Platelet-activating factor increases leukotriene B4 release in stimulated alveolar macrophages from asthmatic patients

Abstract

This study was designed to examine further the role of platelet-activating factor (PAF) in asthma, comparing leukotriene B4 (LTB4) release, 5-lipoxygenase activity and intracellular calcium levels ([Ca2+]i) in macrophages. LTB4 and other lipoxygenase metabolites in macrophages in bronchoalveolar lavage fluids obtained from 23 asthmatic patients and 20 control subjects were measured by reverse-phase high-performance liquid chromatography. [Ca2+]i was monitored using the fluorescent probe fura-2. The basal LTB4 release of resting macrophages was not different between groups (0.02+/-0.01 versus 0.05+/-0.02 ng x 10(-6) cells). When stimulated with calcium ionophore A23187 (2.5 microM), however, macrophages from asthmatic patients released more LTB4 than cells from control subjects (30.2+/-3.4 versus 13.7+/-2.1 ng x 10(-6) cells). Although PAF alone did not alter LTB4 release, it enhanced the response to subsequent A23187 stimulation. This effect was noted following short treatment (i.e., 5 min) at concentrations of > or =1.0 microM PAF, with the maximal effect noted after treatment with 5.0 microM PAF + 2.5 microM A23187 (105.1+/-6.7 versus 15.3+/-2.6 ng x 10(-6) cells). Treatment of macrophages with PAF also increased 5-lipoxygenase activity and [Ca2+]i more in cytosols from asthmatic patients than in cytosols from control subjects. These findings support a role of intracellular calcium in the activation of 5-lipoxygenase which, in turn, augments the release of leukotriene B4. Because levels of platelet-activating factor may be increased in the lung during asthma and can increase the subsequent release of a chemotactic mediator leukotriene B4, from macrophages, these findings suggest that platelet-activating factor may prime the constitutive cells of the lung to augment inflammatory effects important in the pathogenesis of asthma.

Neutrophil airway influx by platelet-activating factor in asthma: role of adhesion molecules and LTB4expression

“…This hypothesis is further supported by a previous study showing that, after platelet-activating factor exposure, neutrophils and macrophages isolated from bronchoalveolar fluid in asthmatics released a greater amount of leukotriene B 4 than in healthy individuals [25,37]. In summary, both b 2 -integrins, macrophage-1 antigen and lymphocyte function-associated antigen-1, are involved in platelet-activating factor-induced neutrophil lung sequestration in patients with mild asthma, a process that may be modulated by leukotriene B 4 release within the airways.…”

Formoterol protects against platelet-activating factor-induced effects in asthma

“…This suggests that other mediators, such as LTs, may be involved. Moreover, LTs may be involved secondarily in the production of some of the systemic and pulmonary effects caused by PAF in asthmatic patients [5,6]. PAF can increase the subsequent release of chemotactic mediator LTB 4 [5], which is significantly increased in the sputum of patients with asthma challenged with PAF [21].…”

“…These gas exchange abnormalities include the development of areas with low ventilation-perfusion (V9A/Q9) units identical to those shown in patients with spontaneous acute asthma [2][3][4]. PAF potentiates its effects by generating secondary release of other inflammatory mediators, such as leukotrienes (LT)s, via the activation of phospholipase A 2 [5,6].…”

Leukotriene B4release by human lung macrophagesviareceptor- not voltage-operated Ca2+channels

“…There is little information regarding the regulation of LTB 4 production by macrophages in COPD; however, macrophages from asthmatic subjects release increased levels of LTB 4 following stimulation with calcium ionophore compared with cells from nonasthmatic subjects [12]. PAF also stimulates [Ca 2+ ] i in human alveolar macrophages [15] suggesting that control of Ca 2+ influx in these cells could regulate LTB 4 release and, thus, modulate neutrophilic inflammation.…”

“…The precise source of LTB 4 in the airways is unknown; however, it is considered the predominant neutrophil chemoattractant released by human alveolar macrophages [9] and can be released following stimulation with ionophore and arachidonic acid [10]. Human alveolar macrophages respond to platelet-activating factor (PAF) by releasing superoxide anions and LTB 4 [11,12]. Notably, alveolar macrophages from smokers are more sensitive to PAF-stimulation with respect to superoxide production compared with cells from nonsmokers [11].…”