Adipocyte differentiation-related protein reduces the lipid droplet association of adipose triglyceride lipase and slows triacylglycerol turnover

Abstract

Although neutral lipid storage droplets are ubiquitous in eukaryotic cells, very little is known about how their synthesis and turnover are controlled. Adipocyte differentiation-related protein (ADRP; also known as adipophilin) is found on the surface of lipid droplets in most mammalian cell types. To learn how ADRP affects lipid storage, we stably expressed the protein in human embryonic kidney 293 (HEK 293) cells, which express little endogenous ADRP. As expected, ADRP was targeted to the surface of lipid droplets and caused an increase in triacylglycerol (TAG) mass under both basal and oleate-supplemented conditions. At least part of the increased mass resulted from a 50% decrease in the rate of TAG hydrolysis in ADRPexpressing cells. Furthermore, ADRP expression increased the fraction of total cellular TAG that was stored in lipid droplets. ADRP expression induced a striking decrease in the association of adipose triglyceride lipase (ATGL) and mannose-6-phosphate receptor tail-interacting protein of 47 kDa with lipid droplets and also decreased the lipid droplet association of several other unknown proteins. Transient expression of ADRP in two other cell lines also reduced the lipid droplet association of catalytically inactive ATGL. We conclude that the reduced lipid droplet association of ATGL and/or other lipases may explain the decrease in TAG turnover observed in ADRP-expressing HEK 293 cells. Eukaryotes store lipid in cytosolic lipid droplets, which consist of neutral lipid cores surrounded by phospholipid monolayers (1-3). In mammals, lipid droplets are most abundant in adipose tissue, where stored triacylglycerol (TAG) provides the primary energy reserve for the organism. Lipid droplets in steroidogenic cells contain cholesteryl esters used in the synthesis of steroid hormones. Most other mammalian cells contain smaller lipid droplets, whose function remains unclear. They may serve as local energy reserves or sources of lipid for membrane synthesis. Furthermore, they may protect cells from the harmful effects of excess lipid accumulation by sequestering toxic lipid species away from pathways that lead to cell death (4, 5).Mechanisms controlling the synthesis and turnover of lipid droplets are only partially understood. According to one model of lipid droplet biogenesis, newly synthesized neutral lipids accumulate inside the endoplasmic reticulum membrane, forming a disk that eventually buds into the cytoplasm surrounded by an endoplasmic reticulumderived phospholipid monolayer (2, 6). Conversely, lipid droplet turnover occurs via the hydrolysis of stored neutral lipids by cytosolic lipases. Much of what we know about the regulation of lipolysis stems from studies in adipocytes. In response to hormone stimulation, protein kinase A phosphorylates two key substrates: hormone-sensitive lipase (HSL) (7) and perilipins (8, 9). Phosphorylation of HSL stimulates both its activity and its association with lipid droplets, in a manner that depends on perilipins.Perilipins regulate TAG hydrolysis in two...

Cytoplasmic lipid droplets are translocated into the lumen of the Chlamydia trachomatis parasitophorous vacuole

“…Although these PLDs have been postulated to hydrolyze PC, their substrate specificity in vivo is unknown (31). An alternative means of processing neutral lipids in LDs may be provided by the organelle itself, which comes prepacked with TAG lipases like ATGL, whose activity is normally repressed by ADRP (15). Similarly, MVBmediated protein traffic could deliver endosomal neutral lipid lipases to the inclusion (32).…”

“…The Perilipin-Adipophilin/ADRP-TIP47 (PAT) family of LD-associated proteins regulate the basal rate of lipolysis by forming a protective ''coat'' that limits the access of lipases to the LD core (14). Overexpression of perilipin or ADRP increases triacylglyceride levels and the formation of LDs (15). In adipocytes, extracellular signals lead to the redistribution of PAT proteins and the subsequent recruitment of neutral-lipid lipases (14) to generate fatty acids for de novo membrane biosynthesis and ␤-oxidation by peroxisomes and mitochondria (16).…”

Lipid homeostasis and the formation of macrophage-derived foam cells in atherosclerosis

“…However, detailed studies to elucidate the mechanistic role of ADRP in the formation of macrophage-derived foam cells have not been performed, and it is possible that ADRP, as an LD-associated protein, could protect LDs against the hydrolysis of lipases. In HEK293 cells, overexpression of ADRP has been shown to correlate with decreased association of the major TG lipase named ATGL with LDs and increased TG accumulation as a result of inhibition of TG hydrolysis (Listenberger et al, 2007). ADRP overexpression also increased TG content in THP-1-derived macrophages by promoting the incorporation of fatty acyl-CoAs into TGs and inhibiting fatty acid oxidation (Larigauderie et al, 2006).…”

Not Just Fat: The Structure and Function of the Lipid Droplet

“…OXPAT/MLDP/ LSDP5 enhances lipid storage by curbing lipolysis (Dalen et al 2007) and it also increases b-oxidation by recruiting CGI-58 to LDs and thereby enhancing the ATGL activity (Granneman et al 2009). Microsomal TG content is increased or decreased by deletion or overexpression of ADRP, respectively, suggesting that ADRP promotes LD formation at the expense of microsomal storage Listenberger et al 2007). This result suggests that ADRP not only protects lipid esters from lipases, but it is also intimately related to the process of LD formation.…”

“…Overexpression of ADRP increases lipid ester storage in LDs, and this effect is thought to occur by limiting ATGL access to the esters (Listenberger et al 2007). TIP47 appears to substitute the function of ADRP at least partially, and when both ADRP and TIP47 are deficient, incorporated fatty acids are used to make phospholipids rather than TG (Sztalryd et al 2006).…”