Atherosclerotic lesions are characterized by the accumulation of oxidized LDL (OxLDL) and the infiltration of macrophages and T cells. Cytokine expression in the microenvironment of evolving lesions can profoundly contribute to plaque development. While the pro‐atherogenic effect of T helper (Th) 1 cytokines, such as IFN‐γ, is well established, the role of Th2 cytokines is less clear. Therefore, we characterized the role of the Th2 cytokine interleukin (IL)‐13 in murine atherosclerosis. Here, we report that IL‐13 administration favourably modulated the morphology of already established atherosclerotic lesions by increasing lesional collagen content and reducing vascular cell adhesion molecule‐1 (VCAM‐1)‐dependent monocyte recruitment, resulting in decreased plaque macrophage content. This was accompanied by the induction of alternatively activated (M2) macrophages, which exhibited increased clearance of OxLDL compared to IFN‐γ‐activated (M1) macrophages in vitro. Importantly, deficiency of IL‐13 results in accelerated atherosclerosis in LDLR^−/− mice without affecting plasma cholesterol levels. Thus, IL‐13 protects from atherosclerosis and promotes a favourable plaque morphology, in part through the induction of alternatively activated macrophages.

Perfluoroalkyl (PFAS) substances are widespread in the environment and in organisms. The fact that exposure to PFAS is associated with elevated cholesterol levels is a major concern for human health. Previous investigations, in which bovine serum albumin was frequently studied, indicate that PFOS, PFOA and PFNA bind to serum albumin. However, it is critical to know whether these and other PFAS have a preference for the protein or the lipid fraction in native human blood fractions. For this reason, blood samples from four young healthy volunteers (two women, two men, 23-31 years old) were used for protein size separation and fractionation by the Cohn method in combination with serial ultracentrifugation. The plasma fractions were analyzed for 11 PFAS using high-performance tandem mass spectrometry (HPLC-MS/MS). Although the data are based on a small sample, they clearly show that albumin is the most important carrier protein for PFOS, PFOA, PFHxS, PFNA and PFDA in native human plasma. These five compounds have very little or no affinity for lipoproteins. The confirmation of their transport through albumin is important for the epidemiology of PFAS. The present results must be verified by the examination of a larger number of persons.

The scavenger receptor-BI (SR-BI) delivers sterols from circulating lipoproteins to tissues, but the relative potency of individual lipoproteins and the transported cholesterol has not been studied in detail. In this study, we used Chinese hamster ovary cells that express recombinant mouse SR-BI but have no functional low density lipoprotein (LDL) receptors (ldlA7-SRBI cells) to compare the fate of lipids transferred from high or low density lipoproteins to cells by SR-BI. HDL and LDL were equally effective in mediating the transfer of [³H]cholesterol to cells. Only 5% of the free cholesterol transferred to cells was esterified, in direct contrast to the findings in the cells that express LDL receptors in which 50% of the transported cholesterol was esterified. Almost all the free cholesterol transferred from lipoproteins to cells was rapidly excreted when the ldlA7-SRBI cells were switched to media containing unlabeled lipoproteins. SR-BI expression was associated with an increase in selective cholesteryl ester uptake from both lipoproteins, but HDL was a more effective donor. HDL and LDL were equally effective in delivering cholesterol to the intracellular regulatory pool via SR-BI. These data indicate that SR-BI is able to exchange cholesterol rapidly between lipoproteins and cell membranes and can mediate the uptake of cholesteryl esters from both classes of lipoproteins.

The high density lipoprotein (HDL) receptor, scavenger receptor class B, type I (SR-BI), mediates selective cholesteryl ester uptake from lipoproteins into liver and steroidogenic tissues but also cholesterol efflux from macrophages to HDL. Recently, we demonstrated the uptake of HDL particles in SR-BI overexpressing Chinese hamster ovarian cells (ldlA7-SRBI) using ultrasensitive microscopy. In this study we show that this uptake of entire HDL particles is followed by resecretion. After uptake, HDL is localized in endocytic vesicles and organelles en route to the perinuclear area; many HDL-positive compartments were classified as multivesiculated and multilamellated organelles by electron microscopy. By using 125I-labeled HDL, we found that approximately 0.8% of the HDL added to the media is taken up by the ldlA7-SRBI cells within 1 h, and almost all HDL is finally resecreted. 125I-Labeled low density lipoprotein showed a very similar association, uptake, and resecretion pattern in ldlA7-SRBI cells that do not express any low density lipoprotein receptor. Moreover, we demonstrate that the process of HDL cell association, uptake, and resecretion occurs in three physiologically relevant cell systems, the liver cell line HepG2, the adrenal cell line Y1BS1, and phorbol myristate acetate-differentiated THP-1 cells as a model for macrophages. Finally, we present evidence that HDL retroendocytosis represents one of the pathways for cholesterol efflux.

Scavenger receptor, class B, type I (SR-BI) is a cell surface glycoprotein that mediates selective uptake and efflux of sterols from high density lipoproteins (HDL) to cells. A Chinese hamster ovary cell line that is deficient in functional LDL receptors, but has high expression levels of recombinant SR-BI (ldlA7-SR-BI), was used to examine the effect of SR-BI on the trafficking of sterols between lipoproteins and cells. To monitor the fate of sterols transported by SR-BI into cells, we measured the incorporation of [¹⁴C]oleate into cholesterol esters by acyl-CoA:cholesteryl acyltransferase in the endoplasmic reticulum. We show that incubation of ldlA7-SRBI cells with either LDL or HDL resulted in an equally dramatic increase in the formation of [¹⁴C]oleate-labeled cholesterol esters. The lipoprotein-stimulated, SR-BI-dependent increase in cholesterol esterification was inhibited by chloroquine. The uptake of sterols and their incorporation into cholesterol esters by SR-BI from LDL was largely a selective process. The addition of free cholesterol to ldlA7-SRBI cells also stimulated cholesterol ester formation in a chloroquine-sensitive fashion. We also show that SR-BI mediates the efflux of endogenously synthesized sterols from the cell membrane. From these studies we conclude that, in the absence of the LDL receptor, overexpression of SR-BI can mediate significant transport of sterols between lipoproteins and the endoplasmic reticulum of cells.

HDL removes excess cholesterol from peripheral tissues and delivers it to the liver and steroidogenic tissues via selective lipid uptake without catabolism of the HDL particle itself. In addition, endocytosis of HDL holo-particles has been debated for nearly 40years. However, neither the connection between HDL endocytosis and selective lipid uptake, nor the physiological relevance of HDL uptake has been delineated clearly. This review will focus on HDL endocytosis and resecretion and its relation to cholesterol transfer. We will discuss the role of HDL endocytosis in maintaining cholesterol homeostasis in tissues and cell types involved in atherosclerosis, focusing on liver, macrophages and endothelium. We will critically summarize the current knowledge on the receptors mediating HDL endocytosis including SR-BI, F1-ATPase and CD36 and on intracellular HDL transport routes. Dependent on the tissue, HDL is either resecreted (retro-endocytosis) or degraded after endocytosis. Finally, findings on HDL transcytosis across the endothelial barrier will be summarized. We suggest that HDL endocytosis and resecretion is a rather redundant pathway under physiologic conditions. In case of disturbed lipid metabolism, however, HDL retro-endocytosis represents an alternative pathway that enables tissues to maintain cellular cholesterol homeostasis.