Endogenous adipocyte apolipoprotein E is colocalized with caveolin at the adipocyte plasma membrane

Abstract

plex organ with a major role in regulation of organismal metabolism ( 3,4 ). Different from many cell types, adipocytes have adapted to store large amounts of lipid and experience substantial lipid fl ux as part of their differentiated function. This cell type expresses two proteins, caveolin-1 (cav-1) and apolipoprotein (apo)E, at high levels that are likely important for this specialization. Cav-1 is a sterolbinding integral membrane protein that is highly expressed in adipocytes and that specifi es the organization of the adipocyte plasma membrane into ultra-structurally distinct lipid-rich domains termed caveolae ( 5-8 ). It is been estimated that up to 30% of adipocyte plasma membrane is found in caveolae. These structures serve an endocytic function and may be important for insulin signal transduction and Glut-4 translocation ( 5-12 ). A subset of caveolae in adipocytes has been shown to synthesize triglyceride (TG) and to form lipid droplets, and caveolae are an important site of fatty acid internalization by adipocytes ( 6, 13 ). Cav-1 knockout mice have decreased adipose tissue mass, small lipid-poor adipocytes, and are resistant to diet-induced obesity ( 10 ). At the same time, they have increased circulating lipids. These in vivo observations are consistent with an inability to accumulate adipose tissue lipid in cav-1 knockout mice.ApoE is a phospholipid binding protein that is well characterized as a secreted protein from hepatocytes and macrophages and that has an important role in systemic lipoprotein metabolism and vessel wall homeostasis. Its high-level expression by adipocytes was demonstrated two decades ago ( 14 ). More recently, additional information regarding these issues has become available. Nutritional status, peptide hormones, liver X receptor agonists, and peroxisome proliferator-activated receptor g agonists regulate Abstract Apolipoprotein (apo)E is well established as a secreted protein that plays an important role in systemic lipoprotein metabolism and vascular wall homeostasis. Recently, endogenous expression of apoE in adipocytes has been shown to play an important role in adipocyte lipoprotein metabolism and gene expression consistent with a nonsecreted cellular itinerary for apoE. We designed studies to evaluate if adipocyte apoE was retained as a constituent protein in adipocytes and to identify a cellular retention compartment. Using confocal microscopy, coimmunoprecipitation, and sucrose density cellular fractionation, we establish that endogenous apoE shares a cellular itinerary with the constituent protein caveolin-1. Altering adipocyte caveolar number by modulating cellular cholesterol fl ux or altering caveolin expression regulates the distribution of cellular apoE between cytoplasmic and plasma membrane compartments. A mechanism for colocalization of apoE with caveolin was established by demonstrating a noncovalent interaction between an aromatic amino acid-enriched apoE N-terminal domain with the caveolin scaffolding domain. Absent apoE expression in adipocytes alters ...

Expression of the human apoE2 isoform in adipocytes: altered cellular processing and impaired adipocyte lipogenesis

“…This is the precise defect identifi ed in the ineffective lipogenesis measured in EKO adipocytes, and in these cells appears to be related to alterations in caveolin function ( 4,7 ). By comparing EKO and wild-type adipocytes, we have previously established that endogenous expression of adipocyte apoE has an important impact on adipocyte caveolin expression and caveolar number ( 7 ).…”

“…The Quencher based technology (QBT) assay kit was obtained from Molecular Devices (Sunnyvale, CA). Other reagents were from previously identifi ed sources (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14).…”

“…Recently, the unexpected observation was made that the endogenous expression of apoE in adipose tissue and adipocytes importantly infl uences adipocyte differentiated function. Adipocytes that do not express endogenous apoE are smaller, contain less lipid, display a defect in lipogenesis and substrate acquisition from extracellular lipoproteins, and have alterations in the expression of genes controlling adipocyte lipid turnover (4)(5)(6)(7). Moreover, these defects cannot be corrected by the provision of extracellular apoE in vitro or in vivo, but can be corrected by adenoviral-mediated expression of endogenous apoE in adipocytes ( 4,5 ).…”

New Targets and Opportunities at the Level of the Endothelium

“…Also, although not yet proven, it seems likely that endothelial health contributes to the maintenance of large vessel elasticity. As such, given the crucial role played by the endothelium in cardiovascular health, 2 recent publications from Yue et al 1,2 at the University of Illinois are particularly exciting. Their latest study 1 shows that, in cultured human umbilical vein endothelial cells, the phospholipid binding protein apolipoprotein E (ApoE) reverses the inhibitory effect of caveolin 1 on endothelial NO synthase.…”

Secreted caveolin-1 enhances periodontal inflammation by targeting gingival fibroblasts

“…Next, after HGFs were treated with 100 or 1,000 ng/mL of Cav-1 for 10 min, the total cell lysates were collected to examine the intracellular signaling pathway. Total cell lysates were extracted with a lysis buffer: 10 mM Tris-HCl, pH 7.4, 50 mM cholesterol transport (32) and endocytosis (18). In addition, Cav-1 seems to modulate the signal transduction by linking the signaling molecules, leading to regulating their downstream activity (33).…”