The Roles of Cytoplasmic Lipid Droplets in Modulating Intestinal Uptake of Dietary Fat.

Dietary fat absorption is required for health but also contributes to hyperlipidemia and metabolic disease when dysregulated. One step in the process of dietary fat absorption is the formation of cytoplasmic lipid droplets (CLDs) in small intestinal enterocytes; these CLDs serve as dynamic triacylglycerol storage organelles that influence the rate at which dietary fat is absorbed. Recent studies have uncovered novel factors regulating enterocyte CLD metabolism that in turn influence the absorption of dietary fat. These include peroxisome proliferator-activated receptor α activation, compartmentalization of different lipid pools, the gut microbiome, liver X receptor and farnesoid X receptor activation, obesity, and physiological factors stimulating CLD mobilization. Understanding how enterocyte CLD metabolism is regulated is key in modulating the absorption of dietary fat in the prevention of hyperlipidemia and its associated metabolic disorders.

Characterization of cytoplasmic lipid droplets in each region of the small intestine of lean and diet-induced obese mice in response to dietary fat.

The absorptive cells of the small intestine, namely, enterocytes, contribute to postprandial blood lipid levels by secreting dietary triacylglycerol in chylomicrons. The rate and amount of dietary triacylglycerol absorbed vary along the length of the small intestine. Excess dietary triacylglycerol not immediately secreted in chylomicrons can be temporarily stored in cytoplasmic lipid droplets (CLDs) and repackaged in chylomicrons at later times. The characteristics of CLDs, including their size, number per cell, and associated proteins, may influence CLD metabolism and reflect differences in lipid processing or storage in each intestinal region. However, it is unknown whether the characteristics or proteomes of CLDs differ in enterocytes of each intestine region in response to dietary fat. Furthermore, it is unclear if obesity influences the characteristics or proteomes of CLDs in each intestine region. To address this, we used transmission electron microscopy and shotgun liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis to assess the characteristics and proteome of CLDs in the proximal, middle, and distal regions of the small intestine of lean and diet-induced obese (DIO) mice 2 h after an oil gavage. We identified differences in lipid storage along the length of the small intestine and between lean and DIO mice, as well as distinct CLD proteomes reflecting potentially unique roles of CLDs in each region. This study reveals differences in lipid processing along the length of the small intestine in response to dietary fat in lean and DIO mice and reflects distinct features of the proximal, middle, distal region of the small intestine.NEW & NOTEWORTHY This study reflects the dynamics of fat absorption along the length of the small intestine in lean and obese mice in the physiological response to dietary fat. We identified unique features of cytoplasmic lipid droplets (CLDs) in the proximal, middle, and distal regions of the small intestine of lean and obese mice that may contribute to regional differences in dietary fat processing, absorption, or CLD metabolism.

Albumin knockout mice exhibit reduced plasma free fatty acid concentration and enhanced insulin sensitivity.

Circulating albumin is expected to play a significant role in the trafficking of plasma free fatty acids (FFA) between tissues, such as FFA transfer from adipose tissue to the liver. However, it was not yet known how disrupting FFA binding to albumin in circulation would alter lipid metabolism and any resulting impacts upon control of glycemia. To improve understanding of metabolic control, we aimed to determine whether lack of serum albumin would decrease plasma FFA, hepatic lipid storage, whole body substrate oxidation, and glucose metabolism. Male and female homozygous albumin knockout mice and C57BL/6J wild type controls, each on a standard diet containing a moderate fat content, were studied at 6-8 weeks of age. Indirect calorimetry, glucose tolerance testing, insulin tolerance testing, exercise performance, plasma proteome, and tissue analyses were performed. In both sexes of albumin knockout mice compared to the wild type mice, significant reductions (p < 0.05) were observed for plasma FFA concentration, hepatic triacylglycerol and diacylglycerol content, blood glucose during the glucose tolerance test, and blood glucose during the insulin tolerance test. Albumin deficiency did not reduce whole body fat oxidation over a 24-h period and did not alter exercise performance in an incremental treadmill test. The system-level phenotypic changes in lipid and glucose metabolism were accompanied by reduced hepatic perilipin-2 expression (p < 0.05), as well as increased expression of adiponectin (p < 0.05) and glucose transporter-4 (p < 0.05) in adipose tissue. The results indicate an important role of albumin and plasma FFA concentration in lipid metabolism and glucoregulation.

Proteome and phosphoproteome characterization of liver in the postprandial state from diet-induced obese and lean mice.

A metabolic consequence of obesity is hepatosteatosis, which can develop into more serious diseases in the non-alcoholic fatty liver disease (NAFLD) spectrum. The goal of this study was to identify the protein signature of liver in the postprandial state in obesity compared to leanness. The postprandial state is of interest due to the central role of the liver in regulating macronutrient and energy homeostasis during the fed-fast cycle and lack of previously reported controlled studies in the postprandial state. Therefore, we assessed the proteome and phosphoproteome of liver in the postprandial state from diet-induced obese (DIO) and lean mice using untargeted LC-MS/MS analysis. We identified significant alterations in the levels of proteins involved in fatty acid oxidation, activation, and transport, as well as proteins involved in energy metabolism including ketogenesis, tricarboxylic acid cycle, and electron transport chain in liver of DIO compared to lean mice. Additionally, phosphorylated proteins in liver of DIO and lean mice reflect possible regulatory mechanisms controlling fatty acid metabolism and gene expression that may contribute to hepatic metabolic alterations in obesity. Our data indicates PPARα-mediated transcriptional regulation of lipid metabolism and adaptation to hepatic lipid overload. The results of this study expand our knowledge of the molecular changes that occur in liver in the postprandial state in obesity compared to leanness. SIGNIFICANCE: Proteome and phosphoproteome studies of liver in a controlled postprandial state in obesity and leanness are lacking; however, this information is crucial to understanding how obesity-associated hepatosteatosis influences postprandial nutrient and energy metabolism. In this global shotgun proteome and phosphoproteome analysis, we identified unique protein signatures defining obesity and leanness in liver in the postprandial state and identified potential mechanisms contributing to hepatic metabolic alterations in obesity. The results of this study provide a foundation to focus future experiments on the contribution of altered protein and phosphorylation patterns to postprandial metabolism in obesity-associated hepatosteatosis.

Proteomic Characterization of Cytoplasmic Lipid Droplets in Human Metastatic Breast Cancer Cells.

One of the characteristic features of metastatic breast cancer is increased cellular storage of neutral lipid in cytoplasmic lipid droplets (CLDs). CLD accumulation is associated with increased cancer aggressiveness, suggesting CLDs contribute to metastasis. However, how CLDs contribute to metastasis is not clear. CLDs are composed of a neutral lipid core, a phospholipid monolayer, and associated proteins. Proteins that associate with CLDs regulate both cellular and CLD metabolism; however, the proteome of CLDs in metastatic breast cancer and how these proteins may contribute to breast cancer progression is unknown. Therefore, the purpose of this study was to identify the proteome and assess the characteristics of CLDs in the MCF10CA1a human metastatic breast cancer cell line. Utilizing shotgun proteomics, we identified over 1500 proteins involved in a variety of cellular processes in the isolated CLD fraction. Interestingly, unlike other cell lines such as adipocytes or enterocytes, the most enriched protein categories were involved in cellular processes outside of lipid metabolism. For example, cell-cell adhesion was the most enriched category of proteins identified, and many of these proteins have been implicated in breast cancer metastasis. In addition, we characterized CLD size and area in MCF10CA1a cells using transmission electron microscopy. Our results provide a hypothesis-generating list of potential players in breast cancer progression and offers a new perspective on the role of CLDs in cancer.

Diet Induced Obesity Alters Intestinal Cytoplasmic Lipid Droplet Morphology and Proteome in the Postprandial Response to Dietary Fat.

Dietary fat absorption by the small intestine is an efficient, multistep process that regulates the uptake and delivery of essential nutrients and energy. Fatty acids taken up by enterocytes, the absorptive cells of the small intestine, are resynthesized into triacylglycerol (TAG) and either secreted in chylomicrons or temporarily stored in cytoplasmic lipid droplets (CLDs). Proteins that associate with CLDs are thought to regulate the dynamics of TAG storage and mobilization. It is currently unclear what effect diet induced obesity (DIO) has on the balance between dietary fat storage and secretion. Specifically, there is limited knowledge of how DIO affects the level and diversity of proteins that associate with CLDs and regulate CLD dynamics. In the current study, we characterize CLDs from lean and DIO mice through histological and proteomic analyses. We demonstrate that DIO mice have larger intestinal CLDs compared to lean mice in response to dietary fat. Additionally, we identified 375 proteins in the CLD fraction isolated from enterocytes of lean and DIO mice. We identified a subgroup of lipid related proteins that are either increased or unique to the DIO CLD proteome. These proteins are involved in steroid synthesis, TAG synthesis, and lipolysis. This analysis expands our knowledge of the effect of DIO on the process of dietary fat absorption in the small intestine (D'Aquila, 2016).