Lipid droplets (LDs) are intracellular organelles that store neutral lipids within cells. Over the last two decades there has been a dramatic growth in our understanding of LD biology and, in parallel, our understanding of the role of LDs in health and disease. In its simplest form, the LD regulates the storage and hydrolysis of neutral lipids, including triacylglycerol and/or cholesterol esters. It is becoming increasingly evident that alterations in the regulation of LD physiology and metabolism influence the risk of developing metabolic diseases such as diabetes. In this review we provide an update on the role of LD-associated proteins and LDs in metabolic disease. Overview of lipid dropletsLipid droplets (LDs) are intracellular organelles that store neutral lipids within cells. Over the last two decades there has been a dramatic growth in our understanding of LD biology and, in parallel, our understanding of the role they play in health and disease. LDs regulate the storage and hydrolysis of neutral lipids, including triacylglycerol (TAG) and/or cholesterol esters. For example, adipocytes, the major reservoir of TAG in the body, store their TAG within LDs, and TAG storage in adipocytes is increased in obese animals and humans. The rates of adipocyte lipolysis in many obese individuals are constitutively increased, resulting in elevated levels of circulating fatty acids, which may be stored as TAG in LDs within skeletal muscle and liver. Both local and circulating free fatty acids are thought to be important etiologic agents in the development of insulin resistance, hyperlipidemia, inflammation, and hepatic steatosis (1-3). In this article, we will briefly review the basic characteristics of LDs and then focus on our present knowledge of the current view of the role of LDs in metabolic disease.Cells have developed the capacity to store fatty acids as neutral lipids within LDs for several reasons. An important role of LDs in adipocytes is to store fatty acids as TAG to serve as a reservoir of energetic substrates that can be released when food is scarce. The detrimental effects associated with excess fatty acid entry into cells are often termed "lipotoxicity." Cells protect themselves from these effects by either oxidizing the fatty acids or sequestering them as TAG within LDs. Consistent with this hypothesis, activation of PPARα, which increases the expression of genes that encode oxidative proteins, also increases the expression of LDs and LD-associated proteins (4, 5). PPARγ and PPARδ, along with other transcription factors, can also promote droplet formation (4, 6). As noted above, when fatty acids exceed the oxidative capacity of cells, they not only enhance LD formation, but may also induce apoptosis. An example of the protection LD formation provides was demonstrated in an experiment in which exogenous oleic acid added to fibroblasts deficient in diacylglycerol acyltransferase 1 (DGAT1) promoted lipotoxic cell death. Expression of DGAT1, the terminal step in TAG synthesis, in fibroblasts channeled exces...

The mammary gland of the lactating mouse synthesizes and secretes milk lipid equivalent to its entire body weight in a single 20-day lactation cycle, making it one of the most active lipid synthetic organs known. We test the hypothesis that multiple control points and potential regulatory mechanisms regulate milk lipid synthesis at the level of gene expression. The mammary transcriptome of 130 genes involved in glucose metabolism was examined at late pregnancy and early lactation, utilizing data obtained from microarray analysis of mammary glands from quadruplicate FVB mice at pregnancy day 17 and lactation day 2. To correlate changes with physiological parameters, the metabolome obtained from magnetic resonance spectroscopy of flash-frozen glands at day 17 of pregnancy was compared with that at day 2 of lactation. A significant increase in carbohydrates (glucose, lactose, sialic acid) and amino acids (alanine, aspartate, arginine, glutamate) with a moderate increase in important osmolytes (myoinositol, betaine, choline derivatives) were observed in the lactating gland. In addition, diets containing 8% or 40% lipid were fed from lactation days 5-10 and mammary glands and livers of triplicate FVB mice prepared for microarray analysis. The results show that substantial regulation of lipid synthesis occurs at the level of mRNA expression and that some of the regulation points differ substantially from the liver. They also implicate the transcription factor SREBP-1c in regulation of part of the pathway. lipid synthesis; microarray; metabolomics; dietary lipid; magnetic resonance spectroscopy THE MOUSE MAMMARY GLAND OFFERS an outstanding model system for examining developmental regulation of metabolic processes. Pregnancy in this species lasts ϳ19 days including an intensive proliferative phase followed by a differentiation phase marked by an increase in milk protein gene expression, lipid droplet formation, and stromal adaptations (42,43). A fall in progesterone ϳday 18 initiates secretory activation, a programmed series of changes in the epithelium that leads to the copious secretion of very rich milk consisting of ϳ12% protein, 30% lipid, and 5% lactose. Lipid synthesis is particularly remarkable: The mammary gland of the FVB mouse must synthesize an amount of triacylglycerol (TAG) equivalent to the entire weight of the mouse during a 20-day lactation, generally while the mouse is eating a diet containing Ͻ8% of the calories as fat (37).Coordinate transcriptional regulation of many of the lipid synthesis enzymes occurs in other organs such as the liver (12, 17) and adipose tissue (39) as well as pancreatic ␤-cells (2). We hypothesized that coordinate regulation of the same pathways might be integral to the remarkable increase in lipid and lactose synthesis during the activation of secretion (sometimes called lactogenesis II) in the mammary gland. We wished to identify both the key enzymes that change during the initiation of this program and the transcriptional regulators involved.Our strategy was first to utilize...

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Diabetic kidney disease has been associated with the presence of lipid deposits, but the mechanisms for the lipid accumulation have not been fully determined. In the present study, we found that db/db mice on the FVB genetic background with loss-of-function mutation of the leptin receptor (FVB-Lepr db mice or FVB db/db ) develop severe diabetic nephropathy, including glomerulosclerosis, tubulointerstitial fibrosis, increased expression of type IV collagen and fibronectin, and proteinuria, which is associated with increased renal mRNA abundance of transforming growth factor-␤, plasminogen activator inhibitor-1, and vascular endothelial growth factor. Electron microscopy demonstrates increases in glomerular basement membrane thickness and foot process (podocyte) length. We found that there is a marked increase in neutral lipid deposits in glomeruli and tubules by oil red O staining and biochemical analysis for cholesterol and triglycerides. We also detected a significant increase in the renal expression of adipocyte differentiation-related protein (adipophilin), a marker of cytoplasmic lipid droplets. We examined the expression of sterol regulatory element-binding protein (SREBP)-1 and -2, transcriptional factors that play an important role in the regulation of fatty acid, triglyceride, and cholesterol synthesis. We found significant increases in SREBP-1 and -2 protein levels in nuclear extracts from the kidneys of FVB db/db mice, with increases in the mRNA abundance of acetyl-CoA carboxylase, fatty acid synthase, and 3-hydroxy-3-methylglutaryl-CoA reductase, which mediates the increase in renal triglyceride and cholesterol content. Our results indicate that in FVB db/db mice, renal triglyceride and cholesterol accumulation is mediated by increased activity of SREBP-1 and -2. Based on our previous results with transgenic mice overexpressing SREBP-1 in the kidney, we propose that increased expression of SREBPs plays an important role in causing renal lipid accumulation, glomerulosclerosis, tubulointerstitial fibrosis, and proteinuria in mice with type 2 diabetes.

Primary and acquired resistance to the breast cancer drug trastuzumab (Herceptin) is a significant clinical problem. Here, we report enhanced activation of downstream signaling pathways emanating from the growth factor receptors erbB2, erbB3, and insulin-like growth factor-I receptor (IGF-IR) in trastuzumab-resistant breast cancer cells. Interactions between IGF-IR and erbB2 or erbB3 occur exclusively in trastuzumab-resistant cells, where enhanced erbB2-erbB3 interactions are also observed. Moreover, these three receptors form a heterotrimeric complex in resistant cells. erbB3 or IGF-IR knockdown by short hairpin RNA-mediated strategies upregulates p27 kip1 , inactivates downstream receptor signaling, and resensitizes resistant cells to trastuzumab. Our findings reveal a heterotrimer complex with a key role in trastuzumab resistance. On the basis of our results, we propose that trastuzumab resistance in breast cancer might be overcome by therapeutic strategies that jointly target erbB3, erbB2, and IGF-IR.

To characterize the molecular mechanisms by which progesterone withdrawal initiates milk secretion, we examined global gene expression during pregnancy and lactation in mice, focusing on the period around parturition. Trajectory clustering was used to profile the expression of 1358 genes that changed significantly between pregnancy day 12 and lactation day 9. Predominantly downward trajectories included stromal and proteasomal genes and genes for the enzymes of fatty acid degradation. Milk protein gene expression increased throughout pregnancy, whereas the expression of genes for lipid synthesis increased sharply at the onset of lactation. Examination of regulatory genes with profiles similar or complementary to those of lipid synthesis genes led to a model in which progesterone stimulates synthesis of TGF-beta, Wnt 5b, and IGFBP-5 during pregnancy. These factors are suggested to repress secretion by interfering with PRL and IGF-1 signaling. With progesterone withdrawal, PRL and IGF-1 signaling are activated, in turn activating Akt/PKB and the SREBPs, leading to increased lipid synthesis.

The transition from pregnancy to lactation is a critical event in the survival of the newborn since all the nutrient requirements of the infant are provided by milk. While milk contains numerous components, including proteins, that aid in maintaining the health of the infant, lactose and milk fat represent the critical energy providing elements of milk. Much of the research to date on mammary epithelial differentiation has focused upon expression of milk protein genes, providing a somewhat distorted view of alveolar differentiation and secretory activation. While expression of milk protein genes increases during pregnancy and at secretory activation, the genes whose expression is more tightly regulated at this transition are those that regulate lipid biosynthesis. The sterol regulatory element binding protein (SREBP) family of transcription factors is recognized as regulating fatty acid and cholesterol biosynthesis. We propose that SREBP1 is a critical regulator of secretory activation with regard to lipid biosynthesis, in a manner that responds to diet, and that the serine/threonine protein kinase Akt influences this process, resulting in a highly efficient lipid synthetic organ that is able to support the nutritional needs of the newborn.

This article is available online at http://www.jlr.org type 2 diabetes, and nonalcoholic fatty liver disease (NAFLD) ( 2 ). Obesity-associated NAFLD in particular is associated with development of hyperlipidemia, insulin resistance, hyperglycemia ( 3 ), and increased risk of liver cancer ( 4, 5 ). In tissues, neutral lipids such as triacylglycerol are stored in cytoplasmic lipid droplets (CLDs), which are specialized organelle-like structures that function in intracellular traffi cking and metabolism of lipids ( 6 ). Alterations in CLD properties are increasingly recognized to be associated with metabolic abnormalities and disruptions of normal cellular and tissue functions ( 7 ).Perilipins (PLINs) are a family of genes encoding fi ve CLD-associated proteins in mammals that are implicated in the regulation of storage and hydrolysis of neutral lipids ( 7,8 ). Prior studies investigating reduced expression of perilipin1 (Plin1) in mice demonstrated that it is required for normal adipose lipid storage ( 9-11 ), whereas reduced perilipin-2 (Plin2) expression prevents hepatic lipid accumulation in response to high-fat diet (HFD) feeding (12)(13)(14). Although Plin1-null mice are also resistant to obesity and many of its attendant metabolic abnormalities ( 9, 10 ), there is limited understanding of the contributions of Plin2, or other PLIN family members, to obesity or other metabolic disorders.Defi ning the roles of Plin2 in obesity and metabolism has been complicated by observations that existing Plin2-defi cient mice produce a truncated product that is biologically active ( 15 ), and that the short-term-diet models used previously did not adequately investigate Plin2 regulation Abstract The cytoplasmic lipid droplet (CLD) protein perilipin-2 (Plin2) is expressed in multiple nonadipose tissues, where it is thought to play a role in regulating their lipid storage properties. However, the extent to which Plin2 functions in nutrient utilization and metabolism, or how it infl uences the consequences of over-feeding, remains unclear. In this study, we demonstrate that the absence of Plin2 prevents high-fat diet(HFD)-induced obesity in male and female mice. This response is associated with increased formation of subcutaneous beige adipocyte cells with uncoupling protein 1 expression, and amelioration of infl ammatory foci formation in white adipose tissue and steatosis in the liver. Experiments demonstrate that Plin2 loss results in reduced energy intake and increased physical activity in response to HFD feeding. Our study provides the fi rst evidence that Plin2 contributes to HFD-induced obesity by modulating food intake, and that its absence prevents obesity-associated adipose tissue infl ammatory foci and liver steatosis. Obesity, resulting from increased calorie consumption and decreased energy expenditure, remains a major public health concern for children and adults in the United States ( 1 ), through its strong etiological links to several metabolic complications, including metabolic syndrome,