Retro-endocytosis of low density lipoprotein by cultured bovine aortic smooth muscle cells.

Cultured bovine aortic smooth muscle cells, pretreated with 125I-labelled low density lipoprotein (LDL), rapidly released significant amounts of the lipoprotein as trichloroacetic acid-precipitable material during a subsequent chase period. The time and temperature dependence of this release process and its insensitivity to heparin-pretreatment of equilibrated cells suggest that LDL was regurgitated from cells by a rapid process that we have termed 'retro-endocytosis'. The total amount of lipoprotein released from cells equilibrated at 37 degrees C with 125I-labelled LDL was approximately 20% of the amount degraded, pointing to the existence of a small pool of material which was distinct from the lysosomal pathway. To quantify the flux of LDL through retro-endocytosis, the fate of surface-bound lipoproteins was analyzed. Cells, pretreated with 125I-labelled LDL at 4 degrees C, regurgitated about 50% of the initial surface-bound LDL during a chase period at 37 degrees C and degraded the remainder more slowly through the lysosomal pathway. The involvement of LDL-receptors was implicated because retro-endocytosis was a saturable process and was affected by up- and down-regulation. The apolipoprotein of the released LDL showed little proteolytic modification as analyzed by gel filtration. We conclude that in a steady-state situation the fraction of LDL that passes through the retro-endocytosis pathway is of the same order as that which is directed through the lysosomal system.