Phospholipid transfer protein can transform reconstituted discoidal HDL into vesicular structures

Niacin Increases Atherogenic Proteins in High-Density Lipoprotein of Statin-Treated Subjects

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The regulation of glucose and lipid homeostasis via PLTP as a mediator of BAT-liver communication

While brown adipose tissue (BAT) is well-recognized for its ability to dissipate energy in the form of heat, recent studies suggest multifaced roles of BAT in the regulation of glucose and lipid homeostasis beyond stimulating thermogenesis. One of the functions involves interorgan communication with metabolic organs, such as the liver, through BAT-derived secretory factors, a.k.a., batokine. However, the identity and the roles of such mediators remain insufficiently understood. Here, we employed proteomics and transcriptomics in human thermogenic adipocytes and identified previously unappreciated batokines, including phospholipid transfer protein (PLTP). We found that increased circulating levels of PLTP, via systemic or BAT-specific overexpression, significantly improve glucose tolerance and insulin sensitivity, increased energy expenditure, and decrease the circulating levels of cholesterol, phospholipids, and sphingolipids. Such changes were accompanied by increased bile acids in the circulation, which in turn enhances glucose uptake and thermogenesis in BAT. Our data suggest that PLTP is a batokine that contributes to the regulation of systemic glucose and lipid homeostasis as a mediator of BAT-liver interorgan communication.

Spontaneous remodeling of HDL particles at acidic pH enhances their capacity to induce cholesterol efflux from human macrophage foam cells

HDL particles may enter atherosclerotic lesions having an acidic intimal fluid. Therefore, we investigated whether acidic pH would affect their structural and functional properties. For this purpose, HDL(2) and HDL(3) subfractions were incubated for various periods of time at different pH values ranging from 5.5 to 7.5, after which their protein and lipid compositions, size, structure, and cholesterol efflux capacity were analyzed. Incubation of either subfraction at acidic pH induced unfolding of apolipoproteins, which was followed by release of lipid-poor apoA-I and ensuing fusion of the HDL particles. The acidic pH-modified HDL particles exhibited an enhanced ability to promote cholesterol efflux from cholesterol-laden primary human macrophages. Importantly, treatment of the acidic pH-modified HDL with the mast cell-derived protease chymase completely depleted the newly generated lipid-poor apoA-I, and prevented the acidic pH-dependent increase in cholesterol efflux. The above-found pH-dependent structural and functional changes were stronger in HDL(3) than in HDL(2). Spontaneous acidic pH-induced remodeling of mature spherical HDL particles increases HDL-induced cholesterol efflux from macrophage foam cells, and therefore may have atheroprotective effects.

Role of plasma phospholipid transfer protein in lipid and lipoprotein metabolism

The understanding of the physiological and pathophysiological role of PLTP has greatly increased since the discovery of PLTP more than a quarter of century ago. A comprehensive review of PLTP is presented on the following topics: PLTP gene organization and structure; PLTP transfer properties; different forms of PLTP; characteristics of plasma PLTP complexes; relationship of plasma PLTP activity, mass and specific activity with lipoprotein and metabolic factors; role of PLTP in lipoprotein metabolism; PLTP and reverse cholesterol transport; insights from studies of PLTP variants; insights of PLTP from animal studies; PLTP and atherosclerosis; PLTP and signal transduction; PLTP in the brain; and PLTP in human disease. PLTP's central role in lipoprotein metabolism and lipid transport in the vascular compartment has been firmly established. However, more studies are needed to further delineate PLTP's functions in specific tissues, such as the lung, brain and adipose tissue. Furthermore, the specific role that PLTP plays in human diseases, such as atherosclerosis, cancer, or neurodegenerative disease, remains to be clarified. Exciting directions for future research include evaluation of PLTP's physiological relevance in intracellular lipid metabolism and signal transduction, which undoubtedly will advance our knowledge of PLTP functions in health and disease. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

The impact of phospholipid transfer protein (PLTP) on HDL metabolism

High-density lipoproteins (HDL) play a major protective role against the development of coronary artery disease. Phospholipid transfer protein (PLTP) is a main factor regulating the size and composition of HDL in the circulation and plays an important role in controlling plasma HDL levels. This is achieved via both the phospholipid transfer activity of PLTP and its capability to cause HDL conversion. The present review focuses on the impact of PLTP on HDL metabolism. The basic characteristics and structure of the PLTP protein are described. The two main functions of PLTP, PLTP-mediated phospholipid transfer and HDL conversion are reviewed, and the mechanisms and control, as well as the physiological significance of these processes are discussed. The relationship between PLTP and the related cholesteryl ester transfer protein (CETP) is reviewed. Thereafter other functions of PLTP are recapitulated: the ability of PLTP to transfer cholesterol, alpha-tocopherol and lipopolysaccharide (LPS), and the suggested involvement of PLTP in cellular cholesterol traffic. The discussion on PLTP activity and mass in (patho)physiological settings includes new data on the presence of two forms of PLTP in the circulation, one catalytically active and the other inactive. Finally, future directions for PLTP research are outlined.

Secretion of phospholipid transfer protein by human hepatoma cell line, Hep G2, is enhanced by sodium butyrate

Hep G2 cells were used to study the synthesis and secretion of phospholipid transfer protein (PLTP). Upon incubation of the cells at confluence with serum-free Dulbecco's modified Eagle's medium (DMEM), phosphatidylcholine (PC) transfer activity was found to accumulate in the culture media. The PC transfer activity in the media was effectively inhibited by rabbit anti-human PLTP immunoglobulin (Ig)G, thus indicating that the PC transfer activity was due to secreted PLTP. The molecular weight of Hep G2 PLTP was approximately 78 kDa by Western blot analysis, in agreement with the molecular weight obtained for purified human plasma PLTP. The PLTP secreted by Hep G2 also possessed an HDL conversion activity similar to that of human plasma PLTP. The addition of butyrate to the cell culture media resulted in a marked increase in the secretion of PLTP. After 24 h incubation with 4 mmol/L sodium butyrate, a more than twofold increase (P < 0.01) of PC transfer activity in the cell-conditioned media was obtained. The dose-dependent increase in the PC transfer activity in the media upon butyrate treatment was well correlated (r = 0.80, P < 0.01) with that of PLTP mass as determined by immuno-slot blot analysis of cell-conditioned media. The increased secretion of PLTP by Hep G2 treated with sodium butyrate was accompanied by a greater increase in the level of PLTP mRNA in the cells as determined by ribonuclease protection assay. In the presence of 4 mmol/L sodium butyrate, a fourfold increase (P < 0. 01) in mRNA level was obtained at 24 h. No stabilizing effect of butyrate on PLTP mRNA was apparent upon treatment of the cultured cells with the RNA synthesis inhibitor, actinomycin D. Thus, the up-regulatory effect of butyrate on PLTP gene expression seemed to have occurred at the transcriptional level.

Unique lipoproteins secreted by primary astrocytes from wild type, apoE (-/-), and human apoE transgenic mice

Composition of central nervous system lipoproteins affects the metabolism of lipoprotein constituents within the brain. The epsilon4 allele of apolipoprotein E (apoE) is a risk factor for Alzheimer's disease via an unknown mechanism(s). As glia are the primary central nervous system cell type that synthesize apoE, we characterized lipoproteins secreted by astrocytes from wild type (WT), apoE (-/-), and apoE transgenic mice expressing human apoE3 or apoE4 in a mouse apoE (-/-) background. Nondenaturing size exclusion chromatography demonstrates that WT, apoE3, and apoE4 astrocytes secrete particles the size of plasma high density lipoprotein (HDL) composed of phospholipid, free cholesterol, and protein, primarily apoE and apoJ. However, the lipid:apoE ratio of particles containing human apoE is significantly lower than WT. ApoE localizes across HDL-like particle sizes. ApoJ localizes to the smallest HDL-like particles. ApoE (-/-) astrocytes secrete little phospholipid or free cholesterol despite comparable apoJ expression, suggesting that apoE is required for normal secretion of astrocyte lipoproteins. Further, particles were not detected in apoE (-/-) samples by electron microscopy. Nondenaturing immunoprecipitation experiments indicate that apoE and apoJ reside predominantly on distinct particles. These studies suggest that apoE expression influences the unique structure of astrocyte lipoproteins, a process further modified by apoE species.