Lipoxins are bioactive eicosanoids that are immunomodulators. In human myeloid cells, lipoxin (LX) A sub(4) actions are mediated by interaction with a G protein--coupled receptor. To explore ...functions of LXA sub(4) and aspirin-triggered 5(S),6(R),15(R)-trihydroxy-7,9,13-trans-11-cis-eicosatetraenoic acid (15-epi-LXA sub(4)) in vivo, we cloned and characterized a mouse LXA sub(4) receptor (LXA sub(4)R). When expressed in Chinese hamster ovary cells, the mouse LXA sub(4)R showed specific binding to super(3)HLXA sub(4) (K sub(d) approximately 1.5 nM), and with LXA sub(4) activated GTP hydrolysis. Mouse LXA sub(4)R mRNA was most abundant in neutrophils. In addition to LXA sub(4) and 15-epi-LXA sub(4), bioactive LX stable analogues competed with both super(3)HLXA sub(4) and super(3)Hleukotriene D sub(4) (LTD sub(4))-specific binding in vitro to neutrophils and endothelial cells, respectively. Topical application of LXA sub(4) analogues and novel aspirin-triggered 15-epi-LXA sub(4) stable analogues to mouse ears markedly inhibited neutrophil infiltration in vivo as assessed by both light microscopy and reduced myeloperoxidase activity in skin biopsies. The 15(R)-16-phenoxy-17,18,19,20-tetranor-LXA sub(4) methyl ester (15-epi-16-phenoxy-LXA sub(4)), an analogue of aspirin triggered 15-epi-LXA sub(4), and 15(S)-16-phenoxy-17,18,19,20-tetranor-LXA sub(4) methyl ester (16-phenoxy-LXA sub(4)) were each as potent as equimolar applications of the anti-inflammatory, dexamethasone. Thus, we identified murine LXA sub(4)R, which is highly expressed on murine neutrophils, and showed that both LXA sub(4) and 15-epi-LXA sub(4) stable analogues inhibit neutrophil infiltration in the mouse ear model of inflammation. These findings provide direct in vivo evidence for an anti-inflammatory action for both aspirin-triggered LXA sub(4) and LXA sub(4) stable analogues and their site of action in vivo.
The hydrolysis of leukotriene A4 and two epoxytetraenes was examined in the presence and absence of liposomes. When added to liposomes in suspension, the stability of LTA4 was increased in a time- ...and dose-dependent fashion. At 10 min, the half-life of LTA4 was increased 67.1 +/- 6.8% in the presence of liposomes which was comparable to that observed with albumin (10 mg/ml): 68.3 +/- 6.9%. Phosphatidylcholine, in a non-bilayer configuration, was also effective in enhancing the half-life of LTA4, albeit to a lesser extent than liposomes. At equal molar concentrations, the enhanced stability of eicosanoid epoxides with liposomes gave a rank order with leukotriene A4 greater than 5(6)epoxytetraene greater than 14(15)epoxytetraene. Results indicate that phospholipid bilayers can protect leukotriene A4 and 5(6)epoxytetraene from non-enzymatic hydrolysis. Moreover, they suggest that the biological half-life of intermediates involved in the formation of both leukotrienes and lipoxins can be increased by their association with membranes.
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