Trafficking of the epidermal growth factor receptor and transferrin in three hepatocytic endosomal fractions

Ca2+ and Annexins - Emerging Players for Sensing and Transferring Cholesterol and Phosphoinositides via Membrane Contact Sites

Maintaining lipid composition diversity in membranes from different organelles is critical for numerous cellular processes. However, many lipids are synthesized in the endoplasmic reticulum (ER) and require delivery to other organelles. In this scenario, formation of membrane contact sites (MCS) between neighbouring organelles has emerged as a novel non-vesicular lipid transport mechanism. Dissecting the molecular composition of MCS identified phosphoinositides (PIs), cholesterol, scaffolding/tethering proteins as well as Ca²⁺ and Ca²⁺-binding proteins contributing to MCS functioning. Compelling evidence now exists for the shuttling of PIs and cholesterol across MCS, affecting their concentrations in distinct membrane domains and diverse roles in membrane trafficking. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at the plasma membrane (PM) not only controls endo-/exocytic membrane dynamics but is also critical in autophagy. Cholesterol is highly concentrated at the PM and enriched in recycling endosomes and Golgi membranes. MCS-mediated cholesterol transfer is intensely researched, identifying MCS dysfunction or altered MCS partnerships to correlate with de-regulated cellular cholesterol homeostasis and pathologies. Annexins, a conserved family of Ca²⁺-dependent phospholipid binding proteins, contribute to tethering and untethering events at MCS. In this chapter, we will discuss how Ca²⁺ homeostasis and annexins in the endocytic compartment affect the sensing and transfer of cholesterol and PIs across MCS.

Annexin A6 in the liver: From the endocytic compartment to cellular physiology

Annexin A6 (AnxA6) belongs to the conserved annexin family - a group of Ca²⁺-dependent membrane binding proteins. AnxA6 is the largest of all annexins and highly expressed in smooth muscle, hepatocytes, endothelial cells and cardiomyocytes. Upon activation, AnxA6 binds to negatively charged phospholipids in a wide range of intracellular localizations, in particular the plasma membrane, late endosomes/pre-lysosomes, but also synaptic vesicles and sarcolemma. In these cellular sites, AnxA6 is believed to contribute to the organization of membrane microdomains, such as cholesterol-rich lipid rafts and confer multiple regulatory functions, ranging from vesicle fusion, endocytosis and exocytosis to programmed cell death and muscle contraction. Growing evidence supports that Ca²⁺ and Ca²⁺-binding proteins control endocytosis and autophagy. Their regulatory role seems to operate at the level of the signalling pathways that initiate autophagy or at later stages, when autophagosomes fuse with endolysosomal compartments. The convergence of the autophagic and endocytic vesicles to lysosomes shares several features that depend on Ca²⁺ originating from lysosomes/late endosomes and seems to depend on proteins that are subsequently activated by this cation. However, the involvement of Ca²⁺ and its effector proteins in these autophagic and endocytic stages still remains poorly understood. Although AnxA6 makes up almost 0.25% of total protein in the liver, little is known about its function in hepatocytes. Within the endocytic route, we identified AnxA6 in endosomes and autophagosomes of hepatocytes. Hence, AnxA6 and possibly other annexins might represent new Ca²⁺ effectors that regulate converging steps of autophagy and endocytic trafficking in hepatocytes. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Regulation of intracellular signaling and function by caveolin

Caveolae, flask-like invaginations of the plasma membrane, were discovered nearly 60 years ago. Originally regarded as fixation artifacts of electron microscopy, the functional role for these structures has taken decades to unravel. The discovery of the caveolin protein in 1992 (by the late Richard G.W. Anderson) accelerated progress in defining the contribution of caveolae to cellular physiology and pathophysiology. The three isoforms of caveolin (caveolin-1, -2, and -3) are caveolae-resident structural and scaffolding proteins that are critical for the formation of caveolae and their localization of signaling entities. A PubMed search for "caveolae" reveals ∼280 publications from their discovery in the 1950s to the early 1990s, whereas a search for "caveolae or caveolin" after 1990, identifies ∼7000 entries. Most work on the regulation of biological responses by caveolae and caveolin since 1990 has focused on caveolae as plasma membrane microdomains and the function of caveolin proteins at the plasma membrane. By contrast, our recent work and that of others has explored the localization of caveolins in multiple cellular membrane compartments and in the regulation of intracellular signaling. Cellular organelles that contain caveolin include mitochondria, nuclei and the endoplasmic reticulum. Such intracellular localization allows for a complexity of responses to extracellular stimuli by caveolin and the possibility of novel organelle-targeted therapeutics. This review focuses on the impact of intracellular localization of caveolin on signal transduction and cell regulation.

Rab1a regulates sorting of early endocytic vesicles

We previously reported that Rab1a is associated with asialoorosomucoid (ASOR)-containing early endocytic vesicles, where it is required for their microtubule-based motility. In Rab1a knockdown (KD) cell lines, ASOR failed to segregate from its receptor and, consequently, did not reach lysosomes for degradation, indicating a defect in early endosome sorting. Although Rab1 is required for Golgi/endoplasmic reticulum trafficking, this process was unaffected, likely due to retained expression of Rab1b in these cells. The present study shows that Rab1a has a more general role in endocytic vesicle processing that extends to EGF and transferrin (Tfn) trafficking. Compared with results in control Huh7 cells, EGF accumulated in aggregates within Rab1a KD cells, failing to reach lysosomal compartments. Tfn, a prototypical example of recycling cargo, accumulated in a Rab11-mediated slow-recycling compartment in Rab1a KD cells, in contrast to control cells, which sort Tfn into a fast-recycling Rab4 compartment. These data indicate that Rab1a is an important regulator of early endosome sorting for multiple cargo species. The effectors and accessory proteins recruited by Rab1a to early endocytic vesicles include the minus-end-directed kinesin motor KifC1, while others remain to be discovered.

The intracellular trafficking pathway of transferrin

BACKGROUND

Transferrin (Tf) is an iron-binding protein that facilitates iron-uptake in cells. Iron-loaded Tf first binds to the Tf receptor (TfR) and enters the cell through clathrin-mediated endocytosis. Inside the cell, Tf is trafficked to early endosomes, delivers iron, and then is subsequently directed to recycling endosomes to be taken back to the cell surface.

SCOPE OF REVIEW

We aim to review the various methods and techniques that researchers have employed for elucidating the Tf trafficking pathway and the cell-machinery components involved. These experimental methods can be categorized as microscopy, radioactivity, and surface plasmon resonance (SPR).

MAJOR CONCLUSIONS

Qualitative experiments, such as total internal reflectance fluorescence (TIRF), electron, laser-scanning confocal, and spinning-disk confocal microscopy, have been utilized to determine the roles of key components in the Tf trafficking pathway. These techniques allow temporal resolution and are useful for imaging Tf endocytosis and recycling, which occur on the order of seconds to minutes. Additionally, radiolabeling and SPR methods, when combined with mathematical modeling, have enabled researchers to estimate quantitative kinetic parameters and equilibrium constants associated with Tf binding and trafficking.

GENERAL SIGNIFICANCE

Both qualitative and quantitative data can be used to analyze the Tf trafficking pathway. The valuable information that is obtained about the Tf trafficking pathway can then be combined with mathematical models to identify design criteria to improve the ability of Tf to deliver anticancer drugs. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

The IL-15/IL-15Ralpha on cell surfaces enables sustained IL-15 activity and contributes to the long survival of CD8 memory T cells

We previously described unique features of the IL-15 receptor (IL-15R)alpha. IL-15Ralpha by itself forms stable complexes with IL-15 on cell surfaces and presents IL-15 in trans to neighboring natural killer/T cells. Moreover, the membrane IL-15/IL-15Ralpha complexes (membIL-15) undergo endosomal internalization but survive lysosomal degradation, allowing the complexes to recycle back to the cell surface. Here, we show that membIL-15+ cells act as a persistent source of IL-15 for the surrounding microenvironment (intercellular reservoir effect). Additionally, membIL-15+ cells give rise to augmented retention of IL-15 in the circulation as well as in tissues. Curiously, IL-15 retention was particularly associated with lungs, rather than with lymph nodes, in normal unstimulated mice, which correlated with the preferential homing of antigen-specific CD8 T cells to lungs during their contraction phase in an IL-15Ralpha-dependent manner. Furthermore, membIL-15, unlike soluble IL-15, caused sustained IL-15 signal transduction in the target cells. Collectively, these characteristics define IL-15 as a unique cytokine with prolonged in vivo survival and sustained biological action on the target cells, which may account for the proposed persistent action of IL-15 that helps the long-term survival of functional CD8 memory T cells in vivo.

cAMP-Coupled riboflavin trafficking in placental trophoblasts: a dynamic and ordered process

Riboflavin (RF, vitamin B(2)), an essential micronutrient central to cellular metabolism through formation of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) cofactors, is internalized, at least in part, via a proposed receptor-mediated endocytic (RME) process. The purpose of this study was to delineate the cellular RF distribution using human placental trophoblasts and evaluate the regulatory role of cAMP in this process. Subcellular fractionation and three-dimensional confocal microscopy analyses were carried out to define the RF accumulation profile. Biochemical assays evaluating the cAMP dependence of this pathway were also performed. This study records an intracellular RF distribution pattern that shows dynamic accumulation of the ligand predominantly in the endosomal and lysosomal compartments and to a lesser extent in the Golgi and mitochondria. In contrast, transferrin (TF) colocalizes rapidly within endosomes with minimal accumulation in the other organelles. The temporal and spatial distribution of RF and TF colocalized with unique markers of the endocytic machinery provides added morphological evidence in support of the RME process with ultimate translocation to the mitochondrial domain. Colocalized staining with the Golgi also suggests a possible recycling or exocytic mechanism for this ligand. Furthermore, this study demonstrates cAMP regulation of the putative ligand-bound RF receptor and its association into endocytic vesicles. Delineating the dynamics of the process governing cellular RF homeostasis presents an untapped resource that can be further exploited in improving our current understanding of nutritional biology and fetal growth and development, and perhaps in targeting the endogenous system for developing novel therapeutic approaches.

The secreted form of dengue virus nonstructural protein NS1 is endocytosed by hepatocytes and accumulates in late endosomes: implications for viral infectivity

The flavivirus nonstructural protein NS1 is expressed as three discrete species in infected mammalian cells: an intracellular, membrane-associated form essential for viral replication, a cell surface-associated form that may be involved in signal transduction, and a secreted form (sNS1), the biological properties of which remain elusive. To determine the distribution of the dengue virus (DEN) sNS1 protein in vivo, we have analyzed by immunohistological means the tissue tropism of purified DEN sNS1 injected intravenously into adult mice. The sNS1 protein was found predominantly associated with the liver, where hepatocytes appeared to represent a major target cell. We further showed that sNS1 could be efficiently endocytosed by human Huh7 and HepG2 hepatocytes in vitro. After its internalization, the protein was detected intracellularly for at least 48 h without being substantially degraded. Colocalization studies of sNS1 with markers of the endolysosomal compartments revealed that the protein was specifically targeted to lysobisphosphatidic acid-rich structures reminiscent of late endosomes, as confirmed by electron microscopy. Intracellular accumulation of sNS1 in Huh7 cells enhanced the fluid phase uptake of rhodamine-labeled dextran. Furthermore, preincubation of Huh7 cells with sNS1 increased dengue virus production after infection with the homologous strain of DEN-1 virus. Our results demonstrate that the accumulation of DEN sNS1 in the late endosomal compartment of hepatocytes potentializes subsequent dengue virus infection in vitro, raising the possibility that sNS1 may contribute to viral propagation in vivo.

Macropinocytosis of Polyplexes and Recycling of Plasmid via the Clathrin-Dependent Pathway Impair the Transfection Efficiency of Human Hepatocarcinoma Cells

Knowledge of the entry mechanism and intracellular routing of polyplexes is of major importance for designing efficient gene delivery systems. We therefore investigated the internalization and trafficking of polyplexes in HepG2 cells. pDNA encoding the luciferase was complexed with histidylated polylysine (His-pLK), a polymer that requires acidic pH for pDNA endosomal release. Fluoresceinylated polyplexes (F-His-pLK or F-pDNA) were internalized by clathrin-dependent and -independent pathways. The latter most likely occurred by macropinocytosis since it was stimulated by phorbol myristate and blocked by dimethylamiloride. Intracellular routing of the plasmid was analyzed by confocal microscopy and flow cytometry. These data revealed that: (i) one part of the plasmid was present in vesicles that were not labeled with any known organelle-specific marker, (ii) the other part was in transferrin receptor-positive vesicles, and (iii) the plasmid was not transferred to late endosomes/lysosomes. Using luciferase activity as a readout for gene expression, we found that it was strongly reduced when macropinocytosis was stimulated, whereas macropinocytosis inhibitors had no effect. However, blocking clathrin-dependent internalization by chlorpromazine completely prevented gene expression. These findings demonstrate that: (i) macropinocytosis of polyplexes and (ii) plasmid recycling impair the transfection efficiency and (iii) clathrin-dependent endocytosis is the most productive route for transfection of HepG2 cells.

Intracellular trafficking during liver regeneration. Alterations in late endocytic and transcytotic pathways

BACKGROUND/AIMS

Liver growth, induced by partial hepatectomy of the organ is a precisely regulated process during which a radical reorganisation of metabolism occurs as the hepatocytes become committed to enter the cell cycle. Recent studies have shown the importance of the endocytic compartment in the control of lipid and protein intracellular trafficking but also in the control of the signal transduction events, which eventually will trigger the initiation of DNA synthesis and the subsequent cell division.

METHODS

We isolated endosomes at different times after partial hepatectomy in male rats and compared with endosomes isolated from sham-operated animals. Also, bile was collected and analysed by 2D-gel electrophoresis.

RESULTS

The amount of late endosomes isolated from regenerating livers decreased, concomitant with decreased cathepsin D specific enzyme activity. Furthermore, secretion of horseradish peroxidase, pIgA and transferrin increased in the pre-replicative phase of liver regeneration.

CONCLUSIONS

At the early stages of liver regeneration, the hepatocellular transport pathway towards degradation (late endosomes and lysosomal pathway) decreases, but the transcytosis and the bile secretion of several major proteins increases.

Endocytosis of hepatic lipase and lipoprotein lipase into rat liver hepatocytes in vivo is mediated by the low density lipoprotein receptor-related protein

In isolated cell studies, the internalization and degradation of hepatic lipase (HL) has been linked to its binding to the low density lipoprotein receptor-related protein (LRP). We have utilized the receptor-associated protein (RAP), a universal inhibitor of high affinity ligand binding to LRP, to evaluate the participation of LRP in the endocytosis of HL and lipoprotein lipase (LPL). We isolated a total endosome fraction from rat livers after a 30-min infusion of recombinant RAP, administered as a glutathione S-transferase conjugate (GST-RAP). GST-RAP infusion had no effect on the concentration of HL in liver homogenates, but its concentration in blood plasma increased progressively by 20%, and enrichment over homogenate of HL in endosomes was reduced by 50% as compared with infusion of GST alone. The concentrations of LPL in liver and plasma were 1.4 and 0.5%, respectively, those of HL, but endosomal enrichment of the two enzymes was similar ( approximately 10-fold). GST-RAP infusion had no effect on the concentration of LPL in liver but increased its concentration in blood plasma by 250% and reduced its endosomal enrichment by 95% or greater. GST-RAP infusion also reduced endosomal enrichment of LRP by 40%, but enrichment of several other endocytic receptors was unaffected. Endosomal enrichment of several membrane trafficking proteins associated with the endocytic pathway in hepatocytes was unaffected by GST-RAP with the exception of early endosome endosome antigen 1, which was reduced by 85%. We conclude that HL is partially and LPL almost exclusively taken up into rat hepatocytes after binding to the endocytic receptor LRP.

A fragment of anthrax lethal factor delivers proteins to the cytosol without requiring protective antigen

Anthrax protective antigen (PA) is a 735-aa polypeptide that facilitates the exit of anthrax lethal factor (LF) from the endosome to the cytosol where the toxin acts. We recently found, however, that a fusion protein of the detoxified N-terminal domain of lethal factor (LFn) with a foreign peptide could induce CD8 T cell immune responses in the absence of PA. Because CD8 T cells recognize peptides derived from proteins degraded in the cytosol, this result suggests that lethal factor may be capable of entering the cytosol independently of PA. To investigate this further, the intracellular trafficking of an LFn-enhanced green fluorescent protein fusion protein (LFn-GFP) in the presence or absence of PA was examined by using confocal microscopy. LFn-GFP is able to enter the cytosol without PA. Moreover, it efficiently colocalizes with the proteosome 20s subunit, which degrades proteins into peptides for presentation to CD8 T cells by the MHC class I pathway. We further demonstrate that in the presence of an immune adjuvant LFn fusion protein without PA is able to effectively elicit anti-HIV cytotoxic T lymphocyte in inbred mice. These results indicate that LFn may be used without PA in a protein vaccine as a carrier to deliver antigens into the cytosol for efficient induction of T lymphocyte responses. Furthermore, these results enable us to propose a modified molecular mechanism of anthrax lethal toxin.

Tyrosine phosphorylation of Eps15 is required for ligand-regulated, but not constitutive, endocytosis

Membrane receptors are internalized either constitutively or upon ligand engagement. Whereas there is evidence for differential regulation of the two processes, little is known about the molecular machinery involved. Previous studies have shown that an unidentified kinase substrate is required for endocytosis of the epidermal growth factor receptor (EGFR), the prototypical ligand-inducible receptor, but not of the transferrin receptor (TfR), the prototypical constitutively internalized receptor. Eps15, an endocytic protein that is tyrosine phosphorylated by EGFR, is a candidate for such a function. Here, we show that tyrosine phosphorylation of Eps15 is necessary for internalization of the EGFR, but not of the TfR. We mapped Tyr 850 as the major in vivo tyrosine phosphorylation site of Eps15. A phosphorylation-negative mutant of Eps15 acted as a dominant negative on the internalization of the EGFR, but not of the TfR. A phosphopeptide, corresponding to the phosphorylated sequence of Eps15, inhibited EGFR endocytosis, suggesting that phosphotyrosine in Eps15 serves as a docking site for a phosphotyrosine binding protein. Thus, tyrosine phosphorylation of Eps15 represents the first molecular determinant, other than those contained in the receptors themselves, which is involved in the differential regulation of constitutive vs. regulated endocytosis.

Apolipoprotein E is resistant to intracellular degradation in vitro and in vivo. Evidence for retroendocytosis

Apolipoprotein E (apoE) is an important determinant for the uptake of triglyceride-rich lipoproteins and emulsions by the liver, but the intracellular pathway of apoE following particle internalization is poorly defined. In the present study, we investigated whether retroendocytosis is a unique feature of apoE as compared with apoB by studying the intracellular fate of very low density lipoprotein-sized apoE-containing triglyceride-rich emulsion particles and LDL after LDLr-mediated uptake. Incubation of HepG2 cells with [(3)H]cholesteryl oleate-labeled particles at 37 degrees C led to a rapid release of [(3)H]cholesterol within 30 min for both LDL and emulsion particles. In contrast, emulsion-derived (125)I-apoE was more resistant to degradation (>/=120 min) than LDL-derived (125)I-apoB (30 min). Incubation at 18 degrees C, which allows endosomal uptake but prevents lysosomal degradation, with subsequent incubation at 37 degrees C resulted in a time-dependent release of intact apoE from the cells (up to 14% of the endocytosed apoE at 4 h). The release of apoE was accelerated by the presence of protein-free emulsion (20%) or high density lipoprotein (26%). Retroendocytosis of intact particles could be excluded since little intact [(3)H]cholesteryl oleate was released (<3%). In contrast, the degradation of LDL was complete with virtually no secretion of intact apoB into the medium. The intracellular stability of apoE was also demonstrated after hepatic uptake in C57Bl/6 mice. Intravenous injection of (125)I-apoE and [(3)H]cholesteryl oleate-labeled emulsions resulted in efficient LDLr-mediated uptake of both components by the liver (45-50% of the injected dose after 20 min). At 1 h after injection, only 15-20% of the hepatic (125)I-apoE was degraded, whereas 75% of the [(3)H]cholesteryl oleate was hydrolyzed. From these data we conclude that following LDLr-mediated internalization by liver cells, apoE can escape degradation and can be resecreted. This sequence of events may allow apoE to participate in its hypothesized intracellular functions such as mediator of the post-lysosomal trafficking of lipids and very low density lipoprotein assembly.

Cellubrevin is present in the basolateral endocytic compartment of hepatocytes and follows the transcytotic pathway after IgA internalization

The endocytic compartment of polarized cells is organized in basolateral and apical endosomes plus those endocytic structures specialized in recycling and transcytosis, which are still poorly characterized. The complexity of the various populations of endosomes has been demonstrated by the exquisite repertoire of endogenous proteins. In this study we examined the distribution of cellubrevin in the endocytic compartment of hepatocytes, since its intracellular location and function in polarized cells are largely unknown. Highly purified rat liver endosomes were isolated from estradiol-treated rats, and the early/sorting endosomal fraction was further subfractionated in a multistep sucrose density gradient, and studied. Analysis of dissected endosomal fractions showed that cellubrevin was located in early/sorting endosomes, with Rab4, annexins II and VI, and transferrin receptor, but in a specific subpopulation of these early endosomes with the same density range as pIgA and Raf-1. Interestingly, only in those isolated endosomal fractions, endosomes enriched in transcytotic structures (of livers loaded with IgA), the polymeric immunoglobulin receptor specifically co-immunoprecipitated with cellubrevin. In addition, confocal and immuno-electron microscopy identification of cellubrevin in tubular structures underneath the sinusoidal plasma membrane together with the re-organization of cellubrevin, in the endocytic compartment, after the IgA loading, strongly suggest the involvement of cellubrevin in the transcytosis of pIgA.

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

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.

Chimeric forms of furin and TGN38 are transported with the plasma membrane in the trans-Golgi network via distinct endosomal pathways

Furin and TGN38 are menbrane proteins that cycle between the plasma membrane and the trans-Golgi network (TGN), each maintaining a predominant distribution in the TGN. We have used chimeric proteins with an extracellular Tac domain and the cytoplasmic domain of TGN38 or furin to study the trafficking of these proteins in endosomes. Previously, we demonstrated that the postendocytic trafficking of Tac-TGN38 to the TGN is via the endocytic recycling pathway (Ghosh, R.N.,W.G. Mallet,T.T. Soe,T.E.McGraw, and F.R. Maxfield.1998.J. Cell Biol.142:923-936). Here we show that internalized Tac-furin is delivered to the TGN through late endosomes, bypassing the endocytic recycling compartment. The transport of Tac-furin from late endosomes to the TGN appears to proceed via an efficient, single-pass mechanism. Delivery of Tac-furin but not Tac-TGN38 to the TGN is blocked by nocodazole, and the two pathways are also differentially affected by wortmannin. These studies demonstrate the existence of two independentpathways for endosomal transport of proteins to the TGN from the plasma membrane.

The "early-sorting" endocytic compartment of rat hepatocytes is involved in the intracellular pathway of caveolin-1 (VIP-21)

The sinusoidal plasma membrane of the hepatocyte is organized into functional and structural microdomains whose origin, maintenance, and functioning are closely related with the endocytic compartment. Three different subcellular fractions, from rat liver, containing caveolin-1, the structural protein of caveolae, were morphologically and biochemically characterized. A caveolae-enriched plasma membrane fraction (CEF), contains large membrane structures surrounding attached internal plasmalemmal vesicles; the receptor-recycling compartment (RRC), contains tubules and vesicles with similar morphology to the internal vesicles observed by electron microscopy in CEF; and finally, caveolin-1 was also detected in early-sorting endosomes (CURL, compartment of uncoupling receptors and ligands). In this study, we show that following an intravenous administration of retinol-binding protein (RBP), there was a redistribution of caveolin-1 from the plasma membrane (CEF) to intracellular endocytic compartments (RRC and early-sorting endosomes). Thus, these results indicate that, in the hepatocyte, caveolae are dynamic structures actively interacting with the endocytic compartment.

Recycling of apolipoprotein E in mouse liver

Following the internalization of low density lipoprotein (LDL) by the LDL receptor within cells, both the lipid and the protein components of LDL are completely degraded within the lysosomes. Remnant lipoproteins are also internalized by cells via the LDL receptor as well as other receptors, but the events following the internalization of these complexes, which use apolipoprotein E (apoE) as their ligand for receptor capture, have not been defined. There is evidence that apoE-containing beta-very low density lipoproteins follow differential intracellular routing depending on their size and apoE content and that apoE internalized with lipoproteins can be resecreted by cultured hepatocytes and fibroblasts. In the present studies, we addressed the question of apoE sparing or recycling as a physiologic phenomenon. Remnant lipoproteins (d < 1.019 g/ml) from normal mouse plasma were iodinated and injected into normal C57BL/6 mice. Livers were collected at 10, 30, 60, and 120 min after injection, and hepatic Golgi fractions were prepared for gel electrophoresis analysis. Golgi preparations were analyzed for galactosyltransferase enrichment (>40-fold above cell homogenate) and by appearance of the Golgi stacks and vesicles on electron microscopy. Iodinated apoE was consistently found in the Golgi fractions peaking at 10 min and disappearing by 2 h after injection. Although traces of apoB48 were present in the Golgi fractions, the apoE/apoB ratio in the Golgi was 50-fold higher compared with serum. Quantitatively similar results were obtained when the very low density lipoprotein remnants were injected into mice deficient in either apoE or the LDL receptor, indicating that the phenomenon of apoE recycling is not influenced by the production of endogenous apoE and is not dependent on the presence of LDL receptors. In addition, radioactive apoE in the Golgi fractions was part of d = 1.019-1.21 g/ml complexes, indicating an association of recycled apoE with either newly formed lipoproteins or the internalized complexes. These studies show that apoE recycling is a physiologic phenomenon in vivo and establish the presence of a unique pathway of intracellular processing of apoE-containing remnant lipoproteins.

Isolated endosomes from quiescent rat liver contain the signal transduction machinery. Differential distribution of activated Raf-1 and Mek in the endocytic compartment

In this study we identify the molecules involved in the MAPK signal transduction pathway (Ras, Raf-1, Mek, Mek-P and MAPK) in highly purified endosomal fractions isolated from rat liver. Biochemical analysis shows that only the early-sorting endocytic compartment contains activated Raf-1 and Mek. Finally, the exogenous administration of EGF led to redistribution of Raf-1 from the caveolin-enriched plasma membrane into the endosomes.

Effect of brefeldin A and lysosomotropic reagents on intracellular trafficking of epidermal growth factor and transferrin in Madin-Darby canine kidney epithelial cells

To regulate intracellular sorting of epidermal growth factor (EGF) or transferrin (Tf), the effect of brefeldin A (BFA) or lysosomotropic reagents was investigated. To examine the effect of them on the net transcellular transport of 125I-EGF or 125I-Tf, their transcytosis was investigated in the presence or absence of reagents. For the investigation of their fate after internalization, radiolabeled ligands were internalized at 37 degreesC, followed by extensive washing and subsequent incubation at 37 degreesC in the ligand-free medium (pulse-chase study). BFA enhanced transcytosis of 125I-Tf, but had no effect on 125I-EGF. Kinetic analysis in the pulse-chase study showed that BFA does not affect cell-surface binding or intracellular sorting of EGF, while it only increases the transcytosis rate constant of Tf. From the lysosomotropic reagents study, both ammonium chloride and monensin suppressed transcytosis and recycling as well as the degradation of EGF, while both chloroquine and bafilomycin A selectively suppressed the degradation process with only a minimal effect on transcytosis, resulting in an increase in the amount transcytosed. It is concluded the that enhancement effect of BFA on transcytosis depends upon the type of receptor targeted. Lysosomotropic reagents can be divided into two types as far as the specificity of the effect on the net amount of EGF transcytosed in Madin-Darby canine kidney (MDCK) cells is concerned.

Polyamine deficiency alters EGF receptor distribution and signaling effectiveness in IEC-6 cells

Cell growth and migration are essential processes for the differentiation, maintenance, and repair of the intestinal epithelium. Epidermal growth factor (EGF) is an important factor in the reorganization of the cytoskeleton required for both processes. Because we had previously found significant changes in the cytoskeleton during polyamine deficiency, it was of interest to know whether those changes could prevent EGF from stimulating growth and migration. Polyamine biosynthesis in IEC-6 cells was interrupted by treatment with alpha-difluoromethylornithine (DFMO), a specific inhibitor of ornithine decarboxylase, the primary rate-limiting enzyme of polyamine biosynthesis. DFMO halted cell proliferation and inhibited cell migration, and neither function could be normally stimulated by EGF. Immunocytochemistry of the transferrin receptor (used as a marker for the endocytic pathway) revealed an abnormal distribution of the EGF receptor (EGFR) 10 min after binding EGF. Polyamine deficiency depleted the cells of interior microfilaments, thickened the actin cortex, and prevented the prompt association of EGF-bound EGFR with actin. EGF-stimulated 170-kDa protein tyrosine phosphorylation and the kinase activity of purified membrane EGFR were reduced by 50%. Immunoprecipated EGFR protein concentration, however, was not reduced by polyamine deficiency. All of these changes could be prevented by supplementation with putrescine. Cytoskeletal disruption, reduced EGFR phosphorylation and kinase activity, aberrant intracellular EGFR distribution, and delayed association with actin filaments suggest a partial explanation for the dependence of epithelial cell growth and migration on polyamines.

Membrane transport in rat liver endocytic pathways: preparation, biochemical properties and functional roles of hepatic endosomes

The endocytic compartment has emerged as a major regulator of the uptake and processing of circulating ligands, and has been extensively studied during the last decade. In this work, the polypeptides of the three endosomal fractions: compartment of uncoupling receptors and ligands (CURL), multivesicular bodies (MVB) and receptor recycling compartment (RRC), isolated from livers of estradiol-treated rats, were analyzed by two-dimensional gel electrophoresis. Silver-stained gels revealed that although the three endosomal fractions shared a generally similar pattern of approximately 120 components, qualitative and quantitative differences between the three endocytic fractions could be demonstrated. The polypeptide composition of the bile was also studied and compared with ligands and proteins identified in the different endosomal fractions. One- and two-dimensional gel electrophoresis and Western blotting were used to investigate the protein composition of the three isolated endocytic fractions and 39 proteins were identified. The distribution of identified receptors, ligands and structural proteins among the three endosomal fractions was in agreement with their expected functionalities and with the different endocytic pathways in the hepatocyte.

The polymeric immunoglobulin receptor is the major calmodulin-binding protein in an endosome fraction from rat liver enriched in recycling receptors

Rat liver endosomes contain one major high-affinity calmodulin-binding protein (CaMBP) that now has been identified as the polymeric immunoglobulin receptor (pIgR). In isolated endosomes pIgR was enriched in the receptor-recycling compartment (RRC); lesser enrichment was found in 'early' endosome (CURL) and much less in 'late' endosome fractions (multivesicular bodies, MVB). The distribution of the major CaMBP, shown by Western blotting or by overlay with I125-calmodulin in the isolated fractions, was consistent with rapid accumulation of I125-immunoglobulin A (IgA) in RRC and CURL after intravenous injection into rats. The receptor was also found in sinusoidal plasma membranes but not in cell fractions containing apical (bile canalicular) or lateral plasma membrane domains of the hepatocyte. The interaction of pIgR with calmodulin was shown by direct binding assays and by affinity chromatography. Thus, calmodulin is the first cytoplasmic protein shown to interact with the pIgR. We postulate that calmodulin regulates pIgA trafficking in rat liver. In addition, the receptor recycling fraction emerges as an endosomal subcompartment involved in pIgA transport via pIgR.

Impact of receptor downregulation on clearance of two human EGFs with different receptor binding activity

Human epidermal growth factor [hEGF(1-53)] has been thought to be cleared mainly via an EGF receptor (EGFR) endocytosis pathway. Pretreatment of rats with hEGF(1-53) has been shown previously to cause a dramatic reduction in clearance of the peptide contributable to EGFR downregulation. The impact of receptor downregulation has raised concerns for rational design of dosage regimen for this potential wound-healing therapeutic peptide. However, following a similar protocol, we could not reproduce the dramatic reduction in clearance reported previously mediated by an i.v. bolus acute dose. As EGFR downregulation may be sensitive to the length of exposure and to the activation of the receptor tyrosine kinase activity, two other pretreatment protocols were also evaluated: a 4-h i.v. infusion (prolonged exposure) of the peptide and an i.v. bolus of a potent synthetic kinase inhibitor pretreatment were evaluated for effects on clearance. However, neither pretreatment affected the peptide's clearance profile. Further, no effects on clearance and other kinetic parameters were observed for any pretreatment paradigms with a truncated analogue hEGF (1-48), whose EGF receptor binding activity is much weaker but plasma clearance is much higher than hEGF (1-53). In addition, a study in a second rat strain showed no difference in clearance profile of hEGF-(1-53) following pretreatment. Results of the present investigation suggest that receptor binding does not have a direct relationship with plasma clearance, and that the EGF clearance mechanisms is highly refractory with EGF receptors possibly recovering rapidly from downregulation through the recycling process.

Evidence that secretase cleavage of cell surface Alzheimer amyloid precursor occurs after normal endocytic internalization

Three different treatments (methylamine, colchicine, and 18 degrees C temperature block), known to disrupt normal endocytic internalization, significantly reduced the secretory cleavage of cell surface-derived Alzheimer amyloid precursor (APP) in non-transfected C6 cell cultures. Conversely, treatments with methylamine or colchicine had no significant effect on the secretion of total APP. Treatment of these cells with the lysosomotropic amine chloroquine resulted in a significant increase in the levels of both cell surface full-length APP and cell surface-derived secreted nexin II (NXII). Immunofluorescence analysis of C6 glioma cells transfected with APP751 indicated that under normal conditions, cell surface APP was internalized, and within 30 minutes was localized in discrete intracellular vesicles. These vesicles contained the endocytic tracer Texas red-conjugated ovalbumin and probably represented late endosomes or lysosomes. However, treatment of the transfected C6 cultures with methylamine or colchicine prevented localization of cell surface APP in intracellular vesicles, suggesting that these treatments altered the normal intracellular trafficking of cell surface-derived APP. Both the biochemical and immunofluorescence data are compatible with the suggestion that inhibition of normal endocytic internalization reduces the secretory cleavage of cell surface APP. Furthermore, our results suggest that following internalization, cell surface APP is cleaved by secretase(s) and secreted or routed to the lysosomes where it is degraded.

Arginine vasopressin receptor internalization and recycling in rat renal collecting tubules

Arginine vasopressin (AVP) binds to two distinct receptors to initiate vasopressor (V1 receptor) and hydroosmotic actions (V2 receptor). Internalization and recycling of the V1 receptor in cultured vascular smooth muscle cells and hepatocytes have recently been demonstrated. However, the receptor cycle of the AVP V2 receptor in the renal collecting tubules has not yet been well defined. Therefore, the present study was undertaken to investigate the AVP V2 receptor cycle, including AVP binding to the surface receptor, internalization and potential recycling in isolated outer medullary collecting tubules. The maximal AVP surface binding was reached in 10 min, and 25 micrograms/ml trypsin completely inhibited the surface binding. A Scatchard plot of 125I-AVP surface binding indicated a single population of V2 receptors with a Kd of 1.92 x 10(-9) M and a Bmax of 1.77 x 10(-11) M or 590 fmoles/mg protein. 81.7% (72-85%) of specific bound receptor was internalized (specific surface binding: 742.8 +/- 111.1 vs internalized binding: 607.3 +/- 27.8 fmoles bound/mg protein). More than 90% of surface bound receptor was recycled to the cell surface after internalization (control surface binding: 584.0 +/- 64.0 vs recycled surface binding: 546.6 +/- 32.0 fmoles bound/mg protein). Cycloheximide (40 micrograms/ml) did not inhibit the receptor recycling (control recycled surface binding: 546.6 +/- 32.0 vs cycloheximide recycled surface binding: 505.0 +/- 54.8 fmoles bound/mg protein), thus suggesting that the receptors were not resynthesized after dissociation from the receptor-ligand complex. These studies therefore demonstrate that the AVP V2 receptor is internalized and recycled in the rat renal collecting tubule.

Dissection of compartments in rat hepatocytes involved in the intracellular trafficking of high-density lipoprotein particles or their selectively internalized cholesteryl esters

The trafficking of apolipoprotein E-deficient high-density lipoprotein particles and of their component cholesteryl esters in rat hepatocytes was studied. Human high-density lipoprotein 3, labeled with two nondegradable, intracellularly trapped tracers in their apolipoprotein A-I and their cholesteryl esters, were injected into rats, and five subcellular hepatocytic fractions were isolated at various time intervals. In control experiments with homologous lipoproteins, doubly labeled rat high-density lipoproteins depleted of apolipoprotein E were used. In endosomes and lysosomes the two labels were recovered at near unity, indicating that high-density lipoproteins are endocytosed as particles, transported to early and late endosomes and finally subjected to lysosomal degradation. No significant amounts of label were found in receptor-recycling endosomes. In contrast to label of those of low-density lipoproteins, label of component protein and cholesteryl esters of high-density lipoproteins from isolated endosomes floated at different densities in gradient ultracentrifugation, indicating early disintegration of high-density lipoprotein particles. In contrast to the endocytic organelles, in the whole liver, label of high-density lipoprotein-associated cholesteryl esters exceeded the label of high-density lipoprotein-associated apolipoprotein A-I twofold to threefold. This finding is compatible with selective uptake of high-density lipoprotein cholesteryl esters in addition to uptake of high-density lipoprotein particles. The excess cholesteryl esters accumulated in a nonendosomal fraction, whose major proteins differed from the integral proteins of endosomes. These data suggest two distinct intracellular routes of hepatocytic high-density lipoprotein trafficking in vivo. High-density lipoproteins free of apolipoprotein E are internalized intact by hepatocytes, are predominantly transported to early and late endosomes and are finally subjected to lysosomal degradation. High-density lipoprotein particles do not undergo retroendocytosis in hepatocytes. In addition, high-density lipoprotein-associated cholesteryl esters can be taken up by hepatocytes selectively. They, however, accumulate in a nonendosomal, nonlysosomal compartment.

Hepatic accumulation of lysosomes and defective transcytotic vesicular pathways in cirrhotic rat liver

To investigate the potential role of lysosomes in cirrhosis, we analyzed the activity of lysosomal enzymes in rats exposed long-term to phenobarbital and carbon tetrachloride. The activity of lysosomal enzymes was markedly increased in the homogenate of cirrhotic livers (e.g., arylsulfatase 9 +/- S.D.2 vs. 16 +/- 6 nmoles.min-1.mg-1 in control rats and cirrhotic rats, respectively; p less than 0.001). The corresponding plasma levels were also increased (7 +/- 1 vs. 12 +/- 3 nmoles.min-1.mg-1; p less than 0.01), whereas biliary excretion was diminished (16 +/- 7 vs. 7 +/- 2 pmol.min-1.gm liver-1; p less than 0.05) in cirrhotic rats. Stereological quantification of lysosomes visualized cytochemically revealed an increase of pericanalicular lysosomes averaging 1.5 +/- 0.4 around a canaliculus in controls and 3.7 +/- 1.0 in cirrhotic rats (p less than 0.01). Because this suggested a defect in the transcellular vesicular pathway, we investigated the biliary excretion of horseradish peroxidase and epidermal growth factor in perfused livers. Bile flow and total horseradish peroxidase excretion were similar in control rats and cirrhotic rats. However, the early peak of biliary horseradish peroxidase excretion--usually taken as evidence of paracellular transport--was increased in cirrhotic rats (13 +/- 7 vs. 57 +/- 22%; p less than 0.01), whereas the second peak--reflecting the transcellular vesicular pathway(s)--was markedly reduced (87 +/- 7 vs. 43 +/- 22%; p less than 0.001). A similar reduction in the biliary excretion of intact epidermal growth factor and of its degradation products was found. These results demonstrate an increased number of lysosomes in hepatocytes of cirrhotic livers; this appears to be the result of accumulation rather than proliferation, in view of the reduced transcellular vesicular movement of different markers into bile.