Lipodystrophy is a disorder characterized by a loss of adipose tissue often accompanied by severe hypertriglyceridemia, insulin resistance, diabetes, and fatty liver. It can be inherited or acquired. The most severe inherited form is Berardinelli-Seip Congenital Lipodystrophy Type 2, associated with mutations in the BSCL2 gene. BSCL2 encodes seipin, the function of which has been entirely unknown. We now report the identification of yeast BSCL2/seipin through a screen to detect genes important for lipid droplet morphology. The absence of yeast seipin results in irregular lipid droplets often clustered alongside proliferated endoplasmic reticulum (ER); giant lipid droplets are also seen. Many small irregular lipid droplets are also apparent in fibroblasts from a BSCL2 patient. Human seipin can functionally replace yeast seipin, but a missense mutation in human seipin that causes lipodystrophy, or corresponding mutations in the yeast gene, render them unable to complement. Yeast seipin is localized in the ER, where it forms puncta. Almost all lipid droplets appear to be on the ER, and seipin is found at these junctions. Therefore, we hypothesize that seipin is important for droplet maintenance and perhaps assembly. In addition to detecting seipin, the screen identified 58 other genes whose deletions cause aberrant lipid droplets, including 2 genes encoding proteins known to activate lipin, a lipodystrophy locus in mice, and 16 other genes that are involved in endosomallysosomal trafficking. The genes identified in our screen should be of value in understanding the pathway of lipid droplet biogenesis and maintenance and the cause of some lipodystrophies. BSCL2 ͉ lipid bodies

Lipid droplets are accumulations of neutral lipids surrounded by a monolayer of phospholipids and associated proteins. Recent proteomic analysis of isolated droplets suggests that they are part of a dynamic organelle system that is involved in membrane traffic as well as packaging and distributing lipids in the cell. To gain a better insight into the function of droplets, we used a combination of mass spectrometry and NMR spectroscopy to characterize the lipid composition of this compartment. In addition to cholesteryl esters and triacylglycerols with mixed fatty acid composition, we found that ?10-20% of the neutral lipids were the ether lipid monoalk(en)yl diacylglycerol. Although lipid droplets contain only 1-2% phospholipids by weight, .160 molecular species were identified and quantified. Phosphatidylcholine (PC) was the most abundant class, followed by phosphatidylethanolamine (PE), phosphatidylinositol, and ether-linked phosphatidylcholine (ePC). Relative to total membrane, droplet phospholipids were enriched in lysoPE, lysoPC, and PC but deficient in sphingomyelin, phosphatidylserine, and phosphatidic acid. These results suggest that droplets play a central role in ether lipid metabolism and intracellular lipid traffic. Lipid droplets are recognized by their conserved structural organization, which consists of a hydrophobic matrix of neutral lipid covered by a monolayer of phospholipids and associated proteins (1). Although traditionally regarded as a simple repository for stored carbon reserves, emerging evidence suggests that droplets function as dynamic organelles with a central role in cellular lipid metabolism, membrane trafficking, and cell signaling (2). Because lipid droplets can be found in bacteria, yeast, plant, and animal cells, over the years they have acquired a variety of names. Recently, we proposed that this diverse collection of names be replaced with the designation adiposome (3). Thus, an adiposome is an organelle that is specialized for packaging and distributing lipids in cells. In this nomenclature, the droplet is simply the most visible stage in the complex life cycle of an adiposome.During the past few years, a number of reports have focused on the protein composition of lipid droplets isolated from yeast (4), plant (5), and animal (3, 6, 7) cells. A consensus view from these studies is that droplets contain structural proteins, proteins involved in the biosynthesis and breakdown of lipids, and proteins that mediate membrane traffic. Thus, the proteome indicates that droplets are actively engaged in membrane traffic, perhaps for the purpose of maintaining the proper lipid composition of different membrane compartments. In contrast to the proteins, surprisingly little is known about the lipid composition of animal cell droplets. Generally, droplets are rich in neutral lipids such as triacylglycerol (TAG) and cholesteryl esters that have a diverse population of esterified fatty acids (8). Here, we report an analysis of the lipid composition of droplets purified from variou...

Patients with chronic kidney disease (CKD) develop increased levels of the phosphate-regulating hormone, fibroblast growth factor (FGF) 23, that are associated with a higher risk of mortality. Increases in inflammatory markers are another common feature of CKD that predict poor clinical outcomes. Elevated FGF23 is associated with higher circulating levels of inflammatory cytokines in CKD, which can stimulate osteocyte production of FGF23. Here, we studied whether FGF23 can directly stimulate hepatic production of inflammatory cytokines in the absence of α-klotho, an FGF23 co-receptor in the kidney that is not expressed by hepatocytes. By activating FGF receptor isoform 4 (FGFR4), FGF23 stimulated calcineurin signaling in cultured hepatocytes, which increased the expression and secretion of inflammatory cytokines, including C-reactive protein. Elevating serum FGF23 levels increased hepatic and circulating levels of C-reactive protein in wild-type mice, but not in FGFR4 knockout mice. Administration of an isoform-specific FGFR4 blocking antibody reduced hepatic and circulating levels of C-reactive protein in the 5/6 nephrectomy rat model of CKD. Thus, FGF23 can directly stimulate hepatic secretion of inflammatory cytokines. Our findings indicate a novel mechanism of chronic inflammation in patients with CKD and suggest that FGFR4 blockade might have therapeutic anti-inflammatory effects in CKD.

Lipid droplet is a cellular organelle with a neutral lipid core surrounded by a phospholipid monolayer and coated with structural as well as functional proteins. The determination of these proteins, especially their functional regulations and dynamic movement on and off droplets, holds a key to resolving the biological functions of the cellular organelle. To address this, we carried out a comprehensive proteomic study that includes a complete proteomic, a phosphoprotein proteomic, and a comparative proteomic analysis using purified lipid droplets and mass spectrometry techniques. The complete proteome identified 125 proteins of which 70 proteins had not been identified on droplets of mammalian cells previously. In phosphoprotein proteomic analysis, 7 functional lipid droplet proteins were determined to be phosphorylated, including adipose differentiation related protein (ADRP/ADFP), two Rab proteins, and four lipid metabolism enzymes, including adipose triglyceride lipase (ATGL). To understand the dynamics of lipid droplets, GTP-dependent protein recruitment was analyzed by comparative proteomics. Arf1 and some of its coatomers, three other Arfs, several other small G-proteins including 3 Rabs, and several lipid synthetic enzymes were recruited from cytosol to purified droplets. Together, the present study suggests that lipid droplet is an active and dynamic cellular organelle that governs lipid homeostasis and intracellular trafficking through protein phosphorylation as well as GTP-regulated protein translocation.