Lipophagy modifies the catabolism of intracellular triglycerides by preferentially targeting small-sized lipid droplets in bovine mammary epithelial cells.

The size of fat globules in milk, which is influenced mainly by the size of lipid droplets (LDs) in bovine mammary epithelial cells, affects the quality of the milk. The present study aimed to explore how lipophagy affected the catabolism of LDs in bovine mammary epithelial cells. Lipophagy was initiated by 24 h of starvation with serum-free culture medium. Compared with those in the control group (CT), the level of intracellular triglycerides and xanthine dehydrogenase (XDH) protein were lower, whereas the expression of perilipin 2 (PLIN2), peroxisome proliferator-activated receptor gamma (PPAR γ) and sterol regulatory element-binding protein 1 (SREBP1) was greater in response to starvation. The number of LDs was lower, whereas the average diameter of LDs and the distribution of large LDs (diameter > 3 μm) was greater in response to starvation. Inhibition of the lipophagic flux induced the accumulation of small LDs (diameter < 1 μm). Starvation enhanced the expression of adipose triglyceride lipase (ATGL), the inhibition of which increased average diameter of LDs and the accumulation of large LDs. Furthermore, colocalization of ATGL and large LDs was observed in cells subjected to starvation. Inhibition of lysosomal acid lipase decreased the size of LDs, and the colocalization of small LDs and autolysosomes was also observed in bovine mammary epithelial cells subjected to starvation. The results of the present study suggested that nutrient deficiency induced the formation of autophagosomes, which selectively delivered small lipid droplets to lysosomes for degradation and affected the catabolism of triglycerides in bovine epithelial cells.