ADRP/adipophilin is degraded through the proteasome-dependent pathway during regression of lipid-storing cells

Abstract

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...

Docosahexaenoic Acid Induces Adipose Differentiation-Related Protein through Activation of Retinoid X Receptor in Human Choriocarcinoma BeWo Cells

“…[3][4][5][6][7] Addition of high concentrations of fatty acids to various cells in culture up-regulates both Adrp mRNA and ADRP protein, 8) while ADRP is degraded via the ubiquitin-proteasome mechanism when cellular triacylglycerol (TG) is reduced. 9,10) These observations suggest that ADRP has a role in cellular fatty acid uptake and storage.…”

“…complexes with BSA as previously described. 9) To examine the effect of nuclear receptors, PA024 and HX630 (RXR agonist), 21,22) PA452 and HX531 (RXR antagonist), 23,24) GW1929 (PPARg agonist; Sigma), and GW9662 (PPARg antagonist; Alexis) were used. These reagents dissolved in dimethylsufoxide were added to the cells (10 ml DMSO/5 ml cell culture dish).…”

A lipid-droplet-targeted O-GlcNAcase isoform is a key regulator of the proteasome

“…Oleic acid is known to induce synthesis of perilipin-2, initiating nascent lipid droplets, whereas it is gradually replaced by perilipin-1 during lipid droplet maturation in adipocytes (Bickel et al, 2009). This transition from perilipin-2 to perilipin-1 is regulated by a proteasome-sensitive mechanism (Xu et al, 2005;Xu et al, 2006;Masuda et al, 2006). Interestingly, OGA-S demonstrates a robust effect on proteasomal activity and we have pursued the hypothesis that OGA-S could regulate perilipin-2 stability by activating proteasomes located at the lipid droplet surface.…”

Physiologic Roles of Hepatic Lipid Droplets and Involvement of Peroxisome Proliferator-Activated Receptor .ALPHA. in Their Dynamism

“…Notably, our pulse-chase experiment suggested that 17b-HSD11 is redistributed to preexisting LDs via the ER. 15) Furthermore, expression levels of adipose differentiation-related protein (ADRP) are tightly coupled with the amounts of LDs in all three different types of cells (liver-, intestine-, and adipose-derived cell lines) as reported, 17) whereas both basal and induced expressions of 17b-HSD11 were only detected in the hepatoma Fao cells (Fig. 2).…”