Disturbed granulocyte macrophage-colony stimulating factor priming of phosphatidylinositol 3,4,5-trisphosphate accumulation and Rac activation in fMLP-stimulated neutrophils from patients with myelodysplasia

The clinical consequences of neutrophil priming

PURPOSE OF REVIEW

Neutrophils priming has been long studied in vitro. Recent studies describe it in vivo. In pathophysiological conditions, complex, heterogeneous characteristics of priming are described in the last few years.

RECENT FINDINGS

Priming can occur systemically when insults such as sepsis or trauma result in an array of circulating mediators and circulating primed neutrophils seem to exert detrimental effects either directly, or indirectly by interacting with other cells, thereby contributing to the development of organ dysfunction. Local priming of neutrophils augments their ability to clear infection, but may also lead to local bystander tissue injury, for example, in the inflamed joint. The complexity, heterogeneity and dynamic nature of inflammatory responses and the accessibility of cells from local sites make neutrophil priming challenging to study in human disease; however, recent advances have made significant progress to this field.

SUMMARY

Herein, we summarize the literature regarding neutrophil priming in selected conditions. In some diseases and in the setting of specific genetic influences, the priming repertoire seems to be restricted, with only some neutrophil functions upregulated. A greater understanding of the nature of neutrophil priming and its role in human disease is required before this process becomes tractable to therapeutic intervention.

PAK2 is an effector of TSC1/2 signaling independent of mTOR and a potential therapeutic target for Tuberous Sclerosis Complex

Tuberous sclerosis complex (TSC) is caused by inactivating mutations in either TSC1 or TSC2 and is characterized by uncontrolled mTORC1 activation. Drugs that reduce mTOR activity are only partially successful in the treatment of TSC, suggesting that mTOR-independent pathways play a role in disease development. Here, kinome profiles of wild-type and Tsc2^−/− mouse embryonic fibroblasts (MEFs) were generated, revealing a prominent role for PAK2 in signal transduction downstream of TSC1/2. Further investigation showed that the effect of the TSC1/2 complex on PAK2 is mediated through RHEB, but is independent of mTOR and p21RAC. We also demonstrated that PAK2 over-activation is likely responsible for the migratory and cell cycle abnormalities observed in Tsc2^−/− MEFs. Finally, we detected high levels of PAK2 activation in giant cells in the brains of TSC patients. These results show that PAK2 is a direct effector of TSC1-TSC2-RHEB signaling and a new target for rational drug therapy in TSC.

Erythropoietin receptor signaling is membrane raft dependent

Upon erythropoietin (Epo) engagement, Epo-receptor (R) homodimerizes to activate JAK2 and Lyn, which phosphorylate STAT5. Although recent investigations have identified key negative regulators of Epo-R signaling, little is known about the role of membrane localization in controlling receptor signal fidelity. Here we show a critical role for membrane raft (MR) microdomains in creation of discrete signaling platforms essential for Epo-R signaling. Treatment of UT7 cells with Epo induced MR assembly and coalescence. Confocal microscopy showed that raft aggregates significantly increased after Epo stimulation (mean, 4.3±1.4(SE) vs. 25.6±3.2 aggregates/cell; p≤0.001), accompanied by a >3-fold increase in cluster size (p≤0.001). Raft fraction immunoblotting showed Epo-R translocation to MR after Epo stimulation and was confirmed by fluorescence microscopy in Epo stimulated UT7 cells and primary erythroid bursts. Receptor recruitment into MR was accompanied by incorporation of JAK2, Lyn, and STAT5 and their activated forms. Raft disruption by cholesterol depletion extinguished Epo induced Jak2, STAT5, Akt and MAPK phosphorylation in UT7 cells and erythroid progenitors. Furthermore, inhibition of the Rho GTPases Rac1 or RhoA blocked receptor recruitment into raft fractions, indicating a role for these GTPases in receptor trafficking. These data establish a critical role for MR in recruitment and assembly of Epo-R and signal intermediates into discrete membrane signaling units.

Therapeutic potential of SH2 domain-containing inositol-5'-phosphatase 1 (SHIP1) and SHIP2 inhibition in cancer

Many tumors present with increased activation of the phosphatidylinositol 3-kinase (PI3K)-PtdIns(3,4,5)P(3)-protein kinase B (PKB/Akt) signaling pathway. It has long been thought that the lipid phosphatases SH2 domain-containing inositol-5'-phosphatase 1 (SHIP1) and SHIP2 act as tumor suppressors by counteracting with the survival signal induced by this pathway through hydrolysis or PtdIns(3,4,5)P(3) to PtdIns(3,4)P(2). However, a growing body of evidence suggests that PtdInd(3,4)P(2) is capable of, and essential for, Akt activation, thus suggesting a potential role for SHIP1/2 enzymes as proto-oncogenes. We recently described a novel SHIP1-selective chemical inhibitor (3α-aminocholestane [3AC]) that is capable of killing malignant hematologic cells. In this study, we further investigate the biochemical consequences of 3AC treatment in multiple myeloma (MM) and demonstrate that SHIP1 inhibition arrests MM cell lines in either G0/G1 or G2/M stages of the cell cycle, leading to caspase activation and apoptosis. In addition, we show that in vivo growth of MM cells is blocked by treatment of mice with the SHIP1 inhibitor 3AC. Furthermore, we identify three novel pan-SHIP1/2 inhibitors that efficiently kill MM cells through G2/M arrest, caspase activation and apoptosis induction. Interestingly, in SHIP2-expressing breast cancer cells that lack SHIP1 expression, pan-SHIP1/2 inhibition also reduces viable cell numbers, which can be rescued by addition of exogenous PtdIns(3,4)P(2). In conclusion, this study shows that inhibition of SHIP1 and SHIP2 may have broad clinical application in the treatment of multiple tumor types.

Insulin-like growth factor binding protein-1 activates integrin-mediated intracellular signaling and migration in oligodendrocytes

In multiple sclerosis (MS), oligodendrocytes in lesions are lost, leaving damaged tissue virtually devoid of these myelin-producing cells. Our group has recently demonstrated enhanced expression of insulin-like growth factor (IGF) binding protein-1 (IGFBP-1) in oligodendrocytes (CNPase(+)) localized adjacent to MS lesions. In the present study, we demonstrate IGF-1-independent actions of IGFBP-1 on OLN-93 oligodendroglial cells, including activation of kinases ERK1/2, focal adhesion kinase and p21-activated kinase as well as small monomeric GTPases Rac and Ral. Activation of these intracellular signaling components was inhibited by GRGDS peptide, indicating signaling through integrin receptors. While both IGF-1 and IGFBP-1 demonstrated rapid induction of actin polymerization, IGFBP-1 proved to be a more potent inducer of migration than IGF-1, inducing a threefold increased migration rate. Furthermore, through integrin receptor signaling IGFBP-1 induced rapid transient translocalization of intracellular Rac toward punctuated structures followed by translocation of Rac to the plasma membrane. Our results suggest that up-regulation of IGFBP-1 in oligodendrocytes in MS may serve two functions: (i) regulate IGF-1 actions, (ii) exert IGF-independent effects through its RGD sequence.

Bone marrow stromal cell interaction reduces syndecan-1 expression and induces kinomic changes in myeloma cells

CD138 (Syndecan 1) is a heparan sulfate proteoglycan that concentrates heparan sulfate-binding growth factors on the surface of normal and malignant plasma cells (multiple myeloma, MMC). Recent studies have shown the presence of a CD138-negative fraction of MMC within myelomatous bone marrow (BM). We employed kinome array technology to characterize this fraction at a molecular level, using a myeloma cell line model. Compared to CD138-positive cells, CD138-negative MMC showed (i) a reduced activity of kinases involved in cell cycle progression, in agreement with a decreased labeling index and (ii) reduced Rho signaling to F-actin. Interestingly, CD138 mRNA and protein expression was reduced upon interaction of MM cells with stromal cell lines and primary mesenchymal cultures, which was accompanied by the acquisition of an increased Bcl6/Blimp1 ratio. Co-culture induced an increased activity of kinases involved in adhesion and a decreased S-phase transition in both CD138-positive and -negative fractions. In addition, CD138-negative MMC demonstrated an increased STAT3 and ERK1/2 activation compared to CD138+ MMC, in agreement with a lower sensitivity to compound exposure. The presence of a less mature, more resistant CD138-negative myeloma cell fraction within bone marrow microniches might contribute to high incidence of relapse of Myeloma patients.

Myelodysplasia and anemia of chronic disease in human tumor necrosis factor-alpha transgenic mice

TNF-alpha is a pleitropic cytokine that expresses both pro- and anti-inflammatory activity and transgenic mice expressing human tumor necrosis factor-alpha (TNF-alpha) exhibit a progressive polyarthritis that models rheumatoid arthritis (RA). One of the common comorbidities of RA is anemia of chronic disease (ACD). The purpose of these experiments was to study the changes in the bone marrow and peripheral blood that accompany polyarthritis in TNF-alpha transgenic mice in an effort to better understand the pathogenesis of myelodysplasia and ACD. Polychromatic cytometry, hematology and serum cytokine analysis were used to study the pathogenesis of ACD in human TNF-alpha transgenic mice. Our hematological evaluation revealed a mild, compensated, microcytic hypochromic anemia, and monocytosis. In the bone marrow, we observed alterations in cell kinetics, decreased relative expression of transferrin receptor and increased apoptosis and cell death in several late precursor cell populations. Although significant levels of human TNF-alpha were found in the serum, neither change in serum murine erythropoietin nor any significant difference observed in serum levels of murine IL-beta, IL-5, IL-6, IL-10, IL-12(p70), IL-17, TNF-alpha, IFNgamma, GM-CSF, MIP-1alphaJE, MCP-5 was observed. Tg197 mice develop a compensated, microcytic, hypochromic anemia, and a functional iron deficiency by 9 weeks of age. Changes in peripheral blood are reflected in alterations in cell kinetics, transferrin receptor expression and markedly increased apoptosis and cell death in the bone marrow indicating that TNF-alpha may contribute to myelodysplasia in ACD. Moreover, since human TNF-alpha can interact only with murine TNFR1, our data suggest that TNFR1 may play an important role in the development of ACD.

Expansion of normal and leukemic human hematopoietic stem/progenitor cells requires Rac-mediated interaction with stromal cells

OBJECTIVE

To determine the involvement of Rac signaling in self-renewal and expansion on bone marrow stroma of normal CD34+ cells vs leukemic CD34+ cells from acute myeloid leukemia (AML) patients.

MATERIALS AND METHODS

Rac signaling was modulated by retroviral introduction of Racl-N17, Racl-V12, or by using the Rac inhibitor NSC23766. In long-term MS5 cocultures (leukemic) expansion, migration, adhesion, and presence of stem/progenitor cells were monitored in both normal as well as leukemic CD34+ cells.

RESULTS

Inhibition of Rac signaling impaired migration and adhesion of cord blood (CB) CD34+ cells on MS5 stroma. Long-term inhibition of Rac during a 5-week coculture period on stroma prevented association of hematopoietic progenitors with the bone marrow stromal cells and resulted in a dramatic decrease in the primitive stem cell frequency (long-term culture-initiating cell) in a dose-dependent manner. Many of these phenotypes were reversed in the presence of activated Racl-V12, including improved migration toward, and association with, MS5 cells. CD34+ AML cells were characterized by elevated levels of Rac activity (five of seven patients) and enhanced migration and adhesion to MS5 bone marrow stroma as compared to CB CD34+ cells. A dramatic decrease was observed in the formation of leukemic cobblestone area-forming cells as well as strongly diminished clonal expansion in the presence of the Rac inhibitor NSC23766.

CONCLUSION

Our data indicate that Rac signal transduction is required for the maintenance and expansion of both normal as well as leukemic stem/progenitor cells by mediating their interaction with stromal cells.

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.

Impaired interleukin-8- and GROα-induced phosphorylation of extracellular signal-regulated kinase result in decreased migration of neutrophils from patients with myelodysplasia

Patients with myelodysplasia suffer from recurrent bacterial infections as a result of differentiation defects of the myeloid lineage and a disturbed functioning of neutrophilic granulocytes. Important physiological activators of neutrophils are the cytokines interleukin-8/CXC chemokine ligand 8 (IL-8/CXCL8), which activates CXC chemokine receptor 1 and 2 (CXCR1 and CXCR2), and growth-related oncogene (GROalpha)/CXCL1, which stimulates only CXCR2. In this study, we show that migration toward IL-8/GROalpha gradients is decreased in myelodysplastic syndrome (MDS) neutrophils compared with healthy donors. We investigated the signal transduction pathways involved in IL-8/GROalpha-induced migration and showed that specific inhibitors for extracellular signal-regulated kinase (ERK)1/2 and phosphatidylinositol-3 kinase (PI-3K) abrogated neutrophil migration toward IL-8/GROalpha. In accordance with these results, we subsequently showed that IL-8/GROalpha-stimulated activation of ERK1/2 was substantially diminished in MDS neutrophils. Activation of the PI-3K downstream target protein kinase B/Akt was disturbed in MDS neutrophils when cells were activated with IL-8 but normal upon GROalpha stimulation. IL-8 stimulation resulted in higher migratory behavior and ERK1/2 activation than GROalpha stimulation, suggesting a greater importance of CXCR1. We then investigated IL-8-induced activation of the small GTPase Rac implicated in ERK1/2-dependent migration and found that it was less efficient in neutrophils from MDS patients compared with healthy donors. In contrast, IL-8 triggered a normal activation of the GTPases Ras and Ral, indicating that the observed defects were not a result of a general disturbance in CXCR1/2 signaling. In conclusion, our results demonstrate a disturbed CXCR1- and CXCR2-induced neutrophil chemotaxis in MDS patients, which might be the consequence of decreased Rac-ERK1/2 and PI-3K activation within these cells.