Leukotriene C4 biosynthesis during polymorphonuclear leukocyte-vascular cell interactions

Angiogenesis in paediatric airway disease

A number of characteristic changes occur in the bronchial wall in paediatric airway diseases. The process of remodelling is usually associated with specific changes to the vasculature, resulting in an increase in vessel numbers, vasodilatation, vessel leakage and cellular margination with transmigration to target tissues. This combined action in conditions such as asthma, cystic fibrosis and bronchiolitis lead to airway wall thickening and reduced airflow. Each component of the vascular response has been shown to be controlled by a range of inflammatory mediators and growth factors. These factors are regulated by a complex process involving gene expression, transcription and translation at the molecular level, protein release, binding to matrix elements and receptors on endothelial cells, then the endothelial response itself. A number of commonly used airway medications are potentially capable of modulating the vascular response to inflammatory stimuli. New therapies may be able to improve airflow through better regulation of vessel growth, dilatation and leakage in the airway wall.

Regulation of leukotriene A4 hydrolase activity in endothelial cells by phosphorylation

Endothelial cells contain leukotriene (LT) A4 hydrolase (LTA-H) as detected by Northern and Western blotting, but several studies have been unable to detect the activity of this enzyme. Since LTA-H could play a key role in determining what biologically active lipids are generated by activated endothelium during the inflammatory process, we studied possible mechanisms by which this enzyme may be regulated. We find that LTA-H is phosphorylated under basal conditions in human endothelial cells and in this state does not exhibit epoxide hydrolase activity (i.e. conversion of LTA4 to LTB4). LTA-H purified from endothelial cells is efficiently dephosphorylated by incubation with protein phosphatase-1 in the presence of an LTA-H peptide substrate and not at all in the absence of substrate. Under conditions that lead to dephosphorylation, protein phosphatase-1 activates the epoxide hydrolase activity of LTA-H. Using peptide mapping and site-directed mutagenesis, we have identified serine 415 as the site of phosphorylation of LTA-H by a kinase found in endothelial cell cytosol. In parallel, we have studied a human lung carcinoma cell line that expresses active LTA-H. Although these cells have cytosolic kinases that phosphorylate recombinant LTA-H, they do not target serine 415 and thus do not inhibit LTA-H activity. We believe that LTA-H is regulated in intact cells by a kinase/phosphatase cycle and further that the kinase in endothelial cells specifically recognizes and phosphorylates a regulatory site in the LTA-H.

Inhibition of lipoxygenase and cyclooxygenase augments cardiac injury by H2O2

The role of arachidonic acid metabolites in the cardiac effects of toxic oxygen metabolites (TOM) was investigated in buffer-perfused rat hearts (Langendorff model). Hydrogen peroxide (H2O2, 200 microM) was given for 10 min to generate TOM, followed by 30 min recovery. H2O2 reduced left ventricular developed pressure (LVDP), increased left ventricular end-diastolic pressure (LVEDP), and increased coronary flow (CF). The hydroxyl radical scavenger thiourea inhibited the H2O2-induced effects. Perfusion with three lipoxygenase inhibitors, AA861, BWA4C, and diethylcarbamazine, in addition to H2O2, augmented the decrease of LVDP and the increase of LVEDP induced by H2O2. The cyclooxygenase inhibitor indomethacin had the same effects. The H2O2-induced increase in CF was not influenced by diethylcarbamazine, but inhibited by all other drugs. Control perfusion with drugs alone did not influence cardiac function. In conclusion, inhibition of lipoxygenase and cyclooxygenase augmented the depression of cardiac function induced by TOM. Leukotrienes and prostanoids appear to be protective against H2O2-induced cardiac injury.