Characterization of platelet-activating factor binding to human airway epithelial cells: modulation by fatty acids and ion-channel blockers

Platelet-activating factor-induced chloride channel activation is associated with intracellular acidosis and apoptosis of intestinal epithelial cells

Platelet-activating factor (PAF) is a phospholipid inter- and intracellular mediator implicated in intestinal injury primarily via induction of an inflammatory cascade. We find that PAF also has direct pathological effects on intestinal epithelial cells (IEC). PAF induces Cl(-) channel activation, which is associated with intracellular acidosis and apoptosis. Using the rat small IEC line IEC-6, electrophysiological experiments demonstrated that PAF induces Cl(-) channel activation. This PAF-activated Cl(-) current was inhibited by Ca(2+) chelation and a calcium calmodulin kinase II inhibitor, suggesting PAF activation of a Ca(2+)-activated Cl(-) channel. To determine the pathological consequences of Cl(-) channel activation, microfluorimetry experiments were performed, which revealed PAF-induced intracellular acidosis, which is also inhibited by the Cl(-) channel inhibitor 4,4'diisothiocyanostilbene-2,2'disulfonic acid and Ca(2+) chelation. PAF-induced intracellular acidosis is associated with caspase 3 activation and DNA fragmentation. PAF-induced caspase activation was abolished in cells transfected with a pH compensatory Na/H exchanger construct to enhance H(+) extruding ability and prevent intracellular acidosis. As ClC-3 is a known intestinal Cl(-) channel dependent on both Ca(2+) and calcium calmodulin kinase II phosphorylation, we generated ClC-3 knockdown cells using short hairpin RNA. PAF induced Cl(-) current; acidosis and apoptosis were all significantly decreased in ClC-3 knockdown cells. Our data suggest a novel mechanism of PAF-induced injury by which PAF induces intracellular acidosis via activation of the Ca(2+)-dependent Cl(-) channel ClC-3, resulting in apoptosis of IEC.

Expression and localization of platelet-activating factor receptor in human nasal mucosa

BACKGROUND

Platelet-activating factor (PAF) has been thought to be a potent mediator of allergic rhinitis because PAF was recovered from the nasal lavage fluid of patients with allergic rhinitis after allergen provocation. Furthermore, PAF receptor antagonist attenuates the antigen-induced increase in nasal airway resistance and nasal vascular permeability in sensitized guinea pigs.

OBJECTIVE

To clarify the expression of PAF receptor in human nasal mucosa by investigating PAF receptor messenger RNA (mRNA) expression and its protein localization using polymerase chain reaction (PCR) and immunohistochemical analyses, respectively.

METHODS

Human turbinates were obtained after turbinectomy from 6 patients with nasal obstruction refractory to medical therapy. Total RNA was isolated from human nasal mucosa, and PAF receptor mRNA was detected in these tissues by using reverse transcriptase-PCR analysis. To identify the cells expressing PAF receptor protein, double immunostaining was performed using anti-PAF receptor antibody and monoclonal antileukocyte antibodies.

RESULTS

Reverse transcriptase-PCR analysis of total nasal RNA demonstrated the expression of PAF receptor mRNA. The immunohistochemical studies revealed that anti-PAF receptor antibody-labeled eosinophils, macrophages, neutrophils, mast cells, lymphocytes, vascular endothelial cells, epithelial cells, and submucosal glands in nasal mucosa.

CONCLUSIONS

These results may have important clinical implications for understanding the role of PAF receptor on upper airway diseases such as allergic and nonallergic rhinitis.

Platelet-activating factor regulates chloride transport in colonic epithelial cell monolayers

Platelet-activating factor (PAF) has been implicated in the pathogenesis of gastrointestinal diseases such as necrotizing enterocolitis, Crohn's disease, and ulcerative colitis. However, neither the physiologic role of PAF in the intestine, nor the mechanisms by which PAF participates in the pathogenesis of disease are well understood. The aim of the present study was to determine the direct effect of PAF on intestinal epithelial cell ion transport, and to delineate the mechanisms of regulation. Ion transport was evaluated by measuring short circuit current (I(sc)) in HT29-CL19A cell monolayers using Ussing chambers. PAF receptor polarity was assessed using domain-selective biotinylation followed by immunoprecipitation and streptavidin blotting of intact epithelial monolayers. PAF (1-200 microM) stimulated I(sc) that followed the direction of a Cl(-) gradient and was specifically inhibited by the Cl(-) channel blockers glybenclamide, 2,2' iminodibenzoic acid and 4,4' diisothiocyanostilbene-2, 2' disulfonic acid, but was unaffected by the inhibition of prostaglandin synthesis with indomethacin. Stimulated I(sc) was only detected after apical addition of PAF, correlating with the results of biotinylation experiments indicating an exclusive apical polarity of the PAF receptor. PAF receptor antagonists CV6209 and octylonium bromide abolished PAF-stimulated I(sc). Thus, mucosal acting PAF directly and specifically stimulates ion transport via activation of an apical Cl(-) channel in intestinal epithelial cell monolayers independent of prostaglandin biosynthesis.

PAF mediates cigarette smoke-induced goblet cell metaplasia in guinea pig airways

Goblet cell metaplasia is an important morphological feature in the airways of patients with chronic airway diseases; however, the precise mechanisms that cause this feature are unknown. We investigated the role of endogenous platelet-activating factor (PAF) in airway goblet cell metaplasia induced by cigarette smoke in vivo. Guinea pigs were exposed repeatedly to cigarette smoke for 14 consecutive days. The number of goblet cells in each trachea was determined with Alcian blue-periodic acid-Schiff staining. Differential cell counts and PAF levels in bronchoalveolar lavage fluid were also evaluated. Cigarette smoke exposure significantly increased the number of goblet cells. Eosinophils, neutrophils, and PAF levels in bronchoalveolar lavage fluid were also significantly increased after cigarette smoke. Treatment with a specific PAF receptor antagonist, E-6123, significantly attenuated the increases in the number of airway goblet cells, eosinophils, and neutrophils observed after cigarette smoke exposure. These results suggest that endogenous PAF may play a key role in goblet cell metaplasia induced by cigarette smoke and that potential roles exist for inhibitors of PAF receptor in the treatment of hypersecretory airway diseases.

Platelet-activating factor receptor mRNA is localized in eosinophils and epithelial cells in rat small intestine: regulation by dexamethasone and gut flora

Platelet-activating factor (PAF) is a potent mediator involved in bowel injury. We investigated PAF receptor transcription and its mRNA localization in the small intestine of normal (conventionally fed) and germ-free rats, by competitive polymerase chain reaction (PCR) and in situ hybridization. A dose of PAF (1.5 microg/kg, i.v.) insufficient to cause gross bowel injury was injected into rats. Some rats were pretreated with dexamethasone (1 mg/kg). We found: (1) PAF receptor (PAF-R) mRNA localized predominantly in lamina propria eosinophils and in epithelial cells; (2) PAF increased PAF-receptor signals in the epithelial cells; (3) Dexamethasone depleted eosinophils in the intestine and markedly decreased PAF-receptor transcripts; the response to PAF was also weaker than control rats; (4) Germ-free rats had less PAF-R mRNA than normal rats, and showed a weaker response to PAF than conventionally fed rats. Thus, we conclude: (1) PAF receptor mRNA is constitutively expressed in the epithelium and in lamina propria eosinophils in the intestine. (2) PAF-R transcription is up-regulated by PAF and gut flora, mostly in the epithelium. (3) PAF-R transcription is down-regulated by glucocorticoids, mainly as a result of eosinophil depletion. These results suggest a functional role for PAF receptors both in host defence and the inflammatory response in the small intestine.

Anticonvulsant effect of polyunsaturated fatty acids in rats, using the cortical stimulation model

Recent studies have shown that long-chain polyunsaturated fatty acids can prevent cardiac arrhythmias, attributed to the reduction in excitability of cardiomyocytes, owing mainly to a shift in hyperpolarizing direction of the inactivation curves of both Na+ and Ca2+ currents and to a slowed recovery from inactivation. Qualitatively similar effects of polyunsaturated fatty acids on inactivation parameters have been observed in freshly isolated hippocampal neurons. Since the same effects are presumed to underlie the action of some established anticonvulsant drugs, polyunsaturated fatty acids might have an anticonvulsant action as well. We have investigated this for eicosapentaenoic acid, docosahexaenoic acid, linoleic acid and oleic acid, employing cortical stimulation in rats, a seizure model allowing the determination of the full anticonvulsant effect-time profile in freely moving, individual animals. I.v. infusion of 40 micromol of eicosapentaenoic acid or docosahexaenoic acid over a period of 30 min, modestly increased the threshold for localized seizure activity after 6 h by 73 +/- 13 microA (mean +/- S.E.M.; n = 7) and 77 +/- 17 microA (n = 7), respectively, and the threshold for generalized seizure activity by 125 +/- 20 and 130 +/- 19 microA, respectively (P < 0.001). The thresholds remained elevated for 6 h after infusion, but returned to baseline the next day. Free plasma concentrations in rats treated with eicosapentaenoic acid or docosahexaenoic acid, averaged 5.7 +/- 1.6 microM (n = 4) for eicosapentaenoic acid and 12.9 +/- 1.8 microM (n = 5) for docosahexaenoic acid at the end of infusion, but declined to undetectable levels within 3 h. Linoleic acid and oleic acid were less effective. Possible mechanisms for the modest anticonvulsant effect but of long duration with the polyunsaturated fatty acids are discussed.

Platelet-activating factor induction of activator protein-1 signaling in bronchial epithelial cells

Platelet-activating factor (PAF) has been implicated in the pathogenesis of allergic and inflammatory events in the airway. In the present study, we sought to determine if PAF receptors are present on human bronchial epithelial cells and whether PAF binding to these receptors leads to activation of activator protein-1 (AP-1)-mediated transcription. Radioligand binding studies demonstrated specific binding sites for the PAF antagonist [3H]WEB 2086 (3-[4-(2-chlorophenyl)-9-methyl-6H-thieno[3,2-f]-[1,2,4]triazolo[4,3- a][1,4]diazepine-2-yl]-1-(4-morpholinyl)-1-propanone) on primary bronchial epithelial cells with an equilibrium dissociation constant (Kd) = 9.8 nM and maximal density of binding sites (Bmax) = 42.4 fmol/mg of protein. The expression of PAF receptors in these cells was further confirmed by reverse transcriptase-polymerase chain reaction, which revealed amplification products derived from PAF receptor mRNA corresponding to transcripts 1 and 2. In the bronchial epithelial cell line BEAS-2B transfected with an expression plasmid for the human PAF receptor, PAF stimulation increased AP-1 DNA binding activity as determined by electrophoretic mobility shift assays. The Fos and Jun family proteins were identified as components of the DNA-protein complexes by anti-peptide antibodies in gel supershift assays. Additionally, PAF significantly induced AP-1 mediated transcription which was dependent on the expression of PAF receptors. The PAF antagonist WEB 2086 blocked the PAF effect but not that induced by 12-O-tetradecanoyl phorbol-13-acetate, indicating the specificity of the PAF response. These results indicate that activation of airway epithelial cells through stimulation of PAF receptors includes up-regulation of the nuclear transcription factor AP-1 and AP-1 transcriptional activity.

Essential fatty acid deficiency and predisposition to lung disease in cystic fibrosis

Essential fatty acid (EFA) deficiency is a predisposing factor for pulmonary infection with Staphylococcus aureus and Pseudomonas aeruginosa, the two major pathogenic microorganisms in cystic fibrosis (CF).

OBJECTIVE

The goal of this study was to investigate the essential fatty acid status of CF patients from infancy to 20 years old.

MATERIALS AND METHODS

Plasma fatty acid profiles for phospholipid (PL) were determined for cord (n = 6), 4 months (n = 40), 16 months (n = 25), 3 y (n = 8), 5-10 y (n = 10), and 10-20 y (n = 10) aged CF patients and compared to their respective control; cord (n = 22), 1-36 months (n = 38) and adult (n = 100). Significance was established by Student's t-test (p < 0.05).

RESULTS

The plasma PL fatty acid profile for all CF patients, except cord, revealed consistent deficiency in omega 3 and omega 6 EFAs. These deficiencies were most marked at infancy and more pronounced for patients with meconium ileus.

CONCLUSIONS AND RELEVANCE

EFA deficiency may contribute to the predisposition of CF infants to develop respiratory disease and to the excess cytotoxic activity found in bronchoalveolar lavage fluid at 2 months of age in the majority of screened infants.

Polyunsaturated fatty acids exert antiarrhythmic actions as free acids rather than in phospholipids

Previous studies have shown that exogenous free n-3 polyunsaturated fatty acids (PUFA) can prevent tachyarrhythmias caused by specific agents in isolated cardiac myocytes. However, the question as to whether incorporation of the n-3 PUFA into membrane phospholipids has the same immediate protective effects remained to be answered. To answer this question, we increased the content of n-3 PUFA in the phospholipids of cultured neonatal rat myocytes by growing them 2-3 d in a culture to which eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) in 15 microM concentration was added. Analysis of the fatty acid composition of membrane phospholipids revealed a significantly higher level of EPA and DHA (from 0.2 to 7.6% and from 1.2 to 6.5%) in cells supplemented with EPA or DHA, respectively. The responses of the myocytes grown in normal media or in media enriched with the PUFA to arrhythmogenic agents were examined after free fatty acids were removed from the medium and the cells. The arrhythmogenic agents used were the beta-adrenergic agonist isoproterenol or an elevated extracellular concentration of calcium. The results showed that there was no significant difference in the induction of tachyarrhythmias by isoproterenol or by elevated [Ca2+]o in cells grown in media enriched with PUFA, as compared with cells grown in normal media in the absence of the free PUFA. Under the conditions of this study, only the unesterified PUFA were able to protect the cardiomyocytes against induced arrhythmias. There was no antiarrhythmic effect due to an increased fraction of EPA or DHA in membrane phospholipids.