Visualization of the in vivo uptake and processing of native and modified low-density-lipoproteins in rat liver
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The in vivo uptake and processing of native and modified low density lipoproteins (LDL) was visualized using an indirect immuno goldlabeling technique on ultrathin cryo-sections. Human LDL or acetylated LDL (AcLDL) was injected intravenously into rats and was allowed to circulate for different time intervals. Subsequently the livers were fixed and processed for lightmicroscopical (LM) or electronmicroscopical (EM) visualization.
AcLDL was associated with endothelial cells in coated pits and in large vacuoles of variable electran density. These large vacuoles were demonstrated not to be in continuity with the cell membrane, suggesting uptake via the coated pits and processing via the large vacuoles. Inhibition of lysosomal breakdown by pretreatment with chloroquine or leupeptin in combination with immune-double labeling of the lysosomal compartments clearly showed localization of AcLDL in lysosomal structures.
Native LDL was localized in Kupffercells and parenchymal cells. Native LDL is rapidly taken up and catabolized by Kupffer cells since we could only demonstrate immuno label inside endosomes and lysosomal organelles after inhibition of the degradative process. In parenchymal cells from estradiol treated rats we demonstrated next to the biochemically well characterized lysosomal pathway, a LDL receptor mediated non-lysosomal route. This transcytotic pathway results in an excretion of a significant proportion of immuno reactive apo B fragments into the bile.
It is concluded that uptake of AcLDL by endothelial cells, and uptake of LDL by Kupffer cells is coupled to lipoprotein de gradation in lysosomes. while an additional exrralysosomal route of LDL. directing apo B to bile is only evident in parenchymal cells.
AcLDL was associated with endothelial cells in coated pits and in large vacuoles of variable electran density. These large vacuoles were demonstrated not to be in continuity with the cell membrane, suggesting uptake via the coated pits and processing via the large vacuoles. Inhibition of lysosomal breakdown by pretreatment with chloroquine or leupeptin in combination with immune-double labeling of the lysosomal compartments clearly showed localization of AcLDL in lysosomal structures.
Native LDL was localized in Kupffercells and parenchymal cells. Native LDL is rapidly taken up and catabolized by Kupffer cells since we could only demonstrate immuno label inside endosomes and lysosomal organelles after inhibition of the degradative process. In parenchymal cells from estradiol treated rats we demonstrated next to the biochemically well characterized lysosomal pathway, a LDL receptor mediated non-lysosomal route. This transcytotic pathway results in an excretion of a significant proportion of immuno reactive apo B fragments into the bile.
It is concluded that uptake of AcLDL by endothelial cells, and uptake of LDL by Kupffer cells is coupled to lipoprotein de gradation in lysosomes. while an additional exrralysosomal route of LDL. directing apo B to bile is only evident in parenchymal cells.
TNO Identifier
268243
Publisher
the Kupffer Cell Foundation
Source title
Cells of the hepatic sinusoid
Editor(s)
Wisse, E.
Knook, D.L.
McCuskey, R.S.
Knook, D.L.
McCuskey, R.S.
Place of publication
Leiden
Pages
489-493
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