Abstract
Milk fat globule size is determined by the size of its precursors—intracellular lipid droplets—and is tightly associated with its composition. We examined the relationship between phospholipid composition of mammary epithelial cells and the size of both intracellular and secreted milk fat globules. Primary culture of mammary epithelial cells was cultured in medium without free fatty acids (control) or with 0.1 mM free capric, palmitic or oleic acid for 24 h. The amount and composition of the cellular lipids and the size of the lipid droplets were determined in the cells and medium. Mitochondrial quantity and expression levels of genes associated with mitochondrial biogenesis and polar lipid composition were determined. Cells cultured with oleic and palmitic acids contained similar quantities of triglycerides, 3.1- and 3.8-fold higher than in controls, respectively (P < 0.0001). When cultured with oleic acid, 22% of the cells contained large lipid droplets (>3 μm) and phosphatidylethanolamine concentration was higher by 23 and 63% compared with that in the control and palmitic acid treatments, respectively (P < 0.0001). In the presence of palmitic acid, only 4% of the cells contained large lipid droplets and the membrane phosphatidylcholine concentration was 22% and 16% higher than that in the control and oleic acid treatments, respectively (P < 0.0001). In the oleic acid treatment, approximately 40% of the lipid droplets were larger than 5 μm whereas in that of the palmitic acid treatment, only 16% of the droplets were in this size range. Triglyceride secretion in the oleic acid treatment was 2- and 12-fold higher compared with that in the palmitic acid and control treatments, respectively. Results imply that membrane composition of bovine mammary epithelial cells plays a role in controlling intracellular and secreted lipid droplets size, and that this process is not associated with cellular triglyceride content.
Highlights
We examined the relationship between phospholipid composition of mammary epithelial cells and the size of both intracellular and secreted milk fat globules
Milk lipids are present in the form of milk fat globules (MFGs) that are secreted in a wide range of sizes, from 200 nm to over 15 μm, depending on the animal [1,2], genetic disposition [3], lactation stage [4], season [5] and diet [6,7]
The first examined whether culturing with different free fatty acids (FFAs) induces changes in cellular Tg content and if so, whether these alterations are associated with membrane composition
Summary
Milk lipids are present in the form of milk fat globules (MFGs) that are secreted in a wide range of sizes, from 200 nm to over 15 μm, depending on the animal [1,2], genetic disposition [3], lactation stage [4], season [5] and diet [6,7]. The size of MFGs is determined by their formation and secretion processes. MFG precursors, comprised of a triglyceride (Tg) core covered with a single layer of phospholipids, appear in the cytosol of mammary epithelial cells as microlipid droplets ranging in size from less than 0.5 μm to more than 4 μm [11,12]. The droplets can grow substantially in size upon fusion with other lipid droplets [13]
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