Caveolae mediated endocytosis of VLDL particles in macrophages requires NPC1 and STARD3 for further lysosomal processing

Abstract

Macrophages accumulate triglycerides under certain pathological conditions such as atherosclerosis. Triglycerides are carried in the bloodstream as part of very low-density lipoproteins (VLDL) and chylomicrons. How macrophages take up and process VLDL-lipids is not very well known. Here, using VLDL-sized triglyceride-rich emulsion particles, we aimed to study the mechanism by which VLDL-triglycerides are taken up, processed, and stored in macrophages. Our results show that macrophage uptake of emulsion particles mimicking VLDL (VLDLm) is dependent on lipoproteins lipase (LPL) and requires the lipoprotein-binding C-terminal domain of LPL but not the catalytic N-terminal domain. Subsequent internalization of VLDLm-triglycerides by macrophages is carried out by caveolae-mediated endocytosis, followed by triglyceride hydrolysis catalyzed by lysosomal acid lipase. Transfer of lysosomal fatty acids to the ER for subsequent storage as triglycerides is mediated by Stard3, whereas NPC1 was found to promote the extracellular efflux of fatty acids from lysosomes. Our data provide novel insights into how macrophages process VLDL-derived triglycerides and suggest that macrophages have the remarkable capacity to excrete part of the internalized triglycerides as fatty acids. SummaryTriglyceride-rich lipoproteins and their remnants contribute to atherosclerosis, possibly by carrying remnant cholesterol and/or by exerting a pro-inflammatory effect on macrophage. Nevertheless, little is known about how macrophages process triglyceride-rich lipoproteins. We show that uptake by macrophages of VLDL-like particles is dependent on the enzyme lipoproteins lipase via its C-terminal domain. Subsequent internalization of VLDL-triglycerides by macrophages is carried out by caveolae-mediated endocytosis, followed by hydrolysis by lysosomal acid lipase. Transfer of lysosomal fatty acids to the ER for lipid storage is mediated by Stard3, while NPC1 promotes the extracellular efflux of fatty acids. Our data provide novel insights into how macrophages process VLDL-derived triglycerides and suggest that macrophages have the remarkable capacity to excrete internalized triglycerides as fatty acids. O_FIG_DISPLAY_L [Figure 1] M_FIG_DISPLAY C_FIG_DISPLAY