Entry of polyunsaturated fatty acids into the brain: evidence that high-density lipoprotein-induced methylation of phosphatidylethanolamine and phospholipase A₂ are involved

The conversion of phosphatidylethanolamine (PE) into phosphatidylcholine (PC) by a sequence of three transmethylation reactions is shown to be stimulated by the apolipoprotein E-free subclass of high-density lipoprotein (HDL₃) in isolated bovine brain capillary (BBC) membranes. HDL₃-induced stimulation of BBC membranes pulsed with [methyl-¹⁴C]methionine causes a transient increase in each methylated phospholipid, i.e. phosphatidyl-N-monomethylethanolamine (PMME), phosphatidyl-NN-dimethylethanolamine (PDME) and PC. PC substrate arising from the activation of PE N-methyltransferase (PEMT) is hydrolysed by a phospholipase A₂ (PLA₂), as demonstrated by the accumulation of lysophosphatidylcholine (lyso-PC). When PE containing [¹⁴C]arachidonic acid in the sn-2 position ([¹⁴C]PAPE) is incorporated into BBC membranes, HDL₃ stimulation induces the formation of PMME, PDME, PC and lyso-PC and the release of [¹⁴C]arachidonic acid, which correlates with the previous production of lyso-PC, suggesting that HDL₃ stimulates a PLA₂ that can release polyunsaturated fatty acids (PUFA). Both PEMT and PLA₂ activities depend on a HDL₃ concentration in the range 0–50 μg/ml and are strictly dependent on HDL₃ binding, because HDL₃ modified by tetranitromethane is no longer able to bind to specific receptors and to trigger PEMT and PLA₂ activation. Moreover, HDL₃ prelabelled with [¹⁴C]PAPE can stimulate PDME and lyso-PC synthesis in BBC membranes in the presence of S-adenosylmethionine, suggesting that HDL₃ can supply BBC membranes in polyunsaturated PE and can activate enzymes involved in PE N-methylation and PUFA release. The results support the hypothesis of a close relationship between HDL₃ binding, PE methylation and PUFA release, and suggest that the PC pool arising from PE could be used as a pathway for the supply of PUFA to the brain.