Retro-endocytosis of low density lipoprotein by cultured human skin fibroblasts.

A fraction of the low-density lipoprotein (LDL) internalized by cells via receptor-mediated endocytosis follows a short-circuit pathway, termed "retro-endocytosis," that results in the rapid exocytosis of ligand. Results from the current study suggest that retro-endocytosis of LDL in human fibroblasts is caused by resurfacing of endocytotic vesicles that contain both free and receptor-bound ligand, resulting in discharge of vesicular contents and in spontaneous dissociation of LDL from its receptor. The bulk of the released LDL particles had the same size, density, and immunogenic properties as native LDL, indicating that they were discharged intact. Some of the retro-endocytosed LDL was larger than native LDL, and some exhibited altered sedimentation properties. When fusion of endosomes with lysosomes was inhibited by chilling cells to 18 degrees C, the proportion of intracellular LDL subsequently released was unaffected, suggesting that retro-endocytosis does not require lysosomal participation. Furthermore, the shorter the internalization phase the greater was the proportion of LDL subsequently released, suggesting that LDL was discharged from compartments formed early in endocytosis. Retro-endocytosis of LDL was stimulated by agents that neutralize acid intracellular compartments, such as ionophores (monensin) and weak bases (chloroquine and methylamine). Monensin increased the proportion of intracellular LDL released, suggesting that it had a direct effect on retro-endocytosis. The effect of weak bases appeared to be secondary to their ability to promote cellular accumulation of undegraded LDL. Thus, retro-endocytosis of LDL becomes a major pathway when intracellular compartments fail to maintain a low pH or where the intracellular concentration of LDL reaches abnormal levels.