Characterisation of novel lipids generated by activated human platelets via COX-1
mesheuropmc: lipids (amino acids, peptides, and proteins)
Initially, prostaglandins (PGs) were considered to only exist as free acid mediators. Although, formation of PG glycerol esters and PG ethanolamides by cellular cyclooxygenase (COX)-2 has been reported, generation of complex oxidised lipids via COX-1 has not been considered. In this study, formation of sixteen unique PG-containing phospholipids generated by agonist-activated human platelets is demonstrated using lipidomic approaches. Precursor scanning-tandem mass spectrometry identified a group of specific lipids comprising PGE2, PGD2 and two previously undescribed PG-like molecules (named PGb and PGc), attached to four phosphatidylethanolamine (PE) phospholipids (16:0p/, 18:1p/, 18:0p/ and 18:0a/). PGb and PGc were also detected as free eicosanoids and their structures remain to be characterised. These novel lipids formed within 2-5 minutes of platelet activation by thrombin, collagen or ionophore and required activation of several intracellular signalling intermediates, including cytosolic phospholipase A2 (cPLA2), p38 mitogen-activated protein kinase (MAPK), src tyrosine kinases, phospholipase C (PLC) and cytosolic calcium. Unlike free PGs that are secreted, PG-PEs remain cell associated, suggesting an autocrine mode of action. Aspirin supplementation in vivo (75 mg/day) or in vitro (1 mM) blocked their generation, indicating that COX-1 is required. Pharmacological studies using inhibitors of fatty acyl re-esterification significantly reduced formation of PG-PEs. Furthermore, purified COX-1 was unable to directly oxidise PE in vitro. Collectively, these indicate that PG-PEs are initially formed as free PGs via COX-1, and then rapidly esterified into PEs. In summary, this is the first demonstration of acute generation of PG-PEs in agonist-activated human platelets from endogenous substrate via COX-1. These unique lipids may represent additional bioactive molecules from this key platelet enzyme.
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