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.
D000818:Animals; D001011:Aorta / Q000196:embryology / Q000378:metabolism; D002417:Cattle; D002478:Cells, Cultured; D011232:Chemical Precipitation; D004705:Endocytosis; D007700:Kinetics; D008077:Lipoproteins, LDL / Q000378:metabolism*; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D014238:Trichloroacetic Acid
D008077:Lipoproteins, LDL; D014238:Trichloroacetic Acid