that PI3Kδ accelerated leukemia progression in vivo. However, the absence of PI3Kδ also affected NK cell–mediated tumor surveillance. PI3Kδ-deficient NK cells failed to lyse a large variety of target cells because of defective degranulation, as also documented by capacitance recordings. Accordingly, transplanted leukemic cells killed PI3Kδ-deficient animals more rapidly. As a net effect, no difference in disease latency in vivo was detected if both leukemic cells and NK cells lack PI3Kδ. Other tumor models confirmed that PI3Kδ-deficient mice succumbed more rapidly when challenged with T- or B-lymphoid leukemic or B16 melanoma cells. Thus, the action of PI3Kδ in the NK compartment is as relevant to survival of the mice as the delayed tumor progression. This dual function must be taken into account when using PI3Kδ inhibitors as antileukemic agents in clinical trials.

Stat1 phosphorylation determines Ras oncogenicity by regulating p27 kip1

Inactivation of p27 Kip1 is implicated in tumorigenesis and has both prognostic and treatment-predictive values for many types of human cancer. The transcription factor Stat1 is essential for innate immunity and tumor immunosurveillance through its ability to act downstream of interferons. Herein, we demonstrate that Stat1 functions as a suppressor of Ras transformation independently of an interferon response. Inhibition of Ras transformation and tumorigenesis requires the phosphorylation of Stat1 at tyrosine 701 but is independent of Stat1 phosphorylation at serine 727. Stat1 induces p27 Kip1 expression in Ras transformed cells at the transcriptional level through mechanisms that depend on Stat1 phosphorylation at tyrosine 701 and activation of Stat3. The tumor suppressor properties of Stat1 in Ras transformation are reversed by the inactivation of p27 Kip1. Our work reveals a novel functional link between Stat1 and p27 Kip1, which act in coordination to suppress the oncogenic properties of activated Ras. It also supports the notion that evaluation of Stat1 phosphorylation in human tumors may prove a reliable prognostic factor for patient outcome and a predictor of treatment response to anticancer therapies aimed at activating Stat1 and its downstream effectors.

Origin and pathogenesis of nodular lymphocyte-predominant Hodgkin lymphoma as revealed by global gene expression analysis

The pathogenesis of nodular lymphocyte–predominant Hodgkin lymphoma (NLPHL) and its relationship to other lymphomas are largely unknown. This is partly because of the technical challenge of analyzing its rare neoplastic lymphocytic and histiocytic (L&H) cells, which are dispersed in an abundant nonneoplastic cellular microenvironment. We performed a genome-wide expression study of microdissected L&H lymphoma cells in comparison to normal and other malignant B cells that indicated a relationship of L&H cells to and/or that they originate from germinal center B cells at the transition to memory B cells. L&H cells show a surprisingly high similarity to the tumor cells of T cell–rich B cell lymphoma and classical Hodgkin lymphoma, a partial loss of their B cell phenotype, and deregulation of many apoptosis regulators and putative oncogenes. Importantly, L&H cells are characterized by constitutive nuclear factor κB activity and aberrant extracellular signal-regulated kinase signaling. Thus, these findings shed new light on the nature of L&H cells, reveal several novel pathogenetic mechanisms in NLPHL, and may help in differential diagnosis and lead to novel therapeutic strategies.

Ups and downs: the STAT1:STAT3 seesaw of Interferon and gp130 receptor signalling

Downstream of cytokine or growth factor receptors, STAT3 counteracts inflammation and promotes cell survival/proliferation and immune tolerance while STAT1 inhibits proliferation and favours innate and adaptive immune responses. STAT1 and STAT3 activation are reciprocally regulated and perturbation in their balanced expression or phosphorylation levels may re-direct cytokine/growth factor signals from proliferative to apoptotic, or from inflammatory to anti-inflammatory. Here we review the functional canonical and non-canonical effects of STAT1/3 activation and discuss the hypothesis that perturbation of their expression and/or activation levels may provide novel therapeutic strategies in different clinical settings and particularly in cancer.

The tumor suppressor semaphorin 3B triggers a prometastatic program mediated by interleukin 8 and the tumor microenvironment

Semaphorins are a large family of evolutionarily conserved morphogenetic molecules originally identified for their repelling role in axonal guidance. Intriguingly, semaphorins have recently been implicated in cancer progression (Neufeld, G., T. Lange, A. Varshavsky, and O. Kessler. 2007. Adv. Exp. Med. Biol. 600:118-131). In particular, semaphorin 3B (SEMA3B) is considered a putative tumor suppressor, and yet we found that it is expressed at high levels in many invasive and metastatic human cancers. By investigating experimental tumor models, we confirmed that SEMA3B expression inhibited tumor growth, whereas metastatic dissemination was surprisingly increased. We found that SEMA3B induced the production of interleukin (IL) 8 by tumor cells by activating the p38-mitogen-activated protein kinase pathway in a neuropilin 1-dependent manner. Silencing the expression of endogenous SEMA3B in tumor cells impaired IL-8 transcription. The release of IL-8, in turn, induced the recruitment of tumor-associated macrophages and metastatic dissemination to the lung, which could be rescued by blocking IL-8 with neutralizing antibodies. In conclusion, we report that SEMA3B exerts unexpected functions in cancer progression by fostering a prometastatic environment through elevated IL-8 secretion and recruitment of macrophages coupled to the suppression of tumor growth.

Roles of unphosphorylated STATs in signaling

The seven members of the signal transducer and activator of transcription (STAT) family of transcription factors are activated in response to many different cytokines and growth factors by phosphorylation of specific tyrosine residues. The STAT1 and STAT3 genes are specific targets of activated STATs 1 and 3, respectively, resulting in large increases in the levels of these unphosphorylated STATs (U-STATs) in response to the interferons (STAT1) or ligands that active gp130, such as IL-6 (STAT3). U-STATs drive gene expression by novel mechanisms distinct from those used by phosphorylated STAT (P-STAT) dimers. In this review, we discuss the roles of U-STATs in transcription and regulation of gene expression.

A detailed protocol for bacterial artificial chromosome recombineering to study essential genes in stem cells

Bacterial artificial chromosome (BAC) recombineering is a novel technique for DNA manipulation. It starts from an original chromosomal gene locus that is modified to introduce a transgene under the expression control of the original gene locus. In most cases a cell type specific promoter is chosen and the transgene is placed in a way that the exon containing the start codon is replaced. Alternatively, BACs such as the Rosa26 BAC are chosen because of their known open chromatin and ubiquitous promoter activity that allows a broad expression profile of the transgene in the whole body. Thus, transgenes can be overexpressed within their natural transcriptional regulatory circuit. BAC transgenes have a high tendency to maintain their appropriate chromatin status because the endogenous locus was expressed in different cell types. Here, we give a detailed protocol based on the original idea to choose a BAC approach until the injection of the modified BAC DNA that leads to the generation of novel transgenic mouse lines. As an example for a BAC mouse model suitable for the analysis of stem cell or hematopoietic stem cell functions, we chose modification of the locus for the transcription factor Stat3. Stat3 variants replace the wild-type Stat3 gene to study their function in particular in the earliest cell types of the body.

NADPH oxidase promotes pancreatic cancer cell survival via inhibiting JAK2 dephosphorylation by tyrosine phosphatases

BACKGROUND & AIMS

Growth factors, such as insulin-like growth factor-1 (IGF-I), protect pancreatic cancer (PaCa) cells from death. We recently showed that reactive oxygen species (ROS) produced by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase Nox4 mediate the antiapoptotic effect of growth factors. Here, we examine the mechanisms of the antiapoptotic role of NADPH oxidase. We hypothesized that ROSs produced by NADPH oxidase inhibit key protein tyrosine phosphatases (PTPs) and thus sustain the activation of kinases mediating antiapoptotic pathways in PaCa cells.

METHODS

Transfections and pharmacologic inhibition were used to assess the effects of NADPH oxidase on Janus kinase 2 (JAK2) kinase, the low molecular weight-protein tyrosine phosphatase (LMW-PTP), and apoptosis.

RESULTS

We found that 1 target of ROSs is JAK2, an important antiapoptotic kinase in PaCa cells. Both serum-induced and IGF-I biphasic JAK2 phosphorylation, with a rapid (minutes) and transient first phase, and a slow and sustained (24-72 hours) second phase. Nox4 mediated the sustained phase of JAK2 phosphorylation, which was required for the antiapoptotic effects of IGF-I and serum. Transfection experiments identified the LMW-PTP as a negative regulator of sustained JAK2 phosphorylation. Growth factors inhibited LMW-PTP through its oxidation by NADPH oxidase. LMW-PTP colocalizes with Nox4 both in PaCa cells and in human pancreatic adenocarcinoma.

CONCLUSIONS

The results suggest a novel signaling pathway, in which NADPH oxidase activation results in inhibition of PTPs, such as LMW-PTP, leading, in turn, to enhanced and sustained phosphorylation of kinases, such as JAK2, and suppression of apoptosis. This pathway mediates the prosurvival effect of ROSs and suggests new targets for pancreatic cancer treatment.

Epstein-Barr virus induces a distinct form of DNA-bound STAT1 compared with that found in interferon-stimulated B lymphocytes

Since 'constitutive activation' of STAT1 was first described in Epstein-Barr virus (EBV)-immortalized lymphoblastoid cell lines (LCLs), there has been controversy regarding the molecular identity of the STAT1 DNA-binding complex found in these cells. The post-translational modifications of STAT1 in LCLs have been analysed and an LMP1-induced STAT1 DNA-binding complex, different from that generated by alpha interferon (IFN) stimulation and not involving tyrosine phosphorylation, is demonstrated. STAT1 is serine-phosphorylated downstream of PI3K and MEK in LCLs and this modification restricts IFN-stimulated STAT1-DNA binding. These data suggest that EBV induces a distinct form of DNA-bound STAT1 in virus-infected cells.

Epstein-Barr virus-encoded EBNA1 regulates cellular gene transcription and modulates the STAT1 and TGFbeta signaling pathways

The Epstein-Barr virus (EBV)-encoded EBNA1 protein is expressed in all virus-associated tumors where it plays an essential role in the maintenance, replication and transcription of the EBV genome. Transcriptional profiling of EBNA1-expressing carcinoma cells demonstrated that EBNA1 also influences the expression of a range of cellular genes including those involved in translation, transcription and cell signaling. Of particular interest was the ability of EBNA1 to enhance expression of STAT1 and sensitize cells to interferon-induced STAT1 activation with resultant enhancement of major histocompatibility complex expression. A negative effect of EBNA1 on the expression of TGFbeta1-responsive betaig-h3 and PAI-1 genes was confirmed at the protein level in EBV-infected carcinoma cells. This effect resulted from the ability of EBNA1 to repress TGFbeta1-induced transcription via a reduction in the interaction of SMAD2 with SMAD4. More detailed analysis revealed that EBNA1 induces a lower steady-state level of SMAD2 protein as a consequence of increased protein turnover. These data show that EBNA1 can influence cellular gene transcription resulting in effects that may contribute to the development of EBV-associated tumors.

NUP98 Dysregulation in Myeloid Leukemogenesis

Nucleoporin 98 (NUP98) is a component of the nuclear pore complex that facilitates mRNA export from the nucleus. It is mapped to 11p15.5 and is fused to a number of distinct partners, including nine members of the homeobox family as a consequence of leukemia-associated chromosomal translocations. NUP98-HOXA9 is associated with the t(7;11)(p15;p15) translocation in acute myeloid leukemia (AML), myelodysplastic syndrome, and blastic crisis of chronic myeloid leukemia. Expression of NUP98-HOXA9 in murine bone marrow resulted in a myeloproliferative disease progressing to AML by 7-8 months. Transduction of NUP98 fusion genes into human CD34(+) cells confers a proliferative advantage in long-term cytokine-stimulated and stromal cocultures and in NOD-SCID engrafted mice, associated with a five- to eight-fold increase in hematopoietic stem cells. NUP98-HOXA9 expression inhibited erythroid and myeloid differentiation but enhanced serial progenitor replating. NUP98-HOXA9 upregulated a number of homeobox genes of the A and B cluster as well as MEIS1 and Pim-1, and downmodulated globin genes and C/EBPalpha. The HOXA9 component of the NUP98-HOXA9 fusion protein was protected from cullin-4A-mediated ubiquitination and subsequent proteasome-dependent degradation. In NUP98-HOX-transduced CD34(+) cells and cells from AML patients with t(7;11)(p15;p15) NUP98 was no longer associated with the nuclear pore complex but formed intranuclear aggregation bodies. Analysis of NUP98 allelic expression in AML and myelodysplastic syndrome showed loss of heterozygosity observed in 29% of the former and 8% of the latter. This was associated with poor prognosis.