Platelet activating factor and asthma

Effect of lipoxygenase inhibition on mucous glycoprotein secretion from chinchilla middle ear epithelial cells in vitro

Lipoxygenase is an enzyme that metabolizes arachidonic acid down to leukotrienes. Recent studies have shown that the enzyme is implicated in mucous glycoprotein (MGP) secretion stimulated by inflammatory mediators in the airways, suggesting its possible role in secretion of MGP from middle ear epithelial cells. To investigate a correlation between MGP secretion and the arachidonic acid metabolites, we examined the effects of nordihydroguaretic acid (NDGA, both a cyclooxygenase and lipoxygenase inhibitor), low-dose indomethacin (an inhibitor of cyclooxygenase), and A63162 (an inhibitor of lipoxygenase) on MGP secretion in cultured chinchilla middle ear epithelial cells. It was found that lipoxygenase inhibition led to reduction of MGP secretion from cultured chinchilla middle ear epithelial cells, while cyclooxygenase inhibition did not. Both cyclooxygenase and lipoxygenase inhibition resulted in profound blockage of MGP secretion in baseline and platelet activating factor-stimulated MGP secretion. It was concluded, therefore, that MGP secretion was linked to arachidonic acid metabolites, especially lipoxygenase products.

Interactions between respiratory epithelial cells and cytokines: relationships to lung inflammation

Epithelial cells lining respiratory airways can participate in inflammation in a number of ways. They can act as target cells, responding to exposure to a variety of inflammatory mediators and cytokines by altering one or several of their functions, such as mucin secretion, ion transport, or ciliary beating. Aberrations in any of these functions can affect local inflammatory responses and compromise pulmonary defense. For example, oxidant stress can increase secretion of mucin and depress ciliary beating efficiency, thereby affecting the ability of the mucociliary system to clear potentially pathogenic microbial agents. Recent studies have indicated that airway epithelial cells also can act as "effector" cells, synthesizing and releasing cytokines, lipid mediators, and reactive oxygen species in response to a number of pathologically relevant stimuli, thereby contributing to inflammation. Many of these epithelial-derived substances can act locally, affecting both neighboring cells and tissues, or, via autocrine or paracrine mechanisms, affect structure and function of the epithelial cells themselves. Studies in our laboratories utilized cell cultures of both human and guinea pig tracheobronchial and nasal epithelial cells, and isolated human nasal epithelial cells, to investigate activity of respiratory epithelial cells in vitro as sources of cytokines and inflammatory mediators. Primary cultures of guinea pig and human tracheobronchial and nasal epithelial cells synthesize and secrete low levels of IL-6 and IL-8 constitutively. Production and release of these cytokines increases substantially after exposure to specific inflammatory stimuli, such as TNF or IL-1, and after viral infection.

Platelet-activating factor provokes release of mucin-like glycoproteins from guinea pig respiratory epithelial cells via a lipoxygenase-dependent mechanism

Primary cultures of guinea pig tracheal epithelial cells maintained in an air/liquid interface system that maintains differentiated characteristics were grown to near confluence and exposed for 1 h to platelet-activating factor (PAF) on both apical and basal sides. PAF provoked release of high-molecular-weight mucin-like glycoproteins (MLG) from the cells, with maximal stimulation occurring at 10(-8) and 10(-9) M. The inactive form of PAF, lyso-PAF, was without effect. Indomethacin, the cyclooxygenase inhibitor, did not affect secretion stimulated by PAF, but nordihydroguiaretic acid (NDGA), a mixed cyclooxygenase and lipoxygenase inhibitor, attenuated secretion stimulated by PAF in a concentration-dependent manner. High performance liquid chromatography assay of the culture medium after addition of PAF revealed increased production of 15-, 12-, and 5-hydroxyeicosatetraenoic acids (15-, 12-, and 5-HETEs). The stimulatory effect of PAF on both mucin secretion and formation of HETEs was inhibited by the PAF receptor antagonists, CV-3988 and Ro 19 3704, with Ro 19 3704 acting at a concentration 10-fold lower than CV-3988 in inhibiting both effects. When added exogenously to the cell cultures, the combination of 5-, 12-, and 15-HETEs stimulated MLG release in a concentration-dependent manner. The results suggest that PAF stimulates release of MLG by guinea pig airway epithelium in vitro by a mechanism involving binding of PAF to receptors on epithelial cell surfaces, stimulation of lipoxygenase metabolism of arachidonic acid to HETEs within the epithelium, and stimulation of secretion by these epithelial-derived HETEs via an autocrine or paracrine mechanism.

Kinetic studies of human and rat neutrophil lysoPAF acetyltransferase using lysoPAF and dansyllysoPAF as substrates

Enzyme kinetic studies of lysoPAF acetyltransferase from microsomal preparations of human and rat neutrophils were carried out using lysoPAF or dansyllysoPAF as substrate. With the human enzyme, incomplete conversion of the substrate into the product was observed at 37 degrees C with both substrates. The acetyltransferase was inactivated at 37 degrees C in the absence of substrate with a half-life of 7.5 min. However, the initial rate of product formation under the assay conditions was linear up to 10 min. Both enzymes were optimally active at 40 microM concentration with either substrate, but enzyme activity was inhibited at higher substrate levels. At a constant substrate concentration (40 microM), the Km (microM) and Vmax (nmol product/min/mg protein) values for the human acetyltransferase, with respect to acetyl-CoA were 132 and 23.1, respectively, with lysoPAF as substrate, and 105 and 26.7, respectively, when dansyllysoPAF was used. The Km and Vmax values for the rat enzyme were 105 and 6.5, respectively, with lysoPAF as substrate, and 120 and 5.4, respectively, when dansyllysoPAF was used. Under our standard conditions, lysoPAF required 1 mg of BSA per mL in the assay, whereas full activity of both enzymes was seen with dansyllysoPAF even in the absence of BSA. The results show that dansyllysoPAF can replace lysoPAF in the assay without any significant changes in kinetic parameters.

Nutrition, immune function, and inflammation: an overview

The collective evidence suggests that nutritional insult to both cell-mediated and humoral immunity in the presence of protein-energy malnutrition contributes to abnormalities of inflammation. The primary goal of nutritional support in inflammatory disease is to provide adequate energy and protein to meet endogenous requirements for tissue repair, IL-1 production, and restored cellular function, thus preventing secondary infection. Substrate provision should aim at improving the acute phase of injury while avoiding immune dysfunction. This goal may be achieved by altering the eicosanoid pathway toward a more regulated inflammatory state. In the context of allograft response, macrophages are central to the initiation of allosensitization by virtue of their ability to present antigen to T-cells. Activated T-cells may further modulate macrophage function by the secretion of lymphokines. Manipulation of macrophage eicosanoid production by dietary omega-3 PUFA may reduce cellular immune response. (table; see text) Nutritional support should also focus on providing essential micronutrients, with their potentially immunomodulating role, as adjunctive therapy in order to protect the host from toxic effects of free-radicals and chemicals released during inflammatory events. (Feeding regimens currently under investigation and development are presented in Table 4.) By integrating dietary immunotherapy with the use of recombinant hormones, monoclonal antibodies, and various available monokines, an optimal outcome for each patient may be achieved. However, effective application of immunotherapy to nutritional supplementation will require accurate monitoring of immune function in individual patients in order to avoid inappropriate treatment.

Analysis of lyso-platelet activating factor by negative ion gas chromatography/mass spectrometry of the nitrobenzal acetal derivatives

This paper describes the methods that have been employed for the quantification of platelet activating factor (PAF) and its principal metabolite, in biological matrices. In plasma PAF is rapidly hydrolysed to lyso-PAF, which is also the major precursor of PAF. Measurement of lyso-PAF has been accomplished by mass spectrometric methods using cyclic acetal derivatives of the alkyl glycerol produced after removal of the polar head group. We describe the preparation of a novel electron capture derivative for this glycerol and its behaviour under electron impact and negative ion mass spectrometry. High sensitivities have been achieved using the procedures described here.

(+/-)-trans-2-(3-Methoxy-5-methylsulfonyl-4-propoxyphenyl)-5-(3,4,5- trimethoxyphenyl)tetrahydrofuran (L-659,989), a novel, potent PAF receptor antagonist

The title compound, L-659,989, is a highly potent, competitive, and selective antagonist of the binding of [3H]PAF to its receptors in platelet membranes from rabbits and humans. It exhibits equilibrium inhibition constants for PAF binding of 1.1 nM (rabbit) to 9.0 nM (human), values that are at least 1-2 orders of magnitude lower than those of other PAF antagonists tested. L-659,989 potently inhibits PAF-induced aggregation of rabbit platelets and degranulation of rat (ED50 4.5 nM) and human (ED50 10 nM) neutrophils. L-659,989 inhibits PAF-induced extravasation and lysosomal enzyme release in rats, and is active orally in female rats (ED50 0.2 mg/kg) with an extraordinary oral duration of action of 12 to 16 hours at 1.0 mg/kg p.o.

Platelet activating factor (PAF) is a potent stimulator of porcine tracheal fluid secretion in vitro

Mucus hypersecretion is a major clinical feature of chronic obstructive lung diseases such as asthma. The possible role of the inflammatory and bronchoconstrictor ether lipid PAF (platelet activating factor) has been studied in isolated porcine trachea with the tantalum 'hillock' technique used to visualize fluid production from tracheal submucosal glands. PAF caused a rapid, dose-dependent (0.001-1 nM) stimulation of fluid secretion which could be detected after 5 min and which increased with time up to at least 15 min. The PAF-induced fluid secretion was unaffected by both antagonists of histamine, acetylcholine and leukotriene D4 and inhibitors of prostaglandin and leukotrienes synthesis. A purported PAF receptor antagonist (CV 3988) inhibited the PAF responses in a dose-dependent manner implying a receptor-mediated event. These results may be of relevance to the mucus hypersecretion seen in chronic airway diseases.