Stromal STAT5-Mediated Trophic Activity Regulates Hematopoietic Niche Factors

Signal transducer and activator of transcription 5 (STAT5a and STAT5b) are intrinsically critical for normal hematopoiesis but are also expressed in stromal cells. Here, STAT5ab knockout (KO) was generated with a variety of bone marrow hematopoietic and stromal Cre transgenic mouse strains. Vav1-Cre/+STAT5abfl/fl, the positive control for loss of multipotent hematopoietic function, surprisingly dysregulated niche factor mRNA expression, and deleted STAT5ab in CD45neg cells. Single-cell transcriptome analysis of bone marrow from Vav1-Cre/+ wild-type or Vav1-Cre/+STAT5abfl/fl mice showed hematopoietic stem cell (HSC) myeloid commitment priming. Nes+ cells were detected in both CD45neg and CD45+ clusters and deletion of STAT5ab with Nes-Cre caused hematopoietic repopulating defects. To follow up on these promiscuous Cre promoter deletions in CD45neg and CD45+ bone marrow cell populations, more stroma-specific Cre strains were generated and demonstrated a reduction in multipotent hematopoietic progenitors. Functional support for niche-supporting activity was assessed using STAT5-deficient mesenchymal stem cells (MSCs). With Lepr-Cre/+STAT5abfl/fl, niche factor mRNAs were downregulated with validation of reduced IGF-1 and CXCL12 proteins. Furthermore, advanced computational analyses revealed a key role for STAT5ab/Cish balance with Cish strongly co-expressed in MSCs and HSCs primed for differentiation. Therefore, STAT5ab-associated gene regulation supports the bone marrow microenvironment.

Gab2 and Gab3 Redundantly Suppress Colitis by Modulating Macrophage and CD8+ T-Cell Activation

Inflammatory Bowel Disease (IBD) is a multi-factorial chronic inflammation of the gastrointestinal tract prognostically linked to CD8⁺ T-cells, but little is known about their mechanism of activation during initiation of colitis. Here, Grb2-associated binding 2/3 adaptor protein double knockout mice (Gab2/3^−/−) were generated. Gab2/3^−/− mice, but not single knockout mice, developed spontaneous colitis. To analyze the cellular mechanism, reciprocal bone marrow (BM) transplantation demonstrated a Gab2/3^−/− hematopoietic disease-initiating process. Adoptive transfer showed individual roles for macrophages and T-cells in promoting colitis development in vivo. In spontaneous disease, intestinal intraepithelial CD8⁺ but much fewer CD4⁺, T-cells from Gab2/3^−/− mice with rectal prolapse were more proliferative. To analyze the molecular mechanism, reduced PI3-kinase/Akt/mTORC1 was observed in macrophages and T-cells, with interleukin (IL)-2 stimulated T-cells showing increased pSTAT5. These results illustrate the importance of Gab2/3 collectively in signaling responses required to control macrophage and CD8⁺ T-cell activation and suppress chronic colitis.

Global loss of leucine carboxyl methyltransferase-1 causes severe defects in fetal liver hematopoiesis

Leucine carboxyl methyltransferase-1 (LCMT-1) methylates the C-terminal leucine α-carboxyl group of the catalytic subunits of the protein phosphatase 2A (PP2A) subfamily of protein phosphatases, PP2Ac, PP4c, and PP6c. LCMT-1 differentially regulates the formation and function of a subset of the heterotrimeric complexes that PP2A and PP4 form with their regulatory subunits. Global LCMT-1 knockout causes embryonic lethality in mice, but LCMT-1 function in development is unknown. In this study, we analyzed the effects of global LCMT-1 loss on embryonic development. LCMT-1 knockout causes loss of PP2Ac methylation, indicating that LCMT-1 is the sole PP2Ac methyltransferase. PP2A heterotrimers containing the Bα and Bδ B-type subunits are dramatically reduced in whole embryos, and the steady-state levels of PP2Ac and the PP2A structural A subunit are also down ∼30%. Strikingly, global loss of LCMT-1 causes severe defects in fetal hematopoiesis and usually death by embryonic day 16.5. Fetal livers of homozygous lcmt-1 knockout embryos display hypocellularity, elevated apoptosis, and greatly reduced numbers of hematopoietic stem and progenitor cell-enriched Kit+Lin-Sca1+ cells. The percent cycling cells and mitotic indices of WT and lcmt-1 knockout fetal liver cells are similar, suggesting that hypocellularity may be due to a combination of apoptosis and/or defects in specification, self-renewal, or survival of stem cells. Indicative of a possible intrinsic defect in stem cells, noncompetitive and competitive transplantation experiments reveal that lcmt-1 loss causes a severe multilineage hematopoietic repopulating defect. Therefore, this study reveals a novel role for LCMT-1 as a key player in fetal liver hematopoiesis.

Stat5 deficiency decreases transcriptional heterogeneity and supports emergence of hematopoietic sub-populations

Aging is associated with significant changes in hematopoiesis, including clonal dominance, anemia, myeloid malignancies, and reduced activation of signal transducer and activator of transcription 5 (Stat5). In previous studies, Stat5 deletion surprisingly amplified FLT3/ITD+ myeloid expansion or Myc-driven lymphoid expansion. Here we show that Stat5 deficiency has a strong impact upon transcriptional heterogeneity in single sorted c-Kit+Lin-Sca-1+ (KLS) cells or CD150+CD48- KLS long-term repopulating hematopoietic stem cells (LT-HSC). Single cell polymerase chain reaction (PCR) was performed on selected regulators of multi-lineage hematopoiesis. At least two dominant sub-populations were identified by increased expression of cell cycle regulatory and leukemia-associated genes. Furthermore, in the top expressing quartile of cells, the majority of genes were proportionally overrepresented. In wild-type KLS cells, Stat5 mRNA levels were also strongly correlated with several genes. Since heterogeneity decreases with age or inflammatory or oncogenic stress, these results provide a potential mechanistic linkage to Stat5 expression.

MLLT11/AF1q boosts oncogenic STAT3 activity through Src-PDGFR tyrosine kinase signaling

Constitutive STAT3 activation by tyrosine phosphorylation of mutated or amplified tyrosine kinases (pYSTAT3) is critical for cancer initiation, progression, invasion, and motility of carcinoma cells. We showed that AF1q is associated with STAT3 signaling in breast cancer cells. In xenograft models, enhanced AF1q expression activated STAT3 and promoted tumor growth and metastasis in immunodeficient NSG mice. The cytokine secretory phenotype of MDA-MB-231LN breast cancer cells with altered AF1q expression revealed changes in expression of platelet-derived growth factor subunit B (PDGF-B). AF1q-induced PDGF-B stimulated motility, migration, and invasion of MDA-MB-231LN cells, and AF1q up-regulated platelet-derived growth factor receptor (PDGFR) signaling. Further, AF1q-induced PDGFR signaling enhanced STAT3 activity through Src kinase activation, which could be blocked by the Src kinase inhibitor PP1. Moreover, AF1q up-regulated tyrosine kinase signaling through PDGFR signaling, which was blockable by imatinib. In conclusion, we demonstrated that enhanced AF1q expression contributes to persistent and oncogenic pYSTAT3 levels in invasive carcinoma cells by activating Src kinase through activation of the PDGF-B/PDGFR cascade. Therefore, AF1q plays an essential role as a cofactor in PDGF-B-driven STAT3 signaling.

Synergistic cell death in FLT3-ITD positive acute myeloid leukemia by combined treatment with metformin and 6-benzylthioinosine

Current therapy for acute myeloid leukemia (AML) primarily includes high-dose cytotoxic chemotherapy with or without allogeneic stem cell transplantation. Targeting unique cellular metabolism of cancer cells is a potentially less toxic approach. Monotherapy with mitochondrial inhibitors like metformin have met with limited success since escape mechanisms such as increased glycolytic ATP production, especially in hyperglycemia, can overcome the metabolic blockade. As an alternative strategy for metformin therapy, we hypothesized that the combination of 6-benzylthioinosine (6-BT), a broad-spectrum metabolic inhibitor, and metformin could block this drug resistance mechanism. Metformin treatment alone resulted in significant suppression of ROS and mitochondrial respiration with increased glycolysis accompanied by modest cytotoxicity (10-25%). In contrast, 6-BT monotherapy resulted in inhibition of glucose uptake, decreased glycolysis, and decreased ATP with minimal changes in ROS and mitochondrial respiration. The combination of 6-BT with metformin resulted in significant cytotoxicity (60-70%) in monocytic AML cell lines and was associated with inhibition of FLT3-ITD activated STAT5 and reduced c-Myc and GLUT-1 expression. Therefore, although the anti-tumor and metabolic effects of metformin have been limited by the metabolic reprogramming within cells, the novel combination of 6-BT and metformin targets this bypass mechanism resulting in reduced glycolysis, STAT5 inhibition, and increased cell death.

Stat5-deficient hematopoiesis is permissive for Myc-induced B-cell leukemogenesis

Despite being an attractive molecular target for both lymphoid and myeloid leukemias characterized by activated tyrosine kinases, the molecular and physiological consequences of reduced signal transducer and activator of transcription-5 (Stat5) during leukemogenesis are not well known. Stat5 is a critical regulator of mouse hematopoietic stem cell (HSC) self-renewal and is essential for normal lymphocyte development. We report that pan-hematopoietic deletion in viable adult Vav1-Cre conditional knockout mice as well as Stat5ab^null/null fetal liver transplant chimeras generated HSCs with reduced expression of quiescence regulating genes (Tie2, Mpl, Slamf1, Spi1, Cited2) and increased expression of B-cell development genes (Satb1, Dntt, Btla, Flk2). Using a classical murine B-cell acute lymphoblastic leukemia (B-ALL) model, we demonstrate that these HSCs were also poised to produce a burst of B-cell precursors upon expression of Bcl-2 combined with oncogenic Myc. This strong selective advantage for leukemic transformation in the background of Stat5 deficient hematopoiesis was permissive for faster initiation of Myc-induced transformation to B-ALL. However, once established, the B-ALL progression in secondary transplant recipients was Stat5-independent. Overall, these studies suggest that Stat5 can play multiple important roles that not only preserve the HSC compartment but can limit accumulation of potential pre-leukemic lymphoid populations.

STAT5 activation in B-cell acute lymphoblastic leukemia: damned if you do, damned if you don't

A significant role of the microenvironment in leukemogenesis is beginning to emerge. The leukemia cell microenvironment consists of not only the stromal and endothelial cell components but also the normal hematopoietic cells. Signal transducer and activator of transcription 5 (STAT5) is a latent transcription factor that is normally transiently activated by phosphorylation in response to microenvironmental signals. In hematopoietic cells, persistently activated STAT5 via aberrant receptor signaling, Janus kinases (JAKs), or intracellular tyrosine kinases is a bona fide driver of leukemogenesis. However, active IL-7/STAT5 signaling also protects the early B-cell genome by suppressing error-prone recombination and vulnerability to transformation. Along these lines, we have reported that lymphocyte development from transplanted STAT5-deficient fetal liver cells was blocked at the pre-pro-B-cell stage but when combined with transgenic Myc and Bcl-2 promoted faster initiation of B-ALL. Furthermore, inflammatory responses may also be involved in leukemia initiation in both pediatric and adult patients which are associated with decreased phosphorylation of STAT5. Likewise, additional targeted agents continue to be developed for precision medicine that prominently suppress signaling pathways. A common theme of all of these perturbations is potential risk for dysregulating hematopoiesis through general transcriptional modulation. Here we discuss the potential for STAT5 inhibition as a double edged sword in certain hematologic disorders, such as early B-cell lymphoblastic leukemias. Considering the rapid pace of understanding of the pre-leukemic decrease in poly-clonality that precedes leukemia, the functional changes associated with microenvironmental influences are thus of potential clinical significance.

AF1q is a novel TCF7 co-factor which activates CD44 and promotes breast cancer metastasis

AF1q is an MLL fusion partner that was identified from acute myeloid leukemia (AML) patients with t (1; 11) (q21; q23) chromosomal abnormality. The function of AF1q is not yet fully known, however, elevated AF1q expression is associated with poor clinical outcomes in various malignancies. Here, we show that AF1q specifically binds to T-cell-factor-7 (TCF7) in the Wnt signaling pathway and results in transcriptional activation of CD44 as well as multiple downstream targets of the TCF7/LEF1. In addition, enhanced AF1q expression promotes breast cancer cell proliferation, migration, mammosphere formation, and chemo-resistance. In xenograft models, enforced AF1q expression in breast cancer cells also promotes liver metastasis and lung colonization. In a cohort of 63 breast cancer patients, higher percentages of AF1q-positive cancer cells in primary sites were associated with significantly poorer overall survival (OS), disease-free survival (DFS), and brain metastasis-free survival (b-MFS). Using paired primary/metastatic samples from the same patients, we demonstrate that AF1q-positive breast cancer cells become dynamically dominant in the metastatic sites compared to the primary sites. Our findings indicate that breast cancer cells with a hyperactive AF1q/TCF7/CD44 regulatory axis in the primary sites may represent "metastatic founder cells" which have invasive properties.

The hematopoietic stem cell landscape

Hematopoietic stem cells (HSCs) play critical roles in regulating normal blood cell development. Although initially these cells were mysterious and difficult to study in isolation, those obstacles have progressively been rolled away in just a few decades to reveal a heterogeneity of repopulating activity, cell proliferation, and energy metabolism within defined stem cell populations based on drug transporter and cell surface marker expression. A wide range of new technologies have driven innovative discovery of the regulators of HSCs and continued to move the field forward toward a full view of the landscape of single HSCs at the gene and protein levels. It is the goal of this overview chapter to summarize the array of techniques included in the third edition of Hematopoietic Stem Cell Protocols which will aid investigators in the field.

Nanoproteomic assays on hematopoietic stem cells

Dysregulation of cytokine signaling pathways is associated with benign and malignant hematologic disorders. Improvements in therapy rely on understanding the biology of the pathways and the proteins involved. Studying these pathways in patient samples is challenging as samples are difficult to obtain, contain fewer cells, and are heterogeneous in nature. To address some of these difficulties, we have utilized the technique of microcapillary electrophoresis. Using the NanoPro 1000 system (ProteinSimple) which is built on an automated, capillary-based immunoassay platform, we have developed rapid and quantitative assays for specific proteins from relatively small sample sizes. The NanoPro provides precise and quantitative data of the phosphorylation states of a specific protein of interest. We describe our experience with NanoPro assay development and optimization with specific application toward understanding aberrant cytokine signaling in human leukemia cells.

A novel IL-25 signaling pathway through STAT5

IL-25 is a member of the IL-17 family of cytokines that promotes Th2 cell-mediated inflammatory responses. IL-25 signals through a heterodimeric receptor (IL-25R) composed of IL-17RA and IL-17RB, which recruits the adaptor molecule Act1 for downstream signaling. Although the role of IL-25 in potentiating type 2 inflammation is well characterized by its ability to activate the epithelium as well as T cells, the components of its signaling cascade remain largely unknown. In this study, we found that IL-25 can directly activate STAT5 independently of Act1. Furthermore, conditional STAT5 deletion in T cells or epithelial cells led to a defective IL-25-initiated Th2 polarization as well as defective IL-25 enhancement of Th2 responses. Finally, we found that STAT5 is recruited to the IL-25R in a ligand-dependent manner through unique tyrosine residues on IL-17RB. Together, these findings reveal a novel Act1-independent IL-25 signaling pathway through STAT5 activation.

STAT5 N-domain deleted isoforms are naturally occurring hypomorphs partially rescued in hematopoiesis by transgenic Bcl-2 expression

Signal transducer and activator of transcription 5 (STAT5) is a critical regulator of normal and leukemic lympho-myeloid development through activation downstream of early-acting cytokines, their receptors, and JAKs. Truncation of STAT5 can be mediated through alternative translation initiation from an internal start codon giving rise to N-terminally deleted isoforms. To determine whether these isoforms could be detected naturally in normal murine tissues, Western blot analyses were performed on heart, lung, brain, spleen, liver, and kidney. Relative expression of full-length to truncated STAT5 was variable among tissues. Since we have previously demonstrated that STAT5abΔN lacks the ability to effectively upregulate pro-survival signals and bcl-2 expression, we used a transgenic mouse approach to next determine whether constitutive expression of human Bcl-2 in STAT5ab(ΔN/ΔN) mouse hematopoietic cells could restore normal hematopoiesis. Transgenic H2K-Bcl-2 expression in hypomorphic STAT5ab(ΔN/ΔN) mice largely rescued peripheral B and T lymphocyte numbers whereas multilineage donor contribution was only rescued to levels about 10% of normal. At the hematopoietic stem cell level, direct competitive repopulation with H2K-Bcl-2/STAT5ab(ΔN/ΔN) against STAT5ab(ΔN/ΔN) competitor showed a corrective effect of Bcl-2 expression whether the STAT5ab(ΔN/ΔN) genotype was competed as the donor or as the host versus H2K-Bcl-2/STAT5ab(ΔN/ΔN) genotype bone marrow cells. Therefore, STAT5abΔN isoforms are heterogeneously expressed and lack key functional activities that can be partially rescued by adding back Bcl-2.

Capillary nano-immunoassay for Akt 1/2/3 and 4EBP1 phosphorylation in acute myeloid leukemia

Background

Overall cure rates in acute myeloid leukemia (AML) continue to range between 60-65% with disease relapse being a major cause of mortality. The PI3K-Akt-mTOR kinase pathway plays a vital role in pro-survival signals within leukemic cells and inhibition of this pathway is being investigated to improve patient outcomes. Tracking activation of multiple signaling proteins simultaneously in patient samples can be challenging especially with limiting cell numbers within rare sub-populations.

Methods

The NanoPro 1000 system (ProteinSimple) is built on an automated, capillary-based immunoassay platform and enables a rapid and quantitative analysis of specific proteins and their phosphorylation states. We have utilized this nano-immunoassay to examine activation of Akt 1/2/3 and downstream mTOR target - eukaryotic initiation factor 4E-Binding Protein 1 (4EBP1).

Results

Assays for Akt 1/2/3 and 4EBP1 were standardized using AML cell lines (MV4-11, MOLM-14, OCI-AML3 and HL-60) prior to testing in patient samples. Target inhibition was studied using mTOR 1/2 inhibitor AZD-8055 and results were corroborated by Western blotting. The assay was able to quantify nanogram amounts of 4EBP1 and Akt 1/2/3 in AML cell lines and primary pediatric AML samples and results were quantifiable, consistent and reproducible.

Conclusion

Our data provides a strong basis for testing this platform on a larger scale and our long term aim is to utilize this nano-immunoassay prospectively in de-novo AML to be able to identify poor responders who might benefit from early introduction of targeted therapy.

STAT5 in hematopoietic stem cell biology and transplantation

Signal transducer and activator of transcription 5 (STAT5) regulates normal lympho-myeloid development through activation downstream of early-acting cytokines, their receptors, and Janus kinases (JAKs). Despite a general understanding of the role of STAT5 in hematopoietic stem cell (HSC) proliferation, survival, and self-renewal, the transcriptional targets and mechanisms of gene regulation that control multi-lineage engraftment following transplantation for the most part remain to be understood. In this review, we focus on the role of STAT5 in HSC transplantation and recent developments toward identifying the relevant downstream target genes and their role as part of a pleiotropic STAT5 mediated signaling response.

Genotype-dependent effects of TGF-β1 on mast cell function: targeting the Stat5 pathway

We previously demonstrated that TGF-β1 suppresses IgE-mediated signaling in human and mouse mast cells in vitro, an effect that correlated with decreased expression of the high-affinity IgE receptor, FcεRI. The in vivo effects of TGF-β1 and the means by which it suppresses mast cells have been less clear. This study shows that TGF-β1 suppresses FcεRI and c-Kit expression in vivo. By examining changes in cytokine production concurrent with FcεRI expression, we found that TGF-β1 suppresses TNF production independent of FcεRI levels. Rather, IgE-mediated signaling was altered. TGF-β1 significantly reduced expression of Fyn and Stat5, proteins critical for cytokine induction. These changes may partly explain the effects of TGF-β1, because Stat5B overexpression blocked TGF-mediated suppression of IgE-induced cytokine production. We also found that Stat5B is required for mast cell migration toward stem cell factor, and that TGF-β1 reduced this migration. We found evidence that genetic background may alter TGF responses. TGF-β1 greatly reduced mast cell numbers in Th1-prone C57BL/6, but not Th2-prone 129/Sv mice. Furthermore, TGF-β1 did not suppress IgE-induced cytokine release and did increase c-Kit-mediated migration in 129/Sv mast cells. These data correlated with high basal Fyn and Stat5 expression in 129/Sv cells, which was not reduced by TGF-β1 treatment. Finally, primary human mast cell populations also showed variable sensitivity to TGF-β1-mediated changes in Stat5 and IgE-mediated IL-6 secretion. We propose that TGF-β1 regulates mast cell homeostasis, and that this feedback suppression may be dependent on genetic context, predisposing some individuals to atopic disease.

Mutation of STAT1/3 binding sites in gp130(FXXQ) knock-in mice does not alter hematopoietic stem cell repopulation or self-renewal potential

Interleukin (IL)-6 family cytokine signaling through gp130 and signal transducer and activator of transcription (STAT) activation is believed important for early hematopoiesis. To determine whether gp130/STAT1/3 physical interaction is required, we compared hematopoietic repopulating activities of embryonic day (E)14.5 fetal liver cells from gp130(FXXQ/FXXQ) knock-in mice, which have four mutated STAT1/3 binding sites. In hematopoietic cells, failure to tyrosine phosphorylate STAT3 by gp130 did not cause any significant effects on myeloid progenitor colony forming units (CFU) in vitro and or on competitive multilineage hematopoietic reconstitution. Serial transplantation of fetal liver (FL) cells was unaffected throughout primary, secondary, and tertiary transplants indicating normal self-renewal capacity. Even gp130(FXXQ/FXXQ) on the background of STAT5 deficiency, with known hematopoietic stem cell (HSC) repopulating dysfunction, did not further impair HSCs beyond that of STAT5 alone. Overall, the defective gp130-mediated STAT1/3 signaling is surprisingly dispensable for HSC function. However, since these mice lack both STAT1/3 binding sites there are several possible explanations for this result and these are discussed.

Th17 related cytokines in acute myeloid leukemia

Acute myeloid leukemia (AML) is the most common hematological malignancy in adults, characterized by distorted proliferation and development of myeloid cells and their precursors in blood and bone marrow. Impressive biologic advances have increased our understanding of leukemogenesis, however, little is known about the pathogenic events which lead to the initiation and progression of AML. T helper type 17 (Th17) cells are a unique subset of CD4+ T cells. They play important roles in the pathogenesis of many diseases, including inflammatory diseases, autoimmune diseases, and cancers. A range of cytokines, such as interleukin (IL)-23, transforming growth factor-beta (TGF-beta), IL-1beta, IL-6, IL-17, IL-22, and IL-21, have been shown related to Th17 cells. Some researchers have reported that the levels of Th17 and its related cytokines were different between normal cells and malignant AML cells, suggesting that Th17 might be involved in AML pathogenesis. In this review, we summarize current progress in the mechanisms of Th17 related cytokines in AML pathogenesis.

Integrative molecular and developmental biology of adult stem cells

Stem cells are believed to be important for regeneration of several adult tissues. Recently, adult stem cells with very broad differentiation potential have been identified although whether they represent vestigial primitive pluripotent stem cells or products of extremely rare de-differentiation events involving tissue-specific stem cells is not known. Transdifferentiation of tissue-specific stem cells across lineage boundaries has also been demonstrated but the relative inefficiency of the process in vivo, even in the presence of tissue damage, questions whether such a mechanism is of physiologic relevance. Interestingly, among adult stem cells, the capacity for lineage switching appears to be greatest in stem cells that can be cultured ex vivo for extended periods. If the normal cell fate decisions of diverse adult stem cell types could be reliably redirected at high frequency in situ, possible regenerative therapies for a wide variety of diseases could be envisioned. An integrated understanding of the transcriptional regulatory networks that comprise the various adult stem cell entities as well as the signaling pathways governing their differentiation into therapeutically useful cell types will facilitate clinical application of these exciting findings.

Novel mechanism for Fc{epsilon}RI-mediated signal transducer and activator of transcription 5 (STAT5) tyrosine phosphorylation and the selective influence of STAT5B over mast cell cytokine production

Previous studies indicate that STAT5 expression is required for mast cell development, survival, and IgE-mediated function. STAT5 tyrosine phosphorylation is swiftly and transiently induced by activation of the high affinity IgE receptor, FcεRI. However, the mechanism for this mode of activation remains unknown. In this study we observed that STAT5 co-localizes with FcεRI in antigen-stimulated mast cells. This localization was supported by cholesterol depletion of membranes, which ablated STAT5 tyrosine phosphorylation. Through the use of various pharmacological inhibitors and murine knock-out models, we found that IgE-mediated STAT5 activation is dependent upon Fyn kinase, independent of Syk, PI3K, Akt, Bruton's tyrosine kinase, and JAK2, and enhanced in the context of Lyn kinase deficiency. STAT5 immunoprecipitation revealed that unphosphorylated protein preassociates with Fyn and that this association diminishes significantly during mast cell activation. SHP-1 tyrosine phosphatase deficiency modestly enhanced STAT5 phosphorylation. This effect was more apparent in the absence of Gab2, a scaffolding protein that docks with multiple negative regulators, including SHP-1, SHP-2, and Lyn. Targeting of STAT5A or B with specific siRNA pools revealed that IgE-mediated mast cell cytokine production is selectively dependent upon the STAT5B isoform. Altogether, these data implicate Fyn as the major positive mediator of STAT5 after FcεRI engagement and demonstrate importantly distinct roles for STAT5A and STAT5B in mast cell function.

Serine phosphorylation of the Stat5a C-terminus is a driving force for transformation

Persistent tyrosine phosphorylation of Stat3 and Stat5 is associated with oncogenic activity. Phosphorylation of the conserved tyrosine residue (pTyr) was long believed to be the only essential prerequisite to promote activation and nuclear translocation of Stat proteins. It has become evident, however, that post-translational protein modifications like serine phosphorylation, acetylation, glycosylation as well as protein splicing and processing constitute further regulatory mechanisms to modulate Stat transcriptional activity and to provide an additional layer of specificity to Jak-Stat signal transduction. Significantly, most vertebrate Stat proteins contain one conserved serine phosphorylation site within their transactivation domains. This phosphorylation motif is located within a P(M)SP sequence. Stat transcription factor activity is negatively influenced by mutation of the serine to alanine. Moreover, it was shown for both Stat3 and Stat5 that their capacity to transform cells was diminished. This review addresses recent advances in understanding the regulation and the biochemical and biological consequences of Stat serine phosphorylation. In particular, we discuss their role in persistently activated Stat proteins for cancer research.

Multiple apoptotic defects in hematopoietic cells from mice lacking lipocalin 24p3

The lipocalin mouse 24p3 has been implicated in diverse physiological processes, including apoptosis, iron trafficking, development and innate immunity. Studies from our laboratory as well as others demonstrated the proapoptotic activity of 24p3 in a variety of cultured models. However, a general role for the lipocalin 24p3 in the hematopoietic system has not been tested in vivo. To study the role of 24p3, we derived 24p3 null mice and back-crossed them onto C57BL/6 and 129/SVE backgrounds. Homozygous 24p3(-/-) mice developed a progressive accumulation of lymphoid, myeloid, and erythroid cells, which was not due to enhanced hematopoiesis because competitive repopulation and recovery from myelosuppression were the same as for wild type. Instead, apoptotic defects were unique to many mature hematopoietic cell types, including neutrophils, cytokine-dependent mast cells, thymocytes, and erythroid cells. Thymocytes isolated from 24p3 null mice also displayed resistance to apoptosis-induced by dexamethasone. Bim response to various apoptotic stimuli was attenuated in 24p3(-/-) cells, thus explaining their resistance to the ensuing cell death. The results of these studies, in conjunction with those of previous studies, reveal 24p3 as a regulator of the hematopoietic compartment with important roles in normal physiology and disease progression. Interestingly, these functions are limited to relatively mature blood cell compartments.

MIR29B regulates expression of MLLT11 (AF1Q), an MLL fusion partner, and low MIR29B expression associates with adverse cytogenetics and poor overall survival in AML

MLLT11, an MLL fusion partner, is a poor prognostic biomarker for paediatric acute myeloid leukaemia (AML), adult normal cytogenetics AML, and adult myelodysplastic syndrome. MLLT11 is highly regulated during haematopoietic progenitor differentiation and development but its regulatory mechanisms have not been defined. In this study, we demonstrate by transfection experiments that MIR29B directly regulates MLLT11 expression in vitro. MIR29B expression level was also inversely related to MLLT11 expression in a cohort of 56 AML patients (P<0·05). AML patients with low MIR29B/elevated MLLT11 expression had poor overall survival (P=0·038). Therefore, MIR29B may be a potential prognostic biomarker for AML patients.

Grb2-associated binding (Gab) proteins in hematopoietic and immune cell biology

Grb2-associated binding (Gab) scaffolding/adapter proteins are a family of three members including mammalian Gab1, Gab2, and Gab3 that are highly conserved. Since the discovery of these proteins, there has been an extensive amount of work done to better understand Gab functional roles in multiple signaling pathways, typically acting as a downstream effectors of receptor-tyrosine kinase (RTK)-triggered signal transduction. In addition to their participation in hematopoiesis, Gabs play important roles in regulation of immune response and in also in cancer cell signaling. Gabs may play complex roles and thus a complete understanding of their interactions and how they modulate hematopoietic and immune cell biology remains to be determined. This review will cover the most recent findings including the involvement of Gabs in disease development and signaling which will be important for design of future therapeutic interventions.

Effective targeting of STAT5-mediated survival in myeloproliferative neoplasms using ABT-737 combined with rapamycin

Signal transducer and activator of transcription-5 (STAT5) is a critical transcription factor for normal hematopoiesis and its sustained activation is associated with hematologic malignancy. A persistently active mutant of STAT5 (STAT5a^S711F) associates with Grb2 associated binding protein 2 (Gab2) in myeloid leukemias and promotes growth in vitro through AKT activation. Here we have retrovirally transduced wild-type or Gab2^−/− mouse bone marrow cells expressing STAT5a^S711F and transplanted into irradiated recipient mice to test an in vivo myeloproliferative disease (MPD) model. To target Gab2-independent AKT/mTOR activation, wild-type mice were treated separately with rapamycin. In either case, mice lacking Gab2 or treated with rapamycin displayed attenuated myeloid hyperplasia and modestly improved survival, but the effects were not cytotoxic and were reversible. To improve upon this approach, in vitro targeting of STAT5-mediated AKT/mTOR using rapamycin was combined with inhibition of the STAT5 direct target genes bcl-2 and bcl-X_L using ABT-737. Striking synergy with both drugs was observed in mouse BaF3 cells expressing STAT5a^S711F, TEL-JAK2, or BCR-ABL and in the relatively single agent-resistant human BCR-ABL positive K562 cell line. Therefore, targeting distinct STAT5 mediated survival signals, e.g. bcl-2/bcl-X_L and AKT/mTOR may be an effective therapeutic approach for human myeloproliferative neoplasms.

Direct and differential suppression of myeloid-derived suppressor cell subsets by sunitinib is compartmentally constrained

The antiangiogenic drug sunitinib is a receptor tyrosine kinase inhibitor with significant, yet not curative, therapeutic effects in metastatic renal cell carcinoma (RCC). Sunitinib is also an immunomodulator, potently reversing myeloid-derived suppressor cell (MDSC) accumulation and T-cell inhibition in the blood even of nonresponder RCC patients. We observed that sunitinib similarly prevented MDSC accumulation and restored normal T-cell function to the spleens of tumor-bearing mice, independent of the capacity of sunitinib to inhibit tumor progression (RENCA>CT26>4T1). Both monocytic and neutrophilic splenic MDSC were highly repressible by sunitinib. In contrast, MDSC within the microenvironment of 4T1 tumors or human RCC tumors proved highly resistant to sunitinib and ambient T-cell function remained suppressed. Proteomic analyses comparing tumor to peripheral compartments showed that granulocyte macrophage colony-stimulating factor (GM-CSF) predicted sunitinib resistance and recombinant GM-CSF conferred sunitinib resistance to MDSC in vivo and in vitro. MDSC conditioning with GM-CSF uniquely inhibited signal transducers and activators of transcription (STAT3) and promoted STAT5 activation. STAT5ab(null/null) MDSC were rendered sensitive to sunitinib in the presence of GM-CSF in vitro. We conclude that compartment-dependent GM-CSF exposure in resistant tumors may account for the regionalized effect of sunitinib upon host MDSC modulation and hypothesize that ancillary strategies to decrease such regionalized escape will enhance the potency of sunitinib as an immunomodulator and a cancer therapy.

Abstract 4296A: Myeloproliferative disease induced by leukemogenic Ptpn11 (Shp-2) phosphatase arises from hematopoietic stem cells

Germline and somatic gain-of-function (GOF) mutations in the tyrosine phosphatase PTPN11 (SHP-2) are associated with juvenile myelomonocytic leukemia (JMML), a clonal myeloproliferative disease (MPD) of early childhood. However, the mechanism by which PTPN11 mutations induce this disease is not fully understood. Here we report that the germline mutation Ptpn11D61G increases mouse hematopoietic stem cell (HSC) activity, as evidenced by accelerated cycling, increased stem cell pool, enhanced self-renewal, and elevated short-term and long-term repopulating capabilities. More importantly, the MPD is reproduced in primary and secondary recipient mice transplanted with Ptpn11D61G mutant marrow cells or sorted Lineage- Sca-1+c-Kit+ cells, indicative of the long-term stem cell origin of the disease. The leukemogenic effect of Ptpn11D61G mutation appears to be attributed to enhancing cytokine signaling. Furthermore, HSC homeostasis disrupted by Ptpn11D61G mutation is partially restored by deletion of Gab2, an important interacting protein/target of SHP-2. MPD phenotypes are markedly ameliorated in Ptpn11D61G/+/Gab2−/− double mutant mice. Together, these studies suggest that SHP-2 associated JMML arises from HSCs and that targeting SHP-2/Gab2 mediated signaling in stem cells may be useful for the treatment of this disease.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4296A.

Gab2 promotes hematopoietic stem cell maintenance and self-renewal synergistically with STAT5

Background

Grb2-associated binding (Gab) adapter proteins play major roles in coordinating signaling downstream of hematopoietic cytokine receptors. In hematopoietic cells, Gab2 can modulate phosphatidylinositol–3 kinase and mitogen associated protein kinase activities and regulate the long-term multilineage competitive repopulating activity of hematopoietic stem cells (HSCs). Gab2 may also act in a linear pathway upstream or downstream of signal transducer and activator of transcription-5 (STAT5), a major positive regulator of HSC function. Therefore, we aimed to determine whether Gab2 and STAT5 function in hematopoiesis in a redundant or non-redundant manner.

Methodology/Principal Findings

To do this we generated Gab2 mutant mice with heterozygous and homozygous deletions of STAT5. In heterozygous STAT5 mutant mice, deficiencies in HSC/multipotent progenitors were reflected by decreased long-term repopulating activity. This reduction in repopulation function was mirrored in the reduced growth response to early-acting cytokines from sorted double mutant c-Kit⁺Lin⁻Sca-1⁺ (KLS) cells. Importantly, in non-ablated newborn mice, the host steady-state engraftment ability was impaired by loss of Gab2 in heterozygous STAT5 mutant background. Fetal liver cells isolated from homozygous STAT5 mutant mice lacking Gab2 showed significant reduction in HSC number (KLS CD150⁺CD48⁻), reduced HSC survival, and dramatic loss of self-renewal potential as measured by serial transplantation.

Conclusions/Significance

These data demonstrate new functions for Gab2 in hematopoiesis in a manner that is non-redundant with STAT5. Furthermore, important synergy between STAT5 and Gab2 was observed in HSC self-renewal, which might be exploited to optimize stem cell-based therapeutics.

Conditional deletion of STAT5 in adult mouse hematopoietic stem cells causes loss of quiescence and permits efficient nonablative stem cell replacement

Currently, there is a major need in hematopoietic stem cell (HSC) transplantation to develop reduced-intensity regimens that do not cause DNA damage and associated toxicities and that allow a wider range of patients to receive therapy. Cytokine receptor signals through c-Kit and c-Mpl can modulate HSC quiescence and engraftment, but the intracellular signals and transcription factors that mediate these effects during transplantation have not been defined. Here we show that loss of one allele of signal transducer and activator of transcription 5 (STAT5) in nonablated adult mutant mice permitted engraftment with wild-type HSC. Conditional deletion of STAT5 using Mx1-Cre caused maximal reduction in STAT5 mRNA (> 97%) and rapidly decreased quiescence-associated c-Mpl downstream targets (Tie-2, p57), increased HSC cycling, and gradually reduced survival and depleted the long-term HSC pool. Host deletion of STAT5 was persistent and permitted efficient donor long-term HSC engraftment in primary and secondary hosts in the absence of ablative conditioning. Overall, these studies establish proof of principle for targeting of STAT5 as novel transplantation conditioning and demonstrate, for the first time, that STAT5, a mitogenic factor in most cell types, including hematopoietic progenitors, is a key transcriptional regulator that maintains quiescence of HSC during steady-state hematopoiesis.

Elevated AF1q expression is a poor prognostic marker for adult acute myeloid leukemia patients with normal cytogenetics

Nearly half of the patients with newly diagnosed acute myeloid leukemia have normal cytogenetics (NC-AML) and are classified as intermediate risk, but their 5-year overall survival (OS) ranges from 24 to 42%. Therefore, molecular biomarkers to identify poor-risk patients are needed. Elevated AF1q expression in the absence of specific poor cytogenetics is associated with poor outcomes in pediatric patients with AML and adult patients with myelodysplastic syndrome. We examined AF1q expression in 290 patients with NC-AML. We found that patients with low AF1q (n = 73) expression (AF1q(low)) have better OS (P = 0.026), disease-free survival (P = 0.1), and complete remission rate (P = 0.06) when compared with patients with high AF1q expression (AF1q(high) n = 217). The patients with AF1q(high) had significantly greater incidence of concurrent tyrosine kinase3 internal tandem duplication. A subgroup of the patients with AF1q(high) who received allogeneic stem cell transplantation (SCT) had a significant better relapse-free survival when compared with patients who received chemotherapy/autologous SCT (P = 0.04). This study suggests that high AF1q expression is a poor prognostic marker for adult patients with NC-AML.

IL-3 induces basophil expansion in vivo by directing granulocyte-monocyte progenitors to differentiate into basophil lineage-restricted progenitors in the bone marrow and by increasing the number of basophil/mast cell progenitors in the spleen

Recent work has established important roles for basophils in regulating immune responses. To exert their biological functions, basophils need to be expanded to critical numbers. However, the mechanisms underlying basophil expansion remain unclear. In this study, we established that IL-3 played an important role in the rapid and specific expansion of basophils. We found that the IL-3 complex (IL-3 plus anti-IL-3 Ab) greatly facilitated the differentiation of GMPs into basophil lineage-restricted progenitors (BaPs) but not into eosinophil lineage-restricted progenitors or mast cells in the bone marrow. We also found that the IL-3 complex treatment resulted in approximately 4-fold increase in the number of basophil/mast cell progenitors (BMCPs) in the spleen. IL-3-driven basophil expansion depended on STAT5 signaling. We showed that GMPs but not common myeloid progenitors expressed low levels of IL-3 receptor. IL-3 receptor expression was dramatically up-regulated in BaPs but not eosinophil lineage-restricted progenitors. Approximately 38% of BMCPs expressed the IL-3R alpha-chain. The up-regulated IL-3 receptor expression was not affected by IL-3 or STAT5. Our findings demonstrate that IL-3 induced specific expansion of basophils by directing GMPs to differentiate into BaPs in the bone marrow and by increasing the number of BMCPs in the spleen.

An ENU-induced recessive mutation in Mpl leads to thrombocytopenia with overdominance

OBJECTIVE

The aim of this study was to identify and characterize the causative mutation in the thrombocytopenic mouse strain HLB219 that was generated at the Jackson Laboratory as part of a large-scale N-ethyl-N-nitrosourea mutagenesis screen.

MATERIALS AND METHODS

The HLB219 mutation was identified by interval mapping of F2 mice generated from intercross breeding of HLB219 to both BALB/cByJ (BALB) and 129/SvImJ (129/Sv). Mpl was identified as a candidate gene and sequenced. The mutation was characterized in vivo in mouse hematopoietic stem/progenitor cell assays and in cell culture by expression in Ba/F3 cells.

RESULTS

A novel mutation in the thrombopoietin (TPO) receptor Mpl in HLB219 mice caused a Cys-->Arg substitution at codon 40 in the extracellular region of the receptor. Mice homozygous for the Mpl(hlb219) mutation had an 80% decrease in the number of platelets in comparison to the wild-type C57BL/6J strain, low numbers of bone marrow megakaryocytes, high TPO levels, and decreased competitive repopulating ability, consistent with a loss-of-function mutation in the receptor. Mice heterozygous for Mpl(hlb219) however, showed an overdominance effect with a significant increase in platelet number. Functional analysis in vitro demonstrated that Ba/F3 cells expressing the mutant MPL(hlb219) protein failed to activate extracellular signal-regulated kinase and signal transducers and activators of transcription 5, but proliferated in the absence of TPO and required constitutive phosphorylation of RAC-alpha serine/threonine protein kinase (AKT) for cytokine-independent growth.

CONCLUSION

Thrombocytopenia in HLB219 mice is caused by a recessive mutation in Mpl that abrogates mitogen-activated protein kinase-extracellular signal regulated kinase and janus kinase-signal transducers and activators of transcription signaling.

STAT3- and STAT5-dependent pathways competitively regulate the pan-differentiation of CD34pos cells into tumor-competent dendritic cells

The clinical outcomes of dendritic cell (DC)-based immunotherapy remain disappointing, with DCs often displaying a tenuous capacity to complete maturation and DC1 polarization in the tumor host. Surprisingly, we observed that the capacity for successful DC1 polarization, including robust IL12p70 production, could be regulated by STAT-dependent events even prior to DC differentiation. Exposure of CD34(pos) cells to single-agent granulocyte-macrophage colony-stimulating factor (GMCSF) induced multilineage, STAT5-dependent differentiation, including DCs that failed to mature in the absence of further exogenous signals. In contrast, Flt3L induced nearly global differentiation of CD34(pos) cells into spontaneously maturing DCs. IL-6 synergized with Flt3L to produce explosive, STAT3-dependent proliferation of phenotypically undifferentiated cells that nevertheless functioned as committed DC1 precursors. Such precursors not only resisted many tumor-associated immunosuppressants, but also responded to tumor contact or TGFbeta with facilitated DC maturation and IL12p70 production, and displayed a superior capacity to reverse tumor-induced T-cell tolerance. GMCSF preempted Flt3L or Flt3L plus IL-6 licensing by blocking STAT3 activation and promoting STAT5-dependent differentiation. Paradoxically, following overt DC differentiation, STAT5 enhanced whereas STAT3 inhibited DC1 polarization. Therefore, nonoverlapping, sequential activation of STAT3 and STAT5, achievable by sequenced exposure to Flt3L plus IL-6, then GMCSF, selects for multilog expansion, programming, and DC1 polarization of tumor-competent DCs from CD34(pos) cells.

Identification of 6-benzylthioinosine as a myeloid leukemia differentiation-inducing compound

As the pathophysiology of acute myelogenous leukemia (AML) involves a block of myeloid maturation, a desirable therapeutic strategy is to induce leukemic cell maturation to increase the efficacy and to avoid the side effects of traditional chemotherapeutics. Through a compound library screen, 6-benzylthioinosine (6BT) was identified as a promising differentiation-inducing agent. 6BT induces monocytic differentiation of myeloid leukemia cell lines such as HL-60 and OCI-AML3, as well as primary patient samples as evidenced by morphology, immunophenotyping, and nitroblue tetrazolium reduction. Not only can 6BT induce differentiation but a subset of AML cell lines such as MV4-11 and HNT34 instead undergo 6BT-mediated cell death. Despite inducing cell death in some leukemic cells, 6BT exhibits extremely low toxicity on several nonmalignant cells such as fibroblasts, normal bone marrow, and endothelial cells. This toxicity profile may relate to the function of 6BT as an inhibitor of the nucleoside transporter, ent1, which is thought to prevent it from entering many cell types. In contrast, 6BT likely enters at least some leukemic cell lines as shown by its requirement for phosphorylation for its differentiation activity. 6BT is also able to synergize with currently used myeloid differentiation agents such as ATRA and decitabine. Early studies indicate that the mechanism of action of this compound may involve ATP depletion that leads to growth inhibition and subsequent differentiation. Besides in vitro activity, 6BT also shows the ability to impair HL-60 and MV4-11 tumor growth in nude mice. 6BT is a promising new monocytic differentiation agent with apparent leukemic cell-specific activity.

Hematopoiesis in mice is extremely resilient to wide variation in TIMP/MMP balance

Tissue inhibitors of matrix metalloproteinases (TIMPs) are natural inhibitors of matrix metalloproteinases (MMPs) and are associated with normal and pathologic extracellular matrix turnover. Because the microenvironment is critical for normal hematopoietic stem/progenitor cell function, we aimed to determine whether alterations in the TIMP/MMP balance impact upon normal hematopoiesis in mice. We have used both overexpression and knockout mouse models to determine whether early hematopoiesis is susceptible to potentially pathologic changes in TIMP/MMP level. These studies used TIMP-1(-/-) mice and retroviral vectors co-expressing human TIMP-1 or TIMP-2 linked with the green fluorescent protein (GFP) transduced into bone marrow (BM) cells and transplanted into lethally-irradiated recipient mice. Loss of TIMP-1 in knockout mice or retroviral overexpression of TIMP-1 or TIMP-2 did not alter hematopoietic stem/progenitor function during steady-state hematopoiesis. Surprisingly, even when applying hematopoietic stress through mobilization, chemotaxis, or myelosuppression, murine hematopoiesis was not adversely affected by TIMP-1 or TIMP-2 level. We conclude that TIMP/MMP balance alone does not exert significant influence on blood cell development and homeostasis. An important corollary of these studies is that specific modulation using MMP inhibitors for cancer or immunologic therapy is unlikely to have adverse hematopoietic side effects.

The expanding tool kit for hematopoietic stem cell research

Hematopoietic stem cells (HSC) play critical roles in maintaining blood cell production for the lifetime of the organism. Considerable progress has been made in their isolation from mouse bone marrow to high levels of purity based on a combination of cell-surface phenotype and functional characteristics. In addition, in vitro assays have been established that provide important tools for study of hematopoietic differentiation from HSC and for differentiation to generate HSC from embryonic stem cells. Although these in vitro studies provide a window on the temporal function and differentiation of HSC progeny, the transplantation assay still serves as the gold standard for quantitative and qualitative analysis of murine HSC biology. There are now many flavors of syngeneic and xenogeneic HSC transplant, all focused on quantitative assessment of repopulating function. As a vehicle for genetic modification of HSC, retroviral-mediated gene transfer followed by transplantation has had a major impact upon our understanding of genetic disorders, gene therapy, and leukemogenesis. This overview chapter summarizes the growing number of tools available for HSC research and specifically ties together the methods in chapters of the second edition of Hematopoietic Stem Cell Protocols.

Retroviral transduction of murine hematopoietic stem cells

Hematopoietic stem cells (HSC) are inherently rare cell types that cannot be obtained in sufficient amounts for classical biochemical characterization. To facilitate functional studies of murine HSC and hematopoietic development, the technique of retroviral-mediated gene transfer provides a useful tool. The generation of high titer retroviral vectors permits transduction of stem cells with a variety of genes and leads to long-term marking in the blood of recipient mice. Optimized promoter/enhancers facilitate high-level transgene expression in mice transplanted with transduced bone marrow (BM) cells. The co-expression of reporter genes along with a gene of interest greatly facilitates tracking donor engraftment of transduced hematopoietic progeny following stem cell transplantation. This methodology can be used to reconstitute defective function in a mutant background or to study protein function during hematopoiesis by overexpression. Despite limitations such as integration site variegation and copy number-dependent effects, this approach is rapid and efficient compared with transgenic mouse technology. In this chapter, we review this broadly applicable technique for achieving high-level murine BM stem cell transduction. We also describe methods for transplantation and subsequent analysis of transplanted mice as a bona fide assay for the stem cell transduction efficiency.

Stat5 is essential for early B cell development but not for B cell maturation and function

The two closely related Stat5 (Stat5A and Stat5B) proteins are activated by a broad spectrum of cytokines. However, with the complication of the involvement of Stat5A/5B in stem cell function, the role of Stat5A/5B in the development and function of lymphocytes, especially B cells, is not fully understood. In this study, we demonstrated that Stat5A/5B(-/-) fetal liver cells had severe diminution of B cell progenitors but clearly had myeloid progenitors. Consistently, the mutant fetal liver cells could give rise to hemopoietic progenitors and myeloid cells but not B cells beyond pro-B cell progenitors in lethally irradiated wild-type or Jak3(-/-) mice. Deletion of Stat5A/5B in vitro directly impaired IL-7-mediated B cell expansion. Of note, reintroduction of Stat5A back into Stat5A/5B(-/-) fetal liver cells restored their abilities to develop B cells. Importantly, CD19-Cre-mediated deletion of Stat5A/5B in the B cell compartment specifically impaired early B cell development but not late B cell maturation. Moreover, the B cell-specific deletion of Stat5A/5B did not impair splenic B cell survival, proliferation, and Ig production. Taken together, these data demonstrate that Stat5A/5B directly control IL-7-mediated early B cell development but are not required for B cell maturation and Ig production.

Cited2 is required for normal hematopoiesis in the murine fetal liver

Cited2 (cAMP-responsive elementbinding protein [CBP]/p300-interacting transactivators with glutamic acid [E] and aspartic acid [D]-rich tail 2) is a newly identified transcriptional modulator. Knockout of the Cited2 gene results in embryonic lethality with embryos manifesting heart and neural tube defects. Cited2-/- fetal liver displayed significant reduction in the numbers of Lin(-)c-Kit+Sca-1+ cells, Lin(-)c-Kit+ cells, and progenitor cells of different lineages. Fetal liver cells from Cited2-/- embryos gave rise to markedly reduced number of colonies in the colony-forming unit assay. Primary and secondary transplantation studies showed significantly compromised reconstitution of T-lymphoid, B-lymphoid, and myeloid lineages in mice that received a transplant of Cited2-/- fetal liver cells. Competitive reconstitution experiments further showed that fetal liver hematopoietic stem cell (HSC) function is severely impaired due to Cited2 deficiency. Microarray analysis showed decreased expression of Wnt5a and a panel of myeloid molecular markers such as PRTN3, MPO, Neutrophil elastase, Cathepsin G, and Eosinophil peroxidase in Cited2-/- fetal livers. Decreased expression of Bmi-1, Notch1, LEF-1, Mcl-1, and GATA2 was also observed in Cited2-/- Lin(-)c-Kit+ cells. The present study uncovers for the first time a novel role of Cited2 in the maintenance of hematopoietic homeostasis during embryogenesis and thus provides new insights into the molecular regulation of hematopoietic development.

Complex systems biology approach to understanding coordination of JAK-STAT signaling

In this work, we search for coordination as an organizing principle in a complex signaling system using a multilevel hierarchical paradigm. The objective is to explain the underlying mechanism of Interferon (IFN(gamma)) induced JAK-STAT (specifically JAK1/JAK2-STAT1) pathway behavior. Starting with a mathematical model of the pathway from the literature, we modularize the system using biological knowledge via principles of biochemical cohesion, biological significance, and functionality. The modularized system is then used as a basis for in silico inhibition, knockdown/deletion and perturbation experiments to discover a coordination mechanism. Our analysis shows that a module representing the SOCS1 complex can be identified as the coordinator. Analysis of the coordinator can then be used for the selection of biological experiments for the discovery of 'soft' molecular drug targets, that could lead to the development of improved therapeutics. The coordinator identified is also being investigated to determine its relationship to pathological conditions.

Abnormal hematopoiesis in Gab2 mutant mice

Gab2 is an important adapter molecule for cytokine signaling. Despite its major role in signaling by receptors associated with hematopoiesis, the role of Gab2 in hematopoiesis has not been addressed. We report that despite normal numbers of peripheral blood cells, bone marrow cells, and c-Kit(+)Lin(-)Sca-1(+) (KLS) cells, Gab2-deficient hematopoietic cells are deficient in cytokine responsiveness. Significant reductions in the number of colony-forming units in culture (CFU-C) in the presence of limiting cytokine concentrations were observed, and these defects could be completely corrected by retroviral complementation. In earlier hematopoiesis, Gab2-deficient KLS cells isolated in vitro responded poorly to hematopoietic growth factors, resulting in an up to 11-fold reduction in response to a cocktail of stem cell factor, flt3 ligand, and thrombopoietin. Gab2-deficient c-Kit(+)Lin(-) cells also demonstrate impaired activation of extracellular signal-regulated kinase (ERK) and S6 in response to IL-3, which supports defects in activating the phosphatidylinositol-3 kinase (PI-3K) and mitogen-associated protein kinase (MAPK) signaling cascades. Associated with the early defects in cytokine response, competitive transplantation of Gab2(-/-) bone marrow cells resulted in defective long-term multilineage repopulation. Therefore, we demonstrate that Gab2 adapter function is intrinsically required for hematopoietic cell response to early-acting cytokines, resulting in defective hematopoiesis in Gab2-deficient mice.

Hematopoietic reconstitution with androgenetic and gynogenetic stem cells

Parthenogenetic embryonic stem (ES) cells with two oocyte-derived genomes (uniparental) have been proposed as a source of autologous tissue for transplantation. The therapeutic applicability of any uniparental cell type is uncertain due to the consequences of genomic imprinting that in mammalian uniparental tissues causes unbalanced expression of imprinted genes. We transplanted uniparental fetal liver cells into lethally irradiated adult mice to test their capacity to replace adult hematopoietic tissue. Both maternal (gynogenetic) and paternal (androgenetic) derived cells conveyed long-term, multilineage reconstitution of hematopoiesis in recipients, with no associated pathologies. We also establish that uniparental ES cells can differentiate into transplantable hematopoietic progenitors in vitro that contribute to long-term hematopoiesis in recipients. Hematopoietic tissue in recipients maintained fidelity of parent-of-origin methylation marks at the Igf2/H19 locus; however, variability occurred in the maintenance of parental-specific methylation marks at other loci. In summary, despite genomic imprinting and its consequences on development that are particularly evident in the androgenetic phenotype, uniparental cells of both parental origins can form adult-transplantable stem cells and can repopulate an adult organ.

STAT5 signaling in normal and pathologic hematopoiesis

Hematopoietic development is highly dependent upon cytokine/receptor initiated signaling pathways. Of those activated in hematopoietic cells, the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway plays a major role. This review focuses on the key role of STAT5 activation in hematopoietic stem cells and early hematopoietic progenitor cells of normal and leukemic hematopoiesis. In normal hematopoietic stem cells STAT5 is required for robust competitive repopulation and proliferative responses to early acting cytokines. Activation of STAT5 by many activated receptor tyrosine kinases as well as by JAK2 and JAK3 has also been associated with hematologic malignancies and can result in cytokine-independent cell expansion. The biology of STAT5 function and its potential cooperation with other signaling pathways has become a key area of focus in the new era of molecularly targeted therapeutics for hematologic malignancy. In particular, interactions with Grb2-associated binding protein (Gab2) have linked STAT5 with the phosphatidylinositol-3-kinase pathway and its downstream signaling. Missing is a full understanding of the structure-function relationship of STAT5 activation, including functional targets and cooperating partners required to differentiate normal vs. leukemic STAT5 activation. This review summarizes the latest understanding of leukemogenesis and pathophysiology associated with constitutive STAT5 activation in hematologic malignancies.

Stat5 expression is required for IgE-mediated mast cell function

The mast cell (MC) inflammatory response is now linked not only to atopy, but also to arthritis, multiple sclerosis, heart disease, and resistance to bacterial infection. In the current study, we demonstrate that the signal transducer and activator of transcription 5 (Stat5) is rapidly activated by IgE cross-linkage, and that its expression is critical to the MC response. Stat5-deficient (Stat5KO) MC demonstrated a significant decrease in IgE-mediated degranulation, leukotriene B4 production, cytokine secretion, and survival signals. The defect in cytokine production may be caused by decreased cytokine mRNA stability. Stat5KO MC-induced cytokine mRNAs normally following IgE cross-linkage, but these mRNAs were not sustained over time and were degraded at twice the rate observed in WT cells. Interestingly, the RNA destabilizing protein tristetraprolin was induced following IgE cross-linkage in Stat5KO but not wild-type cells. Moreover, reducing tristetraprolin expression via short hairpin RNA transfection significantly increased IL-13 production in Stat5KO MC. Our work demonstrates that Stat5 is a critical factor in IgE-induced MC activation, acting in part via posttranscriptional control of cytokine mRNA stability. These data have a direct impact on MC-associated inflammatory and autoimmune diseases.

Effects of a leukemia-associated gain-of-function mutation of SHP-2 phosphatase on interleukin-3 signaling

Mutations in SHP-2 phosphatase that cause hyperactivation of its catalytic activity have been identified in human leukemias, particularly juvenile myelomonocytic leukemia, which is characterized by hypersensitivity of myeloid progenitor cells to granulocyte macrophage colony-stimulating factor and interleukin (IL)-3. However, the molecular mechanisms by which gain-of-function (GOF) mutations of SHP-2 induce hematopoietic malignancies are not fully understood. Our previous studies have shown that SHP-2 plays an essential role in IL-3 signal transduction in both catalytic-dependent and -independent manners and that overexpression (5-6-fold) of wild type (WT) SHP-2 attenuates IL-3-mediated hematopoietic cell function through accelerated dephosphorylation of STAT5. These results raised the possibility that SHP-2-associated leukemias are not solely attributed to the increased catalytic activity of GOF mutant SHP-2. GOF mutant SHP-2 must have gained additional capacities. To test this possibility, we investigated effects of a GOF mutation of SHP-2 (SHP-2 E76K) on hematopoietic cell function and IL-3 signal transduction by comparing with those of overexpressed WT SHP-2. Our results showed that SHP-2 E76K mutation caused myeloproliferative disease in mice, while overexpression of WT SHP-2 decreased hematopoietic potential of the transduced cells in recipient animals. The E76K mutation in the N-terminal Src homology 2 domain increased interactions of mutant SHP-2 with Grb2, Gab2, and p85, leading to hyperactivation of IL-3-induced Erk and phosphatidylinositol 3-kinase (PI3K) pathways. In addition, despite the substantial increase in the catalytic activity, dephosphorylation of STAT5 by SHP-2 E76K was dampened. Furthermore, catalytically inactive SHP-2 E76K with an additional C459S mutation retained the capability to increase the interaction with Gab2 and to enhance the activation of the PI3K pathway. Taken together, these studies suggest that in addition to the elevated catalytic activity, fundamental changes in physical and functional interactions between GOF mutant SHP-2 and signaling partners also play an important role in SHP-2-related leukemigenesis.

Bone marrow transplantation combined with gene therapy to induce antigen-specific tolerance and ameliorate EAE

Hematopoietic stem cell (HSC) transplantation is a potential therapy that can offer multiple sclerosis patients a radical, potentially curative treatment. Using experimental autoimmune encephalomyelitis (EAE) as a model, we previously reported that retrovirally transduced B cells expressing myelin basic protein (MBP), MBP Ac1-11, or myelin oligodendrocyte glycoprotein p35-55 induced tolerance and reduced symptoms. Here, we extend our tolerance approach using bone marrow (BM) cells expressing full-length phospholipid protein (PLP) in a model for relapsing, remitting EAE. Using GFP expression as a marker, we found that up to 50% of cells were positive for transgene expression in peripheral blood after 900 rad irradiation and transduced BM transplantation, and expression was stable in hematopoietic lineages for over 10 weeks. Upon challenge, T cell proliferation in response to PLP p139-151 was reduced and EAE was completely abolished in a pretreatment protocol. In addition, protection from EAE could be achieved with PLP-transduced BM cells given on day 12 after immunization, a potential therapeutic protocol. Finally, the protective effect of PLP-expressing BM could also be observed using a nonmyeloablative protocol, albeit with lower efficacy. Our results suggest that HSC may be useful to achieve long-lasting tolerance to protect mice from EAE and possibly to promote CNS repair in ongoing EAE.

Leukemias following retroviral transfer of multidrug resistance 1 (MDR1) are driven by combinatorial insertional mutagenesis

Previous studies have demonstrated leukemic complications in mice after high-copy retroviral gene transfer of the multidrug resistance 1 (MDR1) cDNA, encoding a membrane-located efflux pump expressed in hematopoietic stem cells. In contrast, no such complications or MDR1-associated alterations of hematopoiesis were observed in numerous other studies exploring MDR1 gene transfer into cell lines, mice, dogs, nonhuman primates, and human subjects. Here, we show that leukemias associated with retroviral expression of MDR1 depend on high vector dose, and involve the selection of clones with combinatorial insertional mutagenesis of proto-oncogenes or other signaling genes. Compared with insertion patterns in normal long-term repopulating hematopoietic cells, such hits were overrepresented in leukemic clones, pointing to a causal role. A similar constellation of insertion sites was also observed in a leukemia arising after high-copy retroviral gene transfer of a fluorescent protein. Spectral karyotyping demonstrated additional chromosomal translocations in a subset of cases, indicative of secondary genetic instability. We also show that insertional mutants can be amplified in vitro prior to transplantation. On the basis of these findings, we suggest the use of preclinical dose-escalation studies to define a therapeutic index for retroviral transgene delivery.