Lipid Metabolism in Mammary Epithelial Cells-A Comparison of Common In vitro Models

One of the main cellular functions of mammary epithelial cells (MEC) is the vast production of lipids to provide the new born with the energy and bioactive molecules that are essential for its development and survival. Milk lipids are secreted in a structure termed milk fat globule (MFG) which buds from the MEC, enveloped with the cellular bilayer membrane. MFG membrane composition changes as a function of lactation stage, nutrition and MFG size. In addition, MFG size differs at different stages of lactation and in response to nutritional treatments. Therefore, it is hypothesized that the MEC membrane changes according to metabolic and hormonal signals in the animal's body, and that these changes are associated with milk lipid production and secretion and MFG size. The aim of this work was to find a suitable model for studying lipogenic activity and its relation to membrane composition in the mammary gland. Two in vitro models were compared: mammary gland tissue culture ('explants') and primary culture of MEC. Prolactin treatment increased fat concentration in the explants. In addition, conjugated linoleic acid decreased fat content, but only in the presence of prolactin. Despite these interesting results, the model was not reproducible. Cells of the primary MEC culture showed cytokeratin-18 mRNA and protein expression, which validated the culture's epithelial content. In addition, this model was prolactin-sensitive, as reflected by induction of α-lactalbumin expression in response to prolactin administration. Oleic acid induced the formation of large lipid droplets, and methionine addition to the oleic acid treatment further increased lipid droplet size. In conclusion, primary culture of bovine MEC was found as an appropriate model for studying lipogenic activity with a focus on membrane composition and lipid droplet size in the mammary gland.