Retinol-binding protein and asialo-orosomucoid are taken up by different pathways in liver cells.
The intracellular transport and degradation of in vivo endocytosed retinol-binding protein was compared with that of asialo-orosomucoid, a marker for receptor-mediated endocytosis through coated pits. The transport pathways were studied in rat liver cells by means of subcellular fractionation in Nycodenz and sucrose density gradients and by immunoelectron microscopy. Retinol-binding protein and asialo-orosomucoid were labeled by covalent attachment of radioiodinated tyramine cellobiose, an adduct which is incapable of crossing cellular membranes and thus provides a marker for the organelles where the protein has been taken up and degraded. The data obtained from subcellular fractionation studies, as well as from immunoelectron microscopy, showed that retinol-binding protein and asialo-orosomucoid were initially localized in different endocytic vesicles. Retinol-binding protein co-localized in density gradients with markers for potocytosis, an alternative endocytic pathway which uses internalization through caveolae instead of clathrin-coated pits. Later, retinol-binding protein and asialo-orosomucoid comigrated in the gradients and they were also observed in the same larger vesicles by immunoelectron microscopy. These data suggest that retinol-binding protein is taken up by liver cells by potocytosis and that a fraction of the retinol-binding protein is later transferred to larger vesicles located deeper in the cytoplasm where degradation takes place.
Malaba
L
L
Institute for Nutrition Research, School of Medicine, University of Oslo, Norway.
Smeland
S
S
Senoo
H
H
Norum
K R
KR
Berg
T
T
Blomhoff
R
R
Kindberg
G M
GM
D000818:Animals; D001212:Asialoglycoproteins / Q000378:metabolism*; D051242:Caveolin 1; D022461:Caveolins; D002458:Cell Fractionation; D002499:Centrifugation, Density Gradient; D004705:Endocytosis; D008099:Liver / Q000378:metabolism* / Q000648:ultrastructure; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D009961:Orosomucoid / Q000031:analogs & derivatives* / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012177:Retinol-Binding Proteins / Q000378:metabolism*
D001212:Asialoglycoproteins; D051242:Caveolin 1; D022461:Caveolins; D008565:Membrane Proteins; D009961:Orosomucoid; D012177:Retinol-Binding Proteins; C017030:asialoorosomucoid