Thrombin stimulation of human coronary artery endothelial cells (HCAEC) results in activation of a membrane-associated,
calcium-independent
phospholipase A(2) (
iPLA(2)) that selectively hydrolyzes membrane
plasmalogen phospholipids.
Rupture of an
atherosclerotic plaque and occlusion of the coronary vasculature results in a coronary ischemic event in which HCAEC in the ischemic area would be exposed to dramatic decreases in
oxygen tension in addition to
thrombin exposure. We exposed HCAEC to
hypoxia in the presence or absence of
thrombin stimulation and measured
iPLA(2) activation, membrane
phospholipid hydrolysis, and the accumulation of biologically active
phospholipid metabolites. HCAEC exposed to
hypoxia,
thrombin stimulation, or a combination of the two conditions demonstrated an increase in
iPLA(2) activity and an increase in
arachidonic acid release from
plasmenylcholine.
Thrombin stimulation of normoxic HCAEC did not result in an accumulation of
choline lysophospholipids, but
hypoxia alone and in combination with
thrombin stimulation led to a significant accumulation of
lysoplasmenylcholine (LPlsCho). We propose that the presence of
hypoxia inhibits LPlsCho catabolism, at least in part, as a result of the accumulation of long-chain acylcarnitines. The combination of increased production and decreased catabolism of LPlsCho is necessary for its accumulation. Pretreatment with
bromoenol lactone to inhibit
iPLA(2) blocked membrane
phospholipid hydrolysis and production of membrane
phospholipid-derived metabolites. The increase in
iPLA(2) activity and the subsequent accumulation of membrane
phospholipid-derived metabolites in HCAEC exposed to
hypoxia or
thrombin stimulation alone, and particularly in combination, have important implications in
inflammation and arrhythmogenesis in
atherosclerosis/
thrombosis and subsequent
myocardial ischemia.