Retroendocytosis of low density lipoprotein. Effect of lysosomal inhibitors on the release of undegraded 125I-low density lipoprotein of altered composition from skin fibroblasts in culture.

Retroendocytosis of low density lipoprotein (LDL) is a newly identified pathway of cellular LDL processing in which LDL that has been internalized via LDL receptors is subsequently re-excreted from the cell and not degraded in the lysosomes. In the present study, we examined several characteristics of the retroendocytosis process as well as some of the physical characteristics of the retroendocytosed LDL. Monkey skin fibroblasts were pulse labeled with 125I-LDL for 4 h at 37 degrees C and were then washed extensively to remove both surface and LDL receptor-bound 125I-LDL. Retroendocytosis of LDL was determined by measuring the release of trichloroacetic acid-precipitable 125I-material during a subsequent 2-h chase period. In the presence of the lysosomotropic agents, chloroquine and methylamine, the absolute amount of LDL retroendocytosed was increased up to 3-fold. Without the lysosomotropic agents, less than 10% of the cell-associated 125I-LDL was lost by retroendocytosis in a 2-h chase period; this value increased to as much as 30% in the presence of the lysosomotropic agents. In the presence of lysosomotropic agents, the amount of LDL processed by retroendocytosis was approximately equivalent to the amount of LDL that was degraded. This enhancement of retroendocytosis was seen at 37 degrees C but not at 4 degrees C and was observed only when the lysosomotropic agents were present during the pulse period. Thus, the enhanced retroendocytosis was not a direct effect of the lysosomotropic agents but, instead, was probably secondary to the buildup of undegraded LDL within the cell. As a result, under certain conditions, retroendocytosis of LDL may be a quantitatively important alternate pathway for the cellular processing of LDL. In the absence of lysosomal inhibitors, up to one-third of the retroendocytosed material could have come from 125I-LDL that had not been removed from the cell surface prior to the chase period. In the presence of the lysosomotropic agents, however, less than 10% could have come from the cell surface. The enhancement of retroendocytosis by the lysosomotropic agents was not the result of cytotoxic effects of these agents since release of [3H]leucine-labeled cell proteins was altered only slightly. Thus, the bulk of the retroendocytosed LDL resulted from an active metabolic process for the efflux of LDL that had previously been internalized by the cell.(ABSTRACT TRUNCATED AT 400 WORDS)