Wild-type FLT3 and FLT3 ITD exhibit similar ligand-induced internalization characteristics.

Class III receptor tyrosine kinases control the development of hematopoietic stem cells. Constitutive activation of FLT3 by internal tandem duplications (ITD) in the juxtamembrane domain has been causally linked to acute myeloid leukaemia. Oncogenic FLT3 ITD is partially retained in compartments of the biosynthetic route and aberrantly activates STAT5, thereby promoting cellular transformation. The pool of FLT3 ITD molecules in the plasma membrane efficiently activates RAS and AKT, which is likewise essential for cell transformation. Little is known about features and mechanisms of FLT3 ligand (FL)-dependent internalization of surface-bound FLT3 or FLT3 ITD. We have addressed this issue by internalization experiments using human RS4-11 and MV4-11 cells with endogenous wild-type FLT3 or FLT3 ITD expression, respectively, and surface biotinylation. Further, FLT3 wild-type, or FLT3 ITD-GFP hybrid proteins were stably expressed and characterized in 32D cells, and internalization and stability were assessed by flow cytometry, imaging flow cytometry, and immunoblotting. FL-stimulated surface-exposed FLT3 WT or FLT3 ITD protein showed similar endocytosis and degradation characteristics. Kinase inactivation by mutation or FLT3 inhibitor treatment strongly promoted FLT3 ITD surface localization, and attenuated but did not abrogate FL-induced internalization. Experiments with the dynamin inhibitor dynasore suggest that active FLT3 as well as FLT3 ITD is largely endocytosed via clathrin-dependent endocytosis. Internalization of kinase-inactivated molecules occurred through a different yet unidentified mechanism. Our data demonstrate that FLT3 WT and constitutively active FLT3 ITD receptor follow, despite very different biogenesis kinetics, similar internalization and degradation routes.

Knockout of LASP1 in CXCR4 expressing CML cells promotes cell persistence, proliferation and TKI resistance.

Chronic myeloid leukaemia (CML) is a clonal myeloproliferative stem cell disorder characterized by the constitutively active BCR-ABL tyrosine kinase. The LIM and SH3 domain protein 1 (LASP1) has recently been identified as a novel BCR-ABL substrate and is associated with proliferation, migration, tumorigenesis and chemoresistance in several cancers. Furthermore, LASP1 was shown to bind to the chemokine receptor 4 (CXCR4), thought to be involved in mechanisms of relapse. In order to identify potential LASP1-mediated pathways and related factors that may help to further eradicate minimal residual disease (MRD), the effect of LASP1 on processes involved in progression and maintenance of CML was investigated. The present data indicate that not only overexpression of CXCR4, but also knockout of LASP1 contributes to proliferation, reduced apoptosis and migration as well as increased adhesive potential of K562 CML cells. Furthermore, LASP1 depletion in K562 CML cells leads to decreased cytokine release and reduced NK cell-mediated cytotoxicity towards CML cells. Taken together, these results indicate that in CML, reduced levels of LASP1 alone and in combination with high CXCR4 expression may contribute to TKI resistance.

Isoform localization of Dectin-1 regulates the signaling quality of anti-fungal immunity.

Dectin-1 is recognized as a major receptor for fungal ß-glucans and contributes to anti-fungal immunity. Human monocyte populations express Dectin-1 isoforms A and B, which differ by the presence of a stalk region and its N-linked glycosylation site. Here, we analyzed the expression of both isoforms in human monocyte-derived cells. The cellular localization on cell lines stably expressing either Dectin-1 isoform A or B was studied by flow cytometry and confocal laser scanning microscopy. Intracellular protein signaling and cytokine production were analyzed by immunoblotting and cytometric bead array, respectively. Monocyte-derived cells showed cell type-specific expression of the two isoforms. Glycosylated Dectin-1 isoform A was predominantly localized at the cell surface, non-glycosylated isoform B was retained intracellularly. Inhibition of glycosylation resulted in efficient abrogation of cell surface expression of isoform A. Signaling quality following Dectin-1 stimulation was reduced in isoform B cells. Differential isoform specific cytokine secretion was observed by cytometric bead array. We show here that n-glycosylation of Dectin-1 is crucial for its cell surface expression and consequently signal transduction. Taken together, unique cytokine secretion and varying expression levels of human Dectin-1 isoforms on monocyte-derived cells may indicate distinct isoform usage as a cell type-specific mechanism of regulating anti-fungal immunity.

Phosphoinositide 3-kinase γ ties chemoattractant- and adrenergic control of microglial motility.

Microglial motility is tightly controlled by multitude of agonistic and antagonistic factors. Chemoattractants, released after infection or damage of the brain, provoke directed migration of microglia to the pathogenic incident. In contrast, noradrenaline and other stress hormones have been shown to suppress microglial movement. Here we asked for the signaling reactions involved in the positive and negative control of microglial motility. Using pharmacological and genetic approaches we identified the lipid kinase activity of phosphoinositide 3-kinase species γ (PI3Kγ) as an essential mediator of microglial migration provoked by the complement component C5a and other chemoattractants. Inhibition of PI3Kγ lipid kinase activity by protein kinase A was disclosed as mechanism causing suppression of microglial migration by noradrenaline. Together these data characterize PI3Kγ as a nodal point in the control of microglial motility.

The protein-tyrosine phosphatase DEP-1 promotes migration and phagocytic activity of microglial cells in part through negative regulation of fyn tyrosine kinase.

Microglia cells are brain macrophages whose proper functioning is essential for maintenance and repair processes of the central nervous system (CNS). Migration and phagocytosis are critical aspects of microglial activity. By using genetically modified cell lines and knockout mice we demonstrate here that the receptor protein-tyrosine phosphatase (PTP) DEP-1 (also known as PTPRJ or CD148) acts as a positive regulator of both processes in vitro and in vivo. Notably, reduced microglial migration was detectable in brains of Ptprj-/- mice using a wounding assay. Mechanistically, density-enhanced phosphatase-1 (DEP-1) may in part function by inhibiting the activity of the Src family kinase Fyn. In the microglial cell line BV2 DEP-1 depletion by shRNA-mediated knockdown resulted in enhanced phosphorylation of the Fyn activating tyrosine (Tyr420 ) and elevated specific Fyn-kinase activity in immunoprecipitates. Moreover, Fyn mRNA and protein levels were reduced in DEP-1 deficient microglia cells. Consistent with a negative regulatory role of Fyn for microglial functions, which is inhibited by DEP-1, microglial cells from Fyn-/- mice exhibited elevated migration and phagocytosis. Enhanced microglia migration to a site of injury was also observed in Fyn-/- mice in vivo. Taken together our data revealed a previously unrecognized role of DEP-1 and suggest the existence of a potential DEP-1-Fyn axis in the regulation of microglial functions. GLIA 2017;65:416-428.

Proteinase-activated receptor 2 (PAR2) in hepatic stellate cells - evidence for a role in hepatocellular carcinoma growth in vivo.

BACKGROUND: Previous studies have established that proteinase-activated receptor 2 (PAR2) promotes migration and invasion of hepatocellular carcinoma (HCC) cells, suggesting a role in HCC progression. Here, we assessed the impact of PAR2 in HCC stromal cells on HCC growth using LX-2 hepatic stellate cells (HSCs) and Hep3B cells as model. METHODS: PAR2 expression and function in LX-2 cells was analysed by RT-PCR, confocal immunofluorescence, electron microscopy, and [Ca(2+)]i measurements, respectively. The impact of LX-2-expressed PAR2 on tumour growth in vivo was monitored using HCC xenotransplantation experiments in SCID mice, in which HCC-like tumours were induced by coinjection of LX-2 cells and Hep3B cells. To characterise the effects of PAR2 activation in LX-2 cells, various signalling pathways were analysed by immunoblotting and proteome profiler arrays. RESULTS: Following verification of functional PAR2 expression in LX-2 cells, in vivo studies showed that these cells promoted tumour growth and angiogenesis of HCC xenografts in mice. These effects were significantly reduced when F2RL1 (encoding PAR2) was downregulated by RNA interference (RNAi). In vitro studies confirmed these results demonstrating RNAi mediated inhibition of PAR2 attenuated Smad2/3 activation in response to TGF-ß1 stimulation in LX-2 cells and blocked the pro-mitotic effect of LX-2 derived conditioned medium on Hep3B cells. Furthermore, PAR2 stimulation with trypsin or a PAR2-selective activating peptide (PAR2-AP) led to activation of different intracellular signalling pathways, an increased secretion of pro-angiogenic and pro-mitotic factors and proteinases, and an enhanced migration rate across a collagen-coated membrane barrier. Silencing F2RL1 by RNAi or pharmacological inhibition of Src, hepatocyte growth factor receptor (Met), platelet-derived growth factor receptor (PDGFR), p42/p44 mitogen activated protein kinase (MAPK) or matrix-metalloproteinases (MMPs) blocked PAR2-AP-induced migration. CONCLUSION: PAR2 in HSCs plays a crucial role in promoting HCC growth presumably by mediating migration and secretion of pro-angiogenic and pro-mitotic factors. Therefore, PAR2 in stromal HSCs may have relevance as a therapeutic target of HCC.

Involvement of phosphoinositide 3-kinase class IA (PI3K 110α) and NADPH oxidase 1 (NOX1) in regulation of vascular differentiation induced by vascular endothelial growth factor (VEGF) in mouse embryonic stem cells.

The impact of reactive oxygen species and phosphoinositide 3-kinase (PI3K) in differentiating embryonic stem (ES) cells is largely unknown. Here, we show that the silencing of the PI3K catalytic subunit p110α and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 1 (NOX1) by short hairpin RNA or pharmacological inhibition of NOX and ras-related C3 botulinum toxin substrate 1 (Rac1) abolishes superoxide production by vascular endothelial growth factor (VEGF) in mouse ES cells and in ES-cell-derived fetal liver kinase-1(+) (Flk-1(+)) vascular progenitor cells, whereas the mitochondrial complex I inhibitor rotenone does not have an effect. Silencing p110α or inhibiting Rac1 arrests vasculogenesis at initial stages in embryoid bodies, even under VEGF treatment, as indicated by platelet endothelial cell adhesion molecule-1 (PECAM-1)-positive areas and branching points. In the absence of p110α, tube-like structure formation on matrigel and cell migration of Flk-1(+) cells in scratch migration assays are totally impaired. Silencing NOX1 causes a reduction in PECAM-1-positive areas, branching points, cell migration and tube length upon VEGF treatment, despite the expression of vascular differentiation markers. Interestingly, silencing p110α but not NOX1 inhibits the activation of Rac1, Ras homologue gene family member A (RhoA) and Akt leading to the abrogation of VEGF-induced lamellipodia structure formation. Thus, our data demonstrate that the PI3K p110α-Akt/Rac1 and NOX1 signalling pathways play a pivotal role in VEGF-induced vascular differentiation and cell migration. Rac1, RhoA and Akt phosphorylation occur downstream of PI3K and upstream of NOX1 underscoring a role of PI3K p110α in the regulation of cell polarity and migration.

Features of Ras activation by a mislocalized oncogenic tyrosine kinase: FLT3 ITD signals through K-Ras at the plasma membrane of acute myeloid leukemia cells.

FMS-like tyrosine kinase 3 with internal tandem duplication (FLT3 ITD) is an important oncoprotein in acute myeloid leukemia (AML). Owing to its constitutive kinase activity FLT3 ITD partially accumulates at endomembranes, a feature shared with other disease-associated, mutated receptor tyrosine kinases. Because Ras proteins also transit through endomembranes we have investigated the possible existence of an intracellular FLT3-ITD/Ras signaling pathway by comparing Ras signaling of FLT3 ITD with that of wild-type FLT3. Ligand stimulation activated both K- and N-Ras in cells expressing wild-type FLT3. Live-cell Ras-GTP imaging revealed ligand-induced Ras activation at the plasma membrane (PM). FLT3-ITD-dependent constitutive activation of K-Ras and N-Ras was also observed primarily at the PM, supporting the view that the PM-resident pool of FLT3 ITD engaged the Ras/Erk pathway in AML cells. Accordingly, specific interference with FLT3-ITD/Ras signaling at the PM using PM-restricted dominant negative K-RasS17N potently inhibited cell proliferation and promoted apoptosis. In conclusion, Ras signaling is crucial for FLT3-ITD-dependent cell transformation and FLT3 ITD addresses PM-bound Ras despite its pronounced mislocalization to endomembranes.

Cell transformation by FLT3 ITD in acute myeloid leukemia involves oxidative inactivation of the tumor suppressor protein-tyrosine phosphatase DEP-1/ PTPRJ.

Signal transduction of FMS-like tyrosine kinase 3 (FLT3) is regulated by protein-tyrosine phosphatases (PTPs). We recently identified the PTP DEP-1/CD148/PTPRJ as a novel negative regulator of FLT3. This study addressed the role of DEP-1 for regulation of the acute myeloid leukemia (AML)-related mutant FLT3 internal tandem duplication (ITD) protein. Our experiments revealed that DEP-1 was expressed but dysfunctional in cells transformed by FLT3 ITD. This was caused by enzymatic inactivation of DEP-1 through oxidation of the DEP-1 catalytic cysteine. In intact cells, including primary AML cells, FLT3 ITD kinase inhibition reactivated DEP-1. DEP-1 reactivation was also achieved by counteracting the high levels of reactive oxygen species (ROS) production detected in FLT3 ITD-expressing cell lines by inhibition of reduced NAD phosphate (NADPH)-oxidases, or by overexpression of catalase or peroxiredoxin-1 (Prx-1). Interference with ROS production in 32D cells inhibited cell transformation by FLT3 ITD in a DEP-1-dependent manner, because RNAi-mediated depletion of DEP-1 partially abrogated the inhibitory effect of ROS quenching. Reactivation of DEP-1 by stable overexpression of Prx-1 extended survival of mice in the 32D cell/C3H/HeJ mouse model of FLT3 ITD-driven myeloproliferative disease. The study thus uncovered DEP-1 oxidation as a novel event contributing to cell transformation by FLT3 ITD.

CRISPR/Cas9-Mediated Modification of PTP Expression.

Alteration of protein tyrosine phosphatase (PTP) gene expression is a commonly used approach to experimentally analyze their function in the cell physiology of mammalian cells. Here, exemplified for receptor-type PTPRJ (Dep-1, CD148) and PPTRC (CD45), we provide the CRISPR/Cas9-mediated approaches for their inactivation and transcriptional activation using genome editing. These methods are generally applicable to any other protein of interest.

Protein-tyrosine phosphatase DEP-1 controls receptor tyrosine kinase FLT3 signaling.

Fms-like tyrosine kinase 3 (FLT3) plays an important role in hematopoietic differentiation, and constitutively active FLT3 mutant proteins contribute to the development of acute myeloid leukemia. Little is known about the protein-tyrosine phosphatases (PTP) affecting the signaling activity of FLT3. To identify such PTP, myeloid cells expressing wild type FLT3 were infected with a panel of lentiviral pseudotypes carrying shRNA expression cassettes targeting different PTP. Out of 20 PTP tested, expressed in hematopoietic cells, or presumed to be involved in oncogenesis or tumor suppression, DEP-1 (PTPRJ) was identified as a PTP negatively regulating FLT3 phosphorylation and signaling. Stable 32D myeloid cell lines with strongly reduced DEP-1 levels showed site-selective hyperphosphorylation of FLT3. In particular, the sites pTyr-589, pTyr-591, and pTyr-842 involved in the FLT3 ligand (FL)-mediated activation of FLT3 were hyperphosphorylated the most. Similarly, acute depletion of DEP-1 in the human AML cell line THP-1 caused elevated FLT3 phosphorylation. Direct interaction of DEP-1 and FLT3 was demonstrated by "substrate trapping" experiments showing association of DEP-1 D1205A or C1239S mutant proteins with FLT3 by co-immunoprecipitation. Moreover, activated FLT3 could be dephosphorylated by recombinant DEP-1 in vitro. Enhanced FLT3 phosphorylation in DEP-1-depleted cells was accompanied by enhanced FLT3-dependent activation of ERK and cell proliferation. Stable overexpression of DEP-1 in 32D cells and transient overexpression with FLT3 in HEK293 cells resulted in reduction of FL-mediated FLT3 signaling activity. Furthermore, FL-stimulated colony formation of 32D cells expressing FLT3 in methylcellulose was induced in response to shRNA-mediated DEP-1 knockdown. This transforming effect of DEP-1 knockdown was consistent with a moderately increased activation of STAT5 upon FL stimulation but did not translate into myeloproliferative disease formation in the 32D-C3H/HeJ mouse model. The data indicate that DEP-1 is negatively regulating FLT3 signaling activity and that its loss may contribute to but is not sufficient for leukemogenic cell transformation.

PI3Kγ is a novel regulator of TNFα signaling in the human colon cell line HT29/B6.

Phosphoinositide 3-kinases (PI3Ks) are a family of enzymes phosphorylating phospholipids in the membrane, thereby, promoting the PI3K/AKT signaling cascade. PI3Ks are involved in a variety of fundamental cellular functions, including tumor necrosis factor α (TNFα)-induced tight junction (TJ) impairment-a hallmark of inflammatory bowel diseases. Most of the studies analyzing the role of class I PI3K signaling in epithelial barrier maintenance did not decipher which of the isoforms are responsible for the observed effects. By using wild-type and PI3Kγ-deficient HT-29/B6 cells, we characterized the functional role of PI3Kγ in these cells under inflammatory conditions. Measurement of the transepithelial electrical resistance and the paracellular flux of macromolecules revealed that monolayers of PI3Kγ-deficient cells, compared with wild-type cells, were protected against TNFα-induced barrier dysfunction. This effect was independent of any PI3K activity because treatment with a pan-PI3K inhibitor did not alter this observation. By immunostaining, we found correlative changes in the distribution of the TJ marker ZO-1. Furthermore, the absence of PI3Kγ reduced the basal level of the pore-forming TJ protein claudin-2. Our study suggests a novel noncanonical, kinase-independent scaffolding function of PI3Kγ in TNFα-induced barrier dysfunction.

Disrupting PTPRJ transmembrane-mediated oligomerization counteracts oncogenic receptor tyrosine kinase FLT3 ITD.

The receptor protein tyrosine phosphatase (RPTP) PTPRJ (also known as DEP-1) has been identified as a negative regulator of the receptor tyrosine kinase FLT3 signalling in vitro. The inactivation of the PTPRJ gene in mice expressing the constitutively active, oncogenic receptor tyrosine kinase FLT3 ITD aggravated known features of leukaemogenesis, revealing PTPRJ's antagonistic role. FLT3 ITD mutations resulting in constitutively kinase activity and cell transformation frequently occur in patients with acute myeloid leukaemia (AML). Thus, in situ activation of PTPRJ could be used to abrogate oncogenic FLT3 signalling. The activity of PTPRJ is suppressed by homodimerization, which is mediated by transmembrane domain (TMD) interactions. Specific Glycine-to-Leucine mutations in the TMD disrupt oligomerization and inhibit the Epidermal Growth Factor Receptor (EGFR) and EGFR-driven cancer cell phenotypes. To study the effects of PTPRJ TMD mutant proteins on FLT3 ITD activity in cell lines, endogenous PTPRJ was inactivated and replaced by stable expression of PTPRJ TMD mutants. Autophosphorylation of wild-type and ITD-mutated FLT3 was diminished in AML cell lines expressing the PTPRJ TMD mutants compared to wild-type-expressing cells. This was accompanied by reduced FLT3-mediated global protein tyrosine phosphorylation and downstream signalling. Further, PTPRJ TMD mutant proteins impaired the proliferation and in vitro transformation of leukemic cells. Although PTPRJ's TMD mutant proteins showed impaired self-association, the specific phosphatase activity of immunoprecipitated proteins remained unchanged. In conclusion, this study demonstrates that the destabilization of PTPRJ TMD-mediated self-association increases the activity of PTPRJ in situ and impairs FLT3 activity and FLT3-driven cell phenotypes of AML cells. Thus, disrupting the oligomerization of PTPRJ in situ could prove a valuable therapeutic strategy to restrict oncogenic FLT3 activity in leukemic cells.

Anchoring of FLT3 in the endoplasmic reticulum alters signaling quality.

The mechanism of cell transformation by Fms-like tyrosine kinase 3 (FLT3) in acute myeloid leukemia (AML) is incompletely understood. The most prevalent activated mutant FLT3 ITD exhibits an altered signaling quality, including strong activation of the STAT5 transcription factor. FLT3 ITD has also been found partially retained as a high-mannose precursor in an intracellular compartment. To analyze the role of intracellular retention of FLT3 for transformation, we have generated FLT3 versions that are anchored in the perinuclear endoplasmic reticulum (ER) by appending an ER retention sequence containing a RRR (R3) motif. ER retention of R3, but not of corresponding A3 FLT3 versions, is shown by biochemical, fluorescence-activated cell sorting, and immunocytochemical analyses. ER anchoring reduced global autophosphorylation and diminished constitutive activation of ERK1/2 and AKT of the constitutively active FLT3 versions. ER anchoring was, however, associated with elevated signaling to STAT3. Transforming activity of the FLT3 D835Y mutant was suppressed by ER anchoring. In contrast, ER-anchored FLT3 ITD retained STAT5-activating capacity and was transforming in vitro and in vivo. The findings highlight another aspect of the different signaling quality of FLT3 ITD: It can transform cells from an intracellular location.

PI3Kγ Mediates Microglial Proliferation and Cell Viability via ROS.

(1) Background: Rapid microglial proliferation contributes to the complex responses of the innate immune system in the brain to various neuroinflammatory stimuli. Here, we investigated the regulatory function of phosphoinositide 3-kinase γ (PI3Kγ) and reactive oxygen species (ROS) for rapid proliferation of murine microglia induced by LPS and ATP. (2) Methods: PI3Kγ knockout mice (PI3Kγ KO), mice expressing catalytically inactive PI3Kγ (PI3Kγ KD) and wild-type mice were assessed for microglial proliferation using an in vivo wound healing assay. Additionally, primary microglia derived from newborn wild-type, PI3Kγ KO and PI3Kγ KD mice were used to analyze PI3Kγ effects on proliferation and cell viability, senescence and cellular and mitochondrial ROS production; the consequences of ROS production for proliferation and cell viability after LPS or ATP stimulation were studied using genetic and pharmacologic approaches. (3) Results: Mice with a loss of lipid kinase activity showed impaired proliferation of microglia. The prerequisite of induced microglial proliferation and cell viability appeared to be PI3Kγ-mediated induction of ROS production. (4) Conclusions: The lipid kinase activity of PI3Kγ plays a crucial role for microglial proliferation and cell viability after acute inflammatory activation.

Biocompatibility, uptake and subcellular localization of bacterial magnetosomes in mammalian cells.

Magnetosomes represent biogenic, magnetic nanoparticles biosynthesized by magnetotactic bacteria. Subtle biological control on each step of biomineralization generates core-shell nanoparticles of high crystallinity, strong magnetization and uniform shape and size. These features make magnetosomes a promising alternative to chemically synthesized nanoparticles for many applications in the biotechnological and biomedical field, such as their usage as biosensors in medical diagnostics, as drug-delivery agents, or as contrast agents for magnetic imaging techniques. Thereby, the particles are directly applied to mammalian cells or even injected into the body. In the present work, we provide a comprehensive characterization of isolated magnetosomes as potential cytotoxic effects and particle uptake have not been well studied so far. Different cell lines including cancer cells and primary cells are incubated with increasing particle amounts, and effects on cell viability are investigated. Obtained data suggest a concentration-dependent biocompatibility of isolated magnetosomes for all tested cell lines. Furthermore, magnetosome accumulation in endolysosomal structures around the nuclei is observed. Proliferation rates are affected in the presence of increasing particle amounts; however, viability is not affected and doubling times can be restored by reducing the magnetosome concentration. In addition, we evidence magnetosome-cell interactions that are strong enough to allow for magnetic cell sorting. Overall, our study not only assesses the biocompatibility of isolated magnetosomes, but also evaluates effects on cell proliferation and the fate of internalized magnetosomes, thereby providing prerequisites for their future in vivo application as biomedical agents.

PI3Kgamma controls oxidative bursts in neutrophils via interactions with PKCalpha and p47phox.

Neutrophils release reactive oxygen species (ROS) as part of the innate inflammatory immune response. Phosphoinositide 3-kinase gamma (PI3Kgamma), which is induced by the bacterial peptide N-formylmethionyl-leucyl-phenylalanine (fMLP), has been identified as an essential intracellular mediator of ROS production. However, the complex signalling reactions that link PI3Kgamma with ROS synthesis by NADPH oxidase have not yet been described in detail. We found that activation of neutrophils by fMLP triggers the association of PI3Kgamma with protein kinase Calpha (PKCalpha). Specific inhibition of PI3Kgamma suppresses fMLP-mediated activation of PKCalpha activity and ROS production, suggesting that the protein kinase activity of PI3Kgamma is involved. Our data suggest that the direct interaction of PI3Kgamma with PKCalpha forms a discrete regulatory module of fMLP-dependent ROS production in neutrophils.

Novel Approaches to Target Mutant FLT3 Leukaemia.

Fms-like tyrosine kinase 3 (FLT3) is a member of the class III receptor tyrosine kinases (RTK) and is involved in cell survival, proliferation, and differentiation of haematopoietic progenitors of lymphoid and myeloid lineages. Oncogenic mutations in the FLT3 gene resulting in constitutively active FLT3 variants are frequently found in acute myeloid leukaemia (AML) patients and correlate with patient's poor survival. Targeting FLT3 mutant leukaemic stem cells (LSC) is a key to efficient treatment of patients with relapsed/refractory AML. It is therefore essential to understand how LSC escape current therapies in order to develop novel therapeutic strategies. Here, we summarize the current knowledge on mechanisms of FLT3 activity regulation and its cellular consequences. Furthermore, we discuss how aberrant FLT3 signalling cooperates with other oncogenic lesions and the microenvironment to drive haematopoietic malignancies and how this can be harnessed for therapeutical purposes.

Aberrant Bone Homeostasis in AML Is Associated with Activated Oncogenic FLT3-Dependent Cytokine Networks.

Acute myeloid leukaemia (AML) is a haematopoietic malignancy caused by a combination of genetic and epigenetic lesions. Activation of the oncoprotein FLT3 ITD (Fms-like tyrosine kinase with internal tandem duplications) represents a key driver mutation in 25-30% of AML patients. FLT3 is a class III receptor tyrosine kinase, which plays a role in cell survival, proliferation, and differentiation of haematopoietic progenitors of lymphoid and myeloid lineages. Mutant FLT3 ITD results in an altered signalling quality, which causes cell transformation. Recent evidence indicates an effect of FLT3 ITD on bone homeostasis in addition to haematological aberrations. Using gene expression data repositories of FLT3 ITD-positive AML patients, we identified activated cytokine networks that affect the formation of the haematopoietic niche by controlling osteoclastogenesis and osteoblast functions. In addition, aberrant oncogenic FLT3 signalling of osteogenesis-specific cytokines affects survival of AML patients and may be used for prognosis. Thus, these data highlight the intimate crosstalk between leukaemic and osteogenic cells within the osteohaematopoietic niche.