Differentiation of U937 cells enables a phospholipase D-dependent pathway of cytosolic phospholipase A2 activation

Phospholipase D1 activity is crucial for cytosolic phospholipase A2 -dependent prostaglandin E2 formation in murine osteoblastic MC3T3-E1 cells

In bone, prostaglandin E2 (PGE2) is highly osteogenic and formed by osteoblasts, a key modulatory event in the regulation of bone cell activity. MC3T3-E1 cells are widely used as an in vitro model of osteoblast function. It is still not clear which pathways contribute to the release of AA in these cells. In this study we have focussed on the contribution of phospholipase D (PLD) enzymes to osteoblastic PGE2 formation after stimulation with endothelin-1 (ET-1). Using specific inhibitors of PLD1 and PLD2 we could show that PGE2 formation was strictly dependent on PLD1 but not PLD2 activity and cytosolic phospholipase A2 (cPLA2) was activated by triggering through PLD1. We have identified diacyl glycerol (DAG) as a possible effector molecule which may serve as a triggering signal for PKC activation and subsequent cPLA2 phosphorylation.

Alteration of mitochondrial phospholipid due to the PLA2 activation in rat brains under cadmium toxicity

Cadmium (Cd) is a heavy metal that has received considerable environmental and occupational concern. Cd causes toxic effects due to its accumulation in a variety of tissues, including the kidney, liver and the nervous system (CNS); however, the exact mechanism is poorly understood. In the present study, we tried to explore the impact of acute cadmium exposure on rat brain phospholipids (PLs). Cd exposure significantly reduced PLs in a time dependent manner and the reduction was due to the activation of the Phospholipase A2 enzymes (sPLA2, cPLA2). The release of arachidonic acid from PLs increased during inflammatory conditions by PLA2s. The mRNA expression of cyclooxygenase2 (COX2) and subsequently the pro-inflammatory cytokines, namely, Interleukin 1 (IL-1) and IL-6, were up regulated; however, the expression of anti-inflammatory cytokine IL-10 was reduced in a time dependent manner. The expression of the Tumor necrosis factor alpha (TNF-α), Inducible nitric oxide synthase (iNOS) and Interferon gamma (INF-γ) also experienced increases in the expression. Likewise the mRNA expression of the pro-apoptotic factor, Bcl-2-associated X protein (Bax), was elevated, whereas anti-apoptosis B-cell lymphoma 2 (Bcl2) was down regulated. This present study might help to decipher the effects of cadmium toxicity on rat brain.

Vitamin D3-driven signals for myeloid cell differentiation--implications for differentiation therapy

Primitive myeloid leukemic cell lines can be driven to differentiate to monocyte-like cells by 1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), and, therefore, 1,25(OH)(2)D(3) may be useful in differentiation therapy of myeloid leukemia and myelodysplastic syndromes (MDS). Recent studies have provided important insights into the mechanism of 1,25(OH)(2)D(3)-stimulated differentiation. For myeloid progenitors to complete monocytic differentiation a complex network of intracellular signals has to be activated and/or inactivated in a precise temporal and spatial pattern. 1,25(OH)(2)D(3) achieves this change to the 'signaling landscape' by (i) direct genomic modulation of the level of expression of key regulators of cell signaling and differentiation pathways, and (ii) activation of intracellular signaling pathways. An improved understanding of the mode of action of 1,25(OH)(2)D(3) is facilitating the development of new therapeutic regimens.

Lipopolysaccharide-induced cyclooxygenase-2 expression in human U937 macrophages is phosphatidic acid phosphohydrolase-1-dependent

Cyclooxygenase (COX) has two isoforms, COX-1 and -2, which catalyze the key step in the conversion of cellular arachidonic acid into prostaglandins. In recent years, interest in COX-2 has significantly increased since it has been a target for the development of specific non-steroidal anti-inflammatory drugs. We report that COX-2 expression is up-regulated in phorbol ester (phorbol myristate acetate, PMA)-differentiated human U937 macrophage-like cells stimulated with lipopolysaccharide (LPS), whereas COX-1 is not up-regulated. We show that the LPS-induced up-regulation of COX-2 depends on the activity of the Mg(+2)-dependent phosphatidic acid phosphohydrolase 1 (PAP-1). Inhibition of PAP-1 by bromoenol lactone, propranolol, or ethanol resulted in a decrease in LPS-induced COX-2 mRNA transcript production, COX-2 protein expression, and prostaglandin E(2) release from U937 macrophages. To ensure that these results did not arise because of PMA treatment of the U937 cells, similar experiments were conducted with the P388D(1) cell line, which does not require PMA differentiation. LPS increased the levels of endogenous cellular diacylglycerol (DAG) within 2 min of stimulation. This increase was observed to be sensitive to the PAP-1 inhibitors. Furthermore, phosphatidic acid phosphohydrolase activity assays showed that the bromoenol lactone-sensitive PAP-1 activity was translocated from the cytosolic fraction to the membrane fraction within 2 min of LPS exposure. Finally, DAG add-back experiments demonstrate that LPS-induced COX-2 expression is enhanced by the addition of exogenous DAG.

1Alpha,25-dihydroxyvitamin D3-mediated stimulation of steroid sulphatase activity in myeloid leukaemic cell lines requires VDRnuc-mediated activation of the RAS/RAF/ERK-MAP kinase signalling pathway

1Alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) stimulates the activity of steroid sulphatase (STS) in myeloid cells [Hughes et al., 2001, 2005]. This was attenuated by inhibitors of phospholipase D (PLD) (n-butanol, 2,3-diphosphoglyceric acid, C(2)-ceramide) and phosphatidate phosphohydrolase (PAP) (propranolol and chlorpromazine), but was unaffected by inhibitors of phospholipase C. The 1alpha,25(OH)(2)D(3)-induced STS activity was also attenuated by inhibitors of protein kinase Calpha and protein kinase Cdelta (Go 6976, HBDDE and rottlerin), but not by an inhibitor of protein kinase Cbeta (LY379196). Additionally, 1alpha,25(OH)(2)D(3)-induced STS activity was attenuated by inhibitors of RAS (manumycin A), RAF (GW5074), MEK (PD098059 and U1026) and JNK (SP600125), but not p38 (PD169316). 1alpha,25(OH)(2)D(3) produced a rapid and long lasting stimulation of the ERK-MAP kinase signalling cascade in HL60 myeloid leukaemic cells. This 'non-genomic' effect of 1alpha,25(OH)(2)D(3) blocked by pharmacological antagonists of nuclear vitamin D receptors (VDR(nuc)) and does not appear to require hetero-dimerisation with the retinoid-X receptor (RXR). Inhibitors of the Src tyrosine kinase (PP1), RAS (manumycin A), RAS-RAF interactions (sulindac sulphide and RAS inhibitory peptide), RAF (GW5074 or chloroquine), and protein kinase Calpha (HBDDE) abrogated the 1alpha,25(OH)(2)D(3)-stimulated increase in ERK-MAP kinase activity. Taken together, these results show that 1alpha,25(OH)(2)D(3)/VDR(nuc) activation of the RAS/RAF/ERK-MAP kinase signalling pathway plays an important role in augmenting STS activity in human myeloid leukaemic cell lines.

Peroxisome proliferator-activated receptor gamma agonists inhibit HIV-1 replication in macrophages by transcriptional and post-transcriptional effects

Previous studies have demonstrated that cyclopentenone prostaglandins (cyPG) inhibit human immunodeficiency virus type 1 (HIV-1) replication in various cell types. We investigated the role of PG in the replication of HIV-1 in primary macrophages. The cyPG, PGA(1) and PGA(2), inhibited HIV-1 replication in acutely infected human monocyte-derived macrophages (MDM). Because PGA(1) and PGA(2) have previously been shown to be peroxisome proliferator-activated receptor gamma (PPARgamma) agonists, we examined the effect of synthetic PPARgamma agonists on HIV replication. The PPARgamma agonist ciglitazone inhibited HIV-1 replication in a dose-dependent manner in acutely infected human MDM. In addition, cyPG and ciglitazone reduced HIV replication in latently infected and viral entry-independent U1 cells, suggesting an effect at the level of HIV gene expression. Ciglitazone also suppressed HIV-1 mRNA levels as measured by reverse transcriptase PCR, in parallel with the decrease in reverse transcriptase activity. Co-transfection of PPARgamma wild type vectors and treatment with PPARgamma agonists inhibited HIV-1 promoter activity in U937 cells. Activation of PPARgamma also decreased HIV-1 mRNA stability following actinomycin D treatment. In summary, our experimental findings implicate PPARgamma as an important factor in the suppression of HIV-1 gene expression in MDM by cyPG. Thus natural and synthetic PPARgamma agonists may play a role in controlling HIV-1 infection in macrophages.

Proliferating or differentiating stimuli act on different lipid-dependent signaling pathways in nuclei of human leukemia cells

Previous results have shown that the human promyelocytic leukemia HL-60 cell line responds to either proliferating or differentiating stimuli. When these cells are induced to proliferate, protein kinase C (PKC)-beta II migrates toward the nucleus, whereas when they are exposed to differentiating agents, there is a nuclear translocation of the alpha isoform of PKC. As a step toward the elucidation of the early intranuclear events that regulate the proliferation or the differentiation process, we show that in the HL-60 cells, a proliferating stimulus (i.e., insulin-like growth factor-I [IGF-I]) increased nuclear diacylglycerol (DAG) production derived from phosphatidylinositol (4,5) bisphosphate, as indicated by the inhibition exerted by 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine and U-73122 (1-[6((17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione), which are pharmacological inhibitors of phosphoinositide-specific phospholipase C. In contrast, when HL-60 cells were induced to differentiate along the granulocytic lineage by dimethyl sulfoxide, we observed a rise in the nuclear DAG mass, which was sensitive to either neomycin or propranolol, two compounds with inhibitory effect on phospholipase D (PLD)-mediated DAG generation. In nuclei of dimethyl sulfoxide-treated HL-60 cells, we observed a rise in the amount of a 90-kDa PLD, distinct from PLD1 or PLD2. When a phosphatidylinositol (4,5) bisphosphate-derived DAG pool was generated in the nucleus, a selective translocation of PKC-beta II occurred. On the other hand, nuclear DAG derived through PLD, recruited PKC-alpha to the nucleus. Both of these PKC isoforms were phosphorylated on serine residues. These results provide support for the proposal that in the HL-60 cell nucleus there are two independently regulated sources of DAG, both of which are capable of acting as the driving force that attracts to this organelle distinct, DAG-dependent PKC isozymes. Our results assume a particular significance in light of the proposed use of pharmacological inhibitors of PKC-dependent biochemical pathways for the therapy of cancer disease.

Monocyte chemotactic activity in human abdominal aortic aneurysms: role of elastin degradation peptides and the 67-kD cell surface elastin receptor

BACKGROUND

Chronic inflammation is a characteristic feature of abdominal aortic aneurysms (AAAs), but the molecular signals responsible for recruiting monocytes into the outer aortic wall are unresolved. The purpose of this study was to examine whether AAA tissues elaborate chemotactic activity for mononuclear phagocytes and to determine whether this activity is attributable to interactions between elastin degradation peptides (EDPs) and their cell surface receptor, the 67-kD elastin binding protein (EBP).

MATERIAL AND METHODS

Soluble proteins were extracted from human AAA tissues, and chemotactic activity for differentiated U937 mononuclear phagocytes was measured by use of a modified Boyden chamber. Chemotactic activity induced by N -formyl-Met-Leu-Phe was used as a positive control and checkerboard analysis was used to distinguish chemotaxis from chemokinesis. Inhibition of chemotaxis was tested by peptide competition, blocking antibodies and galactosugar-mediated dissociation of the 67-kD EBP.

RESULTS

AAA extracts stimulated a concentration-dependent increase in monocyte migration that reached up to 24% of the maximal effect induced by N -formyl-Met-Leu-Phe. Checkerboard analysis demonstrated that AAA extracts stimulated chemotaxis without a chemokinetic effect. AAA-derived chemotactic activity was eliminated by competition with Val-Gly-Val-Arg-Pro-Gly (VGVAPG), a repetitive peptide found in human elastin that binds to cellular elastin receptors, and decreased nearly 40% in the presence of BA-4, an antielastin monoclonal antibody that can block EDP-mediated chemotactic activity. Monocyte chemotaxis in response to both VGVAPG and AAA extracts was abolished in the presence of lactose, a galactosugar that specifically dissociates the 67-kD EBP, but it was unaffected by either glucose, fructose, or mannose.

CONCLUSIONS

These findings indicate that soluble EDPs released within human AAA tissue can subsequently attract mononuclear phagocytes through ligand-receptor interactions with the 67-kD EBP, thereby providing a plausible molecular mechanism to explain the inflammatory response that accompanies aneurysmal degeneration. Better understanding of factors regulating inflammatory cell recruitment may lead to novel forms of therapy for early stages of aneurysmal degeneration.

Prostaglandin E(2) inhibits replication of HIV-1 in macrophages through activation of protein kinase A

Since macrophages are a source of increased PGE(2) in AIDS, we investigated the role of PGE(2) in the replication of HIV-1 in these cells. PGE(2) inhibited HIV-1 replication measured by reverse transcriptase in human monocyte-derived macrophage (MDM). Treatment of MDM with the PGE(1) analog misoprostol, the adenylate cyclase activator forskolin, and the cyclic AMP analog dibutyryl-cyclic AMP (db-cAMP) suppressed HIV replication. The protein kinase A (PKA) activator 8-bromo-cyclic AMP also inhibited HIV-1 replication. Similar results were observed with the entry-independent, latently HIV-infected U1 cells. There was a parallel decrease in HIV-1 mRNA levels following PGE(2) treatment. Co-transfection of the HIV-1 promoter LTR.luciferase, with the vector CMV.Calpha, which expresses the PKA catalytic unit increasing PKA activity, reduced HIV-1 promoter activity. Inhibition of PKA activity with the pMT.RAB vector, a mutant regulatory unit of PKA, augmented HIV-1 promoter activity. In summary, PGE(2) inhibits HIV-1 gene expression in MDM through a PKA-dependent mechanism.

Activation and priming of neutrophil nicotinamide adenine dinucleotide phosphate oxidase and phospholipase A(2) are dissociated by inhibitors of the kinases p42(ERK2) and p38(SAPK) and by methyl arachidonyl fluorophosphonate, the dual inhibitor of cytosolic and calcium-independent phospholipase A(2)

Arachidonic acid (AA) generated by phospholipase A(2) (PLA(2)) is thought to be an essential cofactor for phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity. Both enzymes are simultaneously primed by cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor-alpha (TNF-alpha). The possibility that either unprimed or cytokine-primed responses of PLA(2) or NADPH oxidase to the chemotactic agents formyl-methionyl-leucyl-phenylalanine (FMLP) and complement factor 5a (C5a) could be differentially inhibited by inhibitors of the mitogen-activated protein (MAP) kinase family members p42(ERK2) (PD98059) and p38(SAPK) (SB203580) was investigated. PD98059 inhibited the activation of p42(ERK2) by GM-CSF, TNF-alpha, and FMLP, but it did not inhibit FMLP-stimulated superoxide production in either unprimed or primed neutrophils. There was no significant arachidonate release from unprimed neutrophils stimulated by FMLP, and arachidonate release stimulated by calcium ionophore A23187 was not inhibited by PD98059. In contrast, PD98059 inhibited both TNF-alpha- and GM-CSF-primed PLA(2) responses stimulated by FMLP. On the other hand, SB203580 inhibited FMLP-superoxide responses in unprimed as well as TNF-alpha- and GM-CSF-primed neutrophils, but failed to inhibit TNF-alpha- and GM-CSF-primed PLA(2) responses stimulated by FMLP, and additionally enhanced A23187-stimulated arachidonate release, showing that priming and activation of PLA(2) and NADPH oxidase are differentially dependent on both the p38(SAPK) and p42(ERK2) pathways. Studies using C5a as an agonist gave similar results and confirmed the findings with FMLP. In addition, methyl arachidonyl fluorophosphonate (MAFP), the dual inhibitor of c and iPLA(2) enzymes, failed to inhibit superoxide production in primed cells at concentrations that inhibited arachidonate release. These data demonstrate that NADPH oxidase activity can be dissociated from AA generation and indicate a more complex role for arachidonate in neutrophil superoxide production.

Modifications in phospholipase D activity and isoform expression occur upon maturation and differentiation in vivo and in vitro in human myeloid cells

Activation of phospholipase D (PLD) occurs in response to various stimuli and results from the activity of two isozymes, hPLD1 and hPLD2. PLD activity appears to be involved in several myeloid cell processes during their development and activation, including proliferation of myeloblasts in the bone marrow and secretion, phagocytosis and NADPH oxidase activation, essential functions of differentiated neutrophils. The present work studies PLD characteristics, activity and both isozyme expression during maturation and differentiation of myeloid cells by using three different systems: leukemic myeloblasts at different stages of maturation, terminally differentiated neutrophils ex vivo and four human myeloid cell lines, NB4, HL-60, PLB 985 and U937, induced to differentiate with alltrans retinoic acid (ATRA), a cyclic adenosine monophosphate (cAMP) analogue or both agents together. HL-60, a bipotential cell line has also been differentiated along the granulocytic pathway with DMSO and the monocytic pathway with 1,25-dihydroxy vitamin D3. In all these systems, PLD activity increases with maturation and differentiation whatever the inducer used and the granulocytic or monocytic pathways. Increase in basal activity which reflects the expression during development of both hPLD1 and hPLD2 appears to be mainly related to the former isozyme expression. Association of PLD characteristic changes with maturation and differentiation was also confirmed using two NB4 clones resistant to these processes. Comparison between PLD characteristics in myeloblasts during maturation and differentiation ex vivo and in vitro in the different cell lines demonstrated that NB4 induced to differentiate with ATRA represents the best model for further studies on the specific roles of each PLD isoform in various functions of differentiated myeloid cells.