Br. J. Pharmacol. (1991), 103, 1909-1916
Macmillan Press Ltd, 1991
Thrombin-activated platelets promote leukotriene B4 synthesis in polymorphonuclear leucocytes stimulated by physiological agonists Remi Palmantier & 1Pierre Borgeat Unite de Recherche Inflammation et Immunologie-Rheumatologie, Centre Hospitalier de l'Universite Laval, 2705 boul. Laurier, Quebec, G1V 4G2, Canada 1 The addition of 2 x 108 human platelets to 8 x 106 polymorphonuclear leucocytes (PMNL) incubated in presence of 2.5 u ml-' thrombin and 0.1 aM N-formyl-Met-Leu-Phe (FMLP) (or C5a or PAF) led to enhancement of leukotriene B4 (LTB4) synthesis by the PMNL (measured by h.p.l.c. as 20-hydroxy- and 20-carboxy-LTB4) from 4 + 1 pmol (in absence of platelets) to 26 + 4 pmol (mean + s.e.mean, n = 9). Platelets and thrombin were both essential for the enhancement of LTB4 synthesis. 2 Platelets also caused enhancement of LTB4 synthesis from (30 + 12 to 134 + 25 pmol, n = 6) when PMNL pretreated with granulocyte-macrophage colony-stimulating factor were used in similar experiments.
3 Enhancement of LTB4 synthesis was also observed (from 5 + 1.5 to 26.5 + 5 pmol, n = 9) when the supernatants of thrombin-activated platelet suspensions were added to FMLP-stimulated PMNL. 4 Supernatants of platelet suspensions activated by thrombin in presence of cyclo-oxygenase and 12lipoxygenase inhibitors led to greater enhancement (from 5 + 3 to 153.5 + 27.5 pmol, n = 3) of LTB4 synthesis by FMLP-stimulated PMNL, suggesting that arachidonic acid itself, rather than its metabolites was responsible for the effects of platelets. 5 Addition of arachidonic acid to FMLP-stimulated PMNL at a concentration comparable to that measured in thrombin-activated platelet supernatants (0.2 + 0.025iUM, n = 6) mimicked the effect of platelets or platelet supernatants on LTB4 synthesis in FMLP-activated PMNL. 6 The present data indicate that under conditions of cell activation by physiological agonists, platelets can significantly increase the formation of the proinflammatory compound LTB4 in PMNL by providing arachidonic acid. These data lend support to the concept that platelet-PMNL interactions could modulate the inflammatory process. Keywords: LTB4 synthesis by neutrophils; platelet-neutrophil interaction; arachidonic acid; granulocyte-macrophage colony-stimulating factor; lipoxygenase and cyclo-oxygenase inhibitors; inflammation
Introduction Leukotriene B4 (LTB4) is a potent proinflammatory compound derived from arachidonic acid through the 5-lipoxygenase pathway; it is synthesized by polymorphonuclear leucocytes (PMNL) stimulated with ionophore A23187 (Borgeat, 1989). LTB4 is also synthesized by PMNL following receptor-mediated activation by agonists such as the chemotactic peptide N-formyl-Met-Leu-Phe (FMLP), the complement fragment C5a, platelet-activating factor (PAF) or following activation by phagocytosis of particulate stimuli such as zymosan (Claesson et al., 1981; Lin et al., 1982; Clancy et al., 1983; Salari et al., 1985; Borgeat et al., 1988). LTB4 possesses a number of biological activities in vitro and in vivo; it is however generally accepted that the most important activity of this eicosanoid lies in its chemotactic and chemokinetic effects on phagocytes, and it is likely that LTB4 plays an important role in defence mechanisms as well as in inflammatory diseases (FordHutchinson, 1990). Besides the obvious role of platelets in haemostasis, evidence that cooperation between platelets and PMNL might contribute to promote local inflammation in several pathophysiological situations is accumulating (Weksler, 1988). Furthermore, platelets release arachidonic acid metabolites, PAF, platelet-derived growth factor (PDGF) and platelet factor 4 (PF4) which can affect neutrophil functions such as adhesion, chemotactic activity, secretion and production of superoxide anion (Weksler, 1988). Moreover, activated platelets have been shown to adhere to neutrophils (Jungi et 1 Author for correspondence.
al., 1986; Larsen et al., 1989); such platelet-PMNL interaction might be part of a mechanism leading to the elimination of activated platelets (Larsen et al., 1989), but could also favour the recruitment of platelets at inflammatory sites and enable and facilitate the occurrence of metabolic or functional interactions between the two cell types. More specifically concerning the metabolism of arachidonic acid, several cellular cooperations between platelets and PMNL have already been demonstrated. LTA4 released by PMNL is metabolized by platelets into LTC4 (Maclouf et al., 1989) and lipoxins (Serhan, 1990). It was also demonstrated that PMNL stimulated with the ionophore A23187 in the presence of platelets transform platelet-derived 12S-hydroxy-5, 8,10,14-(Z,Z,E,Z)-eicosatetraenoic acid (12-HETE) into 5S,12S-dihydroxy-6,8,10,14(E,Z,E,Z)-eicosatetraenoic acid (5S, 12S-DiHETE) (Borgeat et al., 1982; Marcus et al., 1982), and arachidonic acid into LTB4 (Marcus et al., 1982) via the 5-lipoxygenase pathway. In addition, 12-HETE synthesized in platelets is metabolized to 12S,20-dihydroxy-5,8,10,14(Z,Z,E,Z)eicosatetraenoic acid (12,20-DiHETE) by unstimulated neutrophils (Marcus et al., 1984). In previous studies we have also shown that incubation of PMNL with high concentration of exogenous arachidonic acid in the presence of platelelets results in increased synthesis of 5-lipoxygenase products. The amplification of 5-lipoxygenase product synthesis in this coincubation system was attributed to the synthesis of 12S-hydroperoxy-5,8,10,14(Z,Z,E,Z)-eicosatetraenoic acid (12HpETE) by the platelet 12-lipoxygenase (Maclouf et al., 1982). Amplification of LTB4 synthesis in zymosan-stimulated PMNL was also observed when thrombin-stimulated platelets were present (Del Maschio et al., 1985). These studies clearly indicate that platelets can modulate the synthesis of LTB4 in
R. PALMANTIER & P. BORGEAT
PMNL and possibly contribute to the development of inflammatory reactions. Further investigations were undertaken to assess the hypothesis that platelet-PMNL interactions leading to increased formation of LTB4 could also occur under conditions of cell activation by physiological agonists. In the present study, we show that thrombin-activated platelets strongly increase LTB4 synthesis by PMNL stimulated with FMLP, C5a or PAF. We have also investigated the mechanism of this effect of platelets and established that arachidonic acid released by platelets is determinant to LTB4 synthesis by PMNL stimulated with these physiological agonists.
Materials 5,8,11,14(all cis)-eicosatetraenoic acid (arachidonic acid), fatty acid-free human serum albumin (HSA), FMLP, 1-0hexadecyl-2-acetyl-sn-3-glycerophosphocholine (PAF), prostaglandin B2 (PGB2), soybean lipoxidase (type I) and NaBH4 were obtained from Sigma Chemical Company (Saint Louis, MO, U.S.A.). [14C]-arachidonic acid (1.0 Ci mmol- 1) was purchased from New England Nuclear (Boston, MA, U.S.A.). Thrombin was from Parke-Davis (Scarborough, Ontario, Canada). Hanks' balanced salt solution (HBSS) and N-2hydroxyethylpiperazine-N'-2-ethanesulphonic acid (HEPES) were purchased from GIBCO (Burlington, Ontario, Canada). Ficoll-paque was purchased from Pharmacia (Dorval, Quebec, Canada) and solvents were high performance liquid chromatography (h.p.l.c.) grade from Anachemia (Montreal, Quebec, Canada). Biosynthetic recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) was a generous gift from the Genetics Institute (Cambridge, MA, U.S.A.). GM-CSF was diluted in HBSS containing 2.5% foetal calf serum under sterile conditions to a stock concentration of 100 nm which was endotoxin-free as determined by the limulus amoebocyte assay, and stored at -20'C. Recombinant human C5a was a generous gift from Dr Henry Showell (Pfizer Pharmaceuticals, Groton, CT, U.S.A.); it was kept in HBSS at a stock concentration of 30pM and stored at -200C. Arachidonic acid was purified by silicic acid chromatography before use. Arachidonic acid and PAF were kept in hexane and chloroform, respectively, and aliquots of stock solutions were evaporated and redissolved in dimethylsulphoxide (DMSO) for addition to cell suspensions; the stock solution of FMLP was prepared in DMSO. Cell preparation procedures Venous blood was obtained from healthy donors and collected on citric acid-dextrose anticoagulant. Blood was centrifuged at 200g for 15min at 20°C. The platelet-rich plasma (PRP) was removed and platelets were isolated as described previously (Lagarde et al., 1980). Briefly, the PRP was acidified to pH 6.4 with 0.15 M citric acid and centrifuged at 900g for 10min at 20°C in siliconized tubes. Supernatants were removed, and tube walls and platelet pellet surfaces were gently rinsed with Ca2+- and Mg2"-free HBSS containing 10mM HEPES and buffered to pH 7.4 (HBSS-HEPES). The cells were finally resuspended in the same buffer at 4 x 108
platelets per ml. PMNL were prepared by sequential dextran sedimentation and centrifugation on Ficoll-paque cushions (Boyum, 1968). Contaminating erythrocytes were eliminated by hypotonic lysis. PMNL were then resuspended in Ca2+- and Mg2+-free HBSS-HEPES buffer at the cell concentration of 16 x 106 per ml, and the viability was evaluated by the trypan blue exclusion test. Cell viability was always greater than 95%.
Platelet-polymorphonuclear leucocyte coincubations The mixing of equal volumes of the platelet and PMNL preparations resulted in a cell suspension containing 2 x 108 platelets per ml and 8 x 106 PMNL per ml; CaCl2 and MgCl2 were added at final concentrations of 2 mm and 0.5 mm, respectively, and the cells were pre-incubated 5 min at 37°C. The cells were then incubated 10min at 37°C in presence of various stimuli. In experiments where platelets and PMNL were incubated separately, the platelet and PMNL suspensions were diluted with HBSS-HEPES to obtain final cell concentrations as in the coincubation system. In experiments where GM-CSF was used, PMNL were preincubated with the cytokine at the concentration of 200pM for 60min at 37°C in HBSS-HEPES containing 2mm CaCl2 and 0.5 mm MgCl2 at the cell concentration of 16 x 106 PMNL per ml; coincubation of GM-CSF-primed PMNL with platelets and stimulation were performed as described above. The final concentration of DMSO did not exceed 0.2%. Incubations were stopped by addition of 0.5 ml of ice cold acetonitrile/methanol (1/1, v/v) containing 12.5 ng each of PGB2 and 19-hydroxy-PGB2 as internal standards, and the samples were stored at -20°C. Some coincubation experiments were carried out in the presence of HSA; these incubations were stopped as described above, however the samples were further treated by addition of 3 volumes of acetonitrile (per volume of cell suspension) to achieve complete protein precipitation. The samples were then stored at -20°C until analysed.
Activated platelet supernatants CaCl2 and MgCl2 were added to platelet suspensions (4 x 10' cells per ml in HBSS-HEPES) to achieve final concentrations of 2mm and 0.5mm respectively. The cell suspensions were preincubated 5min at 37°C and stimulated with 2.5umlP1 thrombin for Smin at 37°C. The incubation media were then centrifuged at 2000g for 10min at 4°C and the supernatants were collected and used immediately for incubation with PMNL suspensions, or were denatured by addition of 2 volumes of acetonitrile and stored at -20°C until analysis. In some experiments activated platelet supernatants were prepared in the presence of 7pM 5,8,11,14-eicosatetraynoic acid (ETYA) and 0.1yfM tiaprofenic acid, these two inhibitors being added during the preincubation period, i.e. 5min prior to thrombin activation.
Analysis of 5-lipoxygenase products The denatured samples were centrifuged at 2000g for 20min to remove the precipitate material and the supernatants were analyzed without further treatment by reverse phase (RP) h.p.l.c. as described previously (Borgeat & Picard, 1988) with a minor modification of the mobile phase
(methanol/acetonitrile/water, 23/23/54, v/v/v, was substituted for acetonitrile/water, 30/70, v/v) to improve the separation of the 20-hydroxy and 20-carboxy derivatives of 5S,12S-
DiHETE and LTB4. The denatured samples initially containing HSA were evaporated down to a volume of 2 ml at 40°C under a stream of nitrogen prior to RP-h.p.l.c. analysis. Samples were injected onto a Resolve Cl8 Radial Pak cartridge (5 x 100mm, Spim particles) protected by Guard-Pak cartridges (silica and Resolve C18, 5 pm particles) from Waters Millipore. Elution was performed at 1.5mlmin-'. The lipoxygenase products were detected with fixed wavelength u.v. photometers at 229 and 280 nm. Products were identified on the basis of their comigration with synthetic standards and specificity of absorption at either 229 nm or 280 nm. Product quantification was done by comparison of peak heights to calibrated standards of 20-hydroxy-LTB4, LTB4 and 15S-hydroxy-5,8,1 1,13(Z,Z,Z,E)-
ACTIVATED PLATELETS PROMOTE LTB4 SYNTHESIS IN PMNL
eicosatetraenoic acid (15-HETE), after correction for recovery of 19-hydroxy-PGB2 and PGB2.
Quantification of arachidonic acid Arachidonic acid was quantified in supernatants of thrombin-activated platelets by enzymatic and h.p.l.c. procedures. Briefly, the denatured platelet supernatants were centrifuged (2000 g for 10min) to remove the precipitated material and [14C]-arachidonic acid (1 nCi per sample) was added to the supernatants. Arachidonic acid was purified by the RP-h.p.l.c. system described previously (Borgeat & Picard, 1988), using acetic acid instead of phosphoric acid in the mobile phases. The fractions containing arachidonic acid were collected and evaporated at 40'C under a stream of nitrogen. The residues were first dissolved in 50pl of methanol; then 0.375 ml of 5 mm sodium borate buffer (pH 10) and 100ng of 8,11,14(all cis)-eicosatrienoic acid were added as internal standard to each sample. One thousand units of soybean lipoxidase type I were then added and the reaction mixtures were incubated for 30min at 20'C to allow the conversion of arachidonic acid and the internal standard into the corresponding 15-hydroperoxide derivatives which were subsequently reduced to 15-HETE and 15S-hydroxy-8,11, 13(Z,Z,E)-eicosatrienoic acid (15-HETrE) by addition of 600 ug of NaBH4 in 1 ml of methanol. After 30 min at 20°C, the reaction was stopped by acidifying the reaction mixtures to pH 5 with acetic acid, and 15-HETE and 15-HETrE were analysed by RP-h.p.l.c. with u.v. detection at 229 nm. Arachidonic acid quantification was performed by comparing peak heights of 15-HETE and 15-HETrE after correction for loss of [14C]-arachidonic acid in the course of the h.p.l.c. purification step. Recovery of arachidonic acid varied between 50 and 70%.
CNJ .4 0
Statistical analysis Statistical significance was evaluated by Student's paired data, unless otherwise indicated.
Effect of platelets on leukotriene B4 synthesis by polymorphonuclear leucocytes Figure 1 shows the profiles of lipoxygenase products generated by PMNL suspensions, platelet suspensions and plateletPMNL mixtures stimulated with 2.5 u ml' thrombin and 0.1 lM FMLP. The incubation media of PMNL (alone) contained only traces of the metabolite of LTB4, 20-carboxyLTB4 (Figure la). Platelets (Figure lb) responded to stimulation by producing the 12-lipoxygenase product 12-HETE and the cyclo-oxygenase product HHTrE, while 5lipoxygenase products were not detectable. The absorption peak (280 nm) observed at 16.1-16.2 min elution time on Figure lb was tentatively identified as a 14,15-dihydroxy-5,8, 10,12-eicosatetraenoic acid (14,15-DiHETE), a product of the 15-lipoxygenase (Maas & Brash, 1983; Wong et al., 1985) on the basis of its u.v. spectrum and comparison with the major 14,15-DiHETE isomer generated by platelets incubated with 15-HapETE (data not shown). Other small absorption peaks at 280nm, probably isomeric 14,15-DiHETE and 8,15-DiHETE (Maas & Brash, 1983; Wong et al., 1985), were also present in small amounts. FMLP alone did not stimulate the formation of detectable amounts of arachidonic acid metabolites in platelets, such as thrombin in PMNL (data not shown). However, when platelets and PMNL were coincubated and stimulated with thrombin and FMLP (Figure lc), the formation of 5-lipoxygenase products by the PMNL was strikingly enhanced leading to significant accumulation of 20-hydroxyand 20-carboxy-LTB4. The chromatogram also showed the presence of the platelet products 12-HETE, HHTrE as well as
Elution time (min) Figure 1 Reverse phase (RP) high performance liquid chromatography (h.p.l.c.) chromatograms of the arachidonic acid metabolites generated by polymorphonuclear leucocytes (PMNL) (a), platelets (b) and a platelet-PMNL mixture (c) incubated 10min at 37°C with 2.5uml-' thrombin and 0.1 fM FMLP. The cells were incubated in 1 ml HBSS with 8 x 106 and 200 x 106 per ml for PMNL and platelets respectively. The amount of internal standards added was 12.5 ng each of prostaglandin B2 (PGB2) and 19-hydroxy'PGB2. Attenuation settings of the u.v. photometers were 0.01 and 0.025 absorbance units at full scale at 280 and 229 nm, respectively. For abbreviations, see text.
the products of the platelet-PMNL metabolic cooperation, 12,20-DiHETE, 5S,12S-DiHETE which comigrated with the 14,15-DiHETE, and 5S,12S,20-TriHETE. The presence of contaminating material prevented the possibility of detecting LTC4 synthesis in these experiments. Figure 2a shows the amount of LTB4 synthesized either in PMNL alone which were stimulated with 2.5 u mlP thrombin '
R. PALMANTIER & P. BORGEAT a
z 140 *
, 30 z
I, E 25
20 > x
Tl'°-E 20'T ,a_
Thrombin (u m1-')
CL a) Co
Thrombin + FMLP
2 120 a) 0 100 Cc x60 80
Thrombir + PAF
Figure 2 Synthesis of leukotriene B4 (LTB4) by human polymorphonuclear leucocytes (PMNL) (open columns) or platelet-PMNL mixtures (solid columns) incubated with 0.1 AM FMLP or FMLP and 2.5 u ml-1 thrombin (a) or 0.1 M PAF or PAF and thrombin (b), for 10 min at 37C in HBSS. In the cell mixture, the platelet: PMNL ratio was 25:1. LTB4 was measured by RP-h.p.l.c. and amounts indicated represent the sum of LTB4, 20-hydroxy-LTB4 and 20-carboxy-LTB4. Results are the mean (s.e.mean shown by vertical bars) of 9 experiments (thrombin + FMLP), 3 experiments (FMLP) and 4 experiments (PAF + thrombin). Incubations were carried out in triplicate in each experiment. ** P < 0.001; * P < 0.05 for the comparisons indicated.
and/or 0.1 ,M FMLP, or in PMNL and platelet mixtures stimulated with either FMLP or both stimuli. Amounts of LTB4 produced were significantly higher in platelet-PMNL mixtures stimulated with thrombin and FMLP in comparison to PMNL alone stimulated under the same conditions. In nine different experiments, the average increase of LTB4 (and metabolites) synthesis caused by the coincubation of platelets was 6 fold. The same increment (5 to 6 fold) was observed between platelet-PMNL mixtures stimulated with either FMLP alone or thrombin and FMLP. Furthermore, there was no significant difference in 5-lipoxygenase product synthesis between PMNL alone and platelet-PMNL mixtures stimulated with FMLP only. Leukotriene synthesis was not detectable in unstimulated PMNL or in platelet-PMNL mixtures incubated with thrombin only (data not shown). In these studies the formation of LTB4 was assessed from the cumulative amounts of LTB4 itself (often undetectable) and of its metabolites, 20-hydroxy- and 20-carboxy-LTB4, as LTB4 is rapidly metabolized by co-oxidation in PMNL suspensions (Powell, 1984) (Figures 1-3). Figure 2b shows the results of similar experiments where PAF was used as the PMNL agonist instead of FMLP. The addition of platelets to PMNL incubated in presence of thrombin and PAF also caused a significant increase of LTB4 synthesis by the PMNL, and the addition of thrombin to platelet-PMNL mixtures incubated in presence of PAF had a similar stimulatory effect. PMNL were treated with 200 M GM-CSF for 60min at 37'C and the effects of FMLP and thrombin, separately or in combination, were investigated on the synthesis of LTB4 by platelet-PMNL mixtures. Figure 3 clearly shows that stimulation of the cell mixture with both stimuli induced an 8 fold increase in LTB4 synthesis in comparison to the effect of FMLP alone. Using thrombin alone as a stimulus, leukotriene synthesis was very low. In the same experiments, the addition of platelets to PMNL stimulated with both agonists caused a similar enhancement of LTB4 synthesis. It is noteworthy that LTB4 synthesis by platelet and GM-CSF-primed PMNL mixtures stimulated with thrombin and FMLP was increased by 5 to 6 fold in comparison to LTB4 synthesis observed under the same experimental conditions but using PMNL not previously exposed to GM-CSF (Figures 2 and 3). Comparison of the data presented in Figures 2a and 3 indicated significant differences (P < 0.01, established by Student's t test for unpaired samples) in LTB4 synthesis measured in PMNL or
m a, H
Thrombin + FMLP
Figure 3 Synthesis of leukotriene B4 (LTB4) by GM-CSF-treated polymorphonuclear leucocytes (PMNL) (open column) or plateletPMNL (GM-CSF-treated) mixtures (solid columns) incubated with 0.1AuM FMLP or 2.5uml-' thrombin or both stimuli for 10min at 370C in HBSS buffer. PMNL were preincubated 60min at 37TC in presence of 200 pM GM-CSF. In the cell mixtures the platelet: PMNL ratio was 25:1. LTB4 was measured by RP-h.p.l.c. and the amounts indicated represent the sum of LTB4 and its co-oxidation products. Results are the mean (s.e.mean shown by vertical bars) of 6 experiments (thrombin + FMLP) or 9 experiments (FMLP); in each experiment, incubations were performed in triplicate. The inset shows the synthesis of LTB4 by platelet-PMNL (GM-CSF-treated) mixtures incubated with 0.1 pm FMLP and increasing concentrations of thrombin. The results are from one experiment representative of 2 (incubations were done in triplicate and the values are the mean with s.d. shown by vertical bars). * P < 0.01 for comparison with the three other experimental conditions.
in platelet and PMNL mixtures treated or not with GM-CSF prior to stimulation with thrombin and FMLP. The Figure 3 inset shows the effect of increasing concentrations of thrombin on LTB4 synthesis in platelet and GM-CSF-primed PMNL mixtures stimulated with FMLP. Activation of platelets with increasing concentrations of thrombin lead to a concentration-dependent enhancement of LTB4 synthesis by the PMNL. The effect of thrombin was detectable at 0.1 u ml - l and nearly maximal at 1 u ml -l.
Stimulation of leukotriene B4 synthesis in polymorphonuclear leucocytes by activated platelet supernatants Additional experiments were then undertaken to elucidate the mechanism by which activated platelets stimulated LTB4 synthesis in PMNL. The first series of experiments were aimed at determining whether cell-cell contact was necessary, or alternatively, if active products were released by platelets. Supernatants of thrombin-activated platelet suspensions were obtained as described in Methods. Table 1 shows that the addition of activated platelet supernatants to PMNL stimulated with FMLP or the complement fragment C5a caused significant increase (5 fold or more) of LTB4 synthesis by the PMNL, indicating that substances released by platelets upon activation by thrombin account for the potentiation of LTB4 synthesis by the PMNL.
Effect of 12-lipoxygenase and cyclo-oxygenase inhibitors on the stimulatory action of platelets To test the possibility that products of platelet arachidonic acid metabolism might be involved in the stimulatory effect of activated platelet supernatants, suqh supernatants were generated in the presence or absence of ETYA and tiaprofenic acid.
ACTIVATED PLATELETS PROMOTE LTB4 SYNTHESIS IN PMNL Table 1 Leukotriene B4 (LTB4) synthesis by polymorphonuclear leucocytes (PMNL) in presence or absence of platelet supernatants Stimulus FMLP C5a
LTB4 synthesis (pmol per 8 x 106 PMNL) PMNL PMNL + supernatants 5.1 + 1.3 ND
26.3 ± 4.9* 21.2 + 10.2
PMNL were incubated for 10 min at 37'C in the presence or absence of thrombin-activated platelet supernatants. Values are mean + s.e.mean of 9 and 4 experiments for FMLP- and C5a-stimulated PMNL, respectively. In each experiment, incubations were carried out in triplicate. Values for LTB4 synthesis represent the sum of LTB4 and its .-oxidation products. * P < 0.001 with respect to corresponding value with PMNL. ND, not detectable.
Table 2 shows that the presence of the inhibitors during activation of platelets with thrombin, did not reduce the stimulatory activity of the platelet supernatants on LTB4 synthesis by the FMLP-stimulated PMNL, but rather, strikingly enhanced this effect. These data clearly indicated that platelet arachidonic acid products were not involved in the stimulatory effect of platelets and suggested that free arachidonic acid, which presumably was increased as the consequence of 12-lipoxygenase and cyclo-oxygenase inhibition by ETYA and tiaprofenic acid, could be involved. Table 3 shows the results of the analysis of 12-HETE, HHTrE and arachidonic acid content of thrombin-activated platelet supernatants; whereas 12-HETE and HHTrE synthesis was almost completely abrogated by the two inhibitors, the amount of arachidonic acid was increased by an average of 17 fold in six experiments, supporting the hypothesis that arachidonic acid released by platelets might account for the stimulatory effect of platelets or platelet supernatants on leukotriene synthesis by PMNL.
Effect offatty acid-free human serum albumin on the stimulatory action of platelets In the concentration range of 1 to 5 mg mlP, HSA dosedependently inhibited LTB4 synthesis in platelet-PMNL coincubation in presence of FMLP and thrombin (Figure 4). In similar experimental conditions, HSA did not inhibit FMLPinduced LTB4 synthesis in PMNL alone (data not shown). These data supported the idea that the potentiation of LTB4 synthesis could be due to the release of lipophilic substances
by activated platelets.
Effect of exogenous arachidonic acid on leukotriene B4 synthesis in FMLP-stimulated polymorphonuclear leucocytes Addition of increasing amounts of arachidonic acid to PMNL activated with FMLP led a dose-dependent increase in LTB4 formation (Figure 5). The effect of arachidonic acid was significant (P < 0.05) at 0.1 ,uM, and at a concentration of 0.2,uM, corresponding to the average concentration measured in activated platelet supernatants generated in absence of ETYA and 25
20 co 0D X
Table 2 Effect of inhibitors of platelet arachidonic acid metabolism on leukotriene B4 (LTB4) synthesis by activated polymorphonuclear leucocytes (PMNL) Inhibitors
LTB4 synthesis (pmol per 8 x 106 PMNL) PMNL PMNL + supernatants 3.8 + 1.9 5.1 + 2.9
22.5 ± 5.6* 153 ± 27**t
PMNL were stimulated for 10min at 37°C in the absence or presence of thrombin-activated platelet supernatants generated in the absence or presence of 0.1 1uM tiaprofenic acid and 7pM ETYA. Values are means + s.e.mean of 3 experiments. In each experiment, incubations were carried out in triplicate. Values for LTB4 synthesis represent the sum of LTB4 and its a-oxidation products. * P < 0.05, ** P < 0.01 with respect to PMNL tP < 0.01 to with values; respect PMNL + supernatants without inhibitors.
HSA (mg ml-1) Figure 4 Effect of increasing concentration of fatty acid-free HSA in the incubation media on leukotriene B4 (LTB4) synthesis by platelet polymorphonuclear leucocyte (PMNL) mixtures h incubated with 0.1 fM FMLP and 2.5 u ml- thrombin for 10min at 370C. The figure shows the results of one experiment representative of 3 (incubations were performed in duplicate). :J
x 00I. 0.
Table 3 Arachidonic acid (AA), 12-HETE and HHTrE in thrombin-activated platelet supernatants Products
pmol per 200 x 10o platelets Thrombin Thrombin + inhibitors
AA 12-HETE HHTrE
200 + 25 1580 + 400 1780 + 160
3500 ± 790