Adipose differentiation-related protein (ADRP) is a major protein associated with lipid droplets in various types of cells, including macrophage-derived foam cells and liver cells. However, the role of ADRP in the processes of formation and regression of these cells is not understood. When J774 murine macrophages were incubated with either VLDL or oleic acid, their content of both ADRP and triacylglycerol (TG) increased 3-to 4-fold. Induction of ADRP during TG accumulation was also observed in oleic acid-treated HuH-7 human liver cells. Addition of triacsin C, a potent inhibitor of acyl-CoA synthase, for 6 h decreased the amount of TG in VLDL-induced foam cells and oleic acid-treated liver cells; it decreased the amount of ADRP protein in parallel, indicating the amount of ADRP reduced during regression of the lipid-storing cells. Addition of a proteasome inhibitor during triacsin C treatment abolished the ADRP decrease and accumulated polyubiquitinated ADRP. In addition, the proteasome inhibitor reversed not only the degradation of ADRP but also TG reduction by triacsin C.These results suggest that cellular amounts of ADRP and TG regulate each other and that the ubiquitin-proteasome system is involved in degradation of ADRP during regression of lipid-storing cells. The appearance of cytosolic lipid droplets is observed in many cell types in physiological and pathological conditions. Adipocytes, the major lipid-storing cells in mammals, are full of lipid droplets. Foam cells in atherosclerotic lesions and parenchymal cells in fatty liver are lipid dropletcontaining cells associated with disease states. These cells accumulate massive amounts of cholesteryl ester and/or triacylglycerol (TG) as lipid droplets in their cytoplasmic space. Recently, we and others have shown that there is a distinct set of proteins specifically localized in lipid droplets (1-5), suggesting that the lipid droplet is an organized intracellular structure.Adipose differentiation-related protein (ADRP), also called adipophilin, is known to be a lipid droplet-associating protein. ADRP was originally found as a major protein induced in the early stages of adipocyte differentiation (6), although it is now known that ADRP is expressed in a variety of tissues and cells (7-13) in addition to macrophage-derived foam cells in atherosclerotic lesions (14). ADRP is the most abundant protein in lipid droplets in hepatic cells (3) and macrophages (Y. Fujimoto, et al., unpublished data).Expression of ADRP in macrophages during foam cell formation has been demonstrated by several studies. The gene-chip technique was used to show that ADRP is one of the most strongly induced genes in macrophages after incubation with oxidized LDL (OxLDL) (15). Addition of VLDL stimulated foam cell formation and ADRP mRNA expression in cultured macrophages (16,17). Expression of ADRP mRNA in human atherosclerotic lesions has been demonstrated by in situ hybridization experiments (14). Overexpression of the ADRP gene in cultured cells resulted in increased fatty a...