Sterol carrier protein-2 expression modulates protein and lipid composition of lipid droplets

Despite the critical role lipid droplets play in maintaining energy reserve s and lipid stores for the cell, little is known about the regulation of th e lipid or protein components within the lipid droplet. Although immunofluo rescence of intact cells as well as Western analysis of isolated lipid drop lets revealed that sterol carrier protein-2 (SCP-S) was not associated with lipid droplets, SCP-S expression significantly altered the structure of th e lipid droplet. First, the targeting of fatty acid and cholesterol to the lipid droplets was significantly decreased. Second, the content of several proteins important for lipid droplet function was differentially increased (perilipin A), reduced severalfold (adipose differentiation-related protein (ADRP), vimentin), or almost completely eliminated (hormone-sensitive lipa se and proteins > 93 kDa) in the isolated lipid droplet. Third, the distrib ution of lipids within the lipid droplets was significantly altered. Double labeling of cells with 12-(N-methyl)-N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl )amino]-octadecanoic acid (NBD-stearic acid) and antisera to ADRP showed th at 70, 24, and 13% of lipid droplets contained ADRP, NBD-stearic acid, or b oth, respectively. SCP-2 expression decreased the level of ADRP in the lipi d droplet but increased the proportion wherein ADRP and NBD-stearic acid co localized by 3-fold. SCP-S expression also decreased the lipid droplet fatt y acid and cholesterol mass (nmol/mg protein) by 5.2-and 6.6-fold, respecti vely. Finally, SCP-2 expression selectively altered the pattern of esterifi ed fatty acids in favor of polyunsaturated fatty acids within the lipid dro plet. Displacement studies showed differential binding affinity of ADRP for cholesterol and fatty acids. These data suggested that SCP-2 and ADRP play a significant role in regulating fatty acid and cholesterol targeting to l ipid droplets as well as in determining their lipid and protein components.