Direct detection of omega-3 and omega-6 fatty acid incorporation into cells by surface analysis using water cluster secondary ion mass spectrometry imaging

Water cluster ion beam secondary ion mass spectrometry (SIMS) was used to investigate the fate of essential fatty acids (linoleic acid, LA, and alpha-linolenic acid, ALA) in hormone-receptive (MCF7) and triple-negative (MDA-MB-231) breast cancer cell lines and MCF7 spheroids. SIMS has historically produced low yields of intact lipid signal. However, here, a water gas cluster ion beam (GCIB), that significantly enhances intact ion detection compared to conventional atomic/small cluster ions, was used. In addition to increased molecular signals, the water GCIB produces fragment ions with ratios related to positional isomerism of glycerophospholipids that provided additional direct insights into membrane chemistry. The results show that both cell lines take up both fatty acids and that this uptake leads to an apparent replacement of one fatty acid chain on the different lipids in the cells predominantly in the sn-2 position. The appearance of polyunsaturated triacylglycerides signals indicated that PUFA availability was not the limiting factor here. SIMS imaging of spheroids exposed to the LA and ALA fatty acids showed similar changes in the lipid profile localized to a region of approximately 50-100 µm around the edge of the spheroid.