Involvement of protein kinase C, p38 MAP kinase and ERK in arachidonic acid-stimulated superoxide production in human neutrophils

Arachidonic acid regulates the translocation of 5-lipoxygenase to the nuclear membranes in human neutrophils

Elevation of the intracellular cAMP concentration in agonist-activated human neutrophils (PMN) leads to the concomitant inhibitions of arachidonic acid (AA) release, 5-lipoxygenase (5-LO) translocation, and leukotriene (LT) biosynthesis. We report herein that exogenous AA completely prevents cAMP-dependent inhibition of 5-LO translocation and LT biosynthesis in agonist-activated PMN. Moreover, the group IVA phospholipase A2 inhibitor pyrrophenone and the MEK inhibitor U-0126 inhibited AA release and 5-LO translocation in activated PMN, and these effects were also prevented by exogenous AA, demonstrating a functional link between AA release and 5-LO translocation. Polyunsaturated fatty acids of the C18 and C20 series containing at least three double bonds located from carbon 9 (or closer to the carboxyl group) were equally effective as AA in restoring 5-LO translocation in pyrrophenone-treated agonist-activated PMN. Importantly, experiments with the 5-LO-activating protein inhibitor MK-0591 and the intracellular Ca2+ chelator BAPTA-AM demonstrated that the AA-regulated 5-LO translocation is FLAP- and Ca2+-dependent. Finally, the redox and competitive 5-LO inhibitors L-685,015, L-739,010, and L-702,539 (but not cyclooxygenase inhibitors) efficiently substituted for AA to reverse the pyrrophenone inhibition of 5-LO translocation, indicating that the site of regulation of 5-LO translocation by AA is at or in the vicinity of the catalytic site. This report demonstrates that AA regulates the translocation of 5-LO in human PMN and unravels a novel mechanism of the cAMP-mediated inhibition of LT biosynthesis.

Effect of pulmonary surfactant on TNF-α-activated endothelial cells and neutrophil adhesion in vitro

Pulmonary surfactant given to infants and adults with respiratory failure is metabolized and recycled to a large extent. A small proportion also enters the circulation in cases of increased permeability of the alveolar-capillary membrane. We therefore investigated whether exogenous surfactants such as a natural bovine (natSF) or a synthetic (synSF) preparation had an impact on inflammatory conditions involving the adhesion of neutrophils to endothelial cells. Human umbilical cord vein endothelial cells (HUVEC) were plated on coverslips until confluence, activated by tumor necrosis factor-alpha and incubated with or without surfactant in the media. Human neutrophils passed the HUVEC layer in a flow chamber and interactions were visualized using a video microscope. To test if surfactant affected the expression of cell adhesion molecules, RT-PCR analyses were performed for E-selectin, vascular cell adhesion molecule 1 (VCAM-1) and intercellular adhesion molecule 1 (ICAM-1). Using concentrations between 50 and 300 microg/ml of surfactant in the pre-incubation media the number of adherent neutrophils increased by 10-20% at the higher concentration of the natSF (*P < 0.05) whereas the synSF had no effect. Increased neutrophil adhesion was associated with a significant up-regulation of mRNA levels for E-selectin and VCAM-1; mRNA levels for ICAM-1, however, were not affected by the presence of surfactant. These observations indicate that natSF but not synSF might have pro-inflammatory effects when higher amounts of the exogenous dose reach the circulation. This might be explained by different fatty acid profiles, e.g. the presence of arachidonic acid in the natSF or higher concentrations of surfactant-associated protein-C in the synSF.

Effect of shigatoxin-1 on arachidonic acid release by human glomerular epithelial cells

BACKGROUND

Altered arachidonic acid (AA) metabolism has been implicated in the pathogenesis of renal injury in the hemolytic uremic syndrome (HUS). However, there is very little information of the effect of shigatoxin (Stx; the putative mediator of renal damage in HUS) on AA release or metabolism by renal cells. Since recent studies have demonstrated that glomerular epithelial cells (GECs) may be important early targets of Stx, the current study was undertaken to examine the effects of Stx on AA release and metabolism by GECs.

METHODS

Cultured human GECs were exposed to Stx1 +/- lipopolysaccharide (LPS) for 4 to 48 hours followed by determination of (3)H-arachidonate release, thromboxane A(2) (TxA(2)) and prostacyclin (PGI(2)) production, cyclooxygenase (COX) activity, and Western and Northern analyses for phospholipase A(2) (PLA(2)) and COX protein and mRNA levels, respectively.

RESULTS

Stx1 increased arachidonate release by GECs. LPS alone had no such effect, but increased arachidonate release in response to Stx1. Stx1-stimulated arachidonate release correlated with elevations in cPLA(2) and sPLA(2) protein and cPLA(2) mRNA levels. Stx1 also increased both TxA(2) and PGI(2) production by GECs; LPS alone did not alter eicosanoid production, but augmented Stx1 effects. Both Stx1 and LPS stimulated COX activity; however, these effects were not additive. Although there was an accompanying elevation of COX-1 and COX-2 mRNA, Stx1 decreased and LPS did not change COX1 and COX2 protein levels.

CONCLUSIONS

Stx1 alone or in conjunction with LPS increases arachidonate release and eicosanoid production by human GECs; this effect correlates with increased PLA(2) protein and mRNA levels. To our knowledge, this is the first study identifying the mechanisms of Stx1-stimulated AA release. These results raise the possibility that arachidonate release and metabolism by GECs, and conceivably other renal cell types, are involved in renal injury in HUS.

Protein kinases C translocation responses to low concentrations of arachidonic acid

Arachidonic acid (AA) directly activates protein kinases C (PKC) and may thereby serve as a regulatory signal during cell stimulation. The effect, however, requires a > or =20 microm concentration of the fatty acid. We find that human polymorphonuclear neutrophils (PMN) equilibrated with a ligand for the diacylglycerol receptor on PKC, [(3)H]phorbol dibutyrate (PDB), increased binding of [(3)H]PDB within 15 s of exposure to > or =10-30 nm AA. Other unsaturated fatty acids, but not a saturated fatty acid, likewise stimulated PDB binding. These responses, similar to those caused by chemotactic factors, resulted from a rise in the number of diacylglycerol receptors that were plasma membrane-associated and therefore accessible to PDB. Unlike chemotactic factors, however, AA was fully active on cells overloaded with Ca(2+) chelators. The major metabolites of AA made by PMN, leukotriene B(4) and 5-hydroxyicosatetraenoate, did not mimic AA, and an AA antimetabolite did not block responses to AA. AA also induced PMN to translocate cytosolic PKCalpha, beta(II), and delta to membranes. This response paralleled PDB binding with respect to dose requirements, time, Ca(2+)-independence, resistance to an AA antimetabolite, and induction by another unsaturated fatty acid but not by a saturated fatty acid. Finally, HEK 293 cells transfected with vectors encoding PKCbeta(I) or PKCdelta fused to the reporter enhanced green fluorescent protein (EGFP) were studied. AA caused EGFP-PKCbeta translocation from cytosol to plasma membrane at > or =0.5 microm, and EGFP-PKCdelta translocation from cytosol to nuclear and, to a lesser extent, plasma membrane at as little as 30 nm. We conclude that AA induces PKC translocations to specific membrane targets at concentrations 2-4 orders of magnitude below those activating the enzymes. These responses, at least as they occur in PMN, do not require changes in cell Ca(2+) or oxygenation of the fatty acid. AA seems more suited for signaling the movement than activation of PKC.

Activation of the Phosphatidylinositol 3-Kinase-Akt/Protein Kinase B Signaling Pathway in Arachidonic Acid-stimulated Human Myeloid and Endothelial Cells

Although arachidonic acid has been demonstrated to stimulate a wide variety of cellular functions, the responsible mechanisms remain poorly defined. We now report that arachidonic acid stimulated the activity of class Ia phosphatidylinositol 3-kinase (PI3K) in human umbilical vein endothelial cells, HL60 cells, and human neutrophils. Pretreatment of endothelial cells with AG-1478, an inhibitor of the ErbB receptor family, resulted in the suppression of PI3K activation by arachidonic acid. The fatty acid enhanced the tyrosine phosphorylation of ErbB4 but not of ErbB2 or ErbB3. The ability of arachidonic acid to stimulate PI3K activity in neutrophils was suppressed by indomethacin and nordihydroguaiaretic acid, inhibitors of the cyclooxygenases and lipoxygenases, respectively, but not by 17-octadecynoic acid, an inhibitor of omega-hydroxylation of arachidonic acid by cytochrome P450 monooxygenases. Consistent with this, the activity of PI3K in neutrophils was stimulated by 5-hydroxyeicosatetraenoic acid. Arachidonic acid also transiently stimulated the phosphorylation of Akt on Thr-308 and Ser-473. Although PI3K was not required for the activation of the mitogen-activated protein kinases, ERK1, ERK2, and p38, in arachidonic acid-stimulated neutrophils, the fatty acid acted via PI3K to stimulate the respiratory burst. These results not only define a novel mechanism through which some of the actions of arachidonic acid are mediated but also demonstrate that, in addition to ErbB1 (epidermal growth factor receptor), ErbB4 can also be transactivated by a non-epidermal growth factor-like ligand.

Activation of human neutrophils by technical toxaphene

Toxaphene is a persistent organic pollutant (POP) known to be composed of numerous congeners. Toxaphene technical mixture applied as a pesticide consists of over 800 congeners. Among these, T(2) and T(12) are the two environmentally prevalent forms found in humans. Although toxaphene is known to exert some toxic effects, including potential proinflammatory properties, little is known concerning its action on cells of the human immune system, especially neutrophils. In the present study, we found that toxaphene was not necrotic for human neutrophils incubated for up to 24 h with concentrations ranging from 0.1 to 50 microg/ml. Toxaphene was found to induce neutrophil superoxide production (O(-)(2)) in a concentration-dependent manner. The potency and the kinetics of toxaphene-induced O(-)(2) by neutrophils were found to be similar to that of the classical neutrophil agonists phorbol 12-myristate 13-acetate (PMA). Furthermore, the use of various transduction signal inhibitors (genistein, pertussis toxin, staurosporine, H-7, and HA-1077), suggests that, as for PMA, toxaphene mediates its effect primarily via PKCs and, to a lesser extend, via tyrosine kinases. In this respect, staurosporine, H-7, and genistein were found to inhibit toxaphene- and PMA-induced O(-)(2) production by 52, 72, and 31% and by 63, 62, and 23%, respectively. Toxaphene was also found to significantly enhance neutrophil phagocytosis of opsonized sheep red blood cells and to induce neutrophil apoptosis. The induction of neutrophil apoptosis was paralleled with a decrease in CD16 expression. T(2) and T(12), the two prevalent congeners found in humans, were also found to significantly increase the O(-)(2) production in neutrophils at a concentration of 5 microg/ml. We conclude that neutrophils are important targets for toxaphene, as this POP can activate O(-)(2) production by a PKC- and tyrosine kinase-dependent mechanism, induce phagocytosis, and accelerate the apoptotic rate. This is the first study that focuses on toxaphene/human neutrophil interactions.