Morphological characterization of scavenger receptor-mediated processing of modified lipoproteins by rat liver endothelial cells

Scavenger receptor-mediated processing by rat liver endothelial cells in vivo is studied by using acetylated and oxidized low-density lipoproteins (LDL) as ligands. The cellular localization of acetylated LDL (Ac-LDL) is visualized by both immunohistochemistry and silver enhancement of ultrasmall gold particles conjugated to Ac-LDL. Scavenger receptor-mediated internalization by the endothelial cells only involves coated vesicle formation. Subsequently, three stages of processing are noticed, as represented by (i) large electron lucent vesicles, with ligand in close association to the membrane, (ii) relatively electron-lucent structures, with Ac-LDL dispersed over the vesicular lumen, while tubular membrane extensions did not contain ligand, and (iii) electron-dense vesicles in which the nondegradable gold particles of the conjugate accumulate, while the immunoreactivity for Ac-LDL is low. Addition of chloroquine, an inhibitor of lysosomal degradation, demonstrated that the relatively electron-lucent and electron-dense structures represent subsequent stages of the lysosomal pathway of Ac-LDL, which was also verified by detection of the lysosomal enzyme cathepsin D. Evaluation of the processing of Ac-LDL and oxidized LDL, labeled with different fluorochromes, demonstrated that both ligands follow apparently the same intracellular pathway in the liver endothelial cells, since the fluorescent probes are predominantly localized in the same structures. It is concluded that the scavenger receptor-mediated processing of modified LDL by rat liver endothelial cells involves four morphologically distinguishable stages which represent a highly effective catabolic route sustaining the important role of the liver endothelial cells in the protection against circulating atherogenic lipoproteins.