S6 kinase p90rsk in granulocyte-macrophage colony-stimulating factor-stimulated proliferative and mature hematopoietic cells

RSK Inhibition Induces Apoptosis by Downregulating Protein Synthesis in a Variety of Acute Myeloid Leukemia Cell Lines

Fms-like tyrosine kinase 3 (FLT3) and isocitrate dehydrogenase 1/2 (IDH1/2) mutations drive malignancy in acute myeloid leukemia (AML), which accounts for approximately 40% of AML cases. Treatment with FLT3 or IDH1/2 inhibitors is used for such patients; however, it is not considered for most patients with AML who lack mutations on the respective genes. In this study, p90 ribosomal S6 kinase (RSK) was found to serve as a new therapeutic target in various AMLs with or without FLT3 mutations. BI-D1870, a potent inhibitor of RSK, significantly suppressed the proliferation of AML cell lines, among which three encoded wild-type FLT3 and three contained FLT3 driver mutations, compared with chronic myeloid leukemia K562 cells or other adherent cancer cells. BI-D1870 inhibited protein synthesis by dephosphorylating the p70 S6 kinase and eukaryotic initiation factor 4E-binding protein 1 in all AML cells except KG-1a cells. Meanwhile, the expression of microtubule-associated protein light chain 3B-I and -II increased in KG-1a cells treated with BI-D1870. BI-D1870 induced caspase-dependent apoptosis in all AML cells, including KG-1a cells. We next investigated the synergistic effect of BI-D1870 with cytarabine, a traditional anticancer drug used in AML. Synergistic effects of BI-D1870 and cytarabine were not observed in any of the cell lines. The findings suggested that BI-D1870 alone exerts an adequate antiproliferative effect on AML with or without FLT3 mutations and serves as a novel AML therapeutic agent.

Information theoretic approach to complex biological network reconstruction: application to cytokine release in RAW 264.7 macrophages

BACKGROUND

High-throughput methods for biological measurements generate vast amounts of quantitative data, which necessitate the development of advanced approaches to data analysis to help understand the underlying mechanisms and networks. Reconstruction of biological networks from measured data of different components is a significant challenge in systems biology.

RESULTS

We use an information theoretic approach to reconstruct phosphoprotein-cytokine networks in RAW 264.7 macrophage cells. Cytokines are secreted upon activation of a wide range of regulatory signals transduced by the phosphoprotein network. Identifying these components can help identify regulatory modules responsible for the inflammatory phenotype. The information theoretic approach is based on estimation of mutual information of interactions by using kernel density estimators. Mutual information provides a measure of statistical dependencies between interacting components. Using the topology of the network derived, we develop a data-driven parsimonious input-output model of the phosphoprotein-cytokine network.

CONCLUSIONS

We demonstrate the applicability of our information theoretic approach to reconstruction of biological networks. For the phosphoprotein-cytokine network, this approach not only captures most of the known signaling components involved in cytokine release but also predicts new signaling components involved in the release of cytokines. The results of this study are important for gaining a clear understanding of macrophage activation during the inflammation process.

The potential role for sphingolipids in neuropathogenesis of Alzheimer’s disease

The review discusses the functional role of sphingolipids in the pathogenesis of Alzheimer's disease. Certain evidence exist that the imbalance of sphingolipids such as sphingomyelin, ceramide, sphingosine, sphingosine-1-phosphate and galactosylceramide in the brain of animals and humans, in the cerebrospinal fluid and blood plasma of patients with Alzheimer's disease play a crucial role in neuronal function by regulating growth, differentiation and cell death in CNS. Activation of sphingomyelinase, which leads to the accumulation of the proapoptotic agent, ceramide, can be considered as a new mechanism for AD and may be a prerequisite for the treatment of this disease by using drugs that inhibit sphingomyelinase activity. The role of sphingolipids as biomarkers for the diagnosis of the early stage of Alzheimer's disease and monitoring the effectiveness of treatment with new drugs is discussed.

Single-cell analysis of phosphoinositide 3-kinase and phosphatase and tensin homolog activation

A single-cell assay was developed to measure the activation of phosphoinositide 3-kinase (PI3K) using microanalytical chemical separations and a fluorescently labeled lipid substrate. Phosphatidyl-inositol 4,5 bisphosphate labeled on its acyl chain with Bodipy fluorescein (Bodipy Fl PIP(2)) was utilized as a substrate for both in vitro and cell-based assays. Detection limits for the substrate and product of the PI3K reaction were 10 to 20 zeptomol. In vitro assays with PI3K with and without pharmacologic inhibitors demonstrated that Bodipy Fl PIP(2) was converted to phosphatidyl-inositol 3,4,5 trisphosphate (Bodipy Fl PIP(3)). Bodipy Fl PIP(3) could be back converted to Bodipy Fl PIP(2) by the phosphatase PTEN. When Bodipy Fl PIP(2) was added to a cell lysate, 1.4 fmol of the Bodipy Fl PIP(3) were produced per ng of protein in the cytoplasmic extract in 10 min. Addition of Bodipy Fl PIP(3) to a cell lysate yielded 3 fmol of Bodipy Fl PIP(2) per ng of protein in 8 min. Both Bodipy Fl PIP(2) and Bodipy Fl PIP(3) were measureable in single cells and the two species could be inter-converted. Under the appropriate conditions, a fluorescent diacylglycerol was also detected in single cells. When the FcepsilonR 1 receptor on the cells loaded with the fluorescent lipid was cross-linked, the amount of Bodipy Fl PIP(3) generated per cell increased 4-fold over that of unstimulated cells. This production of Bodipy Fl PIP(3) was blocked by wortmannin. Chemical cytometry utilizing the fluorescent lipids will be of value in understanding lipid metabolism at the single-cell level.

Glucocorticoids promote survival of anti-inflammatory macrophages via stimulation of adenosine receptor A3

Active resolution of inflammation is a previously unrecognized process essential for tissue homeostasis. Monocytes play a pivotal role in the generation as well as resolution of inflammation. Glucocorticoids (GCs) are widely used anti-inflammatory agents. We demonstrate that GCs exhibit antiapoptotic effects in monocytes resulting in differentiation to an anti-inflammatory phenotype. The molecular basis of this novel antiapoptotic effect is a prolonged activation of the extracellular signal regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathway resulting in inhibition of caspase activities and expression of antiapoptotic genes via activation of c-Myc. We identified up-regulation and activation of A3 adenosine receptor (A3AR) as the initial trigger of this antiapoptotic pathway. In summary, we deciphered a novel molecular pathway promoting survival of anti-inflammatory monocytes. Specific activation of A3AR or its downstream signaling pathways may thus be a novel strategy to modulate inflammation in autoimmune disorders with fewer side effects via induction of inflammatory resolution rather than immunosuppression.

Anti-inflammatory effect of interleukin-10 on human neutrophil respiratory burst involves inhibition of GM-CSF-induced p47PHOX phosphorylation through a decrease in ERK1/2 activity

Interleukin-10 (IL-10) exerts its anti-inflammatory properties by down-regulating polymorphonuclear neutrophil (PMN) functions such as reactive oxygen species (ROS) production via NADPH oxidase. The molecular mechanisms underlying this process are unclear. Partial phosphorylation of the NADPH oxidase cytosolic component p47(PHOX) induced by proinflammatory cytokines, such as granulocyte-macrophage colony stimulating factor (GM-CSF) and tumor necrosis factor (TNF)-alpha, is essential for priming ROS production by PMN. The aim of this study was to determine whether IL-10 inhibits GM-CSF- and TNFalpha-induced p47(PHOX) phosphorylation and to investigate the molecular mechanisms involved in this effect. We found that IL-10 selectively inhibited GM-CSF- but not TNFalpha-induced p47PHOX phosphorylation in a concentration-dependent manner. As GM-CSF-induced p47PHOX phosphorylation is mediated by extracellular signal-regulated kinase 1/2 (ERK1/2), we tested the effect of IL-10 on this pathway. We found that IL-10 inhibited GM-CSF-induced ERK1/2 activity in an immunocomplex kinase assay. This inhibitory effect was confirmed by analyzing the phosphorylation status of the endogenous substrate of ERK1/2, p90RSK, in intact PMN. Furthermore, IL-10 decreased ROS production by adherent GM-CSF-treated PMN in keeping with the higher ROS production observed in whole blood from IL-10 knockout mice compared to their wild-type counterparts. Together, these results suggest that IL-10 inhibits GM-CSF-induced priming of ROS production by inhibiting p47PHOX phosphorylation through a decrease in ERK1/2 activity. This IL-10 effect could contribute to the tight regulation of NADPH oxidase activity at the inflammatory site.

The uncovering of a novel regulatory mechanism for PLD2: formation of a ternary complex with protein tyrosine phosphatase PTP1B and growth factor receptor-bound protein GRB2

The regulation of PLD2 activation is poorly understood at present. Transient transfection of COS-7 with a mycPLD2 construct results in elevated levels of PLD2 enzymatic activity and tyrosyl phosphorylation. To investigate whether this phosphorylation affects PLD2 enzymatic activity, anti-myc immunoprecipitates were treated with recombinant protein tyrosine phosphatase PTP1B. Surprisingly, lipase activity and PY levels both increased over a range of PTP1B concentrations. These increases occurred in parallel to a measurable PTP1B-associated phosphatase activity. Inhibitor studies demonstrated that an EGF-receptor type kinase is involved in phosphorylation. In a COS-7 cell line created in the laboratory that stably expressed myc-PLD2, PTP1B induced a robust (>6-fold) augmentation of myc-PLD2 phosphotyrosine content. The addition of growth factor receptor-bound protein 2 (Grb2) to cell extracts also elevated PY levels of myc-PLD (>10-fold). Systematic co-immunoprecipitation-immunoblotting experiments pointed at a physical association between PLD2, Grb2, and PTP1B in both physiological conditions and in overexpressed cells. This is the first report of a demonstration of the mammalian isoform PLD2 existing in a ternary complex with a protein tyrosine phosphatase, PTP1b, and the docking protein Grb2 which greatly enhances tyrosyl phosphorylation of the lipase.

Normal neutrophil maturation is associated with selective loss of MAP kinase activation by G-CSF

Although both GM-CSF and G-CSF activate p42/44 MAPK in neutrophil progenitors, the ability of G-CSF to cause MAPK activation is lost in mature neutrophils, while GM-CSF exposure still causes activation. The mechanism of this differential effect related to maturation status has not been explored. We verified that G-CSF and GM-CSF receptors remain functional on purified mature neutrophils by demonstrating that both cytokines caused phosphorylation of STAT3. However, only GM-CSF was capable of activating MAPK as assessed by gel shift and in vitro kinase assay. Both G-CSF and GM-CSF caused activation of p21 ras in neutrophils, demonstrating that early events in the ras-MAPK pathway remain functional after stimulation by either cytokine. Inhibition of tyrosine phosphatase activity by pervanadate restored the ability of G-CSF to activate MAPK in mature neutrophils. Specific inhibition of the SHP-1 phosphatase, known to be activated by G-CSF but not GM-CSF also restored the ability of G-CSF to activate MAPK in neutrophils. These studies suggest that G-CSF activation of SHP-1 may be an important regulatory step for permitting optimal terminal differentiation during neutrophil production and add to our knowledge of the instructional role of G-CSF and GM-CSF for balancing proliferation and differentiation of neutrophil progenitor cells. This information may prove useful for the understanding of conditions in which neutrophil proliferative/differentiative balancing is dysregulated, such as myeloid leukemia and myelodysplastic disorders.

New aminocyclitols as modulators of glucosylceramide metabolism

A series of 13 aminocyclitol derivatives belonging to two different families is described. Their configuration is governed by the regio- and stereocontrolled epoxide opening of a suitably protected conduritol-B epoxide. Studies on several glycosyl processing enzymes indicate that some of them are good inhibitors of glucosylceramide hydrolase. A rationale to account for preliminary structure-activity relationships is provided.

Bisphosphonates and estrogens inhibit osteocyte apoptosis via distinct molecular mechanisms downstream of extracellular signal-regulated kinase activation

Both estrogens and bisphosphonates attenuate osteocyte apoptosis by activating the extracellular signal-regulated kinases (ERKs). However, whereas estrogens activate ERKs via an extranuclear function of the estrogen receptor, bisphosphonates do so by opening connexin 43 hemichannels. Here, we demonstrated that the signaling events downstream of ERKs induced by these two stimuli are also distinct. Inhibition of osteocyte apoptosis by estrogens requires nuclear accumulation of ERKs and activation of downstream transcription factors. On the other hand, anti-apoptosis induced by bisphosphonates requires neither transcription nor ERK-dependent transcription factors. Instead, the effect of bisphosphonates is abolished when ERKs are restricted to the nucleus by blocking CRM1/exportin1-mediated nuclear protein export or by expressing nuclear-anchored ERKs, but it is unaffected in cells expressing cytoplasmic-anchored ERKs. Connexin 43/ERK-mediated anti-apoptosis induced by bisphosphonates requires the kinase activity of the cytoplasmic target of ERKs, p90(RSK), which in turn phosphorylates the pro-apoptotic protein BAD and C/EBPbeta. Phosphorylation of BAD renders it inactive, whereas phosphorylation of C/EBPbeta leads to binding of pro-caspases, thus inhibiting apoptosis independently of the transcriptional activity of this transcription factor. Consistent with the evidence that estrogens and bisphosphonates phosphorylate diverse targets of ERKs, probably resulting from activation of spatially distinct pools of these kinases, the two agents had additive effects on osteocyte survival.

The role of cyclic-AMP binding protein (CREB) in leukemia cell proliferation and acute leukemias

Leukemia is a result of accumulating genetic alterations. The collaboration of mutations that offer survival and proliferative signals, together with mutations that result in lack of differentiation, is thought to cause a leukemic phenotype. The cyclic-AMP Response Element Binding Protein (CREB) is a transcription factor that is known to be a downstream component of the GM-CSF and IL-3 signaling pathways. We previously showed that CREB is overexpressed in blast cells from patients with acute leukemias. In this paper, we review the role of CREB in hematopoiesis, cell proliferation and acute leukemias.

Ribosomal p70S6K basal activity increases upon induction of differentiation of myelomonocytic leukemic cell lines HL60, AML14 and MPD

The role of ribosomal p70S6K in the cell cycle has been studied extensively, and it is known that this enzyme is crucial for cell advancement through G(1). Conversely, the participation of p70S6K in cell differentiation is not well understood. We have studied the response of p70S6K to the cytokine granulocyte-macrophage colony stimulating factor (GM-CSF) in three differentiation-capable leukemic cell lines (MPD, AML-14 and HL-60) and in normal mature neutrophils. Immature leukemic cells starved for 16 h showed a robust ( approximately 3.5-fold over controls) p70S6K phosphorylation on T(421)/S(424) residues in response to an acute (5 min) 10 nM GM-CSF stimulation. On the other hand, cells that had been induced to differentiate and express granulocytic phenotypes, showed an increased ( approximately 6-fold) basal level of p70S6K T(421)/S(424) phosphorylation over immature cells, as well as an increased baseline tyrosyl phosphorylation of the GM-CSF receptor beta subunit (GM-CSF.Rbeta). However, the differentiated cells displayed a weak ( approximately 1.4-fold over controls) response to GM-CSF even at prolonged incubation times (20 min). In vitro p70S6K enzymatic activity paralleled p70S6K T(421)/S(424) phosphorylation in both high basal, unstimulated, levels in immature cells and a low degree of response to GM-CSF. Lastly, peripheral blood mature neutrophils had low basal GM-CSF.Rbeta and p70S6K activity, with both parameters being robustly stimulated following addition of GM-CSF, a situation in contrast with the cell lines, indicative perhaps of their incomplete terminal differentiation. In summary, these findings show the increase in basal phosphorylation of p70S6K upon granulocytic differentiation of myeloid leukemic cells and their responses to GM-CSF that are closely paralleled with tyrosyl phosphorylation of its receptor, and help in pointing to specific cell signaling molecules that are different in leukemic blasts from normal leukocytes.

Granulocyte-macrophage colony-stimulating factor is a chemoattractant cytokine for human neutrophils: involvement of the ribosomal p70 S6 kinase signaling pathway

GM-CSF stimulates proliferation of myeloid precursors in bone marrow and primes mature leukocytes for enhanced functionality. We demonstrate that GM-CSF is a powerful chemotactic and chemokinetic agent for human neutrophils. GM-CSF-induced chemotaxis is time dependent and is specifically neutralized with Abs directed to either the ligand itself or its receptor. Maximal chemotactic response was achieved at approximately 7 nM GM-CSF, and the EC(50) was approximately 0.9 nM. Both concentrations are similar to the effective concentrations of IL-8 and less than the effective concentrations of other neutrophil chemoattractants such as neutrophil-activating peptide-78, granulocyte chemotactic protein-2, leukotriene B(4), and FMLP. GM-CSF also acts as a chemoattractant for native cells bearing the GM-CSF receptor, such as monocytes, as well as for GM-CSF receptor-bearing myeloid cell lines, HL60 (promyelomonocyte leukemic cell line) and MPD (myeloproliferative disorder cell line), following differentiation induction. GM-CSF induced a rapid, transient increase in F-actin polymerization and the formation of focal contact rings in neutrophils, which are prerequisites for cell migration. The mechanism of GM-CSF-induced chemotaxis appears to involve the cell signaling molecule, ribosomal p70 S6 kinase (p70S6K). Both p70S6K enzymatic activity and T(421)/S(424) and T(389) phosphorylation are markedly increased with GM-CSF. In addition, the p70S6K inhibitor hamartin transduced into cells as active protein, interfered with GM-CSF-dependent migration, and attenuated p70S6K phosphorylation. These data indicate that GM-CSF exhibits chemotactic functionality and suggest new avenues for the investigation of the molecular basis of chemotaxis as it relates to inflammation and tissue injury.

Purification and identification of a protein kinase activity modulated by ionizing radiation

In order to identify novel protein kinases, which are involved in signal transduction processes after exposure of cells to ionizing radiation we screened HL-60 cells using an in-gel renaturation assay. Using this approach we identified a renaturable serine/threonine kinase with an apparent molecular mass of 90 kDa (pK90). The activity of pK90 dropped within minutes after exposure to a dose of 10 Gy. It reached a minimum 15-30 min after irradiation and increased back to pre-treatment values 6h later. A down-regulation of the kinase activity was detectable after a dose of 1 Gy. Failure of H(2)O(2) to down-regulate pK90 activity indicates a requirement for DNA double-strand-breaks to modulate the kinase activity. In contrast to the molecular mass of 90 kDa in SDS-PAGE we found a molecular mass of around 450 kDa for the native protein using gel filtration chromatography, indicating that pK90 forms a multi-protein complex under native conditions. To identify pK90 we partially purified the protein by three affinity chromatography steps (heparin-Sepharose, phosphate metal affinity, and Cibacron-Blue-F3G-A-Sepharose). Mass spectrometric analysis of the purified 90 kDa fraction showed that pK90 is identical to Tlk1, which was verified by immunoprecipitation.

Rapamycin inhibits GM-CSF-induced neutrophil migration

The molecular mechanisms that govern cell movement are the subject of intense study, as they impact biologically and medically important processes such as leukocyte chemotaxis and angiogenesis, among others. We demonstrate that leukocyte chemotaxis is prevented by the macrolide immunosuppressant rapamycin, a specific inhibitor of the mammalian target of rapamycin (mTOR)/ribosomal p70-S6 kinase (p70S6K) pathway. Both neutrophil chemotaxis and chemokinesis elicited by granulocyte-macrophage colony-stimulating factor (GM-CSF) were strongly inhibited by rapamycin with an IC(50) of 0.3 nM. Inhibition, although at a higher dose, was also observed when the chemoattractant was interleukin-8. As for the mechanism, rapamycin targeted the increase of phosphorylation of p70S6K due to GM-CSF treatment, as demonstrated with specific anti-p70S6K immunoprecipitation and subsequent immunoblotting with anti-T(421)/S(424) antibodies. Rapamycin also inhibited GM-CSF-induced actin polymerization, a hallmark of leukocyte migration. The specificity of the effect of rapamycin was confirmed by the use of the structural analog FK506, which did not have a significant effect on chemotaxis but effectively rescued rapamycin-induced p70S6K inhibition. This was expected from a competitive effect of both molecules on FK506-binding proteins (FKBP). Additionally, GM-CSF-induced chemotaxis was completely (>90%) blocked by a combination of rapamycin and the MAPK kinase (MEK) inhibitor PD-98059. In summary, the results presented here indicate for the first time that rapamycin, at sub-nanomolar concentrations, inhibits GM-CSF-induced chemotaxis and chemokinesis. This serves to underscore the relevance of the mTOR/S6K pathway in neutrophil migration.

Mechanism of ribosomal p70S6 kinase activation by granulocyte macrophage colony-stimulating factor in neutrophils: cooperation of a MEK-related, THR421/SER424 kinase and a rapamycin-sensitive, m-TOR-related THR389 kinase

We report here for the first time the detection of the ribosomal p70S6 kinase (p70S6K) in a hematopoietic cell, the neutrophil, and the stimulation of its enzymatic activity by granulocyte macrophage colony-stimulating factor (GM-CSF). GM-CSF modified the Vmax of the enzyme (from 7.2 to 20.5 pmol/min/mg) and induced a time- and dose-dependent phosphorylation on p70S6K residues Thr389 and Thr421/Ser424. The immunosuppressant macrolide rapamycin caused either a decrease in intensity of phospho-Thr389 bands in Western blots, or as a downshift in the relative mobility of phospho-Thr421/Ser424 bands (consistent with the loss of phosphate), but not both simultaneously. The immunosuppressant FK506 failed to inhibit p70S6K activation, but was able to rescue the rapamycin-induced downshift, pointing to a role for the mammalian target of rapamycin (mTOR) kinase. Rapamycin also caused an inhibition (IC50 0.2 nm) of the in vitro enzymatic activity of p70S6K. However, the inhibition of activity was not complete, but only a 40-50%, indicating that neutrophil p70S6K activity has a rapamycin-resistant component. This component was totally inhibited by pre-incubating the cells with the mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor PD-98059 prior to treatment with rapamycin. This indicated that a kinase from the MEK/MAPK pathway also plays a role in p70S6K activation. Thus, GM-CSF causes the dual activation of a rapamycin-resistant, MAPK-related kinase, that targets Thr421/Ser424 S6K phosphorylation, and a rapamycin-sensitive, mTOR-related kinase, that targets Thr389, both of which are needed in cooperation to achieve full activation of neutrophil p70S6K.

Molecular crosstalk between p70S6k and MAPK cell signaling pathways

We report here for the first time that the specific MAPK kinase (MEK) inhibitor, PD-98059, completely knocked out granulocyte-macrophage colony-stimulating factor (GM-CSF)-stimulated MAPK activity but also partially inactivated the ribosomal kinase p70S6K. Since a connection between the two major signaling pathways, Ras/MEK/MAPK and PI3-K/p70S6K was suspected, experiments were designed to prove a molecular crosstalk between those. First, p70S6K protein could be co-immunoprecipitated with anti-MAPK antibodies, MAPK protein was similarly present in anti-p70S6K immunoprecipitates, indicating close spatial proximity of both signaling molecules. Second, p70S6K enzymatic activity was found in anti-MAPK immunoprecipitates and MAPK in anti-p70S6K immunoprecipitates, being the latter activity higher in samples derived from GM-CSF-treated cells. Since an upstream activator of p70S6K, phosphatidylinositol (PI)3-kinase, has been associated to cell movement in phagocytic cells, we studied a possible participation of p70S6K in chemotaxis and whether MAPK had an input. Our data show that functional chemotaxis was inhibited by rapamycin, a specific p70S6K inhibitor, as well as by PD-98059. Thus, a connection between these two kinases extends from the molecular level to cell migration, a key functionality in non-proliferative, mature phagocytes such as neutrophils.

A systematic approach to the complete study of a signaling molecule: ribosomal p90rsk as an example

Ribosomal p90rsk is a kinase of central importance in transducing mitogenic signals from an activated receptor to the cell nucleus and for protein synthesis. Here, we analyze the optimal steps to fully describe this kinase in both normal neutrophils and leukemic cell lines. These are: (i) immunological analyses (immunoblotting and immunoprecipitation); (ii) enzyme activity assays (in vitro and "in-gel"); and (iii) immunobiochemical combination methods (immunoprecipitation/kinase assay, immunoprecipitation/"in-gel" assay and ion exchange chromatography/immunoblotting). For the enzyme assays, we describe a novel method to measure ribosomal p90rsk kinase activity "in-gel", based on a renatured-protein method that allows for the direct quantitation of enzyme activity. Finally, we present an algorithm that can be readily implemented to the quantification of the extent of stimulation of a kinase in response to a particular extracellular stimuli. In our case, it was found that activation of p90rsk was higher in proliferating leukemic cells than in mature neutrophils, indicating that a suppression of key signal transduction links could contribute to the maturational arrest typical of acute leukemia. All the techniques and strategies described here for p90rsk could be easily extrapolated to the study of any signal transduction molecule, provided it has a phosphotransferase activity.

Transcription factors and translocations in lymphoid and myeloid leukemia

Chromosomal translocations involving transcription factors and aberrant expression of transcription factors are frequently associated with leukemogenesis. Transcription factors are essential in maintaining the regulation of cell growth, development, and differentiation in the hematopoietic system. Alterations in the mechanisms that normally control these functions can lead to hematological malignancies. Further characterization of the molecular biology of leukemia will enhance our ability to develop disease-specific treatment strategies, and to develop effective methods of diagnosis and prognosis.

Presence of a phospholipase D (PLD) distinct from PLD1 or PLD2 in human neutrophils: immunobiochemical characterization and initial purification

Utilizing the transphosphatidylation reaction catalyzed by phospholipase D (PLD) in the presence of a primary alcohol and the short-chain phospholipid PC8, we have characterized the enzyme from human neutrophils. A pH optimum of 7.8-8.0 was determined. PIP(2), EDTA/EGTA, and ATP were found to enhance basal PLD activity in vitro. Inhibitory elements were: oleate, Triton X-100, n-octyl-beta-glucopyranoside, divalent cations, GTPgammaS and H(2)O(2). The apparent K(m) for the butanol substrate was 0.1 mM and the V(max) was 6.0 nmol mg(-1) h(-1). Immunochemical analysis by anti-pan PLD antibodies revealed a neutrophil PLD of approximately 90 kDa and other bands recognized minimally by anti-PLD1 or anti-PLD2 antibodies. The 90-kDa protein is tyrosine-phosphorylated upon cell stimulation with GM-CSF and formyl-Met-Leu-Phe. Protein partial purification using column liquid chromatography was performed after cell subfractionation. Based on the enzyme's regulatory and inhibitory factors, and its molecular weight, these data indicate an enzyme isoform that might be different from the mammalian PLD1/2 forms described earlier. The present results lay the foundation for further purification of this granulocyte PLD isoform.

Phospholipase D (PLD) is present in Leishmania donovani and its activity increases in response to acute osmotic stress

We report here that the signaling molecule phospholipase D (PLD) is present in the parasitic protozoan Leishmania donovani. In vitro enzymatic activity is dependent on Ca2+ and Mg2+ ions, its basal activity is stimulated by phosphatidyl-inositol-4,5-bisphosphate (PIP2) and its pH optima are pH 8.0 and pH 6.0. PLD activity increases 3-fold about 5 min after an abrupt decrease in osmolality from 317 mOsm (isosmotic) to 155 mOsm and increases 1.5-fold in response to an abrupt increase in osmolality to 617 mOsM. Cells grown for > 24 h under the anisosmotic conditions showed only marginal changes in activity compared to the controls grown under isosmotic conditions, indicating an adaptation to long-term exposure to hypo- or hyper-osmolarity. Immunologically, two isoforms, PLD1 and PLD2, are present. An analysis of in vitro PLD activity in anti-PLD immunocomplexes revealed that either hypotonic (cell swelling) or hypertonic stress (cell shrinking) causes an increase in PLD1 activation but a reduction in PLD2 activity. The interplay between these two isoforms results in a predominance for PLD1 in the observed increase when measuring total PLD activity. Finally, the increase in enzymatic activity in acute hyposmotic shock is accompanied by tyrosyl phosphorylation of the PLD1 isoform, suggesting a role for protein tyrosine kinase in the control of PLD activity in response to osmotic stress.

MAP kinase upregulation after hematopoietic differentiation: role of chemotaxis

Mitogen-activated protein kinase (MAPK) isoform p42 is known to be active in exponentially growing cells at several points of the cell cycle. A high basal activity was present in three cell lines representative of immature myeloid cells tested: uHL-60, AML-14, and MPD. However, DMSO-induced differentiation of HL-60 cells (dHL-60) and subsequent expression of the neutrophilic phenotype occurred with a concomitant reduction on the basal level of MAPK activity. Simultaneously, extracellular stimuli like the cytokine granulocyte/macrophage colony-stimulating factor (GM-CSF) induced a fast (<10 min) and robust response. In terms of MAPK activity, the more mature the cell was, the higher the corresponding activity, in the three differentiation series considered: AML-14 < 3D10; MPD < G-MPD; uHL-60 < dHL-60 < neutrophils. Interestingly, peripheral blood neutrophils expressed the highest (16-fold) MAPK activation level in response to GM-CSF. Finally, using the specific MAPK inhibitor PD-98059, we demonstrated that MAPK activation is needed for neutrophil chemotaxis toward interleukin-8 and its priming by GM-CSF. Since neutrophils are terminally differentiated cells, GM-CSF does not serve a purpose in proliferation, and it must trigger the recruitment of selective signal transduction pathways particular to that final stage that includes enhanced physiological functions such as chemotaxis.

Human cell line that differentiates to all myeloid lineages and expresses neutrophil secondary granule genes

The aim of this study was to characterize a human leukemic cell line that appears capable of spontaneous differentiation to all myeloid lineages. The MPD cell line was derived using standard tissue culture techniques from the peripheral blood of a patient with an aggressive nonchronic myelogenous leukemia myeloproliferative disorder. Immunophenotyping, cytogenetic analysis, reverse transcriptase polymerase chain reaction, Northern blotting, immunoblotting, and colony assays were used to characterize the line and to assess its ability to express lineage-specific genes representative of advanced differentiation.Light microscopic morphologic analysis of the MPD cell line suggests that it has the unique property of spontaneous differentiation to mature-appearing neutrophils, macrophages, eosinophils, and basophils in proportions that approximate those found in normal bone marrow or peripheral blood. It was demonstrated that this cell line is capable of producing lineage-specific mRNA and granule proteins of at least two myeloid lineages, neutrophil and eosinophil, including neutrophil secondary granule proteins, which are not expressed in other available human cell lines. MPD cells were found to be capable of producing differentiated myeloid colonies (neutrophil, eosinophil, macrophge, mixed) in semisolid medium. The ability of MPD cells to express genetic programs associated with advanced differentiation of multiple myeloid lineages will make it a valuable tool for the study of the processes underlying lineage commitment and the regulation of expression of lineage-specific genes.

Direct inhibition of in vitro PLD activity by 4-(2-aminoethyl)-benzenesulfonyl fluoride

While conducting a purification protocol of phospholipase D (PLD) from human granulocytes, we observed that PLD activity was inhibited by a commonly-used protease inhibitor cocktail. Of the six inhibitors present in the cocktail, the serine protease inhibitor, 4-(2-aminoethyl)-benezensulfonyl fluoride (AEBSF), was found to be the sole inhibitor of PLD. AEBSF caused a loss of neutrophil and purified plant PLD activities in vitro, but not in intact cells at the concentrations used, nor did it affect the related phospholipases A(2) and C, that were utilized as specificity controls. The compound AEBSNH(2), which has the fluoride replaced by an -NH(2) group, failed to affect PLD activity as did other compounds structurally related to AEBSF with known protease inhibitory capabilities. Finally, basal- and agonist-stimulated PLD activity was inhibited in phosphatidylcholine-specific anti-PLD immunoprecipitates (IC(50) = 75 microM). These results suggest that AEBSF, in an effect probably unrelated to its anti-proteolytic ability, directly interferes with PLD enzymatic activity, making it a significant compound to begin analyzing the role of PLD in mammalian cell signaling.

Granulocyte-macrophage colony-stimulating factor stimulation results in phosphorylation of cAMP response element-binding protein through activation of pp90RSK

Granulocyte-macrophage colony-stimulating factor (GM-CSF) activates several kinases and transcription factors through interaction with a heterodimeric receptor complex. We previously demonstrated that phosphorylation of the cyclic adenosine monophosphate (cAMP) response element-binding protein, CREB, occurs through a protein kinase A-independent pathway and is required for GM-CSF–induced transcriptional activation of the immediate early gene, early growth response-1 (egr-1). Recent reports indicate that receptor tyrosine kinases can induce CREB phosphorylation through activation of pp90RSK. We performed immune complex kinase assays in the human myeloid leukemic cell line, TF-1, which revealed that GM-CSF induced pp90RSK activation and phosphorylation of CREB within 5 minutes of stimulation. Transfection with the kinase-defective pp90RSK expression plasmid demonstrated a statistically significant decrease in transcriptional activation of a −116 CAT/egr-1 promoter construct in response to GM-CSF. Furthermore, activation of pp90RSK, CREB and egr-1in GM-CSF–treated cells was inhibited by the presence of the inhibitor, PD98059. In this study, we report that GM-CSF induces CREB phosphorylation and egr-1 transcription by activating pp90RSK through an MEK-dependent signaling pathway.

Granulocyte-macrophage colony-stimulating factor stimulation results in phosphorylation of cAMP response element-binding protein through activation of pp90RSK

Granulocyte-macrophage colony-stimulating factor (GM-CSF) activates several kinases and transcription factors through interaction with a heterodimeric receptor complex. We previously demonstrated that phosphorylation of the cyclic adenosine monophosphate (cAMP) response element-binding protein, CREB, occurs through a protein kinase A-independent pathway and is required for GM-CSF–induced transcriptional activation of the immediate early gene, early growth response-1 (egr-1). Recent reports indicate that receptor tyrosine kinases can induce CREB phosphorylation through activation of pp90RSK. We performed immune complex kinase assays in the human myeloid leukemic cell line, TF-1, which revealed that GM-CSF induced pp90RSK activation and phosphorylation of CREB within 5 minutes of stimulation. Transfection with the kinase-defective pp90RSK expression plasmid demonstrated a statistically significant decrease in transcriptional activation of a −116 CAT/egr-1 promoter construct in response to GM-CSF. Furthermore, activation of pp90RSK, CREB and egr-1in GM-CSF–treated cells was inhibited by the presence of the inhibitor, PD98059. In this study, we report that GM-CSF induces CREB phosphorylation and egr-1 transcription by activating pp90RSK through an MEK-dependent signaling pathway.

Involvement of Shc and Cbl-PI 3-kinase in Lyn-dependent proliferative signaling pathways for G-CSF

Granulocyte colony-stimulating factor (G-CSF) is the major hematopoietic factor which controls the production and differentiation of granulocytes. The G-CSF receptor (G-CSFR) belongs to the superfamily of the cytokine receptors, which transduce signals via the activation of cytosolic protein tyrosine kinases (PTK). To determine the role of specific PTK in G-CSF signaling we expressed the human G-CSFR in cell lines derived from DT40 B cells, which lack either the Src-related Lyn or Syk. Wild-type (wt) and syk-deficient cells underwent increased DNA synthesis in response to G-CSF; lyn-deficient cells did not. The purpose of these studies is to identify Lyn's downstream effectors in mediating DNA synthesis. While G-CSF stimulated Ras activity in all cell lines, G-CSF failed to induce the tyrosine phosphorylation of Shc in lyn-deficient cells. G-CSF induced a statistically significant activation of Erk1/Erk2 Kinase or p90Rsk only in the wt cells. G-CSF induced the tyrosine phosphorylation of Cbl and increased activity of PI 3-kinase in wild-type and syk-deficient, but non in lyn-deficient, cells. Inhibition of Shc by over-expression of its SH2 or PTB domains or PI 3-kinase by either treatment with wortmannin or expression of the CblY731F mutant decreased G-CSF-induced DNA synthesis. Thus, the Lyn, Cbl-PI 3-kinase, and Shc/non-Ras-dependent pathways correlate with the ability of cells to respond to G-CSF with increased DNA synthesis.

Binding of GM-CSF to adherent neutrophils activates phospholipase D

When the hematopoietic growth factor granulocyte-macrophage colony-stimulating factor was incubated with neutrophils adherent to plastic tissue culture plates or plates coated with extracellular matrix proteins, a rapid (3 min) but transient formation of phosphatidic acid was observed. This stimulation was dependent on the dose of GM-CSF, with an EC50 of 140 pM, and was further enhanced (up to 350%) with the PA phosphatase inhibitor propranolol in a dose-dependent manner. Conversely, GM-CSF was unable to trigger any PA formation in neutrophils maintained in suspension, even in the presence of soluble fibronectin. However, GM-CSF did prime the cells for enhanced PA formation in the presence of a secondary stimulus (fMet-Leu-Phe or PAF). GM-CSF also caused a time-dependent stimulation of diacylglycerol formation in adherent, but not suspended, cells and elicited a time-dependent stimulation of phosphatidylethanol formation, with a concomitant decrease in the formation of PA only at early (< 7 min) times. These observations were consistent with a rapid activation of the enzyme phospholipase D in adherent cells stimulated with GM-CSF. Additional data indicated that the source of DAG was PLD coexisting with PLC, especially at later times ( > 7 min) of stimulation with GM-CSF. Finally, the formation of PA and PEt, and to a minor extent, DAG, were inhibited by the protein tyrosine kinase inhibitor erbstatin in conditions in which tyrosine phosphorylation occurred. Taken together the data indicate that GM-CSF rapidly activates PLD in adherent cells, which is responsible for the generation of PA. Thus, PLD activation is an early event in neutrophil signal transduction following exposure of adherent cells to GM-CSF.

The mechanism of IL-5 signal transduction

Cytokines are important regulators of hematopoiesis. They exert their actions by binding to specific receptors on the cell surface. Interleukin-5 (IL-5) is a critical cytokine that regulates the growth, activation, and survival of eosinophils. Because eosinophils play a seminal role in the pathogenesis of asthma and allergic diseases, an understanding of the signal transduction mechanism of IL-5 is of paramount importance. The IL-5 receptor is a heterodimer of alpha- and beta-subunits. The alpha-subunit is specific, whereas the beta-subunit is common to IL-3, IL-5, and granulocyte/macrophage colony-stimulating factor (GM-CSF) receptors and is crucial for signal transduction. It has been shown that there are two major signaling pathways of IL-5 in eosinophils. IL-5 activates Lyn, Syk, and JAK2 and propagates signals through the Ras-MAPK and JAK-STAT pathways. Studies suggest that Lyn, Syk, and JAK2 tyrosine kinases and SHP-2 tyrosine phosphatase are important for eosinophil survival. In contrast to their survival-promoting activity, Lyn and JAK2 appear to have no role in eosinophil degranulation or expression of surface adhesion molecules. Raf-1 kinase, on the other hand, is critical for eosinophil degranulation and adhesion molecule expression. Btk is involved in IL-5 stimulation of B cell function. However, it does not appear to be important for eosinophil function. Thus a clear segregation of signaling molecules based on their functional importance is emerging. This review describes the signal transduction mechanism of the IL-3/GM-CSF/IL-5 receptor system and compares and contrasts IL-5 signaling between eosinophils and B cells.

Phospholipase D: a novel major player in signal transduction

The role of the mammalian phospholipase D (PLD) in the control of key cellular responses has been recognised for a long time, but only recently have there been the reagents to properly study this very important enzyme in the signalling pathways, linking cell agonists with intracellular targets. With the recent cloning of PLD isoenzymes, their association with low-molecular-weight G proteins, protein kinase C and tyrosine kinases, the availability of antibodies and an understanding of the role of PLD product, phosphatidic acid (PA), in cell physiology, the field is gaining momentum. In this review, we will explore the molecular properties of mammalian PLD and its gene(s), the complexity of this enzyme regulation and the myriad physiological roles for PLD and PA and related metabolic products, with particular emphasis on a role in the activation of NADPH oxidase, or respiratory burst, leading to the generation of oxygen radicals.

Glucosylceramide synthase and glycosphingolipid synthesis

In mammalian cells, there are two major classes of sphingolipids---sphingomyelin and glycosphingolipids (GSLs)--both of which are synthesized from the hydrophobic molecule ceramide. The synthesis of most GSLs begins with glucosylation of ceramide to form glucosylceramide (GlcCer), which, in turn, serves as the source of 300-400 GSLs. Although most of these GSLs have been characterized chemically, the biological functions of ceramide glycosylation and GSLs still remain enigmatic. The recent description of a GSL-deficient cell line and isolation of cDNA for GlcCer synthase provide new insights into GSL functions.