Changes in lipid droplets morphometric features in mammary epithelial cells upon exposure to non-esterified free fatty acids compared with VLDL

The effects of the macrostructure of long chain fatty acids on the lipid metabolism and biosynthesis of lipid droplets (LD) was studied in mammary epithelial cells (MEC). MEC were exposed to similar compositions and concentrations of fatty acids in the form of either triglycerides (Tg), as part of the very-low-density lipids (VLDL) isolated from lactating cow plasma, or as non-esterified- free fatty acids (FFA). Exposing MEC to FFA resulted in two distinct processes; each independently could increase LD size: an elevation in Tg production and alterations in phospholipid (PL) composition. In particular, the lower PC/PE ratio in the FFA treatment indicated membrane destabilization, which was concomitant with the biosynthesis of larger LD. In addition, 6 fold increase in the cellular concentration of the exogenously added linoleic acid (C18:2) was found in MEC treated with FFA, implying that long chain fatty acids administrated as FFA have higher availability to MEC, enabling greater PL synthesis, more material for the LD envelope, thereby enhancing LD formation. Availability of long chain fatty acids administrated as VLDL-Tg, is dependent on LPL which its activity can be inhibited by the hydrolysis products. Therefore, we used increasing concentrations of albumin, to reduce the allosteric inhibition on LPL by the hydrolysis products. Indeed, a combined treatment of VLDL and albumin, increased LD size and number, similar to the phenotype found in the FFA treatment. These results reveal the role played by the macrostructure of long chain fatty acids in the regulation of LD size in MEC which determine the size of the secreted MFG.