The origin of H-1 NMR-visible triacylglycerol in human neutrophils - High fatty acid environments result in preferential sequestration of palmitic acid into plasma membrane triacylglycerol

Human neutrophils incubated for 1 h in vitro with 10% commercial pooled, human serum containing high levels of free fatty acids (1141 mu m) displayed a distinct lipid signal, typical of triacylglycerol, in the H-1 NMR spectrum. Concurrently their plasma membrane triacylglycerol mass increased 4.6-fold with a selective rise in the content of palmitic and linoleic acids. Although qualitatively similar, these effects were much greater than those observed after incubating neutrophils with 50 mu g.mL(-1) of lipopolysaccharide in the presence of 10% AB serum with normal free fatty acid content (345 mu m, LPS/S). Incubation of neutrophils with an artificial mixture of free fatty acids at concentrations found in commercial serum, or with the fatty acid fraction isolated from commercial serum increased the H-1 NMR-detectable triacylglycerol. The signal intensity of the H-1 NMR-detectable triacylglycerol depended on the triacylglycerol composition, and correlated with increased membrane triacylglycerol mass. Cellular uptake of H-3-labelled palmitic or oleic acids increased in the presence of commercial serum but not with LPS/S, with little contribution in either case to the triacylglycerol pool that increased in mass. Pulse-chase experiments demonstrated that with LPS/S and commercial serum, radiolabelled palmitic acid was preferentially incorporated into triacylglycerol located in the plasma membrane. This process could occur at the plasma membrane, as cytoplasts efficiently convert exogenous fatty acids into triacylglycerol. We propose that LPS/S and serum containing high levels of free fatty acid, important in conditions of sepsis and inflammation, may facilitate the sequestration of palmitic acid into triacylglycerol by different pathways. This triacylglycerol originates from exogenous and endogenous free fatty acids, is H-1 NMR-visible, and may have a role in regulating apoptosis.