Lipid Droplet Fusion in Mammary Epithelial Cells is Regulated by Phosphatidylethanolamine Metabolism
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Mammary epithelial cells (MEC) secrete fat in the form of milk fat globules (MFG) which are found in milk in diverse sizes. MFG originate from intracellular lipid droplets, and the mechanism underlying their size regulation is still elusive. Two main mechanisms have been suggested to control lipid droplet size. The first is a well-documented pathway, which involves regulation of cellular triglyceride content. The second is the fusion pathway, which is less-documented, especially in mammalian cells, and its importance in the regulation of droplet size is still unclear. Using biochemical and molecular inhibitors, we provide evidence that in MEC, lipid droplet size is determined by fusion, independent of cellular triglyceride content. The extent of fusion is determined by the cell membrane’s phospholipid composition. In particular, increasing phosphatidylethanolamine (PE) content enhances fusion between lipid droplets and hence increases lipid droplet size. We further identified the underlying biochemical mechanism that controls this content as the mitochondrial enzyme phosphatidylserine decarboxylase; siRNA knockdown of this enzyme reduced the number of large lipid droplets threefold. Further, inhibition of phosphatidylserine transfer to the mitochondria, where its conversion to PE occurs, diminished the large lipid droplet phenotype in these cells. These results reveal, for the first time to our knowledge in mammalian cells and specifically in mammary epithelium, the missing biochemical link between the metabolism of cellular complex lipids and lipid-droplet fusion, which ultimately defines lipid droplet size.
KeywordsLipid droplet Phosphatidylethanolamine Fusion Mammary epithelial cell
The authors would like to acknowledge Dr. Sergei Grigoryan for his assistance in confocal microscopy imaging. This research was partially supported by the Nutrigenomics Center of the Hebrew University of Jerusalem, Israel, and by the Israeli Dairy Board grant #8200327.
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Conflict of Interest
The authors declare no conflict of interest.
Lipid droplet fusion. Representative fusion event of two lipid droplets, observed in MEC treated with 100 μM palmitic acid + 10 μM DZA and stained for their lipid droplets with Nile red. Optical sections were recorded with time-lapse imaging and reconstructed as 3D images from deconvolved images. Arrows indicate a pair of fusing lipid droplets. (AVI 78 KB)
Relatively slow movement of lipid droplets in MEC treated with oleic acid + NaN3 + NaF. Time-lapse imaging of Nile red-stained lipid droplets in MEC treated with 360 μM oleic acid + 2.5 mM NaN3 + 10 mM NaF reveals slower movement relative to oleic acid alone (see Online Resource <link rid="Sec22">5</link>). (AVI 199 KB)
Lipid droplet movement in MEC treated with oleic acid. Time-lapse imaging of Nile red-stained lipid droplets in MEC treated with 360 μM oleic acid reveals rapid movement and noticeable fusion between lipid droplets. (AVI 286 KB)
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