Growth Factor-dependent Cell Line Research Articles

A porcine macrophage cell line that supports high levels of replication of OURT88/3, an attenuated strain of African swine fever virus

ABSTRACT The main target cells for African swine fever virus (ASFV) replication in pigs are of monocyte macrophage lineage and express markers typical of the intermediate to late stages of differentiation. The lack of a porcine cell line, which accurately represents these target cells, limits research on virus host interactions and the development of live-attenuated vaccine strains. We show here that the continuously growing, growth factor dependent ZMAC-4 porcine macrophage cell line is susceptible to infection with eight different field isolates of ASFV. Replication in ZMAC-4 cells occurred with similar kinetics and to similar high titres as in primary porcine bone marrow cells. In addition we showed that twelve passages of an attenuated strain of ASFV, OURT88/3, in ZMAC-4 cells did not reduce the ability of this virus to induce protection against challenge with virulent virus. Thus, the ZMAC-4 cells provide an alternative to primary cells for ASFV replication.

CCL-2 Is a KIT D816V-Dependent Modulator of Bone Marrow Remodeling and Microenvironmental Alterations in Systemic Mastocytosis

Systemic mastocytosis (SM) is a myeloproliferative neoplasm (MPN) characterized by mast cell (MC) infiltration in the bone marrow (BM) and/or other organs. Most patients present with the activating KIT mutation D816V. Increased production of cytokines is typically found in patients with classical MPN and has been linked to alterations of the BM microenvironment (e.g. enhanced angiogenesis and fibrosis) that are also commonly found in SM. However, little is known about mechanisms contributing to microenvironment alterations in the BM in SM and the role of MC-derived cytokines.The aim of our study was to identify cytokines that are produced in neoplastic MC in a KIT-dependent manner and contribute to increased BM angiogenesis and fibrosis in SM. In a first step, we screened for KIT D816V-dependent production of cytokines relevant to inflammation and microenvironment alterations using growth factor-dependent human cell lines (TF-1 and Mo7e). In these experiments, expression of CCL-2, IL-8, OSM, and VEGF mRNA was induced by KIT D816V but not by wild type KIT. Based on its pleiotropic effects, we focused on CCL-2, also referred to as monocyte chemotactic protein 1 (MCP-1), a CC chemokine that recruits inflammatory cells to sites of inflammation and enhances angiogenesis. KIT D816V+ HMC-1.2 cell were found to express and secrete substantial amounts of CCL-2. Midostaurin, a multikinase inhibitor suppressing the kinase activity of KIT D816V, was found to reduce expression of CCL-2 similar to RNAi-mediated knockdown of KIT. Furthermore, knockdown of STAT5, a key transcription factor downstream of KIT D816V, reduced expression of CCL-2 in HMC-1.2 cells. These results confirmed that CCL-2 expression in neoplastic MC is dependent on KIT D816V.Since CCL-2 has been reported to promote angiogenesis, we analyzed effects of conditioned medium obtained from HMC-1.2 cells on human umbilical vein endothelial cells (HUVEC) known to express the CCL-2 receptor CCR-2. Indeed, MC-derived conditioned medium induced migration of HUVEC in a wound healing assay (214% ± 38% of control, mean ± SD, p<0.01) as well as in a boyden chamber assay (126% ± 8% of control, p<0.05). Moreover, pre-incubation with a neutralizing antibody against CCL-2 significantly reduced migratory responses (p<0.01), and RNAi-mediated knockdown of CCL-2 in neoplastic MC reduced the effect of conditioned medium to baseline levels (p<0.01). These results confirmed that the migration was CCL-2 dependent. Furthermore, patients with advanced SM often present with marked eosinophilia; and eosinophils are often located in the vicinity of, or even within, BM MC infiltrates. Therefore, we also studied the effect of CCL-2 on eosinophils. Normal human eosinophils as well as the eosinophilic cell line EOL-1 were found to express CCR-2, and to migrate against recombinant CCL-2 in a modified boyden chamber assay. Conditioned medium from KIT D816V+ MC also induced a migratory response in eosinophils (200% ± 30% of control, p<0.05), and a neutralizing antibody against CCL-2 reduced this effect to baseline levels (p<0.05).Finally, SM-patients were found to have significantly elevated serum CCL-2 levels (n=35, p=0.0048, 407.4 ± 42.3 pg/ml, mean ± SEM) compared to controls (274.3 ± 15.9 pg/ml). The highest serum levels of CCL-2 were detected in patients with advanced SM where tissue remodeling in the BM is often a prominent feature. Moreover, CCL-2 levels were found to correlate with the grade of MC infiltration in the BM (r=0.656, p=0.0002). In summary, KIT D816V induces the expression of various cytokines potentially involved in tissue remodeling and microenvironment alterations in SM. Moreover, we have identified CCL-2 as a critical, KIT D816V-dependent, cytokine-mediator that interacts with structural BM cells and thereby may be involved in disease evolution and progression in SM. Whether CCL-2 may also serve as a therapeutic target in SM is currently being examined.This study was supported by Austrian Science Fund (FWF) grant P26079-B13. DisclosuresSperr:Novartis: Honoraria. Valent:Pfizer: Honoraria; Celgene: Honoraria; Ariad: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria; Novartis: Consultancy, Honoraria, Research Funding.

Oncostatin M is a FIP1L1/PDGFRA‐dependent mediator of cytokine production in chronic eosinophilic leukemia

Chronic eosinophilic leukemia (CEL) is a myeloproliferative neoplasm characterized by expansion of neoplastic eosinophils, tissue infiltration, and organ damage. In a subset of these patients, the FIP1L1/PDGFRA (F/P) oncoprotein is detectable. F/P exhibits constitutive tyrosine kinase activity and activates a number of signaling pathways. So far, however, little is known about the role of F/P-dependent proteins in the pathogenesis of CEL. A screen for F/P-dependent cytokines was performed in growth factor-dependent human cell lines lentivirally transduced with F/P. Signal transduction pathways were characterized in Ba/F3 cells with doxycycline-inducible expression of F/P and in EOL-1 cells. Cytokine expression was confirmed in patients' material by immunohistochemistry, immunofluorescence, and confocal microscopy. Gene expression analysis, proliferation assays, and chemotaxis assays were used to elucidate paracrine interactions between neoplastic eosinophils and stromal cells. We show that F/P upregulates expression of oncostatin M (OSM) in various cell line models in a STAT5-dependent manner. Correspondingly, neoplastic eosinophils in the bone marrow were found to overexpress OSM. OSM derived from F/P + cells stimulated proliferation of stromal cells. Moreover, OSM-containing supernatants from F/P + cells were found to upregulate production of stromal cell-derived factor-1 (SDF-1)/CXCL12 in human fibroblasts. SDF-1, in turn, induced migration of EOL-1 cells in a dose-dependent manner. We have identified a F/P-driven paracrine interaction between neoplastic eosinophils and stromal cells that may contribute to tissue fibrosis and accumulation of neoplastic eosinophils in CEL.

Abstract 2282: Rapid identification of targetable CSF3R mutations that define neutrophilic leukemia by combining functional and genomic screens.

Abstract Large scale sequencing efforts are currently being applied to numerous cancers and although significant numbers of mutations are being identified, functional validation to identify driver mutations is lagging. To accelerate progress in the identification of genetic drivers in hematological malignances, we are taking an integrated approach of deep sequencing coupled with functional screens using molecularly targeted drugs and siRNA screening of primary patient leukemia samples. The functional screens allow prioritization of mutations for validation. Using this approach, we identified a CSF3R mutation in a patient with chronic neutrophilic leukemia (CNL). Patients with CNL and the related disorder, atypical chronic myeloid leukemia (aCML), have neoplastic expansion of granulocytic cells with the diagnosis including exclusion of genetic drivers known to occur in other types of World Health Organization-defined myeloproliferative neoplasms (MPN) or myelodysplastic syndrome/MPN overlap disorders. After identifying a CSF3R mutation in our index case, we examined 21 additional cases of CNL and aCML and found that 59% of patients with CNL/aCML harbor CSF3R mutations. Two distinct regions of mutations within the CSF3R gene were identified. Expression of these classes of mutations in a growth factor-dependent myeloid cell line, BaF3, transformed these cells to growth factor independence. Interestingly, these two mutational classes produce preferential downstream kinase signaling (via TNK2/SRC or JAK kinases) and differential sensitivity to kinase inhibitors. Specifically, mutants signaling through TNK2/SRC are sensitive to SRC inhibitors such as dasatinib while mutants signaling thought JAK kinases were sensitive to JAK inhibitors such as ruxolitinib. One patient with CNL carrying a CSF3R mutation that signals through JAK kinases showed a dramatic and durable clinical improvement after treatment with the JAK1/2 kinase inhibitor, ruxolitinib. Mutations in the CSF3R define a large subset of patients with CNL/aCML and testing for these mutations will complement histopathologic diagnosis of CNL and aCML. Patients with CSF3R mutations may benefit from clinical administration of TNK2/SRC inhibitors (such as dasatinib) or JAK inhibitors (such as ruxolitinib). Our studies demonstrate that combining functional screens with genome sequencing can significantly accelerate target validation to define driving mutations that can be matched with therapies. Citation Format: Julia Maxson, Jason Gotlib, Daniel Pollyea, Angela Fleischman, Christopher Eide, Daniel Bottomly, Beth Wilmot, Shannon McWeeney, Cristina Tognon, J. Blake Pond, Robert Collins, Michael Deininger, Bill Chang, Marc Loriaux, Brian Druker, Jeffrey Tyner. Rapid identification of targetable CSF3R mutations that define neutrophilic leukemia by combining functional and genomic screens. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2282. doi:10.1158/1538-7445.AM2013-2282

Location of FLT3-ITDs within the beta1-Sheet of FLT3 Kinase Confers Resistance to Tyrosine Kinase Inhibitors (TKI) in Vitro.

Abstract 2440 Introduction:In AML, the recently described tyrosine kinase domain-1 (TKD1)-ITDs of FLT3 (Breitenbuecher et al., Blood 2009, Kayser-S et al., Blood 2009) are located within the beta1-sheet, nucleotide binding loop and beta2-sheet of tyrosine kinase domain 1 (TKD1), respectively. Multivariate analysis of clinical data revealed that location of FLT3-ITDs within the beta1-sheet of TKD1 is an unfavorable prognostic factor (Kayser-S et al., Blood 2009). Recently, we uncovered a novel mechanism of primary resistance to FLT3 tyrosine kinase inhibitors (TKIs) in a patient displaying an atypical localization within the beta2-sheet-ITD (A627E). Here, we characterized in-vitro sensitivity to FLT3-TKI in growth factor dependent hematopoietic cell lines expressing a representative panel of FLT3-ITDs isolated from patient material. In particular, we compared sensitivity of beta1-sheet ITDs with typical ITDs located in the juxtamembrane domain of FLT3. Methods:FLT3-ITDs isolated from patient material, were sequenced, subcloned in parallel into two expression vectors (pAL and pMSCV-Puromycin-IRES-GFP), and stably expressed in growth-factor dependent hematopoietic Ba/F3 cells and in parallel in 32D cells. Constitutive phosphorylation of FLT3-ITD receptors and of downstream signaling pathways was analyzed by Western-blotting. Transformation of Ba/F3 and 32D cells was investigated by colony formation assays and by withdrawal of IL-3. Induction of apoptosis in response to various concentrations of FLT3-kinase inhibitors midostaurin (PKC412) and quizartinib (AC220) was measured by flow-cytometry at 24h and 48h, respectively. For statistical analysis of replicates t-test was employed. Results:Biological characteristics of FLT3-ITD mutations located in two different structural domains of FLT3-kinase were characterized: (1) beta1-sheet-ITDs E611V(96nt) and Q613E(99nt) and (2) nucleotide binding loop-ITD A620V(84nt). (Our nomenclature used for description of ITDs indicates position of amino-acid where ITD is located, possible exchange of amino-acid residues at this position and nucleotide length of ITD) Hematopoietic cells (32D and Ba/F3) expressing these ITD mutations showed colony formation in methylcellulose medium, growth-factor independent proliferation upon IL-3 withdrawal and constitutive phosphorylation of FLT3 signaling. Three ‘typical' juxtamembrane domain (JMD) ITDs were used as controls; ITD598/599(36nt), ITD598/599(66nt) and ITDK602R(21nt). As compared to these JM-ITDs, we observed significantly less apoptosis of beta1-sheet-ITDs at all concentrations of FLT3-kinase inhibitors applied. However, this difference in sensitivity gradually decreased when incubating with midostaurin or quizartinib for longer periods of time as 48h. Of note, length of FLT3-ITD mutations did not appear to influence sensitivity to TKI treatment. Conclusion:Investigating representative FLT3-ITD-mutations located within the TKD1 revealed that beta1-sheet-ITDs mediate constitutive activation of FLT3 receptors leading to transformation of hematopoietic cell lines 32D and Ba/F3. In comparison to typical JMD-ITDs, beta1-sheet-ITDs analyzed here revealed resistance to FLT3-inhibitors as midostaurin and quizartinib across two cellular reconstitution models (32D and Ba/F3) and using two different expression vectors (pAL and pMSCV). Our results indicate that differential sensitivity is rather an effect of ITD-localization within a functional domain of FLT3 and not necessarily conferred by the length of ITDs. Taken together, our data provide a rationale to prospectively analyze not only the FLT3-ITD mutation status or FLT3-ITD allelic ratio but also location of ITD-mutations in ongoing clinical trials as this may have direct impact on response to therapy (tyrosine kinase inhibitors, chemotherapy, allogeneic stem cell transplantation) in FLT3-ITD positive AML. Disclosures:Heidel:Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees. Fischer:Novartis: Honoraria.

Role of tumor suppressor p53 in megakaryopoiesis and platelet function

The pathobiological role of p53 has been widely studied, however, its role in normophysiology is relatively unexplored. We previously showed that p53 knock-down increased ploidy in megakaryocytic cultures. This study aims to examine the effect of p53 loss on invivo megakaryopoiesis, platelet production, and function, and to investigate the basis for greater ploidy in p53(-/-) megakaryocytic cultures. Here, we used flow cytometry to analyze ploidy, DNA synthesis, and apoptosis in murine cultured and bone marrow megakaryocytes following thrombopoietin administration and to analyze fibrinogen binding to platelets invitro. Culture of p53(-/-) marrow cells for 6 days with thrombopoietin gave rise to 1.7-fold more megakaryocytes, 26.1% ± 3.6% of which reached ploidy classes ≥64 N compared to 8.2% ± 0.9% of p53(+/+) megakaryocytes. This was due to 30% greater DNA synthesis in p53(-/-) megakaryocytes and 31% greater apoptosis in p53(+/+) megakaryocytes by day 4 of culture. Although the bone marrow and spleen steady-state megakaryocytic content and ploidy were similar in p53(+/+) and p53(-/-) mice, thrombopoietin administration resulted in increased megakaryocytic polyploidization in p53(-/-) mice. Although their platelet counts were normal, p53(-/-) mice exhibited significantly longer bleeding times and p53(-/-) platelets were less sensitive than p53(+/+) platelets to agonist-induced fibrinogen binding and P-selectin secretion. In summary, our invivo and exvivo studies indicate that p53 loss leads to increased polyploidization during megakaryopoiesis. Our findings also suggest for the first time a direct link between p53 loss and the development of fully functional platelets resulting in hemostatic deficiencies.

Site-directed mutagenesis reveals a unique requirement for tyrosine residues in IL-7Rα and TSLPR cytoplasmic domains in TSLP-dependent cell proliferation

BackgroundThymic stromal lymphopoietin (TSLP) is an interleukin-7 (IL-7) like cytokine, which plays an important role in the regulation of immune responses to allergens. TSLP binds to a heterodimeric receptor complex composed of the IL-7 receptor α chain (IL-7Rα) and the TSLP receptor (TSLPR, also known as CRLF2). It has previously been suggested that the lone tyrosine residue in the mouse TSLPR cytoplasmic domain is required for cell proliferation using chimeric receptor systems. Also the role of tyrosine residues in the IL-7Rα cytoplasmic domain in TSLP signaling has not yet been investigated. We undertook a systematic analysis to test the role of tyrosine residues of both the IL-7Rα and the TSLPR in inducing cell proliferation in a growth factor dependent cell line, Ba/F3.ResultsA multiple sequence alignment of the IL-7Rα and TSLPR cytoplasmic domains revealed conservation of most, but not all, cytoplasmic tyrosine residues across several species. Our site-directed mutagenesis experiments revealed that the single tyrosine residue in human TSLPR was not required for TSLP-dependent cell proliferation. It has previously been reported that Y449 of human IL-7Rα is required for IL-7 dependent proliferation. Interestingly, in contrast to IL-7 signaling, none of tyrosine residues in the human IL-7Rα cytoplasmic domain were required for TSLP-dependent cell proliferation in the presence of a wild type TSLPR. However, the mutation of all cytoplasmic four tyrosine residues of human IL-7Rα and human TSLPR to phenylalanine residues abolished the proliferative ability of the TSLP receptor complex in response to TSLP.ConclusionThese results suggest that TSLP requires at least one cytoplasmic tyrosine residue to transmit proliferative signals. Unlike other members of IL-2 cytokine family, tyrosine residues in IL-7Rα and TSLPR cytoplasmic domains play a redundant role in TSLP-mediated cell growth.

MODELING and Targeting ERG-INDUCED Acute MYELOID LEUKEMIA (AML).

Abstract 475ERG is an oncogene located on the long arm of human chromosome 21 that encodes an ETS transcription factor that has been reported to be involved in normal and aberrant megakaryopoiesis (Salek-Ardakani et al Cancer Res. 2009;69:4665; Stankiewicz et al Blood 2009;113:3337). High ERG expression has also been reported associated with poor prognosis of cytogenetically normal AML (Marcucci et al J Clin Oncol. 2007;25:3337). Thus, deciphering the molecular targets and oncogenic pathways activated by overexpressed ERG protein is likely to lead to more effective therapy for ERG-related myeloid leukemias. Towards these goals we have combined in-vitro and in-vivo approaches. We have created transgenic mice expressing the ERG3 hematopoietic isoform under the VAV promoter. All these mice die from invasive acute megakaryocytic or undifferentiated myeloid leukemia by the age of 5 months. The leukemias are transplantable, and primary growth factor dependent leukemic cell lines, suitable for pharmacological and molecular studies, have been established. To identify ERG target genes and proteins, we have analyzed gene expression in two “mirror-image” cellular systems – shRNA mediated knockdown of ERG in Meg01 megakaryocytic leukemia cells and overexpression of ERG in K562 erythroleukemia cells. Gene Set Enrichment Analysis (GSEA) demonstrated that the “ERG gene-associated expression signature” in these cell lines is enriched with genes that were also identified in primary human AML characterized by ERG overexpression. By chromatin immunoprecipitation, we then identified specific, direct ERG targets and confirmed their expression in samples from ERG transgenic leukemias and primary human “ERG-overexpressing” AMLs. Surprisingly, we observed that the lymphoid kinase Bruton agammaglobulinemia tyrosine kinase (BTK) is an ERG direct target that is upregulated in both ERG overexpressing mouse and human leukemias. Preliminary experiments have demonstrated activity of a BTK inhibitor on ERG transgenic and ERG overexpressing human AML cells. ERG overexpression also induced marked activation of RAS signaling as supported by elevated levels of RAS-GTP and its downstream targets and significant growth inhibitory activity caused by a novel RAS inhibitor, farnesylthiosalicylic acid (Salirasib), that disrupts the spatiotemporal localization of active Ras (Rotblat et al Methods Enzymol. 2008;439:467) was observed. Dramatic growth inhibitory activity has also been induced by two novel compounds that induce megakaryocytic polyploidization, suggesting a potential role for “differentiation” therapy of ERG-related leukemias. Thus we have created a mouse model for ERG-related AML and characterized several genes and biochemical pathways that are susceptible to pharmacological inhibition. Our studies are not only likely to decipher the mechanisms by which ERG overexpression contributes to leukemogenesis, but also provide a platform for preclinical studies of novel therapeutics of these poor prognosis leukemias. Disclosures:No relevant conflicts of interest to declare.

A Multi-Lineage Oral Hemokine™ HQK-1002 Enhances Neutrophil Recovery in Sub-Lethally Irradiated Mice and in Baboons.

Abstract 2509Poster Board II-486Acute radiation syndrome and therapeutic radiation are associated with cytopenias, notably severe neutropenia, particularly when large regions of bone marrow are exposed. An orally-active neutrophil stimulant which reduces the duration or magnitude of neutropenia would offer therapeutic benefit in radiation therapy and would be useful for acute radiation exposure. Orally-bioavailable short-chain fatty acid derivatives (SCFAD, HemokinesTM HQK-1002 and its S-enantiomer HQK-1002S), which stimulate proliferation of growth factor-dependent hematopoietic cell lines and CFU-GM cultures, were evaluated for in vivo activity in irradiated and 5-FU-treated mice and in normal baboons. In sub-lethal irradiation studies, mice were irradiated (6 Gy single dose) and then treated with saline or HQK-1002 once daily, 5 days/week for 2 weeks, beginning on day 1 after irradiation, and absolute neutrophil counts (ANC) were determined. The mean nadir of absolute neutrophil counts (ANC) <500/mm3 was 6 days in sub-lethally-irradiated mice treated with HQK-1002, and 18 days in the saline treated controls. In HQK-1002 treated mice, ANC did not decline below 200/mm3, while saline-treated animals experienced 10 days of severe neutropenia with ANC <200/mm3. In lethally-irradiated (LD90) mice, either saline or the (S) enantiomer of HQK-1002 was administered daily. ANC and marrow pathology were compared between saline and HQK-1002S-treated animals. The duration of peripheral neutropenia was similar in controls and HQK-1002S-treated animals, but bone marrow pathology demonstrated profound ablation of all mature myeloid series (10-20% cellularity) in the saline controls, while HQK-1002S-treated animals had normal bone marrow cellularity (55–75%) with complete neutrophil maturation within 2 weeks following radiation. In 5FU-treated mice, neutrophil recovery was compared between HQK-1002, saline, or G-CSF treatment for 3 weeks. Recovery from profound neutropenia (nadir at Day 9) occurred by Day 13 in G-CSF- and HQK-1002 treated animals, 5 days before recovery was observed in saline controls. HQK-1002 and HQK-1002S were also administered to non-irradiated, normal juvenile baboons, once daily for 5 days, and CBCs and neutrophil counts were assayed. HQK-1002 or HQK-1002S treatment in 3 unchallenged baboons resulted in a two-fold to three-fold rises in ANC within 7 days. Collectively, these studies strongly suggest that HQK-1002 and HQK-1002S, orally-bioavailable SCFADs, stimulate myelopoiesis in vivo in normal animals and during sub-lethal radiation-induced bone marrow suppression, and merit further evaluation in neutropenia associated with marrow suppression. Disclosures:Perrine:HemaQuest Pharmaceuticals: Employment, Equity Ownership, Patents & Royalties, Research Funding. Boosalis:HemaQuest Pharmaceuticals: Equity Ownership. Thomson:HemaQuest Pharmaceuticals: Employment, Equity Ownership. Berenson:hemaQuest Pharmaceuticals: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Faller:HemaQuest Pharmaceuticals: Consultancy, Equity Ownership, Patents & Royalties, Research Funding.

Hoxa6 potentiates short-term hemopoietic cell proliferation and extended self-renewal

Hemopoietic progenitor cells express clustered homeobox (Hox) genes in a pattern characteristic of their lineage and stage of differentiation. In general, HOX expression tends to be higher in more primitive and lower in lineage-committed cells. These trends have led to the hypothesis that self-renewal of hemopoietic stem/progenitor cells is HOX-dependent and that dysregulated HOX expression underlies maintenance of the leukemia-initiating cell. Gene expression profile studies support this hypothesis and specifically highlight the importance of the HOXA cluster in hemopoiesis and leukemogenesis. Within this cluster HOXA6 and HOXA9 are highly expressed in patients with acute myeloid leukemia and form part of the "Hox code" identified in murine models of this disease. We have examined endogenous expression of Hoxa6 and Hoxa9 in purified primary progenitors as well as four growth factor-dependent cell lines FDCP-Mix, EML, 32Dcl3, and Ba/F3, representative of early multipotential and later committed precursor cells respectively. Hoxa6 was consistently higher expressed than Hoxa9, preferentially expressed in primitive cells and was both growth-factor and cell-cycle regulated. Enforced overexpression of HOXA6 or HOXA9 in FDCP-Mix resulted in increased proliferation and colony formation but had negligible effect on differentiation. In both FDCP-Mix and the more committed Ba/F3 precursor cells overexpression of HOXA6 potentiated factor-independent proliferation. These findings demonstrate that Hoxa6 is directly involved in fundamental processes of hemopoietic progenitor cell development.

Cellular LanthaScreen andβ-Lactamase Reporter Assays for High-Throughput Screening of JAK2 Inhibitors

The Janus kinase (JAK) 2/signal transducer and activator of transcription (STAT) 5 pathway is responsible for regulation of cellular responses to a number of cytokines and growth factors. In hematopoietic cells, growth factors such as granulocyte macrophage-colony stimulating factor, interleukin-3, and erythropoietin induce the activation of JAK2, which leads to the phosphorylation, dimerization, and transactivation of STAT5 proteins. Dysregulation of JAK2 by activating mutations such as JAK2V617F results in constitutive phosphorylation of STAT5 and has been linked to numerous myeloproliferative disorders such as polycythemia vera. A cellular LanthaScreen (Invitrogen Corp., Carlsbad, CA) time-resolved Förster resonance energy transfer assay for wild-type JAK2 activity was developed. This assay utilized the growth factor-dependent human erythroleukemia TF1 cell line engineered to express a green fluorescent protein-STAT5 fusion protein. Furthermore, a complementary beta-lactamase reporter gene assay was developed to analyze the transcriptional activity of STAT5 downstream of JAK2 in TF1 cells. The same technologies were applied to the development of cellular assays for the interrogation of the disease-relevant JAK2V617F activating mutant. A small molecule inhibitor and Stealth (Invitrogen Corp.) RNA interference oligonucleotides were used to confirm the involvement of JAK2. Our results suggest that these cellular assays and validation tools represent powerful integrated methods for the analysis of physiological and disease-relevant JAK/STAT pathways within the physiological cellular context.

Mitotic Arrest Deficiency 2 (Mad2) Is Required for Proliferative Synergy of GM-CSF Plus SCF in Hematopoietic Progenitors Via Effects on Apoptosis and Is Mediated by Ligand-Dependent Association of Mad2 with SCF and GM-CSF Receptors.

Mitotic arrest deficiency 2 (Mad2) is a component of the mitotic spindle checkpoint complex and is essential for accurate chromosome segregation. We previously reported that Mad2 is involved in synergistic proliferation of hematopoietic progenitor cells in response to SCF combined with GM-CSF, but the underlying mechanism remains unclear. Recently, it was reported that Mad2 physically associates with common β chain (βc) of GM-CSF receptor. Because c-Kit constitutively binds to βc, we hypothesized that Mad2 protein physically associates with c-Kit receptor. We examined if Mad2 interacts with c-Kit using human growth factor-dependent cell line, MO7e, which expresses both c-Kit and GM-CSF receptors. Immunoprecipitation assays demonstrated interaction of c-Kit with Mad2. We then examined whether the interaction of Mad2 with c-Kit is cytokine dependent. Mad2 dissociates from c-Kit after stimulation with SCF plus GM-CSF, but not with SCF or GM-CSF alone. Additionally, because bone marrow hematopoietic progenitor cells from Mad2+/− mice lack synergy in response to SCF plus GM-CSF, it is possible that interaction of Mad2 with c-Kit mediates their synergistic proliferative effects. To address this, we examined intracellular cytokine signaling and apoptosis in MO7e cells depleted of Mad2 by RNA interference. We observed no difference between control and Mad2-depleted cells in phosphorylation of Erk1/2 at Thr202/Tyr204 and Akt at Ser473 after synergistic stimulation with SCF plus GM-CSF. In contrast, the percent of apoptosis of Mad2-depleted cells was significantly higher than that of control (26.2±0.7% vs. 20.3±0.4%, P<0.01). This was compatible with the results of apoptosis assays using c-Kit+Lineage− bone marrow cells from Mad2+/− and wild type mice. These results suggest that Mad2 may be involved in synergistic proliferation of hematopoietic progenitor cells via regulation of apoptosis rather than early cytokine signaling. These effects are likely mediated through Mad2 interaction with c-Kit and the beta chain of the GM-CSF receptor.

Gangliosides of the stroma layer participate in the interferon-gamma receptor-dependent controls of myelopoiesis

Stroma-mediated myelopoiesis depends upon growth-factors and an appropriate intercellular microenvironment, whose polarity is relevant for granulocyte-macrophage colony stimulating factor (GM-CSF) mediated myeloid cell proliferation. Here we have studied qualitative and quantitative aspects of ganglioside participation in controls of the microenvironment required to sustain myelopoiesis. We analysed ganglioside synthesis, expression and shedding by two primary liver stromal cell cultures isolated from wild type and interferon-gamma (IFNγ) receptor knockout mice. The latter one has a higher capacity to sustain myelopoiesis. FDC-P1 myeloid growth factor-dependent cell line was used as the reporter system, monitoring the cell survival and proliferation that reflect the bio-availability and the activity of GM-CSF. Although the two stromal cells synthesised the same gangliosides their relative content was quite different. FDC-P1 proliferation decreased in cultures in which ganglioside synthesis was inhibited in the stroma, as well as in presence of stroma cell supernatants in which GM3 was neutralised by the anti-GM3 monoclonal antibody. Addition of exogenous GM3 reverted the inhibition and sustained proliferation of FDC-P1 cells. FDC-P1 cells do not accumulate GM3, but they are able to take up the stroma-produced sphingolipids. Thus, stroma has a double role in sustaining myelopoiesis, providing both growth factor(s) and ganglioside(s) required for the optimal stimulation of the myeloid cell proliferation, and the IFNγ mediated stroma-dependent controls of myelopoiesis are determinant for this cell interaction.

TEL-AML1 Transgenic Zebrafish Model of Acute Lymphoblastic Leukemia (ALL).

TEL-AML1 (ETV6-RUNX1) fusion is the product of the t(12;21) chromosomal translocation, the most common chromosomal rearrangement in childhood cancer. The translocation fuses two highly conserved transcription factors, TEL and AML1 that have essential roles in hematopoiesis. Genetic studies of identical twins with concordant leukemia, the detection of leukemia-specific fusion genes in neonatal blood spots, and the existence of multiple leukemic subclones at ALL diagnosis point to prenatal origin of the fusion and long latency before leukemia development. Additionally, a study in growth factor-dependent cell lines and transgenic mice, and several mouse transplant models suggest that TEL-AML1 is insufficient by itself for leukemic transformation. To determine whether TEL-AML1 has selective transforming impact on a particular stem cell lineage, we established stable zebrafish transgenic lines expressing the TEL-AML1 fusion protein both constitutively, and within the lymphoid progenitors. In these lines, either the zebrafish β-actin (ZBA) promoter or the xenopus elongation factor-1 (XEF1) promoter drives TEL-AML1 expression constitutively, while the zebrafish RAG-2 promoter (RAG2) selectively drives TEL-AML1 expression in committed lymphocyte precursors, but not in earlier multilineage hematopoietic precursors. TEL-AML1 expression, alone or fused to EGFP, was detected at 24-hour post fertilization (hpf) with fluorescent microscopy or RT-PCR and confirmed with western blot analysis. In-Situ Hybridization and confocal microscopy revealed that transgenic zebrafish maintained TEL-AML1 expression throughout adulthood. The expression of TEL-AML1 was associated with accumulation of immature hematopoietic progenitor cells, mostly in the kidney and spleen of several transgenic zebrafish lines, within a few weeks of development, indicating an expansion of the progenitor cell population. To date, a small number (5 out of 391 transgenic fish; 1.3%) of founders and progeny of zebrafish transgenic for ZBA- or XEF1-TEL-AML1 fusion developed an infiltrating lymphoid neoplasm, most likely acute lymphoblastic leukemia with a latency of 8–12 month. The lymphoblasts were negative for myeloperoxidase and PAS staining, with abundant expression of both EGFP in lymphoblasts from EGFP-TEL-AML1 line, and pre-B-cell leukemia transcription factor-1 (PBX1; also called cALLa) indicating that Leukemias originated in a TEL-AML1 expressing cell(s), and mimic the human CD10-positive precursor-B cell ALL. None of 350 wild-type, 125 control EGFP fish or 353 RAG2-TEL-AML1 transgenic zebrafish had any leukemia or hematopoietic changes. Therefore, the expression of TEL-AML1 fusion in our ZBA- and XEF1-TEL-AML1 transgenic lines generates a premalignant state, which appears to require additional genetic events for acquisition of the leukemic phenotype. The fact that none of the RAG2 zebrafish expressing TEL-AML1 in committed lymphoid progenitors developed leukemia or progenitor expansion indicates that the leukemic stem cell associated with TEL-AML1 leukemia is an immature progenitor earlier than the committed lymphoid progenitors where RAG2 expression is detected. The TEL-AML1 transgenic zebrafish lines provide a basis for large scale and targeted mutagenesis screens aimed at identifying mutations and cooperating events that are required for TEL-AML1 induced leukemias.

The polycomb group protein enhancer of zeste homolog 2 (EZH2) is an oncogene that influences myeloma cell growth and the mutant ras phenotype

Three distinct proliferative signals for multiple myeloma (MM) cell lines induce enhancer of zeste homolog 2 (ezh 2) transcript expression. EZH 2 is a polycomb group protein that mediates repression of gene transcription at the chromatin level through its methyltransferase activity. Normal bone marrow plasma cells do not express ezh2; however, gene expression is induced and correlates with tumor burden during progression of this disease. We therefore investigated how EZH 2 expression is deregulated in MM cell lines and determined the consequence of this activity on proliferation and transformation. We found that EZH 2 protein expression is induced by interleukin 6 (IL-6) in growth factor-dependent cell lines and is constitutive in IL-6-independent cell lines. Furthermore, EZH 2 expression correlates with proliferation and B-cell terminal differentiation. Significantly, EZH 2 protein inhibition by short interference RNA treatment results in MM cell growth arrest. Conversely, EZH 2 ectopic overexpression induces growth factor independence. We found that the growth factor-independent proliferative phenotype in MM cell lines harboring a mutant N- or K-ras gene requires EZH 2 activity. Finally, this is the first report to demonstrate that EZH 2 has oncogenic activity in vivo, and that cell transformation and tumor formation require histone methyltransferase activity.

The Combined Expression of AML1-ETO and Oncogenic RAS Is Insufficient To Induce Acute Myeloid Leukemia (AML).

To define the leukemic potential of AML1-ETO, we recently generated a mouse strain with a conditional loxP-stop-loxP (LSL)-AML1-ETO knock-in allele that can be activated in hematopoietic cells by Cre-mediated recombination. Expression of AML1-ETO under these conditions readily induced the immortalization of multipotential hematopoietic progenitors, but failed to induce overt leukemia. Induction of secondary mutations with the alkylating agent ENU, however, resulted in the development of an acute leukemia similar to human t(8;21)-AML. Immortal AML1-ETO expressing non-leukemic cells grew as growth factor-dependent cell lines, where as the murine AML1-ETO expressing leukemias grew as growth factor-independent cell lines. This suggested that alterations in growth factor signaling pathways may cooperate with AML1-ETO to induce full transformation. Consistent with this interpretation, we have recently demonstrated that ~50% of core-binding factor leukemias have activating mutations in NRAS or KRAS. To directly assess the cooperativity of AML1-ETO and oncogenic Ras, we crossed the AML1-ETO / Mx1-Cre mice with a strain containing a LSL-KrasG12D knock-in allele. Expression of these two oncoproteins was induced simultaneously by Cre-mediated deletion of the upstream transcriptional stop cassettes. Importantly, expression levels for both genes depended on their respective endogenous regulatory sequences. As reported by others, activation of the LSL-KrasG12D allele alone resulted in the development of a lethal myeloproliferative disease (MPD), with all mice dying within 120 days (n=17). Triple transgenic mice (LSL-AML1-ETO / Mx1-Cre / LSL-KrasG12D, n=14) also died within 120 days of activation of both conditional alleles. Like the Mx1-Cre / KrasG12D only mice, the triple transgenic mice developed an MPD characterized by leukocytosis, anemia, splenomegaly and hepatic periportal leukocyte infiltrates. Surprisingly, no significant differences were noted in the frequency, latency, or phenotype of the MPD that developed in mice expressing either KrasG12D alone or both AML1-ETO and KrasG12D. Moreover, no AML was observed in the AML1-ETO and KrasG12D expressing mice. We further examined the cooperativity between these two oncoproteins by expressing a human NRASG12D cDNA through an MSCV-based retroviral vector that coexpressed a Cre-GFP fusion protein (MSCV-NRASG12D-IRES-Cre-GFP). Transduced bone marrow cells were transplanted into lethally irradiated mice and the recipients monitored for the development of disease. Expression of mutant NRASG12D by itself efficiently induced the development of a rapidly fatal MPD and a T-cell lymphoma/leukemia in the majority of transplanted mice. However, we again were unable to obtain any evidence for collaboration between NRASG12D and AML1-ETO. The clinical presentation as well as the morphology and immunophenotype of the myeloproliferative process were essentially identical between NRASG12D and AML1-ETO / NRASG12D induced disease. The only difference observed was a reduced incidence of T-cell disease in the AML1-ETO expressing mice, explicable by the previously published finding that AML1-ETO is toxic to developing T-cells. The results demonstrate that despite the frequent occurrence of RAS mutations in AML1-ETO expressing human AML, the coexpression of these two oncoproteins in mice using the experimental approaches described fails to induce an acute myeloid leukemia. These data suggest that these two oncogenic alterations require additional genetic lesions to induce acute leukemia.

Pim Kinases Mediate Viability Signals Downstream of the Tyrosine Kinase Oncogenes BCR-ABL and FLT3-ITD.

Pim1 and Pim2 belong to a family of serine/threonine kinases that are overexpressed in many leukemias. Pim1 has previously been shown to cooperate with the v-myc oncogene to transform hematopoietic cells with murine transforming viruses and overexpression of Pim2 has been shown to induce growth factor independence in a factor dependent murine BaF3 cell line. Protein expression of both Pim1 and Pim2 is low or absent in non-transformed hematopoietic cells but was found to be rapidly increased upon transformation by BCR-ABL or FLT3 with an internal tandem duplication (ITD). The exact contribution of the Pim kinases to transformation in these cells is unknown. BaF3 cell lines were created that stably expressed either BCR-ABL or FLT3-ITD tyrosine kinases, and in which either Pim1 or Pim2 could be induced with doxycycline. Treatment of BCR-ABL or FLT3-ITD expressing cells with small molecule kinase inhibitors specific for either ABL or FLT3 led to inhibition of cell growth, increased apoptosis, and downregulation of Pim expression as expected. However, if Pim2, and to a lesser extent, Pim1 expression was maintained by doxycycline, there was a substantial increase in both viability and cell growth. The molecular mechanisms by which Pim proteins exhibit their effects on target cells are not known. Using parental growth factor dependent BaF3 cell lines with doxycycline inducible Pim1 and Pim2, we show that expression of either of the Pim proteins is sufficient, by itself, to reduce apoptosis and induce modest cell growth. The effects of Pim overexpression on several pathways known to be associated with viability were studied. We found that while Pim over-expression does not activate Akt, it does result in the phosphorylation of a known Akt target, GSK-3β, a regulator of cell cycle progression by targeting the stability of cyclin D proteins. This suggests that Pim proteins may mediate their biological effects through regulation of components in the phosphatidylinositol-3′ kinase signaling cascade, independent of Akt activation. These results further suggest that upregulation of Pim kinases significantly contributes to transformation. Inhibition of Pim kinases could have beneficial therapeutic effects in leukemias.

Sensitivity toward tyrosine kinase inhibitors varies between different activating mutations of the FLT3 receptor

AbstractActivating mutations of FLT3 have been detected in patients with acute myeloid leukemia (AML). Two distinct types of FLT3 mutations are most common: internal tandem duplication (ITD) of sequences coding for the juxtamembrane domain and point mutations at codon 835 (Asp835) within the kinase domain. Both types of mutations constitutively activate the tyrosine kinase activity of FLT3 in experimental systems and result in factor-independent proliferation of Ba/F3 and 32D cells. Recently, novel mutations within the activation loop were identified in patients with AML: deletion of isoleucine 836 (Ile836del) and an exchange of isoleucine 836 to methionine plus an arginine insertion (Ile836Met+Arg). To examine whether the Ile836 mutations result in constitutive activation of the FLT3 receptor, we introduced both mutant FLT3 cDNAs transiently into HEK 293 cells. Both mutant FLT3 receptors were constitutively autophosphorylated in the absence of ligand and kinase activity led to constitutive activation of downstream signaling cascades as determined by activation of the STAT5 (signal transducer and activator of transcription 5) pathway. When stably expressed in the growth factor–dependent cell lines Ba/F3 and 32D, both deletion and insertion mutants led to factor-independent proliferation, indicating that both mutants have transforming capabilities. We then examined the sensitivity of the FLT3 ITD, FLT3 Asp835Tyr, and the novel FLT3 receptor mutants toward the kinase inhibitors AG1296, PKC412, and SU5614. We show that these FLT3 kinase inhibitors have distinct inhibitory potencies against different activating FLT3 receptor mutants. These results suggest that it may be useful to determine the exact kind of FLT3 mutation when applying receptor kinase inhibitors in clinical trials.

The human herpes virus 8–encoded viral FLICE inhibitory protein protects against growth factor withdrawal–induced apoptosis via NF-κB activation

The human herpes virus 8 (HHV8)-encoded viral FLICE (Fas-associating protein with death domain-like interleukin-1-converting enzyme) inhibitory protein (vFLIP) is believed to protect cells against death receptor-mediated apoptosis. In the present study we demonstrate that expression of HHV8 vFLIP in a growth factor-dependent TF-1 leukemia cell line protects against growth factor withdrawal-induced apoptosis. Unlike vector-expressing cells, those expressing HHV8 vFLIP maintain their mitochondrial membrane potential upon withdrawal from growth factor and also exhibit a block in the activation of caspases. The protective effect of HHV8 vFLIP is associated with its ability to activate the nuclear factor-kappa B (NF-kappaB) pathway and is missing in the vFLIP encoded by equine herpes virus 2 that lacks this activity. Inhibition of the NF-kappaB pathway by IkappaB superrepressor, lactacystin, MG132, arsenic trioxide, and phenylarsine oxide reverse the protection against growth factor withdrawal-induced apoptosis conferred by HHV8 vFLIP. HHV8 vFLIP up-regulates the expression of Bcl-x(L), an antiapoptotic member of the Bcl2 family, which is a known target of the NF-kappaB pathway. Collectively, the above results suggest that HHV8 vFLIP-induced NF-kappaB activation may contribute to cellular transformation seen in association with HHV8 infection by preventing the apoptosis of cells destined to die because of growth factor deprivation.

Targeted inhibition of FLT3 overcomes the block to myeloid differentiation in 32Dcl3 cells caused by expression of FLT3/ITD mutations

Internal tandem duplication (ITD) mutations of the juxtamembrane domain-coding sequence of the FLT3 gene are found in up to 34% of patients with acute myeloid leukemia (AML) and are associated with a poor prognosis. FLT3/ITDs result in constitutive activation of the tyrosine kinase domain and transform growth factor-dependent cell lines. FLT3 activation leads to antiapoptotic and proliferative signals, but little is known about the impact of FLT3/ITDs on differentiation. This study was designed to investigate the effect of FLT3/ITD expression on the differentiation of the 32Dcl3 (32D) myeloblastic cell line to neutrophils in response to granulocyte colony-stimulating factor (G-CSF). Expression of FLT3/ITD completely blocked morphologic differentiation and induction of myeloperoxidase (MPO), lysozyme, and CCAAT/enhancer-binding protein epsilon (C/EBPepsilon) in response to G-CSF. Wild-type FLT3 and vector-transfected 32D cells were able to differentiate, although the maturation of FLT3-transfected cells was delayed by FLT3 ligand (FL) stimulation. CEP-701, a potent FLT3 tyrosine kinase inhibitor, overcame the morphologic block in differentiation caused by FLT3/ITD expression and allowed G-CSF induction of myeloid maturation markers. These findings suggest that blocking differentiation may be one of the mechanisms by which FLT3/ITDs contribute to leukemogenesis. CEP-701 and other FLT3 inhibitors may be useful for overcoming the block to differentiation (as well as the block to apoptosis) in the leukemic cells of patients with AML.