Uptake of fatty acids by a single endothelial cell investigated by Raman spectroscopy supported by AFM

In this work, confocal Raman imaging was used to study the formation of lipid droplets (LDs) in vitro in a single endothelial cell upon incubation with polyunsaturated fatty acids (10 or 25 μM) including arachidonic acid (AA) and its deuterated analog (AA-d₈), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Based on the Raman spectra obtained from a single endothelial cell, it was possible to investigate biochemical changes induced by addition of polyunsaturated fatty acids. In particular, the content of lipids in the formed LDs and the unsaturation degree were identified by Raman spectroscopy by marker bands at 1660 cm^-1 due to the C[double bond, length as m-dash]C stretching and at ∼3015 cm^-1 due to the stretching mode of [double bond, length as m-dash]C-H associated with C[double bond, length as m-dash]C double bonds (except for a deuterated form where these bands are shifted respectively). To establish if the exogenous fatty acid was taken up by the cell and stored in LDs, a deuterium labelled polyunsaturated fatty acid was used. AA-d₈ shows characteristic bands at around 2200-2300 cm^-1 assigned to the [double bond, length as m-dash]C-D stretching modes. We established the uptake of AA and the accumulation of EPA into newly formed LDs in the endothelial cells. In contrast, no accumulation of DHA in LDs was observed even though LDs were formed upon DHA incubation. Furthermore, using AFM we demonstrated that the presence of LDs in the endothelium affected endothelial stiffness which could have pathophysiological significance. In summary, the results suggest that the formation of LDs in the endothelium involves exogenous and endogenous polyunsaturated fatty acids, and their relative contribution to the LD formation seems distinct for AA, EPA and DHA.