Dissection of the multifunctional “receptor-recycling” endocytic compartment of hepatocytes

We previously have isolated an endosomal fraction from rat liver, termed receptor-recycling compartment (RRC), which is highly enriched in recycling receptors and in the transcytotic polymeric Ig receptor (pIgR). We now have analyzed the RRC fraction by immunoisolation and found that no uniquely transcytotic elements were present, because recycling receptors and the pIgR were coisolated on the same elements. In addition, RRC was very rich in proteins previously shown to be associated with recycling endosomes, such as rab 11, cellubrevin, and endobrevin, but relatively poor in early endosome antigen 1. As RRC contains mainly tubules and small vesicles, our results indicate that it is enriched in elements of a tubular endosomal compartment involved in receptor sorting. Biochemical analysis showed that the density of recycling receptors and transcytotic pIgR in RRC membranes was similar to that in early endosome membranes. This observation supports the idea that increasing membrane surface area by endosome tubulation is the main mechanism to ensure efficient receptor sorting and, at the same time, locates RRC in a common step of the endocytotic system before final receptor segregation into distinct recycling and transcytotic pathways.

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“…13). These data confirm the suggested involvement of RRC in transcytosis and recycling (18,(20)(21)(22)(23).…”

supporting

confidence: 81%

A tubular endosomal fraction from rat liver: Biochemical evidence of receptor sorting by default

Vergés

Havel

Mostov

1999

Proc. Natl. Acad. Sci. U.S.A.

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“…Consistent with these findings, lysosomal targeting of TRL proteins and apoE was markedly reduced compared with the degradation of cholesteryl oleate in mouse hepatocytes (13). In human hepatoma cells and fibroblasts, the majority of TRL lipids are targeted to the lysosomal compartment, whereas TRL-derived apoE is found in peripheral recycling endosomes, which are distinct from the perinuclear transferrin recycling compartment (14,15). Subsequently substantial amounts of TRL-derived apoE are recycled back to the cell surface and re-secreted (14).…”

mentioning

confidence: 63%

“…Each channel is shown separately in the lower panels as indicated. Arrowheads mark and follow the movement of a complex in the merge (image numbers [2][3][4][5] and in each lower panel: apoE (image numbers 7-10), NBD-cholesterol (image numbers [12][13][14][15], and apoA-I (image numbers 19 and 20). Bar is 2 m. and cholesterol remain associated in EEA1-positive endosomes.…”

Section: Discussionmentioning

confidence: 99%

Recycling of Apoprotein E Is Associated with Cholesterol Efflux and High Density Lipoprotein Internalization

Heeren¹,

Grewal²,

Laatsch³

et al. 2003

Journal of Biological Chemistry

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After receptor-mediated endocytosis of triglyceriderich lipoproteins (TRL) into the liver, TRL particles are immediately disintegrated in peripheral endosomal compartments. Whereas core lipids and apoprotein B are delivered for degradation into lysosomes, TRL-derived apoE is efficiently recycled back to the plasma membrane. This is followed by apoE re-secretion and association of apoE with high density lipoproteins (HDL). Because HDL and apoE can independently promote cholesterol efflux, we investigated whether recycling of TRL-derived apoE in human hepatoma cells and fibroblasts could be linked to intracellular cholesterol transport. In this study we demonstrate that HDL 3 does not only act as an extracellular acceptor for recycled apoE but also stimulates the recycling of internalized TRL-derived apoE. Furthermore, radioactive pulsechase experiments indicate that apoE recycling is accompanied by cholesterol efflux. Confocal imaging reveals co-localization of apoE and cholesterol in early endosome antigen 1-positive endosomes. During apoE re-secretion, HDL 3 -derived apoA-I is found in these early endosome antigen 1, cholesterol-containing endosomes. As shown by time-lapse fluorescence microscopy, apoE recycling involves the intracellular trafficking of apoA-I to pre-existing and TRL-derived apoE/cholesterol-containing endosomes in the periphery. Thus, these studies provide evidence for a new intracellular link between TRL-derived apoE, cellular cholesterol transport, and HDL metabolism.

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“…First of all, CURL represents the early endosomal sorting compartment and contains internalized radiolabeled LDL or TRL that will subsequently be directed to either RRC or MVB vesicles (30). RRC endosomal preparations contain substantial amounts of TRL-derived apoE and LPL (Fig.…”

Section: Discussionmentioning

confidence: 99%

“…The subcellular fractionation developed by Belcher et al (27) separates endosomal fractions on the basis of their morphology: CURL represents the early endocytic compartment involved in the sorting of ligands, whereas MVBs correspond to late endosomes and prelysosomal structures. The third endosomal fraction, the RRC, is characterized by tubular structures that originate from membranous appendages emanating from CURL and MVBs that are enriched in recycling receptors and depleted of ligands (29,30).…”

Section: Intracellular Processing Of Apoe and Lpl Derived From Triglymentioning

confidence: 99%

Recycling of Apolipoprotein E and Lipoprotein Lipase through Endosomal Compartments in Vivo

Heeren¹,

Grewal²,

Jäckle

et al. 2001

Journal of Biological Chemistry

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We have recently described a novel recycling pathway of triglyceride-rich lipoprotein (TRL)-associated apolipoprotein (apo) E in human hepatoma cells. We now demonstrate that not only TRL-derived apoE but also lipoprotein lipase (LPL) is efficiently recycled in vitro and in vivo. Similar recycling kinetics of apoE and LPL in normal and low density lipoprotein receptor-negative human fibroblasts also indicate that the low density lipoprotein receptor-related protein seems to be involved. Intracellular sorting mechanisms are responsible for reduced lysosomal degradation of both ligands after receptor-mediated internalization. Immediately after internalization in rat liver, TRLs are disintegrated, and apoE and LPL are found in endosomal compartments, whereas TRL-derived phospholipids accumulate in the perinuclear region of hepatocytes. Subsequently, substantial amounts of both proteins can be found in purified recycling endosomes, indicating a potential resecretion of these TRL components. Pulse-chase experiments of perfused rat livers with radiolabeled TRLs demonstrated a serum-induced release of internalized apoE and LPL into the perfusate. Analysis of the secreted proteins identified ϳ80% of the recycled TRLderived proteins in the high density lipoprotein fractions. These results provide the first evidence that recycling of TRL-derived apoE and LPL could play an important role in the modulation of lipoproteins in vivo.Triglycerides are transported mainly by two distinct classes of triglyceride-rich lipoproteins (TRLs), 1 the chylomicrons and the very low density lipoproteins (VLDLs). After assembly in the intestine, chylomicrons are transported via lymph into the bloodstream, where they are converted at the endothelial surface to remnant lipoproteins through the catalytic action of lipoprotein lipase (LPL) (for review, see Refs. 1 and 2). After lipolysis, LPL remains associated with the chylomicron remnants and, in concert with apolipoprotein (apo) E (3-5), facilitates their clearance into hepatocytes (6) via LDL receptor (LDLR) and the LDLR-related protein (LRP) (7-10). The essential role for both receptors in TRL removal in vivo has been demonstrated in gene knockout and gene transfer experiments (Refs. 11 and 12; for a recent review, see Ref. 13).Several studies have used different "model particles" to investigate the intracellular processing of TRL constituents. In contrast to the lysosomal degradation of LDL-derived apoB (14), ␤-VLDL-derived apoE was identified in widely distributed vesicles and showed a slow protein degradation in mouse macrophages (15, 16). However, in the same cells, ␤-VLDLderived lipids were delivered to perinuclear, lysosomal compartments (17). Delayed transport and degradation of TRL proteins were also observed in hepatoma cells (18,19). In recent studies, we have been able to demonstrate that the altered transport and retarded degradation of internalized TRLs is due to intracellular disintegration and sorting of TRL components in a peripheral cellular compartment. Whereas lipids are ...

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Dissection of the multifunctional “receptor-recycling” endocytic compartment of hepatocytes

Cited by 19 publications

References 80 publications

A tubular endosomal fraction from rat liver: Biochemical evidence of receptor sorting by default

A tubular endosomal fraction from rat liver: Biochemical evidence of receptor sorting by default

Recycling of Apoprotein E Is Associated with Cholesterol Efflux and High Density Lipoprotein Internalization

Recycling of Apolipoprotein E and Lipoprotein Lipase through Endosomal Compartments in Vivo

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