Co-expression of the collagen receptors leukocyte-associated immunoglobulin-like receptor-1 and glycoprotein VI on a subset of megakaryoblasts

BACKGROUND

The collagen receptor glycoprotein VI generates activating signals through an immunoreceptor tyrosine-based activating motif on the co-associated Fc receptor gamma chain. Leukocyte-associated immunoglobulin-like receptor-1 also ligates collagen but generates inhibitory signals through immunoreceptor tyrosine-based inhibitory motifs. Thus far, the cellular expression of glycoprotein VI and leukocyte-associated immunoglobulin-like receptor-1 appears mutually exclusive.

DESIGN AND METHODS

Using flow cytometry, we studied expression of collagen receptors on differentiating human megakaryocytes. CD34(+) cells were isolated from umbilical cord blood and matured to megakaryocytes in vitro. Freshly isolated bone marrow cells were used to study primary megakaryocytes. Upon cell sorting, cytospins were made to examine cytological characteristics of differentiation.

RESULTS

Megakaryocyte maturation is accompanied by up-regulation of glycoprotein VI and down-regulation of leukocyte-associated immunoglobulin-like receptor-1. Interestingly, both in cultures from hematopoietic stem cells and primary cells obtained directly from bone marrow, we identified a subset of morphologically distinct megakaryocytes which co-express glycoprotein VI and leukocyte-associated immunoglobulin-like receptor-1.

CONCLUSIONS

This is the first report of a primary cell that co-expresses these collagen receptors with opposite signaling properties. Since megakaryocytes mature in the collagen-rich environment of the bone marrow, these findings may point to a role for leukocyte-associated immunoglobulin-like receptor-1 in the control of megakaryocyte maturation/migration.

Human CD34-derived myeloid dendritic cell development requires intact phosphatidylinositol 3-kinase-protein kinase B-mammalian target of rapamycin signaling

Dendritic cells (DCs) are composed of different subsets that exhibit distinct functionality in the induction and regulation of immune responses. The myeloid DC subsets, including interstitial DCs and Langerhans cells (LCs), develop from CD34+ hematopoietic progenitors via direct DC precursors or monocytes. The molecular mechanisms regulating DC development are still largely unknown and mostly studied in mice. Phosphatidylinositol 3-kinase (PI3K) regulates multiple processes in myeloid cells. This study investigated the role of PI3K signaling in the development of human CD34-derived myeloid DCs. Pharmacologic inhibition of PI3K or one of its downstream targets mTOR reduced interstitial DC and LC numbers in vitro. Increased activity of this signaling module by introduction of constitutively active protein kinase B (PKB/c-Akt) increased the yields of human DC precursors in vitro as well as in transplanted beta2-microglobulin-/- NOD/SCID mice in vivo. Signaling inhibition during differentiation did not affect the acquisition of a DC phenotype, whereas proliferation and survival strongly depended on intact PI3K-PKB-mTOR signaling. Interestingly, however, this pathway became redundant for survival regulation upon terminal differentiation, which was associated with an altered expression of apoptosis regulating genes. Although dispensable for costimulatory molecule expression, the PI3K-PKB-mTOR signaling module was required for other important processes associated with DC function, including Ag uptake, LPS-induced cytokine secretion, CCR7 expression, and T cell stimulation. Thus, PI3K-PKB-mTOR signaling plays a crucial role in the development of functional CD34-derived myeloid DCs. These findings could be used as a strategy to manipulate DC subset distribution and function to regulate immunity.

Histone deacetylase inhibition modulates cell fate decisions during myeloid differentiation

BACKGROUND

The clinical use of chromatin-modulating drugs, such as histone deacetylase inhibitors, for the treatment of bone marrow failure and hematopoietic malignancies has increased dramatically over the last few years. Nonetheless, little is currently known concerning their effects on myelopoiesis.

DESIGN AND METHODS

We utilized an ex vivo differentiation system in which umbilical cord blood-derived CD34(+) cells were treated with trichostatin A, sodium butyrate and valproic acid to evaluate the effect of histone deacetylase inhibitor treatment on myeloid lineage development, colony-forming potential, proliferation, and terminal neutrophil differentiation.

RESULTS

Trichostatin A treatment modestly reduced progenitor proliferation, while sodium butyrate and valproic acid resulted in concentration-dependent effects on proliferation and apoptosis. Addition of valproic acid uniquely stimulated CD34(+) proliferation. Sodium butyrate treatment inhibited terminal neutrophil differentiation both quantitatively and qualitatively. Addition of 100 microM valproic acid resulted in increased numbers of mature neutrophils with a block in differentiation at increasing concentrations. Sodium butyrate and valproic acid treatment resulted in increased acetylation of histones 3 and 4 while trichostatin A, sodium butyrate and valproic acid had differential effects on the acetylation of non-histone proteins.

CONCLUSIONS

Individual histone deacetylase inhibitors had specific effects on cell fate decisions during myeloid development. These data provide novel insights into the effects of histone deacetylase inhibitors on the regulation of normal hematopoiesis, which is of importance when considering utilizing these compounds for the treatment of myeloid malignancies and bone marrow failure syndromes.

p38 MAP kinase inhibits neutrophil development through phosphorylation of C/EBPalpha on serine 21

Many extracellular stimuli regulate growth, survival, and differentiation responses through activation of the dual specificity mitogen activated protein kinase (MAPK) kinase three (MKK3) and its downstream effector p38 MAPK. Using CD34+ hematopoietic progenitor cells, here we describe a novel role for MKK3-p38MAPK in the regulation of myelopoiesis. Inhibition of p38MAPK utilizing the pharmacological inhibitor SB203580, enhanced neutrophil development ex vivo, but conversely reduced eosinophil differentiation. In contrast, constitutive activation of MKK3 dramatically inhibited neutrophil differentiation. Transplantation of beta2-microglobulin(-/-) nonobese diabetic/severe combined immune deficient (NOD/SCID) mice with CD34+ cells ectopically expressing constitutively active MKK3 resulted in reduced neutrophil differentiation in vivo, whereas eosinophil development was enhanced. Inhibitory phosphorylation of CCAAT/enhancer binding protein alpha (C/EBPalpha) on serine 21 was induced upon activation of p38MAPK. Moreover, ectopic expression of a non-phosphorylatable C/EBPalpha mutant was sufficient to abrogate MKK3-induced inhibition of neutrophil development. Furthermore, treatment of CD34+ progenitors from patients with severe congenital neutropenia with SB203580 restored neutrophil development. These results establish a novel role for MKK3-p38MAPK in the regulation of lineage choices during myelopoiesis through modulation of C/EBPalpha activity. This signaling module may thus provide an important therapeutic target in the treatment of bone marrow failure.

Ectopic expression of C/EBPalpha and ID1 is sufficient to restore defective neutrophil development in low-risk myelodysplasia

BACKGROUND

In patients with myelodysplasia, a general defect in the multipotent stem-cell compartment results in disturbed proliferation and differentiation of the erythroid, megakaryocytic and myeloid lineages. Although a number of genetic defects in myelodysplastic progenitor cells have been described, the intracellular signaling pathways underlying aberrant regulation of myelopoiesis remain relatively undefined.

DESIGN AND METHODS

Here, an ex vivo differentiation system was used to selectively screen for molecules improving defective hematopoiesis in myelodysplastic CD34(+) progenitor cells.

RESULTS

Bone marrow-derived CD34(+) cells isolated from patients with low-risk myelodysplastic syndrome showed impaired capacity to proliferate and differentiate as well as increased levels of apoptosis. In an attempt to improve the expansion and differentiation of the myelodysplastic CD34(+) progenitors, cells were treated with the p38MAPK pharmacological inhibitor SB203580, or retrovirally transduced to ectopically express active protein kinase B (PKB/c-akt), or the transcriptional regulators STAT5, C/EBPalpha or ID1. Whereas treatment of progenitors with SB203580, PKB or STAT5 did not enhance neutrophil development, ID1- and C/EBPalpha-transduced cells exhibited increased granulocyte/macrophage colony formation. Furthermore, ectopic expression of C/EBPalpha resulted in improved neutrophil maturation.

CONCLUSIONS

These data suggest that targeting the ID1 and C/EBPalpha transcriptional regulators may be of benefit in the design of novel therapies for low-risk myelodysplasia.

Mammalian target of rapamycin activity is required for expansion of CD34+ hematopoietic progenitor cells

BACKGROUND

The mammalian target of rapamycin is a conserved protein kinase known to regulate protein synthesis, cell size and proliferation. Aberrant regulation of mammalian target of rapamycin activity has been observed in hematopoietic malignancies, including acute leukemias and myelodysplastic syndromes, suggesting that correct regulation of mammalian target of rapamycin is critical for normal hematopoiesis.

DESIGN AND METHODS

An ex vivo granulocyte differentiation system was utilized to investigate the role of mammalian target of rapamycin in the regulation of myelopoiesis.

RESULTS

Inhibition of mammalian target of rapamycin activity, with the pharmacological inhibitor rapamycin, dramatically reduced hematopoietic progenitor expansion, without altering levels of apoptosis or maturation. Moreover, analysis of distinct hematopoietic progenitor populations revealed that rapamycin treatment inhibited the expansion potential of committed CD34(+) lineage-positive progenitors, but did not affect early hematopoietic progenitors. Further examinations showed that these effects of rapamycin on progenitor expansion might involve differential regulation of protein kinase B and mammalian target of rapamycin signaling.

CONCLUSIONS

Together, these results indicate that mammalian target of rapamycin activity is essential for expansion of CD34(+) hematopoietic progenitor cells during myelopoiesis. Modulation of the mammalian target of rapamycin pathway may be of benefit in the design of new therapies to control hematologic malignancies.

MAPK signaling pathways in the regulation of hematopoiesis

The MAPKs are a family of serine/threonine kinases that play an essential role in connecting cell-surface receptors to changes in transcriptional programs. MAPKs are part of a three-component kinase module consisting of a MAPK, an upstream MEK, and a MEKK that couples the signals from cell-surface receptors to trigger downstream pathways. Three major groups of MAPKs have been characterized in mammals, including ERKs, JNKs, and p38MAPKs. Over the last decade, extensive work has established that these proteins play critical roles in the regulation of a wide variety of cellular processes including cell growth, migration, proliferation, differentiation, and survival. It has been demonstrated that ERK, JNK, and p38MAPK activity can be regulated in response to a plethora of hematopoietic cytokines and growth factors that play critical roles in hematopoiesis. In this review, we summarize the current understanding of MAPK function in the regulation of hematopoiesis in general and myelopoiesis in particular. In addition, the consequences of aberrant MAPK activation in the pathogenesis of various myeloid malignancies will be discussed.

The transcription of FOXO genes is stimulated by FOXO3 and repressed by growth factors

FOXO (Forkhead box O) transcription factors induce cell growth arrest and apoptosis, which can be prevented by FOXO phosphorylation by AKT in response to growth factors such as platelet-derived growth factors (PDGF) and insulin-like growth factor I (IGF-I). In addition to this well characterized post-translational modification, we showed that FOXO1, FOXO3, and FOXO4 were also regulated at the transcriptional level. PDGF, fibroblast growth factors (FGF), and IGF-I repressed the expression of FOXO genes in human fibroblasts. This process was sensitive to phosphatidylinositol 3-kinase inhibition by LY294002. FOXO1-specific shRNA decreased FOXO1 mRNA expression and enhanced fibroblast proliferation, mimicking the effects of growth factors. Conversely, ectopic FOXO3 activation blocked the proliferation of fibroblasts and induced the expression of FOXO1, FOXO4, and p27-KIP1. Using luciferase reporter assays and chromatin immunoprecipitations, we identified a conserved FOXO-binding site in the promoter of the FOXO1 gene, which was required for regulation by PDGF, and mediated the up-regulation of FOXO1 by itself and by FOXO3. Altogether, our results suggest that the expression of FOXO1 and FOXO4 genes is stimulated by FOXO3 and possibly by other FOXO factors in a positive feedback loop, which is disrupted by growth factors.

The ins and outs of syntenin, a multifunctional intracellular adaptor protein

One of the most challenging issues currently facing cell biologists is how signal specificity and compartmentalization is achieved, allowing extracellular stimulation to result in a unique and pre-defined intracellular outcome. For this to occur, intracellular components must be correctly positioned in both space and time. Adaptor molecules, which contain protein-interaction domains, are often involved in the assembly of multimeric complexes that organize intracellular signal-transduction pathways. One such protein is syntenin, a PDZ-domain-containing molecule that has a surprising variety and diversity of interaction partners. Here we assimilate and discuss current data that support a role for syntenin in regulating transmembrane-receptor trafficking, tumour-cell metastasis and neuronal-synapse function.

FOXO-binding partners: it takes two to tango

Modulation FOXO transcription factor activities can lead to a variety of cellular outputs resulting in changes in proliferation, apoptosis, differentiation and metabolic responses. Although FOXO proteins all contain an identical DNA-binding domain their cellular functions appear to be distinct, as exemplified by differences in the phenotype of Foxo1, Foxo3 and Foxo4 null mutant mice. While some of these differences may be attributable to the differential expression patterns of these transcription factors, many cells and tissues express several FOXO isoforms. Recently it has become clear that FOXO proteins can regulate transcriptional responses independently of direct DNA-binding. It has been demonstrated that FOXOs can associate with a variety of unrelated transcription factors, regulating activation or repression of diverse target genes. The complement of transcription factors expressed in a particular cell type is thus critical in determining the functional end point of FOXO activity. These interactions greatly expand the possibilities for FOXO-mediated regulation of transcriptional programmes. This review details currently described FOXO-binding partners and examines the role of these interactions in regulating cell fate decisions.

Statin synergizes with LPS to induce IL-1beta release by THP-1 cells through activation of caspase-1

Mevalonate kinase deficiency (MKD) is a hereditary syndrome characterized by recurring episodes of fever and inflammation. Peripheral blood mononuclear cells from MKD patients secrete high levels of interleukin (IL)-1beta when stimulated with lipopolysaccharide (LPS), which is thought to be a primary cause of the inflammation. However, the link between a deficient mevalonate kinase and excessive IL-1beta release remains unclear. To investigate this we made use of a model in which monocytic cells (THP-1) were treated with simvastatin. Statins are compounds that inhibit 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase and thereby artificially impair the isoprenoid biosynthesis pathway, mimicking mevalonate kinase deficiency. Our study revealed that LPS-stimulated THP-1 cells treated with simvastatin had an increased caspase-1 mediated processing of proIL-1beta. This increased processing was caused by enhanced autoprocessing of caspase-1, rather than enhanced transcription or translation of caspase-1 or proIL-1beta. Simvastatin-induced activation of caspase-1 was caused by an impairment of non-sterol isoprenoid biosynthesis, as the isoprenyl intermediate GGPP could block activation of caspase-1 and mIL-1beta release. In addition, inhibition of both farnesyl pyrophosphate synthase and geranylgeranyltransferase I also induce mIL-1beta release. Taken together, these results demonstrate that simvastatin augments LPS-induced IL-1beta release post-translationally, by inducing caspase-1 activity. These findings suggest that MKD patients may have overactive caspase-1, causing enhanced IL-1beta processing and subsequent inflammation in response to bacterial components.

Reduced activation of protein kinase B, Rac, and F-actin polymerization contributes to an impairment of stromal cell–derived factor-1–induced migration of CD34+ cells from patients with myelodysplasia

Patients with myelodysplasia (MDS) show a differentiation defect in the multipotent stem-cell compartment. An important factor in stem-cell differentiation is their proper localization within the bone marrow microenvironment, which is regulated by stromal cell–derived factor (SDF-1). We now show that SDF-1–induced migration of CD34+ progenitor cells from MDS patients is severely impaired. In addition, these cells show a reduced capacity to polymerize F-actin in response to SDF-1. We demonstrate a major role for Rac and phosphatidylinositol 3-kinase (PI3K) and a minor role for the extracellular signal-regulated kinase (ERK)1/2 signaling pathway in SDF-1–induced migration of normal CD34+ cells. Furthermore, SDF-1–stimulated activation of Rac and the PI3K target protein kinase B is impaired in CD34+ cells from MDS patients. Lentiviral transduction of MDS CD34+ cells with constitutive active Rac1V12 results in a partial restoration of F-actin polymerization in response to SDF-1. In addition, expression of constitutive active Rac increases the motility of MDS CD34+ cells in the absence of SDF-1, although the directional migration of cells toward this chemoattractant is not affected. Taken together, our results show a reduced migration of MDS CD34+ cells toward SDF-1, as a result of impaired activation of the PI3K and Rac pathways and a decreased F-actin polymerization.

Protein kinase B (c-akt) regulates hematopoietic lineage choice decisions during myelopoiesis

Hematopoiesis is a highly regulated process resulting in the formation of all blood lineages. Aberrant regulation of phosphatidylinositol-3-kinase (PI3K) signaling has been observed in hematopoietic malignancies, suggesting that regulated PI3K signaling is critical for regulation of blood cell production. An ex vivo differentiation system was used to investigate the role of PI3K and its downstream effector, protein kinase B (PKB/c-akt) in myelopoiesis. PI3K activity was essential for hematopoietic progenitor survival. High PKB activity was found to promote neutrophil and monocyte development, while, conversely, reduction of PKB activity was required to induce optimal eosinophil differentiation. In addition, transplantation of β2-microglobulin (−/−) NOD/SCID mice with CD34+ cells ectopically expressing constitutively active PKB resulted in enhanced neutrophil and monocyte development, whereas ectopic expression of dominant-negative PKB induced eosinophil development in vivo. Inhibitory phosphorylation of C/EBPα on Thr222/226 was abrogated upon PKB activation in hematopoietic progenitors. Ectopic expression of a nonphosphorylatable C/EBPα mutant inhibited eosinophil differentiation ex vivo, whereas neutrophil development was induced, demonstrating the importance of PKB-mediated C/EBPα phosphorylation in regulation of granulopoiesis. These results identify an important novel role for PKB in regulation of cell fate choices during hematopoietic lineage commitment.

Molecular mechanisms underlying FIP1L1-PDGFRA-mediated myeloproliferation

An interstitial deletion on chromosome 4q12 resulting in the formation of the FIP1L1-PDGFRA fusion protein is involved in the pathogenesis of imatinib-sensitive chronic eosinophilic leukemia. The molecular mechanisms underlying the development of disease are largely undefined. Human CD34(+) hematopoietic progenitor cells were used to investigate the role of FIP1L1-PDGFRA in modulating lineage development. FIP1L1-PDGFRA induced both proliferation and differentiation of eosinophils, neutrophils, and erythrocytes in the absence of cytokines, which could be inhibited by imatinib. Whereas expression of FIP1L1-PDGFRA in hematopoietic stem cells and common myeloid progenitors induced the formation of multiple myeloid lineages, expression in granulocyte-macrophage progenitors induced only the development of eosinophils, neutrophils, and myeloblasts. Deletion of amino acids 30 to 233 in the FIP1L1 gene [FIP1L1(1-29)-PDGFRA] gave rise to an intermediate phenotype, exhibiting a dramatic reduction in the number of erythrocytes. FIP1L1-PDGFRA and FIP1L1(1-29)-PDGFRA both induced the activation of p38 and extracellular signal-regulated kinase 1/2 (ERK1/2) in myeloid progenitors, whereas signal transducers and activators of transcription 5 (STAT5) and protein kinase B/c-akt were only activated by FIP1L1-PDGFRA. Dominant-negative STAT5 partially inhibited FIP1L1-PDGFRA-induced colony formation, whereas combined inhibition of phosphatidylinositol-3-kinase and ERK1/2 significantly reversed FIP1L1-PDGFRA-induced colony formation. Taken together, these results suggest that expression of FIP1L1-PDFGRA in human hematopoietic progenitors induce a myeloproliferative phenotype via activation of multiple signaling molecules including phosphatidylinositol-3-kinase, ERK1/2, and STAT5.

Chronic protein kinase B (PKB/c-akt) activation leads to apoptosis induced by oxidative stress-mediated Foxo3a transcriptional up-regulation

Increased protein kinase B (PKB; c-Akt) activation is a hallmark of diverse neoplasias providing both proliferative and antiapoptotic survival signals. In this study, we investigated the effect of chronic PKB activation on cellular survival and proliferation using cytokine-dependent bone marrow-derived Ba/F3 cells, in which PKBalpha activation can be directly, and specifically, induced by addition of 4-hydroxytamoxifen (4-OHT). Direct activation of PKB rescued Ba/F3 cells from cytokine withdrawal-induced apoptosis; however, surprisingly, these antiapoptotic effects were short lived, cells only being protected for up to 48 hours. We observed that activation of PKB in survival factor-deprived cells led to a dramatic increase of Foxo3a on both the transcriptional and protein level leading to expression of its transcriptional targets Bim and p27(kip1). High levels of PKB activity result in increased aerobic glycolysis and mitochondrial activity resulting in overproduction of reactive oxygen species. To determine whether oxidative stress might itself be responsible for Foxo3a up-regulation, we utilized hydrogen peroxide (H(2)O(2)) as an artificial inducer of oxidative stress and N-acetylcysteine (NAC), a thiol-containing radical oxygen scavenger. Addition of NAC to the culture medium prolonged the life span of cells treated with 4-OHT and prevented the up-regulation of Foxo3a protein levels caused by PKB activation. Conversely, treatment of Ba/F3 cells with H(2)O(2) caused an increase of Foxo3a on both transcriptional and protein levels, suggesting that deregulated PKB activation leads to oxidative stress resulting in Foxo3a up-regulation and subsequently cell death. Taken together, our data provide novel insights into the molecular consequences of uncontrolled PKB activation.

FOXO3a induces differentiation of Bcr-Abl-transformed cells through transcriptional down-regulation of Id1

Leukemic transformation often requires activation of protein kinase B (PKB/c-Akt) and is characterized by increased proliferation, decreased apoptosis, and a differentiation block. PKB phosphorylates and inactivates members of the FOXO subfamily of Forkhead transcription factors. It has been suggested that hyperactivation of PKB maintains the leukemic phenotype through actively repressing FOXO-mediated regulation of specific genes. We have found expression of the transcriptional repressor Id1 (inhibitor of DNA binding 1) to be abrogated by FOXO3a activation. Inhibition of PKB activation or growth factor deprivation also resulted in strong down-regulation of Id1 promoter activity, Id1 mRNA, and protein expression. Id1 is highly expressed in Bcr-Abl-transformed K562 cells, correlating with high PKB activation and FOXO3a phosphorylation. Inhibition of Bcr-Abl by the chemical inhibitor STI571 resulted in activation of FOXO3a and down-regulation of Id1 expression. By performing chromatin immunoprecipitation assays and promoter-mutation analysis, we demonstrate that FOXO3a acts as a transcriptional repressor by directly binding to the Id1 promoter. STI571 treatment, or expression of constitutively active FOXO3a, resulted in erythroid differentiation of K562 cells, which was inhibited by ectopic expression of Id1. Taken together our data strongly suggest that high expression of Id1, through PKB-mediated inhibition of FOXO3a, is critical for maintenance of the leukemic phenotype.

The reduced GM-CSF priming of ROS production in granulocytes from patients with myelodysplasia is associated with an impaired lipid raft formation

Patients with myelodysplasia (MDS) show an impaired reactive oxygen species (ROS) production in response to fMLP stimulation of GM-CSF-primed neutrophils. In this study, we investigated the involvement of lipid rafts in this process and showed that treatment of neutrophils with the lipid raft-disrupting agent methyl-beta-cyclodextrin abrogates fMLP-induced ROS production and activation of ERK1/2 and protein kinase B/Akt, two signal transduction pathways involved in ROS production in unprimed and GM-CSF-primed neutrophils. We subsequently showed that there was a decreased presence of Lyn, gp91(phox), and p22(phox) in lipid raft fractions from neutrophils of MDS. Furthermore, the plasma membrane expression of the lipid raft marker GM1, which increases upon stimulation of GM-CSF-primed cells with fMLP, was reduced significantly in MDS patients. By electron microscopy, we showed that the fMLP-induced increase in GM1 expression in GM-CSF-primed cells was a result of de novo synthesis, which was less efficient in MDS neutrophils. Taken together, these data indicate an involvement of lipid rafts in activation of signal transduction pathways leading to ROS production and show that in MDS neutrophils, an impaired lipid raft formation in GM-CSF-primed cells results in an impaired ROS production.