Fat-specific protein (FSP)27/Cidec is most highly expressed in white and brown 
adipose tissues and increases in abundance by over 50-fold during adipogenesis. 
However, its function in adipocytes has remained elusive since its discovery 
over 15 years ago. Here we demonstrate that FSP27/Cidec localizes to lipid 
droplets in cultured adipocytes and functions to promote lipid accumulation. 
Ectopically expressed FSP27-GFP surrounds lipid droplets in 3T3-L1 adipocytes 
and colocalizes with the known lipid droplet protein perilipin. Immunostaining 
of endogenous FSP27 in 3T3-L1 adipocytes also confirmed its presence on lipid 
droplets. FSP27-GFP expression also markedly increases lipid droplet size and 
enhances accumulation of total neutral lipids in 3T3-L1 preadipocytes as well as 
other cell types such as COS cells. Conversely, RNA interference-based 
FSP27/Cidec depletion in mature adipocytes significantly stimulates lipolysis 
and reduces the size of lipid droplets. These data reveal FSP27/Cidec as a novel 
adipocyte lipid droplet protein that negatively regulates lipolysis and promotes 
triglyceride accumulation.