Regulation of Lipid Droplet Size in Mammary Epithelial Cells by Remodeling of Membrane Lipid Composition—A Potential Mechanism

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.

Xanthine oxidoreductase mediates membrane docking of milk‐fat droplets but is not essential for apocrine lipid secretion

“…Others have reported lipid release from primary mammary cell culture (Cohen et al . ), but none has visualized the process, nor presented analysis of the structure of the resulting globules. Farkas et al .…”

Relationships between milks differentiated on native milk fat globule characteristics and fat, protein and calcium compositions

“…Recently, studies by Argov-Argaman et al (2013 and, Logan et al (2014) and Cohen et al (2015) have suggested that the phospholipid (PL) to triacylglycerol (TAG) ratio in cytoplasm, linked to diacylglycerol acyltransferase 1 activity, influences MFG diameter. In the PL, phosphatidylethanolamine, when located in the membrane, would play a major role in the process of intracellular fusion of CLD and thereby in MFG diameter (Keenan and Huang, 1972;Mesilati-Stahy et al, 2011;Argov-Argaman et al, 2013).…”

“…A high PL : TAG ratio may also suggest a high membrane turnover and by consequence smaller MFG secreted (Logan et al, 2014). Cohen et al (2015) suggested 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. In our results, the higher contents of UFA in SMFG milk samples might be linked to higher contents of PL and thereby smaller MFG, in accordance with Lopez et al (2014) and Mesilati-Stahy and Argov-Argaman (2014).…”

“…We can hardly suggest explanations for these results. Most of the studies dealing with the effect of the composition of the milk proteins and MFG diameter looked at very specific proteins such as butyrophiline, xanthine oxidoreductase and adipophiline (also known as perilipin-2 or adipose differentiation-related protein) (McManaman et al, 2007;Jeong et al, 2013) or proteins involved in lipid synthesis (Cohen et al, 2015). These three proteins, by forming a bridge between the CLD membrane and the apical membrane, are involved in the membrane enwrapping process during MFG secretion by budding (Ollivier-Bousquet, 2002;Jeong et al, 2013) as well as the phosphatidylethanolamine content in the membrane, as mentioned above (Keenan and Huang, 1972).…”

Symposium review: Intravital imaging of the lactating mammary gland in live mice reveals novel aspects of milk-lipid secretion

“…A third group of lipids, including phosphatidylserine and phosphatidylcholine, have cylindrical structures, which assemble into planar surfaces in the absence of confounding factors ( Figure 7A). In the context of mammary cells, the size of lipid droplets in primary cultures can be manipulated by changing the TG precursors in the medium (Cohen et al, 2015. Culture conditions that favor the intracellular accumulation of phosphatidylethanolamine through the decarboxylation of phosphatidylserine induce the formation and secretion of larger lipid droplets.…”