Contrasting effects of calyculin A and okadaic acid on the respiratory burst of human neutrophils

Pro-oxidant versus anti-oxidant effects of seeds aglycone extracts of Lepidium sativum and Eruca vesicaria Linn., in vitro, and on neutrophil nitro-oxidative functions

This study evaluated the anti-inflammatory and antioxidant properties of seeds aglycone extracts from Lepidium sativum (LS) and Eruca vesicaria (EV) Linn., on oxidative damages in vitro and on neutrophil nitro-oxidative functions. The results showed that LS and EV aglycone extracts attenuated liver microsomal lipids and proteins oxidation through a potent antioxidant effect as attested by the dose dependent quenching of DPPH radical scavenging activity. LS and EV aglycone extracts inhibited dose dependently the production of superoxide anion by BALB/c mice-derived peritoneal neutrophils, whereas they slightly enhanced exocytosis of myeloperoxidase (MPO), a marker of azurophilic granules. Interestingly, only LS replenished glutathione (GSH) and nitric oxide levels, indicating a fine differential effect. This study highlighted the subtle oxidative and antioxidant capacity of LS and EV seeds aglycone extracts. These health promoting compounds could be used to finely modulate critical events involved in microbial infection, inflammation and nitro-oxidative stress.

The NADPH oxidase cytosolic component p67phox is constitutively phosphorylated in human neutrophils: Regulation by a protein tyrosine kinase, MEK1/2 and phosphatases 1/2A

Neutrophils play a key role in host defense and inflammation through the production of superoxide anion and other reactive oxygen species (ROS) by the enzyme complex NADPH oxidase. The cytosolic NADPH oxidase component, p67phox, has been shown to be phosphorylated in human neutrophils but the pathways involved in this process are largely unknown. In this study, we show that p67phox is constitutively phosphorylated in resting human neutrophils and that neutrophil stimulation with PMA further enhanced this phosphorylation. Inhibition of the constitutively active serine/threonine phosphatases type 1 and type 2A (PP1/2A) by calyculin A resulted in the enhancement of p67phox phosphorylation. Constitutive and calyculin A-induced phosphorylation of p67phox was completely inhibited by the protein tyrosine kinase inhibitor genistein and partially inhibited by the MEK1/2 inhibitor PD98059, but was unaffected by GF109203X, wortmannin and SB203580, inhibitors of PKC, PI3K and p38MAP kinase, respectively. Two-dimensional phosphopeptide mapping revealed that constitutive and calyculin A-induced p67phox phosphorylation occurred on the same major sites. Interestingly, calyculin A enhanced formyl-Met-Leu-Phe (fMLP)-induced superoxide production, while genistein inhibited this process. Taken together, these results suggest that (i) p67phox undergoes a continual cycle of phosphorylation/dephosphorylation in resting cells; (ii) p67phox phosphorylation is controlled by MEK1/2 and an upstream tyrosine kinase; (iii) PP1/2A directly or indirectly antagonize this process. Thus, these pathways could play a role in regulating ROS production by human neutrophils at inflammatory sites.

Conventional protein kinase C plays a critical role in negative regulation of CD98-induced homotypic aggregation

CD98, a heterodimeric type II transmembrane protein, is involved in many different cellular events, ranging from amino acid transport to cell-cell adhesion. Little is known about the positive and negative signalling pathways involved in these responses. Therefore, we examined the role of conventional protein kinase C (PKC) isoforms during CD98-induced intracellular signalling and homotypic aggregation of U937 cells. The CD98-induced aggregation was enhanced by the general protein kinase inhibitors GF109203X and staurosporin, and by specific PKC-alpha/-beta peptide inhibitor 19-27, but inhibited by PKC activators such as phorbol 12-myristate 13-acetate (PMA). PMA-inhibition was reversed by PKC inhibitors recognising the ATP-binding site in PKC (e.g. staurosporin, GF109203X and Go6983). Inhibitors which bind to diacylglycerol (DAG) or Ca(2+)-binding sites of PKC (calphostin C and Go6967) had no effect. PMA-induced translocation of conventional PKC (cPKC) isozymes (alpha, beta and gamma), but decreased the expression of PKC-delta, which plays an important role in CD98-induced homotypic aggregation. PMA treatment also suppressed the surface level of CD98 but not CD29, CD18 and CD147, dose- and time-dependently. These data provide evidence that PMA-responsive cPKC isoforms (alpha, beta and gamma) play a key role in negative regulation of CD98 signalling and homotypic aggregation.

Another biological effect of tosylphenylalanylchloromethane (TPCK): it prevents p47phox phosphorylation and translocation upon neutrophil stimulation

TPCK (tosylphenylalanylchloromethane), first discovered as a serine protease inhibitor, has been described to affect in diverse systems a number of physiological events probably unrelated to its antiprotease effect, such as proliferation, apoptosis and tumour formation. In the present study, we focus on its inhibition of the neutrophil respiratory burst, an important element of non-specific immunological defence. The superoxide anion-producing enzyme, NADPH oxidase, is quiescent in resting cells. Upon cell stimulation, the redox component, membrane-bound flavocytochrome b558, is activated when the cytosolic factors (p47phox, p67phox and p40phox, as well as the small GTPase Rac) associate with it after translocating to the membrane. This requires the phosphorylation of several p47phox serine residues. The signal transduction events leading to enzyme activation are not completely understood. In the past, the use of diverse protease inhibitors suggested that proteases were involved in NADPH oxidase activation. We suggested previously that TPCK could prevent enzyme activation by the phorbol ester PMA, not due to inhibition of a protease, but possibly to inhibition of the cytosolic factor translocation [Chollet-Przednowed and Lederer (1993) Eur. J. Biochem. 218, 83-93]. In the present work, we show that TPCK, when added to cells before PMA, prevents p47phox phosphorylation and hence its translocation; moreover, when PMA-stimulated cells are incubated with TPCK, p47phox is dephosphorylated and dissociates from the membrane. These results are in line with previous suggestions that the respiratory burst is the result of a series of continuous phosphorylation and dephosphorylation events. They suggest that TPCK leads indirectly to activation of a phosphatase or inactivation of a kinase, and provide the first clue towards understanding the steps leading to its inhibition of NADPH oxidase activation.

Investigation into the relationship between calyculin A-mediated potentiation of NADPH oxidase activity and inhibition of store-operated uptake of calcium by human neutrophils

The primary objective of the current study was to investigate possible relationships between calyculin A (CA)-mediated potentiation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and inhibition of store-operated uptake of Ca2+ by chemoattractant-activated human neutrophils. Treatment of neutrophils with 100 nM CA, but not at lower concentrations (12.5-50 nM), prior to the addition of the N-formylated chemotactic tripeptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) (1 microM), both potentiated and prolonged the activity of NADPH oxidase which was accompanied by exaggerated membrane depolarisation, delayed and attenuated membrane repolarisation, and inhibition of store-operated Ca2+ influx. Inclusion of diphenylene iodonium chloride (DPI, 10 microM), an inhibitor of NADPH oxidase, antagonised the effects of CA on NADPH oxidase activity and the membrane repolarisation responses of FMLP-activated neutrophils, but failed to restore store-operated influx of Ca2+. Similarly, CA also inhibited store-operated influx of Ca2+ into FMLP-activated neutrophils from a patient with chronic granulomatous disease, a primary immunodeficiency disorder characterised by the absence of a functional NADPH oxidase. CA also inhibited the store-operated influx of Ca2+ into control neutrophils treated with 1 microM thapsigargin, a selective inhibitor of the endomembrane Ca2+-ATPase, which does not activate NADPH oxidase. Taken together, these observations demonstrate that augmentation of NADPH oxidase activity is not primarily involved in CA-mediated inhibition of the store-operated influx of Ca2+ into activated human neutrophils.

Sodium stibogluconate (Pentostam) potentiates oxidant production in murine visceral leishmaniasis and in human blood

Sodium stibogluconate (Sbb), a leishmanicidal drug, was studied for its in vivo effect on the formation of reactive oxygen species (ROS), assessed by chemiluminescence (CL) in the whole blood of mice infected with Leishmania infantum. Stimulation of ROS formation induced ex vivo by zymosan particles or the protein kinase C activator phorbol myristate acetate (PMA) was reduced by approximately 25% (P < 0.05) after infection of mice. Treatment of infected mice with Sbb (50 to 400 mg/kg of body weight) enhanced the blood CL induced by zymosan and PMA (47 to 96%, P < 0.01). The drug potentiation effect also occurred in uninfected mice. In vitro treatment of normal human blood with Sbb (1, 10, or 100 microg/ml) for 1 h primed the CL response to PMA (29 to 54%). The priming effect of Sbb was also observed on the production of superoxide by isolated polymorphonuclear leukocytes stimulated either by PMA and zymosan or by the chemoattractants N-formyl-Met-Leu-Phe and platelet-activating factor. These data provide the first evidence of priming of the phagocyte respiratory burst by Sbb. This novel property of Sbb may contribute to the drug's leishmanicidal effect.

Modulation of luminol chemiluminescence of fMet-Leu-Phe-stimulated neutrophils by affecting dephosphorylation and the metabolism of phosphatidic acid

This paper is addressed to study how PKC-mediated effects and phosphatidic acid interact together in activation of NADPH-oxidase in formyl-methionyl-leucyl-phenylalanine (fMet-Leu-Phe) stimulated neutrophils as detected by luminol chemiluminescence. The early luminescence response in fMet-Leu-Phe-stimulated cells (up to 5 min after stimulation) depends mainly on reactive oxygen species generated extracellularly, whereas all later events are caused by oxidation of luminol inside the cells. The two protein phosphatase inhibitors, okadaic acid and calyculin A, dramatically increased the late luminescence of cells. This enhancement was totally inhibited by the phospholipase D modulator butanol, while the protein kinase C (PKC) inhibitor bisindolylmaleimide I was insensitive. The early luminescence response of the cells was slightly inhibited by both protein phosphatase inhibitors and depended on protein kinase C as well as on phospholipase D activities. Propranolol, an inhibitor of phosphatidate phosphohydrolase, enhanced all parts of luminescence response of fMet-Leu-Phe-stimulated neutrophils at concentrations up to 2.5 x 10(-5) mol/L. While the late luminescence response of propranolol-treated cells was not inhibited by the PKC inhibitor bisindolylmaleimide I, the first response depended on protein kinase C. The inhibitor of diacylglycerol kinase R59949 enhanced the luminescence signal only during the first 4 min in fMet-Leu-Phe-stimulated cells. Only diacylglycerols derived from phospholipase C, such as 1-stearoyl-2-arachidonoyl-sn-glycerol, were able to initiate an oxidative burst in cells. Saturated diacylglycerols (e.g. 1,2-dipalmitoyl-sn-glycerol or 1,2-distearoyl-sn-glycerol) did not yield any luminol chemiluminescence, although they were incorporated into the plasma membrane, as evidenced by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Our results demonstrate that phosphatidic acid produced by phospholipase D is responsible for NADPH-oxidase activity in fMet-Leu-Phe-stimulated neutrophils over the entire measuring time, whereas PKC-mediated processes are only involved during the first 5 min.

Activation of the leukocyte NADPH oxidase by phorbol ester requires the phosphorylation of p47PHOX on serine 303 or 304

The leukocyte NADPH oxidase is an enzyme in phagocytes and B lymphocytes that when activated catalyzes the production of O-2 from oxygen and NADPH. During oxidase activation, serine residues in the C-terminal quarter of the oxidase component p47(PHOX) become extensively phosphorylated, the protein acquiring as many as 9 phosphate residues. In a study of 11 p47(PHOX) mutants, each containing an alanine instead of a serine at a single potential phosphorylation site, we found that all but S379A corrected the defect in O-2 production in Epstein-Barr virus (EBV)-transformed p47(PHOX)-deficient B cells (Faust, L. P., El Benna, J., Babior, B. M., and Chanock, S. J. (1995) J. Clin. Invest. 96, 1499-1505). In particular, O-2 production was restored to these cells by the mutants S303A and S304A. Therefore, apart from serine 379, whose state of phosphorylation in the activated oxidase is unclear, no single potential phosphorylation site appeared to be essential for oxidase activation. We now report that the double mutant p47(PHOX) S303A/S304A was almost completely inactive when expressed in EBV-transformed p47(PHOX)-deficient B cells, even though it was expressed in normal amounts in the transfected cells and was able to translocate to the plasma membrane when the cells were stimulated. In contrast, the double mutant p47(PHOX) S303E/S304E was able to support high levels of O-2 production by EBV-transformed p47(PHOX)-deficient B cells. The surprising discovery that the double mutant S303K/S304K was also able to support considerable O-2 production suggests either that the effect of phosphorylation is related to the increase in hydrophilicity around serines 303 and 304 or that activation involves the formation of a metal bridge between the phosphorylated serines and another region of the protein.

The marine toxin okadaic acid reduces O2- generation and tyrosine phosphorylation in LPS-primed rat neutrophils

Contrasting effects of okadaic acid (OKA) on neutrophil (PMN) superoxide anion (O2-) generation have been reported. In this study, we examined the effect of OKA on phorbol myristate acetate (PMA)-stimulated O2- generation in rat PMNs primed with LPS in vivo (LPS-PMN) and saline-treated rat PMNs (SAL-PMN). The following results were observed: (1) OKA, but neither genistein nor vanadate, markedly reduced O2- generation in a dose and time-dependent manner; (2) genistein, a tyrosine kinase inhibitor, as well as OKA, reduced tyrosine phosphorylation; (3) sodium orthovanadate, a tyrosine phosphatase inhibitor, potently enhanced tyrosine phosphorylation. Our studies suggest that OKA might reduce tyrosine phosphorylation by affecting the activity of tyrosine phosphatases regulated by serine-threonine phosphorylation.

Phosphatase activity regulates superoxide anion generation and intracellular signaling in human neutrophils

Phosphorylation of components of the neutrophil NADPH oxidase plays a critical role in activation and maintenance of superoxide anion (O2-) generation. To investigate the role of dephosphorylation by phosphatases in regulating O2- production, human neutrophils were treated with calyculin A, a potent inhibitor of protein phosphatases 1 and 2A, prior to stimulation. Calyculin A alone did not stimulate O2- production. However, neutrophils exposed to 50 nM calyculin A and the chemotactic peptide formyl-met-leu-phe (FMLP, 100 nM) displayed markedly enhanced O2- production in comparison to cells stimulated with FMLP alone (28.63 +/- 7.00 versus 8.69 +/- 3.69 nmol O2-/1.5 x 10(6) neutrophils/5 min, respectively, n = 18, p < 0.001), with an increased duration of O2- production. In contrast, phosphatase-inhibition decreased oxidative responsiveness to phorbol myristate acetate (PMA, > or = 16 nM). We next examined the effect of calyculin A on products of the phosphatidylcholine-specific phospholipase D (PLD) pathway by assaying the mass levels of phosphatidic acid (PA), choline and diacylglycerol (DAG). Calyculin A increased both PA and choline production to 224 +/- 28% and 315 +/- 61% of FMLP-stimulated controls, respectively (p < 0.01, n = 7) without significantly increasing DAG. Also, membrane protein kinase C activity increased more than 10-fold in FMLP-stimulated cells exposed to calyculin A but decreased in cells stimulated with PMA following calyculin A pre-treatment. These results suggest that phosphatases exert variable and stimulus-dependent effects on pathways leading to O2- production. Further, it appears that phospholipase D activity and PA generation represent important steps in the pathway for NADPH activation triggered by FMLP.

Levels of DNA strand breaks and superoxide in phorbol ester-treated human granulocytes

Phorbol ester treatment of granulocytes triggers release of superoxide (O2.-) and a concomitant burst of DNA strand breaks. The relationship between the amount of O2.- and the number of DNA breaks has not previously been explored. To quantify the relatively large amount of O2.- generated over a 40-min period by 1 x 10(6) granulocytes/mL, a discontinuous "10-min pulse" method employing cytochrome c was used; 140 nmol O2.- per 1 x 10(6) cells was detected. DNA strand breaks were quantified by fluorimetric analysis of DNA unwinding (FADU). To vary the level of O2.- released by cells, inhibitors of the respiratory burst were used. Sodium fluoride (1-10 mM) and staurosporine (2-10 nM) both inhibited O2.- production. In both cases, however, inhibition of strand breakage was considerably more pronounced than inhibition of O2.-. Zinc chloride (50-200 microM) inhibited both O2.- and DNA breaks, approximately equally. Dinophysistoxin-1 (okadaic acid) inhibited O2.- production more effectively than it inhibited DNA breaks. O2.- dismutes to H2O2, a reactive oxygen species known to cause DNA breaks. The addition of catalase to remove extracellular H2O2 had no effect on DNA breakage. Using pulse field gel electrophoresis, few double-stranded breaks were detected compared to the number detected by FADU, indicating that about 95% of breaks were single-stranded. The level of DNA breaks is not directly related to the amount of extracellular O2.- or H2O2 in PMA-stimulated granulocytes. We conclude that either an intracellular pool of these reactive oxygen species is involved in breakage or that the metabolic inhibitors are affecting a novel strand break pathway.

Activation of the leukocyte NADPH oxidase subunit p47phox by protein kinase C. A phosphorylation-dependent change in the conformation of the C-terminal end of p47phox

The leukocyte NADPH oxidase of neutrophils is a membrane-bound enzyme that catalyzes the production of O2- from oxygen using NADPH as electron donor. Dormant in resting neutrophils, the enzyme acquires catalytic activity when the cells are exposed to appropriate stimuli. During activation, the cytosolic oxidase components p47phox and p67phox migrate to the plasma membrane, where they associate with cytochrome b558, a membrane-bound flavohemoprotein, to assemble the active oxidase. An essential element of the activation process is the phosphorylation of p47phox, an event that accompanies oxidase activation in whole cells and can activate the oxidase in a cell-free system. We show here that the phosphorylation of p47phox leads to a substantial decrease in the reactivity of cysteine C378 toward N-ethylmaleimide, indicating the occurrence of a conformational change involving the C-terminal region of p47phox. A similar conformational change occurs when p47phox is exposed to arachidonate, one of a number of anionic detergents that activate the oxidase in the cell-free system. We propose that this change in conformation results in the appearance of a binding site through which p47phox interacts with cytochrome b558 during the activation process.

Regulation of signal transducer and activator of transcription (STAT) 5b activation by the temporal pattern of growth hormone stimulation

Plasma GH profiles, intermittent in adult male and continuous in adult female rats, respectively, activate unique patterns of gene transcription in male and female rat liver. Pulsatile, but not continuous, GH exposure activates liver STAT5 (signal transducer and activator of transcription-5) by tyrosine phosphorylation, leading to nuclear translocation, and is proposed to play a key role in GH pulse-regulated male-specific liver gene expression. The mechanisms underlying the GH pattern dependence of STAT5 activation are presently investigated using a rat hepatocyte-derived cell line. Rat GH stimulated tyrosine phosphorylation followed by serine or threonine phosphorylation, leading to activation of the DNA-binding activity of STAT5b, the major STAT5 form present in these cells. Maximal STAT5b activation required a full 20 min at a receptor-saturating GH concentration of 50 ng/ml, suggesting that hormone binding leading to receptor dimerization is a relatively slow process. Repeat cycles of GH pulsation led to repeat cycles of STAT5b activation followed by deactivation, similar to rat liver in vivo. Full responsiveness to succeeding GH pulses required a minimum GH off-time of > or = 2.5 h, but was independent of new protein synthesis. Continuous GH exposure led to down-regulation of activated STAT5b, consistent with the desensitization of this GH pulse-activated pathway observed in female rat liver. The rapid deactivation of STAT5b after termination of a GH pulse involved phosphotyrosine dephosphorylation as a key first step and could be blocked by pervanadate, a phosphotyrosine phosphatase inhibitor. Unexpectedly, serine/threonine kinase inhibitors also inhibited STAT5b deactivation. These studies establish that STAT5b is responsive to the temporal pattern of GH stimulation and demonstrate a role for both a tyrosine phosphatase and a serine/threonine kinase in resetting this JAK/STAT signaling apparatus so that it may respond to subsequent rounds of GH pulse activation.