Direct Visualization of Lipid Deposition and Reverse Lipid Transport in a Perfused Artery

The major goal of this study was to determine the interactions of VLDL surface and core lipids with the artery wall. We first demonstrated in vitro that surface lipid in VLDL could be traced using the phospholipid-like fluorescent probe 1,1'-dioctadecyl-3,3, 3',3'-tetramethyl-indocarbocyanine (DiI). The core of VLDL particles was traced by fluorescently labeling apolipoprotein B with TRITC. The labeled VLDLs were perfused through rat carotid arteries, and accumulation of the fluorescently labeled VLDL components in the arterial walls was determined by quantitative fluorescence microscopy. Addition of lipoprotein lipase increased the accumulation of both DiI and TRITC by >2.3-fold. Histological examination showed that DiI and TRITC were primarily localized to the endothelial layer; however, DiI also accumulated as small "lakes" deeper in the artery, in a subendothelial position. Addition of HDL to the perfusion decreased the accumulation of surface lipid and apolipoprotein B-containing particles and eliminated the DiI lakes. Moreover, the increase in endothelial layer permeability associated with lipolysis was attenuated 21% by HDL. If VLDL surface lipid first was allowed to accumulate in the arterial wall, its subsequent rate of loss was more than twice as fast if HDL was included in the perfusate. These studies directly demonstrate atherogenic effects of VLDL lipolysis and their inhibition by HDL.