Lipid droplet (LD), a multi-functional organelle, is often found to associate with other cellular membranous structures and vary in size in a given cell, which may be related to their functional diversity. Here we established a method to separate LD subpopulations from isolated CHO K2 LDs into three different size categories. The subpopulation with smallest LDs was nearly free of ER and other membranous structures while those with larger LDs contained intact ER. These distinct subpopulations of LDs differed in their protein composition and ability to recruit proteins. This method was also applicable to LDs obtained from other sources, such as Huh7 cells, mouse liver and brown adipose tissue, et al. We developed an in vitro assay requiring only isolated LDs, Coenzyme A, and ATP to drive lipid synthesis. The LD subpopulation nearly depleted of ER was able to incorporate fatty acids into triacylglycerol and phospholipids. Together, our data demonstrate that LDs in a given cell are heterogeneous in size and function, and suggest that LDs are one of cellular lipid synthetic organelles.

The lipid droplet (LD)-associated protein adipose differentiation-related protein (ADRP or PLIN2) is required for the formation and stability of the LD organelle, whereas its biological roles are still obscure. Herein, we show that PLIN2 is the most abundant protein on the lipid droplets (LDs) of mouse myoblast cell line C2C12. Both the expression of PLIN2 and the accumulation of LDs were up-regulated in a time-and dose-dependent manner when the cells were treated with oleate (OA). The protein level of PLIN2 was positively correlated with the formation of LDs, suggesting that LDs stabilize PLIN2. Furthermore, knocking out PLIN2 in C2C12 cells led to enlarged LDs and higher triacylglycerol hydrolysis activity. The isolated PLIN2 null LDs became closely contact with mitochondria and other cellular organelles. Additionally, mitochondrial activity was suppressed by OA in PLIN2 null cells. Our results reveal the pivotal roles of PLIN2 in governing LD dynamics and their relationship to mitochondria, and suggest a reciprocal stabilization between PLIN2 and LDs.