Liver sinusoidal cells eliminate blood-borne phage K1F

Phage treatment has regained attention due to an increase in multiresistant bacteria. For phage therapy to be successful, phages must reach their target bacteria in sufficiently high numbers. Blood-borne phages are believed to be captured by macrophages in the liver and spleen. Since liver sinusoids also consist of specialized scavenger liver sinusoidal endothelial cells (LSECs) and Kupffer cells (KCs), this study investigated the contribution of both cell types in the elimination of Escherichia coli phage K1Fg10b::gfp (K1Fgfp) in mice. Circulatory half-life, organ, and hepatocellular distribution of K1Fgfp were determined following intravenous administration. Internalization of K1Fgfp and effects of phage opsonization on uptake were explored using primary mouse and human LSEC and KC cultures. When inoculated with 107 virions, >95% of the total K1Fgfp load was eliminated from the blood within 20 min, and 94% of the total retrieved K1Fgfp was localized to the liver. Higher doses resulted in slower elimination, possibly reflecting temporary saturation of liver scavenging capacity. Phage DNA was detected in both cell types, with a KC:LSEC ratio of 12:1 per population following cell isolation. Opsonization with plasma proteins increased time-dependent cellular uptake in both LSECs and KCs in vitro. Internalized phages were rapidly transported along the endocytic pathway to lysosomal compartments. Reduced viability of intracellular K1Fgfp corroborated inactivation following endocytosis. This study is the first to identify phage distribution in the liver at the hepatocellular level, confirming clearance of K1Fgfp performed mostly by KCs with a significant uptake also in LSECs.IMPORTANCEFaced with the increasing amounts of bacteria with multidrug antimicrobial resistance, phage therapy has regained attention as a possible treatment option. The phage field has recently experienced an emergence in commercial interest as research has identified new and more efficient ways of identifying and matching phages against resistant superbugs. Currently, phages are unapproved drugs in most parts of the world. For phages to reach broad clinical use, they must be shown to be clinically safe and useful. The results presented herein contribute to increased knowledge about the pharmacokinetics of the T7-like phage K1F in the mammalian system. The cell types of the liver that are responsible for rapid phage blood clearance are identified. Our results highlight the need for more research about appropriate dose regimens when phage therapy is delivered intravenously and advise essential knowledge about cell systems that should be investigated further for detailed phage pharmacodynamics.

pH-dependent structural transitions in cationic ionizable lipid mesophases are critical for lipid nanoparticle function

Lipid nanoparticles (LNPs) are advanced core-shell particles for messenger RNA (mRNA) based therapies that are made of polyethylene glycol (PEG) lipid, distearoylphosphatidylcholine (DSPC), cationic ionizable lipid (CIL), cholesterol (chol), and mRNA. Yet the mechanism of pH-dependent response that is believed to cause endosomal release of LNPs is not well understood. Here, we show that eGFP (enhanced green fluorescent protein) protein expression in the mouse liver mediated by the ionizable lipids DLin-MC3-DMA (MC3), DLin-KC2-DMA (KC2), and DLinDMA (DD) ranks MC3 ≥ KC2 > DD despite similar delivery of mRNA per cell in all cell fractions isolated. We hypothesize that the three CIL-LNPs react differently to pH changes and hence study the structure of CIL/chol bulk phases in water. Using synchrotron X-ray scattering a sequence of ordered CIL/chol mesophases with lowering pH values are observed. These phases show isotropic inverse micellar, cubic Fd3m inverse micellar, inverse hexagonal [Formula: see text] and bicontinuous cubic Pn3m symmetry. If polyadenylic acid, as mRNA surrogate, is added to CIL/chol, excess lipid coexists with a condensed nucleic acid lipid [Formula: see text] phase. The next-neighbor distance in the excess phase shows a discontinuity at the Fd3m inverse micellar to inverse hexagonal [Formula: see text] transition occurring at pH 6 with distinctly larger spacing and hydration for DD vs. MC3 and KC2. In mRNA LNPs, DD showed larger internal spacing, as well as retarded onset and reduced level of DD-LNP-mediated eGFP expression in vitro compared to MC3 and KC2. Our data suggest that the pH-driven Fd3m-[Formula: see text] transition in bulk phases is a hallmark of CIL-specific differences in mRNA LNP efficacy.

Liver sinusoidal endothelial cells show reduced scavenger function and downregulation of Fc gamma receptor IIb, yet maintain a preserved fenestration in the Glmpgt/gt mouse model of slowly progressing liver fibrosis

Liver sinusoidal endothelial cells (LSECs) are fenestrated endothelial cells with a unique, high endocytic clearance capacity for blood-borne waste macromolecules and colloids. This LSEC scavenger function has been insufficiently characterized in liver disease. The Glmpgt/gt mouse lacks expression of a subunit of the MFSD1/GLMP lysosomal membrane protein transporter complex, is born normal, but soon develops chronic, mild hepatocyte injury, leading to slowly progressing periportal liver fibrosis, and splenomegaly. This study examined how LSEC scavenger function and morphology are affected in the Glmpgt/gt model. FITC-labelled formaldehyde-treated serum albumin (FITC-FSA), a model ligand for LSEC scavenger receptors was administered intravenously into Glmpgt/gt mice, aged 4 months (peak of liver inflammation), 9-10 month, and age-matched Glmpwt/wt mice. Organs were harvested for light and electron microscopy, quantitative image analysis of ligand uptake, collagen accumulation, LSEC ultrastructure, and endocytosis receptor expression (also examined by qPCR and western blot). In both age groups, the Glmpgt/gt mice showed multifocal liver injury and fibrosis. The uptake of FITC-FSA in LSECs was significantly reduced in Glmpgt/gt compared to wild-type mice. Expression of LSEC receptors stabilin-1 (Stab1), and mannose receptor (Mcr1) was almost similar in liver of Glmpgt/gt mice and age-matched controls. At the same time, immunostaining revealed differences in the stabilin-1 expression pattern in sinusoids and accumulation of stabilin-1-positive macrophages in Glmpgt/gt liver. FcγRIIb (Fcgr2b), which mediates LSEC endocytosis of soluble immune complexes was widely and significantly downregulated in Glmpgt/gt liver. Despite increased collagen in space of Disse, LSECs of Glmpgt/gt mice showed well-preserved fenestrae organized in sieve plates but the frequency of holes >400 nm in diameter was increased, especially in areas with hepatocyte damage. In both genotypes, FITC-FSA also distributed to endothelial cells of spleen and bone marrow sinusoids, suggesting that these locations may function as possible compensatory sites of clearance of blood-borne scavenger receptor ligands in liver fibrosis.

Changes in the proteome and secretome of rat liver sinusoidal endothelial cells during early primary culture and effects of dexamethasone

Introduction

Liver sinusoidal endothelial cells (LSECs) are specialized fenestrated scavenger endothelial cells involved in the elimination of modified plasma proteins and tissue turnover waste macromolecules from blood. LSECs also participate in liver immune responses. A challenge when studying LSEC biology is the rapid loss of the in vivo phenotype in culture. In this study, we have examined biological processes and pathways affected during early-stage primary culture of rat LSECs and checked for cell responses to the pro-inflammatory cytokine interleukin (IL)-1β and the anti-inflammatory drug dexamethasone.

Methods

LSECs from male Sprague Dawley rats were cultured on type I collagen in 5% oxygen atmosphere in DMEM with serum-free supplements for 2 and 24 h. Quantitative proteomics using tandem mass tag technology was used to examine proteins in cells and supernatants. Validation was done with qPCR, ELISA, multiplex immunoassay, and caspase 3/7 assay. Cell ultrastructure was examined by scanning electron microscopy, and scavenger function by quantitative endocytosis assays.

Results

LSECs cultured for 24 h showed a characteristic pro-inflammatory phenotype both in the presence and absence of IL-1β, with upregulation of cellular responses to cytokines and interferon-γ, cell-cell adhesion, and glycolysis, increased expression of fatty acid binding proteins (FABP4, FABP5), and downregulation of several membrane receptors (STAB1, STAB2, LYVE1, CLEC4G) and proteins in pyruvate metabolism, citric acid cycle, fatty acid elongation, amino acid metabolism, and oxidation-reduction processes. Dexamethasone inhibited apoptosis and improved LSEC viability in culture, repressed inflammatory and immune regulatory pathways and secretion of IL-1β and IL-6, and further upregulated FABP4 and FABP5 compared to time-matched controls. The LSEC porosity and endocytic activity were reduced at 24 h both with and without dexamethasone but the dexamethasone-treated cells showed a less stressed phenotype.

Conclusion

Rat LSECs become activated towards a pro-inflammatory phenotype during early culture. Dexamethasone represses LSEC activation, inhibits apoptosis, and improves cell viability.

Protocol for Isolation and Culture of Mouse Hepatocytes (HCs), Kupffer Cells (KCs), and Liver Sinusoidal Endothelial Cells (LSECs) in Analyses of Hepatic Drug Distribution

Development of the new generation of drugs (e.g., oligo- and polynucleotides administered intravascularly either as free compounds or as nano-formulations) frequently encounters major challenges such as lack of control of targeting and/or delivery. Uncontrolled or unwanted clearance by the liver is a well-known and particularly important hurdle in this respect. Hence, reliable techniques are needed to identify the type(s) of liver cells, receptors, and metabolic mechanisms that are responsible for unwanted clearance of these compounds.We describe here a method for the isolation and culture of the major cell types from mouse liver: hepatocytes (HCs), Kupffer cells (KCs), and liver sinusoidal endothelial cells (LSECs). The presently described protocol employs perfusion of the liver with a collagenase-based enzyme preparation to effectively transform the intact liver to a single cell suspension. From this initial cell suspension HCs are isolated by specified centrifugation schemes, yielding highly pure HC preparations, and KCs and LSECs are isolated by employing magnetic-activated cell sorting (MACS). The MACS protocol makes use of magnetic microbeads conjugated with specific antibodies that bind unique surface antigens on either KCs or LSECs. In this way the two cell types are specifically and separately pulled out of the initial liver cell suspension by applying a magnetic field, resulting in high purity, yield, and viability of the two cell types, allowing functional studies of the cells.If the drug compound in question is to be studied with respect to liver cell distribution of intravascularly administered drug compounds the isolated cells can be analyzed directly after isolation. Detailed studies of receptor-ligand interactions and/or dynamics of intracellular metabolism of the compound can be conducted in primary surface cultures of HCs, LSECs, and KCs established by seeding the isolated cells on specified growth substrates.

Autofluorescence in freshly isolated adult human liver sinusoidal cells

Autofluorescent granules of various sizes were observed in primary human liver endothelial cells (LSECs) upon laser irradiation using a wide range of wavelengths. Autofluorescence was detected in LAMP-1 positive vesicles, suggesting lysosomal location. Confocal imaging of freshly prepared cultures and imaging flow cytometry of non-cultured cells revealed fluorescence in all channels used. Treatment with a lipofuscin autofluorescence quencher reduced autofluorescence, most efficiently in the near UV-area. These results, combined with the knowledge of the very active blood clearance function of LSECs support the notion that lysosomally located autofluorescent material reflected accumulation of lipofuscin in the intact liver. These results illustrate the importance of careful selection of fluorophores, especially when labelling of live cells where the quencher is not compatible.

The Scavenger Function of Liver Sinusoidal Endothelial Cells in Health and Disease

The aim of this review is to give an outline of the blood clearance function of the liver sinusoidal endothelial cells (LSECs) in health and disease. Lining the hundreds of millions of hepatic sinusoids in the human liver the LSECs are perfectly located to survey the constituents of the blood. These cells are equipped with high-affinity receptors and an intracellular vesicle transport apparatus, enabling a remarkably efficient machinery for removal of large molecules and nanoparticles from the blood, thus contributing importantly to maintain blood and tissue homeostasis. We describe here central aspects of LSEC signature receptors that enable the cells to recognize and internalize blood-borne waste macromolecules at great speed and high capacity. Notably, this blood clearance system is a silent process, in the sense that it usually neither requires or elicits cell activation or immune responses. Most of our knowledge about LSECs arises from studies in animals, of which mouse and rat make up the great majority, and some species differences relevant for extrapolating from animal models to human are discussed. In the last part of the review, we discuss comparative aspects of the LSEC scavenger functions and specialized scavenger endothelial cells (SECs) in other vascular beds and in different vertebrate classes. In conclusion, the activity of LSECs and other SECs prevent exposure of a great number of waste products to the immune system, and molecules with noxious biological activities are effectively “silenced” by the rapid clearance in LSECs. An undesired consequence of this avid scavenging system is unwanted uptake of nanomedicines and biologics in the cells. As the development of this new generation of therapeutics evolves, there will be a sharp increase in the need to understand the clearance function of LSECs in health and disease. There is still a significant knowledge gap in how the LSEC clearance function is affected in liver disease.

Transcriptome and proteome profiling reveal complementary scavenger and immune features of rat liver sinusoidal endothelial cells and liver macrophages

BACKGROUND

Liver sinusoidal endothelial cells (LSECs) and Kupffer cells (KCs; liver resident macrophages) form the body's most effective scavenger cell system for the removal of harmful blood-borne substances, ranging from modified self-proteins to pathogens and xenobiotics. Controversies in the literature regarding the LSEC phenotype pose a challenge when determining distinct functionalities of KCs and LSECs. This may be due to overlapping functions of the two cells, insufficient purification and/or identification of the cells, rapid dedifferentiation of LSECs in vitro, or species differences. We therefore characterized and quantitatively compared expressed gene products of freshly isolated, highly pure LSECs (fenestrated SE-1/FcγRIIb2⁺) and KCs (CD11b/c⁺) from Sprague Dawley, Crl:CD (SD), male rats using high throughput mRNA-sequencing and label-free proteomics.

RESULTS

We observed a robust correlation between the proteomes and transcriptomes of the two cell types. Integrative analysis of the global molecular profile demonstrated the immunological aspects of LSECs. The constitutive expression of several immune genes and corresponding proteins of LSECs bore some resemblance with the expression in macrophages. LSECs and KCs both expressed high levels of scavenger receptors (SR) and C-type lectins. Equivalent expression of SR-A1 (Msr1), mannose receptor (Mrc1), SR-B1 (Scarb1), and SR-B3 (Scarb2) suggested functional similarity between the two cell types, while functional distinction between the cells was evidenced by LSEC-specific expression of the SRs stabilin-1 (Stab1) and stabilin-2 (Stab2), and the C-type lectins LSECtin (Clec4g) and DC-SIGNR (Clec4m). Many immune regulatory factors were differentially expressed in LSECs and KCs, with one cell predominantly expressing a specific cytokine/chemokine and the other cell the cognate receptor, illustrating the complex cytokine milieu of the sinusoids. Both cells expressed genes and proteins involved in antigen processing and presentation, and lymphocyte co-stimulation.

CONCLUSIONS

Our findings support complementary and partly overlapping scavenging and immune functions of LSECs and KCs. This highlights the importance of including LSECs in studies of liver immunity, and liver clearance and toxicity of large molecule drugs and nano-formulations.

Unraveling the transcriptional determinants of liver sinusoidal endothelial cell specialization

Liver sinusoidal endothelial cells (LSECs) are the first liver cells to encounter waste macromolecules, pathogens, and toxins in blood. LSECs are highly specialized to mediate the clearance of these substances via endocytic scavenger receptors and are equipped with fenestrae that mediate the passage of macromolecules toward hepatocytes. Although some transcription factors (TFs) are known to play a role in LSEC specialization, information about the specialized LSEC signature and its transcriptional determinants remains incomplete.Based on a comparison of liver, heart, and brain endothelial cells (ECs), we established a 30-gene LSEC signature comprising both established and newly identified markers, including 7 genes encoding TFs. To evaluate the LSEC TF regulatory network, we artificially increased the expression of the 7 LSEC-specific TFs in human umbilical vein ECs. Although Zinc finger E-box-binding protein 2, homeobox B5, Cut-like homolog 2, and transcription factor EC (TCFEC) had limited contributions, musculoaponeurotic fibrosarcoma (C-MAF), GATA binding protein 4 (GATA4), and MEIS homeobox 2 (MEIS2) emerged as stronger inducers of LSEC marker expression. Furthermore, a combination of C-MAF, GATA4, and MEIS2 showed a synergistic effect on the increase of LSEC signature genes, including liver/lymph node-specific ICAM-3 grabbing non-integrin (L-SIGN) (or C-type lectin domain family member M (CLEC4M)), mannose receptor C-Type 1 (MRC1), legumain (LGMN), G protein-coupled receptor 182 (GPR182), Plexin C1 (PLXNC1), and solute carrier organic anion transporter family member 2A1 (SLCO2A1). Accordingly, L-SIGN, MRC1, pro-LGMN, GPR182, PLXNC1, and SLCO2A1 protein levels were elevated by this combined overexpression. Although receptor-mediated endocytosis was not significantly induced by the triple TF combination, it enhanced binding to E2, the hepatitis C virus host-binding protein. We conclude that C-MAF, GATA4, and MEIS2 are important transcriptional regulators of the unique LSEC fingerprint and LSEC interaction with viruses. Additional factors are however required to fully recapitulate the molecular, morphological, and functional LSEC fingerprint.NEW & NOTEWORTHY Liver sinusoidal endothelial cells (LSECs) are the first liver cells to encounter waste macromolecules, pathogens, and toxins in the blood and are highly specialized. Although some transcription factors are known to play a role in LSEC specialization, information about the specialized LSEC signature and its transcriptional determinants remains incomplete. Here, we show that Musculoaponeurotic Fibrosarcoma (C-MAF), GATA binding protein 4 (GATA4), and Meis homeobox 2 (MEIS2) are important transcriptional regulators of the unique LSEC signature and that they affect the interaction of LSECs with viruses.

Liver sinusoidal endothelial cell ICAM-1 mediated tumor/endothelial crosstalk drives the development of liver metastasis by initiating inflammatory and angiogenic responses

The prometastatic stroma generated through tumor cells/host cells interaction is critical for metastatic growth. To elucidate the role of ICAM-1 on the crosstalk between tumor and primary liver sinusoidal endothelial cells (LSECs) and hepatic stellate cells (HSCs), implicated in tumor adhesion and angiogenesis, we performed in vitro cocultures and an in vivo model of liver metastasis of colorectal cancer (CRC). ICAM-1 blockade in the LSECs decreased the adhesion and transmigration of tumor cells through an LSEC in vitro and vivo. Cocultures of C26 cells and LSECs contained higher amounts of IL-1β, IL-6, PGE-2, TNF-α and ICAM-1 than monocultures. C26 cells incubated with sICAM-1 secreted higher amounts of PGE-2, IL-6, VEGF, and MMPs, while enhanced the migration of LSECs and HSCs. HSCs cultures activated by media from C26 cells pretreated with sICAM-1 contained the largest amounts of VEGF and MMPs. C26 cell activation with sICAM-1 enhanced their metastasizing potential in vivo, while tumor LFA-1 blockade reduced tumor burden and LSECs and HSC-derived myofibroblasts recruitment. In vivo ICAM-1 silencing produced similar results. These findings uncover LSEC ICAM-1 as a mediator of the CRC metastatic cascade in the liver and identifies it as target for the inhibition of liver colonization and metastatic progression.

Evaluating liver uptake and distribution of different poly(2-methyl-2-oxazoline) modified lysine dendrimers following intravenous administration

Residual charge and drug modification determine the cellular distribution in the liver for poly(2-methyl-2-oxazoline) modified lysine dendrimers.

Clearance of Tissue Plasminogen Activator by Mannose and Galactose Receptors in the Liver

The mechanism of uptake of radio-iodinated tissue plasminogen activator (125I-t-PA) was studied in rats. When trace amounts of 125I-t-PA were injected alone, the clearance followed a biphasic pattern in which 65% and 35% were cleared with α- and β-kinetics (t1/2 (α) = 0.6 min, and t1/2 (β) = 6.4 min), respectively. Coinjection with excess unlabelled t-PA or inannan changed the uptake kinetics to the muiiupliasic β-elimination pattern. Mannosylated albumin and ovalbumin, both of which bind to the hepatic mannose receptor, reduced the proportion of t-PA cleared with t1/2 (α) to 48% and 21%, respectively. A corresponding increase in the β-elimination ot t-PA was observed. The t1/2 (α) and t1/2 (β) were unchanged. Studies on the eleaiaucc of 125I-ovalbumin also showed a biphasic elimination with an initial rapid phase, t1/2 (α), accounting for only 39% of the clearance of ovalbumin, as compared to 65% in the case of t PA. Macromolecules with affinity for the galactose-receptor only, such as asialofetuin, or galactosylated albumin, did not significantly affect the clearance kinetics at the concentrations used. Asialoorosomucoid, which also carries galactosyl residues in the terminal position, reduced somewhat (from 65% to 48%) the proportion cleared with α-kinetics. Very high concentrations of galactose and N-acetyl-galactosamine, which are also known to compete for binding to the galactose receptor, lowered the proportion of t-PA cleared in the late β-phase (reduced from 35% to 26% with galactose and to 19% with N-acetyl-galactosamine).

To determine the hepatocellular site of uptake of t-PA, the protein was conjugated with 125I-labelled tyramine cellobiose (125I-TC) and injected intravenously (i.v.). This adduct is nonbiodegradable, and is trapped intralysosomally after endocytosis. I.v. injection of 125I-TC-t-PA and subsequent isolation of the liver cells showed that the Kupffer cells (KC), liver endothelial cells (LEC) and parenchymal cells (PC) contained 11%, 44% and 45%, respectively, of the radioactive label recovered in liver (hepatic uptake 80% of injected dose). The in vivo uptake per cell was about three times higher in KC and LEC than in PC. Injection of 125I-TC-t-PA together with mannose inhibited uptake in LEC and increased uptake in PC. Conversely, co-injection with galactose inhibited the uptake of 125I-TC-t-PA in PC and increased the uptake in LEC. Co-injection with excess amounts of unlabelled t-PA shifted the site of uptake from LEC to PC and changed the clearance kinetics to a monophasic β-elimination. The inhibitors used had only marginal effects on the uptake of 125I-TC-t-PA in KC. Although significant amounts of label were recovered in KC, the total size of the population of these cells is relatively small, so that the main hepatic uptake of 125I-TC-t-PA was in LEC and PC.

In conclusion, the elimination of t-PA from the blood by the liver is strongly dependent on the structure of its carbohydrate side chains. The main cellular sites of clearance are LEC (via mannose receptors), and PC (via galactose receptors and an unsaturable noncarbohydrate uptake mechanism).

Tissue Plasminogen Activator is Endocytosed by Mannose and Galactose Receptors of Rat Liver Cells

Experiments were carried out to charact erize the specificity of uptake of tPA in rat liver cells. Endocytosis in liver endothelial cells of the native carbohydrate variants of tissue plasminogen activator (tPA), and tPA inactivated by diisopropyl fluorophosphate was found to be competitive, suggesting that the determinant being recognized by these cells is different from the active site. Fibronectin and urokinase, which show partial homology with tPA, did not compete with tPA for uptake in liver endothelial cells. Hyaluronic acid, collagen, or IgG, which are endocytosed by specific receptors in liver endothelial cells, did not interfere with the uptake.

Reduced endocytosis by liver endothelial cells was observed with tPA modified in the carbohydrate side chains, suggesting that these structures are important for uptake. Ovalbumin, mannan, mannose, fructose, and EDTA, but not galactose, effectively inhibited uptake in liver endothelial cells of both native and diisopropyl fluorophosphate-inhibited tPA, but had very little effect on the uptake of tPA modified in the carbohydrate side chains.

Endocytosis of native tPA by parenchymal cells could be inhibited by galactose, ovalbumin, and EDTA, but not by mannose.

These results suggest that endocytosis of tPA by liver endothelial cells and parenchymal cells is mediated by the mannose and galactose receptors, respectively.

Uptake and Degradation of Tissue Plasminogen Activator in Rat Liver

The mechanism of uptake of tissue plasminogen activator (tPA) in rat liver was studied. Radio-iodinated tPA was removed from the circulation after intravenous administration in a biphasic mode. The initial half life, t1/2(α), and the terminal phase, t1/2(β), were determined to be 0.5 min and 7.5 min, resp. Separation of the liver cells by collagenase perfusion and density centrifugation, revealed that the uptake per cell was two to three times higher in the non-parenchymal cells than in the parenchymal cells.

Endocytosis of fluorescein isothiocyanate-labelled or 125I-labelled tPA was studied in pure cultures of liver cells in vitro. Liver endothelial cells and parenchymal cells took up and degraded tPA. Endocytosis was more efficient in liver endothelial cells than in parenchymal cells, and was almost absent in Kupffer cells.

Competitivb inhibition experiments showing that excess unlabelled tPA could compete with the uptake and degradation of 125I-tPA, suggested that liver endothelial cells and parenchymal cells interact with the activator in a specific manner. Endocytosis of trace amounts of 125I-tPA in cultures of liver endothelial cells and parenchymal cells was inhibited by 50% in the presence of 19 nM unlabelled tPA. Agents that interfere with one or several steps of the endocytic machinery inhibited uptake and degradation of 125I-tPA in both cell types.

These findings suggest that 1) liver endothelial cells and parenchymal cells are responsible for the rapid hepatic clearance of intravenously administered tPA; 2) the activator is taken up in these cells by specific endocytosis, and 3) endocytosed tPA is transported to the lysosomes where it is degraded.

Delivery is key: lessons learnt from developing splice-switching antisense therapies

The use of splice‐switching antisense therapy is highly promising, with a wealth of pre‐clinical data and numerous clinical trials ongoing. Nevertheless, its potential to treat a variety of disorders has yet to be realized. The main obstacle impeding the clinical translation of this approach is the relatively poor delivery of antisense oligonucleotides to target tissues after systemic delivery. We are a group of researchers closely involved in the development of these therapies and would like to communicate our discussions concerning the validity of standard methodologies currently used in their pre‐clinical development, the gaps in current knowledge and the pertinent challenges facing the field. We therefore make recommendations in order to focus future research efforts and facilitate a wider application of therapeutic antisense oligonucleotides.

Nanocrystal formulations of a poorly soluble drug. 2. Evaluation of nanocrystal liver uptake and distribution after intravenous administration to mice

A stabilized high drug load intravenous formulation could allow compounds with less optimal pharmacokinetic profiles to be developed. Polyethylene glycol (PEG)-ylation is a frequently used strategy for particle delivery systems to avoid the liver, thereby extending blood circulation time. The present work reports the mouse in vivo distribution after i.v. administration of a series of nanocrystals prepared with the bead milling technique and PEG-ylated with DSPE-PEG2000 and Pluronic F127, with and without polyvinylpyrrolidone K30 (PVP)/Aerosol OT (AOT) as primary stabilizers. While all formulations were cleared significantly faster than expected from nanocrystal dissolution alone, purely DSPE-PEG2000 PEG-ylated particles displayed prolonged circulation time (particles elimination half-life of 9min) compared to DSPE-PEG2000/PVP/AOT formulation (half-life of 3min). The two Pluronic F127 stabilized formulations displayed similar half-lives (9min with and without PVP/AOT, respectively). Whole tissue kinetics shows that clearance of particles could be attributed to accumulation in the liver. A separate in vivo study addressed the liver cell distribution after administration. Dissolved compound accumulated in hepatocytes only, while particles were distributed between liver sinusoidal endothelial cells and Kupffer cells. More DSPE-PEG2000/PVP/AOT stabilized particles accumulated in the liver, preferably in Kupffer cells, compared to Pluronic F127/PVP/AOT stabilized particles. The present study extends the understanding of PEG-ylation and "stealth" behaviour to also include nanocrystals.

Pulmonary vascular clearance of harmful endogenous macromolecules in a porcine model of acute liver failure

BACKGROUND

Pulmonary complications are common in acute liver failure (ALF). The role of the lungs in the uptake of harmful soluble endogenous macromolecules was evaluated in a porcine model of ALF induced by hepatic devascularization (n = 8) vs. controls (n = 8). In additional experiments, pulmonary uptake was investigated in healthy pigs. Fluorochrome-labeled modified albumin (MA) was applied to investigate the cellular uptake.

RESULTS

As compared to controls, the ALF group displayed a 4-fold net increased lung uptake of hyaluronan, and 5-fold net increased uptake of both tissue plasminogen activator and lysosomal enzymes. Anatomical distribution experiments in healthy animals revealed that radiolabeled MA uptake (taken up by the same receptor as hyaluronan) was 53% by the liver, and 24% by the lungs. The lung uptake of LPS was 14% whereas 60% remained in the blood. Both fluorescence and electron microscopy revealed initial uptake of MA by pulmonary endothelial cells (PECs) with later translocation to pulmonary intravascular macrophages (PIMs). Moreover, the presence of PIMs was evident 10 min after injection. Systemic inflammatory markers such as leukopenia and increased serum TNF-α levels were evident after 20 min in the MA and LPS groups.

CONCLUSION

Significant lung uptake of harmful soluble macromolecules compensated for the defect liver scavenger function in the ALF-group. Infusion of MA induced increased TNF-α serum levels and leukopenia, similar to the effect of the known inflammatory mediator LPS. These observations suggest a potential mechanism that may contribute to lung damage secondary to liver disease.

Nanocrystal formulations of a poorly soluble drug. 1. In vitro characterization of stability, stabilizer adsorption and uptake in liver cells

In the present work, milled nanocrystals of a poorly soluble compound using different stabilizers were prepared and characterized. The aim of the study was to evaluate a fundamental set of properties of the formulations prior to i.v. injection of the particles. Two polyethylene oxide containing stabilizers; (distearoyl phosphatidylethanol amine (DSPE)) -PEG2000 and the triblock copolymer Pluronic F127, were investigated, with and without polyvinylpyrrolidone K30/Aerosol OT (PVP/AOT) present. The solubility in water was around 10nM for the compound, measured from nanocrystals, but 1000 times higher in 4% human serum albumin. The particles were physically stable during the time investigated. The zeta potential was around -30 and -10mV for DSPE-PEG2000 and Pluronic F127 stabilized particles, respectively, at the conditions selected. The dissolution rate was similar for all four formulations and similar to the theoretically predicted rate. Critical micelle concentrations were determined as 56nM and 1.4μM for DSPE-PEG2000 and Pluronic F127, respectively. The adsorption isotherms for the PEG lipid showed a maximum adsorbed amount of about 1.3mg/m², with and without PVP/AOT. Pluronic F127 showed a higher maximum amount adsorbed, at around 3.1mg/m², and marginally lower with PVP/AOT present. Calculated data showed that the layer of Pluronic F127 was thicker than the corresponding DSPE-PEG2000 layer. The total amount of particles distributed mainly to the liver, and the hepatocellular distribution in vitro (Liver sinusoidal endothelial cells and Kupffer cells), differed depending on the stabilizing mixture on the particles. Overall, DSPE-PEG2000 stabilized nanocrystals (with PVP/AOT) accumulated to a larger degree in the liver compared to particles with Pluronic F127 on the surface. A theoretical model was developed to interpret in vivo pharmacokinetic profiles, explaining the balance between dissolution and liver uptake. With the present, fundamental data of the nanocrystal formulations, the platform for forthcoming in vivo studies was settled.

High-affinity von Willebrand factor binding does not affect the anatomical or hepatocellular distribution of factor VIII in rats

Essentials Von Willebrand factor (VWF) stabilizes factor VIII (FVIII) and prevents its premature clearance. Rat anatomical and hepatocellular distribution studies assessed the VWF effect on FVIII clearance. Hepatocytes and liver sinusoidal endothelial cells play a key role in FVIII clearance. Anatomical and hepatocellular distribution of FVIII is independent of high-affinity VWF binding.

ABSTRACT

Background Von Willebrand factor (VWF) stabilizes factor VIII in the circulation and prevents its premature clearance. Objective To study the effects of VWF on FVIII clearance in rats with endogenous VWF. Methods Anatomical and hepatocellular distribution studies were performed in rats following intravenous administration of glycoiodinated recombinant FVIII (rFVIII) and a FVIII variant, FVIII-Y1680F, lacking high-affinity VWF binding. Radioactivity was quantified in organs, and in distinct liver cell populations. The role of VWF binding was also studied by immunohistochemical staining of rat livers perfused ex vivo with rFVIII alone or with a FVIII-binding VWF fragment. Results The liver was the predominant organ of rFVIII distribution, and a radioactivity peak was also observed in the intestines, suggesting FVIII secretion to the bile by hepatocytes. In the liver, ~60% of recovered radioactivity was associated with hepatocytes, 32% with liver sinusoidal endothelial cells (LSECs), and 9% with Kupffer cells (KCs). When calculated per cell, 1.5-fold to 3-fold more radioactivity was associated with LSECs than with hepatocytes. The importance of hepatocytes and LSECs was confirmed by immunohistochemical staining; strong staining was seen in LSECs, and less intense, punctate staining in hepatocytes. Minor staining in KCs was observed. Comparable anatomical and hepatocellular distributions were observed with rFVIII and FVIII-Y1680F, and the presence of the VWF fragment, D'D3A1, did not change the FVIII staining pattern in intact livers. Conclusions The present data support FVIII clearance via the liver, with hepatocytes and LSECs playing a key role. High-affinity VWF binding did not alter the anatomical or hepatocellular distribution of FVIII.

FITC Conjugation Markedly Enhances Hepatic Clearance of N-Formyl Peptides

In both septic and aseptic inflammation, N-formyl peptides may enter the circulation and induce a systemic inflammatory response syndrome similar to that observed during septic shock. The inflammatory response is brought about by the binding of N-formyl peptide to formyl peptide receptors (FPRs), specific signaling receptors expressed on myeloid as well as non-myeloid cells involved in the inflammatory process. N-formyl peptides conjugated with fluorochromes, such as fluorescein isothiocyanate (FITC) are increasingly experimentally used to identify tissues involved in inflammation. Hypothesizing that the process of FITC-conjugation may transfer formyl peptide to a ligand that is efficiently cleared from the circulation by the natural powerful hepatic scavenging regime we studied the biodistribution of intravenously administered FITC-fNLPNTL (Fluorescein-isothiocyanate- N-Formyl-Nle-Leu-Phe-Nle-Tyr-Lys) in mice. Our findings can be summarized as follows: i) In contrast to unconjugated fNLPNTL, FITC-fNLPNTL was rapidly taken up in the liver; ii) Mouse and human liver sinusoidal endothelial cells (LSECs) and hepatocytes express formyl peptide receptor 1 (FRP1) on both mRNA (PCR) and protein (Western blot) levels; iii) Immunohistochemistry showed that mouse and human liver sections expressed FRP1 in LSECs and hepatocytes; and iv) Uptake of FITC-fNLPNTL could be largely blocked in mouse and human hepatocytes by surplus-unconjugated fNLPNTL, thereby suggesting that the hepatocytes in both species recognized FITC-fNLPNTL and fNLPNTL as indistinguishable ligands. This was in contrast to the mouse and human LSECs, in which the uptake of FITC-fNLPNTL was mediated by both FRP1 and a scavenger receptor, specifically expressed on LSECs. Based on these results we conclude that a significant proportion of FITC-fNLPNTL is taken up in LSECs via a scavenger receptor naturally expressed in these cells. This calls for great caution when using FITC-fNLPNTL and other chromogen-conjugated formyl peptides as a probe to identify cells in a liver engaged in inflammation. Moreover, our finding emphasizes the role of the liver as an important neutralizer of otherwise strong inflammatory signals such as formyl peptides.

Genome-wide analysis of DNA methylation and gene expression patterns in purified, uncultured human liver cells and activated hepatic stellate cells

BACKGROUND & AIMS

Liver fibrogenesis - scarring of the liver that can lead to cirrhosis and liver cancer - is characterized by hepatocyte impairment, capillarization of liver sinusoidal endothelial cells (LSECs) and hepatic stellate cell (HSC) activation. To date, the molecular determinants of a healthy human liver cell phenotype remain largely uncharacterized. Here, we assess the transcriptome and the genome-wide promoter methylome specific for purified, non-cultured human hepatocytes, LSECs and HSCs, and investigate the nature of epigenetic changes accompanying transcriptional changes associated with activation of HSCs.

MATERIAL AND METHODS

Gene expression profile and promoter methylome of purified, uncultured human liver cells and culture-activated HSCs were respectively determined using Affymetrix HG-U219 genechips and by methylated DNA immunoprecipitation coupled to promoter array hybridization. Histone modification patterns were assessed at the single-gene level by chromatin immunoprecipitation and quantitative PCR.

RESULTS

We unveil a DNA-methylation-based epigenetic relationship between hepatocytes, LSECs and HSCs despite their distinct ontogeny. We show that liver cell type-specific DNA methylation targets early developmental and differentiation-associated functions. Integrative analysis of promoter methylome and transcriptome reveals partial concordance between DNA methylation and transcriptional changes associated with human HSC activation. Further, we identify concordant histone methylation and acetylation changes in the promoter and putative novel enhancer elements of genes involved in liver fibrosis.

CONCLUSIONS

Our study provides the first epigenetic blueprint of three distinct freshly isolated, human hepatic cell types and of epigenetic changes elicited upon HSC activation.

Efficient uptake of blood-borne BK and JC polyomavirus-like particles in endothelial cells of liver sinusoids and renal vasa recta

Liver sinusoidal endothelial cells (LSECs) are specialized scavenger cells that mediate high-capacity clearance of soluble waste macromolecules and colloid material, including blood-borne adenovirus. To explore if LSECs function as a sink for other viruses in blood, we studied the fate of virus-like particles (VLPs) of two ubiquitous human DNA viruses, BK and JC polyomavirus, in mice. Like complete virions, VLPs specifically bind to receptors and enter cells, but unlike complete virions, they cannot replicate. ¹²⁵I-labeled VLPs were used to assess blood decay, organ-, and hepatocellular distribution of ligand, and non-labeled VLPs to examine cellular uptake by immunohisto- and -cytochemistry. BK- and JC-VLPs rapidly distributed to liver, with lesser uptake in kidney and spleen. Liver uptake was predominantly in LSECs. Blood half-life (∼1 min), and tissue distribution of JC-VLPs and two JC-VLP-mutants (L55F and S269F) that lack sialic acid binding affinity, were similar, indicating involvement of non-sialic acid receptors in cellular uptake. Liver uptake was not mediated by scavenger receptors. In spleen, the VLPs localized to the red pulp marginal zone reticuloendothelium, and in kidney to the endothelial lining of vasa recta segments, and the transitional epithelium of renal pelvis. Most VLP-positive vessels in renal medulla did not express PV-1/Meca 32, suggesting location to the non-fenestrated part of vasa recta. The endothelial cells of these vessels also efficiently endocytosed a scavenger receptor ligand, formaldehyde-denatured albumin, suggesting high endocytic activity compared to other renal endothelia. We conclude that LSECs very effectively cleared a large fraction of blood-borne BK- and JC-VLPs, indicating a central role of these cells in early removal of polyomavirus from the circulation. In addition, we report the novel finding that a subpopulation of endothelial cells in kidney, the main organ of polyomavirus persistence, showed selective and rapid uptake of VLPs, suggesting a role in viremic organ tropism.

Imaging fenestrations in liver sinusoidal endothelial cells by optical localization microscopy

We demonstrate the use of single molecule localization microscopy for resolving structural details of fenestrations in liver sinusoidal endothelial cells.

Hepatic disposal of advanced glycation end products during maturation and aging

Aging is characterized by progressive loss of metabolic and biochemical functions and accumulation of metabolic by-products, including advanced glycation end products (AGEs), which are observed in several pathological conditions. A number of waste macromolecules, including AGEs are taken up from the circulation by endocytosis mainly into liver sinusoidal endothelial cells (LSECs) and Kupffer cells (KCs). However, AGEs still accumulate in different tissues with aging, despite the presence of this clearance mechanism. The aim of the present study was to determine whether the efficiency of LSECs and KCs for disposal of AGEs changes through aging.

RESULTS

After intravenous administration of (14)C-AGE-albumin in pre-pubertal, young adult, middle aged and old mice, more than 90% of total recovered (14)C-AGE was liver associated, irrespective of age. LSECs and KCs represented the main site of uptake. A fraction of the (14)C-AGE degradation products ((14)C-AGE-DPs) was stored for months in the lysosomes of these cells after uptake. The overall rate of elimination of (14)C-AGE-DPs from the liver was markedly faster in pre-pubertal than in all post-pubertal age groups. The ability to eliminate (14)C-AGE-DPs decreased to similar extents after puberty in LSECs and KCs. A rapid early removal phase was characteristic for all age groups except the old group, where this phase was absent.

CONCLUSIONS

Removal of AGE-DPs from the liver scavenger cells is a very slow process that changes with age. The ability of these cells to dispose of AGEs declines after puberty. Decreased AGE removal efficiency early in life may lead to AGE accumulation.

The scavenger endothelial cell: a new player in homeostasis and immunity

To maintain homeostasis, the animal body is equipped with a powerful system to remove circulating waste. This review presents evidence that the scavenger endothelial cell (SEC) is responsible for the clearance of blood-borne waste macromolecules in vertebrates. SECs express pattern-recognition endocytosis receptors (mannose and scavenger receptors), and in mammals, the endocytic Fc gamma-receptor IIb2. This cell type has an endocytic machinery capable of super-efficient uptake and degradation of physiological and foreign waste material, including all major classes of biological macromolecules. In terrestrial vertebrates, most SECs line the wall of the liver sinusoid. In phylogenetically older vertebrates, SECs reside instead in heart, kidney, or gills. SECs, thus, by virtue of their efficient nonphagocytic elimination of physiological and microbial substances, play a critical role in the innate immunity of vertebrates. In major invertebrate phyla, including insects, the same function is carried out by nephrocytes. The concept of a dual-cell principle of waste clearance is introduced to emphasize that professional phagocytes (macrophages in vertebrates; hemocytes in invertebrates) eliminate larger particles (>0.5 μm) by phagocytosis, whereas soluble macromolecules and smaller particles are eliminated efficiently and preferentially by clathrin-mediated endocytosis in nonphagocytic SECs in vertebrates or nephrocytes in invertebrates. Including these cells as important players in immunology and physiology provides an additional basis for understanding host defense and tissue homeostasis.

The Relationship between fenestrations, sieve plates and rafts in liver sinusoidal endothelial cells

Fenestrations are transcellular pores in endothelial cells that facilitate transfer of substrates between blood and the extravascular compartment. In order to understand the regulation and formation of fenestrations, the relationship between membrane rafts and fenestrations was investigated in liver sinusoidal endothelial cells where fenestrations are grouped into sieve plates. Three dimensional structured illumination microscopy, scanning electron microscopy, internal reflectance fluorescence microscopy and two-photon fluorescence microscopy were used to study liver sinusoidal endothelial cells isolated from mice. There was an inverse distribution between sieve plates and membrane rafts visualized by structured illumination microscopy and the fluorescent raft stain, Bodipy FL C5 ganglioside GM1. 7-ketocholesterol and/or cytochalasin D increased both fenestrations and lipid-disordered membrane, while Triton X-100 decreased both fenestrations and lipid-disordered membrane. The effects of cytochalasin D on fenestrations were abrogated by co-administration of Triton X-100, suggesting that actin disruption increases fenestrations by its effects on membrane rafts. Vascular endothelial growth factor (VEGF) depleted lipid-ordered membrane and increased fenestrations. The results are consistent with a sieve-raft interaction, where fenestrations form in non-raft lipid-disordered regions of endothelial cells once the membrane-stabilizing effects of actin cytoskeleton and membrane rafts are diminished.

Rat liver sinusoidal endothelial cells (LSECs) express functional low density lipoprotein receptor-related protein-1 (LRP-1)

BACKGROUND & AIMS

The low density lipoprotein receptor-related protein-1 (LRP-1) is a large, multifunctional endocytic receptor from the LDL receptor family, highly expressed in liver parenchymal cells (PCs), neurons, activated astrocytes, and fibroblasts. The aim of the study was to investigate if liver sinusoidal endothelial cells (LSECs), highly specialized scavenger cells, express LRP-1.

METHODS

To address this question, experiments were performed in vivo and in vitro to determine if receptor associated protein (RAP) and trypsin-activated α(2)-macroglobulin (α(2)M∗) were endocytosed in LSECs.

RESULTS

Both ligands were cleared from the circulation mainly by the liver. Hepatocellular distribution of intravenously administered ligands, assessed after magnetic bead cell separation using LSEC- and KC-specific antibodies, showed that PCs contained 93% and 82% of liver-associated (125)I-RAP and (125)I-α(2)M∗, whereas 5% and 11% were associated with LSECs. Uptake of RAP and α(2)M∗ in the different liver cell population in vitro was specific and followed by degradation. The uptake of (125)I-RAP was not inhibited by ligands to known endocytosis receptors in LSECs, while uptake of (125)I-α(2)M∗ was significantly inhibited by RAP, suggesting the involvement of LRP-1. Immunofluorescence using LRP-1 antibody showed positive staining in LSECs. Ligand blot analyses using total cell proteins and (125)I-RAP followed by mass spectrometry further confirmed and identified LRP-1 in LSECs.

CONCLUSIONS

LSECs express functional LRP-1. An important implication of our findings is that LSECs contribute to the rapid removal of blood borne ligands for LRP-1 and may thus play a role in lipid homeostasis.

Effects of oxidized low-density lipoproteins on the hepatic microvasculature

Oxidized low-density lipoproteins (oxLDLs) are involved in proinflammatory and cytotoxic events in different microcirculatory systems. The liver is an important scavenger organ for circulating oxLDLs. However, the interaction of oxLDL with the hepatic microcirculation has been poorly investigated. The present study was conducted to examine the effects of differently modified oxLDLs on the hepatic microvasculature. C57Bl/6J mice were injected intravenously with low-density lipoprotein (LDL), or LDL oxidized for 3 h (oxLDL(3)) or 24 h (oxLDL(24)), at doses resembling oxLDL plasma levels in cardiovascular disease patients. Radioiodinated ligands were used to measure blood decay and organ distribution, and nonlabeled ligands to evaluate microcirculatory responses, examined by in vivo microscopy 30-60 min after ligand injection, immunohistochemistry, and scanning and transmission electron microscopy. Mildly oxLDL (oxLDL(3)) was cleared from blood at a markedly slower rate than heavily oxLDL (oxLDL(24)), but significantly faster than LDL (P < 0.01). Injected oxLDLs distributed to liver. OxLDL effects were most pronounced in central areas of the liver lobules where oxLDL(3) elicited a significant (P < 0.05) reduction in perfused sinusoids, and both oxLDL(3) and oxLDL(24) significantly increased the numbers of swollen endothelial cells and adherent leukocytes compared with LDL (P < 0.05). OxLDL-treated livers also exhibited increased intercellular adhesion molecule (ICAM)-1 centrilobular staining. Electron microscopy showed a 30% increased thickness of the liver sinusoidal endothelium in the oxLDL(3) group (P < 0.05) and a reduced sinusoidal fenestration in centrilobular areas with increased oxidation of LDL (P for linear trend <0.05). In conclusion, OxLDL induced several acute changes in the liver microvasculature, which may lead to sinusoidal endothelial dysfunction.

Endocytosis of advanced glycation end-products in bovine choriocapillaris endothelial cells

OBJECTIVES

Age-related changes in Bruch's membrane (BM), situated between the retina and the choroid, include the accumulation of advanced glycation end-products (AGEs) and other modified macromolecules. An important clearance mechanism for such molecules is endocytosis via macrophages and specialized endothelial cells. This study examined the endocytic clearance of AGEs in choriocapillaris endothelial (CCE) cells, which are strategically located beneath the BM.

MATERIALS AND METHODS

Bovine CCE cultures were incubated with radiolabeled or fluorescently labeled AGE-modified bovine serum albumin (AGE-BSA). Cells and tissues were examined for the expression of two AGE-binding scavenger receptors, stabilin-1 and -2, by immunohistochemistry and Western blotting, and with antibody inhibition studies.

RESULTS

CCE cells effectively endocytosed AGE-BSA via a scavenger receptor-mediated pathway. A polyclonal antibody against stabilin-2 significantly inhibited endocytosis (40%). Tissue sections stained positive for stabilin-2 and -1 in the choriocapillaris layer, which was confirmed by immunoblots of cell extracts. Colocalization of stabilin-1 and -2 and internalized AGE-BSA was seen in early endosomes.

CONCLUSIONS

CCEs actively endocytose AGE-BSA and express the scavenger receptors, stabilin-1 and -2, of which at least stabilin-2 is involved in AGE-BSA uptake. Impairment of this stabilin-mediated clearance function may contribute to depositions of waste macromolecules in BM and subsequent retinopathy.

Role of liver sinusoidal endothelial cells and stabilins in elimination of oxidized low-density lipoproteins

Atherogenesis is associated with elevated levels of low-density lipoprotein (LDL) and its oxidized form (oxLDL) in the blood. The liver is an important scavenger organ for circulating oxLDLs. The present study aimed to examine endocytosis of mildly oxLDL (the major circulating form of oxLDLs) in liver sinusoidal endothelial cells (LSECs) and the involvement of the scavenger receptors stabilin-1 and stabilin-2 in this process. Freshly isolated LSECs, Kupffer cells (KCs), and stabilin-1- and stabilin-2-transfected human embryonic kidney cells were incubated with fluorescently labeled or radiolabeled oxLDLs [oxidized for 3 h (oxLDL(3)), 6 h, or 24 h (oxLDL(24))] to measure endocytosis. The intracellular localization of oxLDLs and stabilins in LSECs was examined by immunofluorescence and immunogold electron microscopy. Whereas oxLDL(24) was endocytosed both by LSECs and KCs, oxLDL(3) (mildly oxLDL) was taken up by LSECs only. The LSEC uptake of oxLDLs was significantly inhibited by the scavenger receptor ligand formaldehyde-treated serum albumin. Uptake of all modified LDLs was high in stabilin-1-transfected cells, whereas stabilin-2-transfected cells preferentially took up oxLDL(24), suggesting that stabilin-1 is a more important receptor for mildly oxLDLs than stabilin-2. Double immunogold labeling experiments in LSECs indicated interactions of stabilin-1 and stabilin-2 with oxLDL(3) on the cell surface, in coated pits, and endocytic vesicles. LSECs but not KCs endocytosed mildly oxLDL. Both stabilin-1 and stabilin-2 were involved in the LSEC endocytosis of oxLDLs, but experiments with stabilin-transfected cells pointed to stabilin-1 as the most important receptor for mildly oxLDL.

Age-related changes in scavenger receptor-mediated endocytosis in rat liver sinusoidal endothelial cells

Liver sinusoidal endothelial cells (LSECs) play an essential role in systemic waste clearance by effective endocytosis of blood-borne waste macromolecules. We aimed to study LSECs' scavenger function during aging, and whether age-related morphological changes (eg, defenestration) affect this function, in F344/BN F1 rats. Endocytosis of the scavenger receptor ligand formaldehyde-treated serum albumin was significantly reduced in LSECs from old rats. Ligand degradation, LSEC protein expression of the major scavenger receptors for formaldehyde-treated serum albumin endocytosis, stabilin-1 and stabilin-2, and their staining patterns along liver sinusoids, was similar at young and old age, suggesting that other parts of the endocytic machinery are affected by aging. Formaldehyde-treated serum albumin uptake per cell, and cell porosity evaluated by electron microscopy, was not correlated, indicating that LSEC defenestration is not linked to impaired endocytosis. We report a significantly reduced LSEC endocytic capacity at old age, which may be especially important in situations with increased circulatory waste loads.

Colon carcinoma cell interaction with liver sinusoidal endothelium inhibits organ-specific antitumor immunity through interleukin-1-induced mannose receptor in mice

Mannose receptor (ManR)-mediated liver sinusoidal endothelial cell (LSEC) endocytosis plays a role in antigen presentation and innate immunity, but its role in hepatic metastasis is unknown. We studied ManR-mediated endocytosis during C26 colorectal cancer cell interaction with LSECs and its implications in metastasis. Uptake of labeled ManR ligands (mannan and ovalbumin) and immunohistochemistry were used to study ManR endocytosis and expression. Several interleukin (IL)-1 inhibitors and the cyclooxygenase-2 (COX-2) inhibitor celecoxib were used to analyze the role of IL-1 and COX-2 in ManR regulation. Anti-mouse ManR antibodies and ManR knockout (ManR(-/-)) mice were used to identify ManR-dependent mechanisms during antitumor immune response of liver sinusoidal lymphocytes (LSLs) interacting with tumor-activated LSECs. ManR expression and endocytosis increased in tumor-activated LSECs through a two-step mechanism: (1) Release of COX-2-dependent IL-1-stimulating factors by lymphocyte function-associated antigen-1-expressing C26 cells in response to intercellular adhesion molecule-1 (ICAM-1), which was expressed and secreted by tumor-activated LSECs; and (2) widespread up-regulation of ManR in LSECs through tumor-induced IL-1. In addition, LSLs that had interacted with tumor-activated LSECs in vivo decreased their antitumor cytotoxicity and interferon (IFN)-gamma secretion while they increased IL-10 release ex vivo. IFN-gamma/IL-10 ratio also decreased in the hepatic blood from tumor-injected mice. Immunosuppressant effects of tumor-activated LSECs on LSLs were abrogated in both LSECs from ManR(-/-) mice and tumor-activated LSECs given anti-mouse ManR antibodies.

CONCLUSION

ICAM-1-induced tumor COX-2 decreased antitumor activity during hepatic metastasis through IL-1-induced ManR. ManR constituted a common mediator for prometastatic effects of IL-1, COX-2, and ICAM-1. A rise in hepatic IFN-gamma/IL-10 ratio and antitumor cytotoxicity by way of ManR blockade is consistent with the antimetastatic effects of IL-1, COX-2, and ICAM-1 inhibitors. These data support ManR and ManR-stimulating factors as targets for hepatic colorectal metastasis therapy.

Porcine liver sinusoidal endothelial cells contribute significantly to intrahepatic ammonia metabolism

Ammonia metabolism in the liver has been largely credited to hepatocytes (HCs). We have shown that liver nonparenchymal cells that include liver sinusoidal endothelial cells (LSECs) produce ammonia. To address the limited knowledge regarding a role for LSECs in ammonia metabolism, we investigated the ammonia metabolism of isolated LSECs and HCs under three different conditions: (1) bioreactors containing LSECs (LSEC-bioreactors), (2) bioreactors containing HCs (HC-bioreactors), and (3) separate bioreactors containing LSECs and HCs connected in sequence (Seq-bioreactors). Our results showed that LSEC-bioreactors released six-fold more ammonia (22.2 nM/hour/10(6) cells) into the growth media than HC-bioreactors (3.3 nM/hour/10(6) cells) and Seq-bioreactors (3.8 nM/hour/10(6) cells). The glutamate released by LSEC-bioreactors (32.0 nM/hour/10(6) cells) was over four-fold larger than that released by HC-bioreactors and Seq-bioreactors (<7 nM/hour/10(6) cells). LSEC-bioreactors and HC-bioreactors consumed large amounts of glutamine (>25 nM/hour/10(6) cells). Glutaminase is known for catalyzing glutamine into glutamate and ammonia. To determine if this mechanism may be responsible for the large levels of glutamate and ammonia found in LSEC-bioreactors, immunolabeling of glutaminase and messenger RNA expression were tested. Our results demonstrated that glutaminase was present with colocalization of an LSEC-specific functional probe in lysosomes of LSECs. Furthermore, using a nucleotide sequence specific for kidney-type glutaminase, reverse-transcription polymerase chain reaction revealed that this isoform of glutaminase was expressed in porcine LSECs.

CONCLUSION

LSECs released large amounts of ammonia, perhaps due to the presence of glutaminase in lysosomes. The ammonia and glutamate released by LSECs in Seq-bioreactors were used by hepatocytes, suggesting an intrahepatic collaboration between these two cell types.

Hepatic sinusoidal cells in health and disease: update from the 14th International Symposium

This review aims to give an update of the field of the hepatic sinusoid, supported by references to presentations given at the 14th International Symposium on Cells of the Hepatic Sinusoid (ISCHS2008), which was held in Tromsø, Norway, August 31-September 4, 2008. The subtitle of the symposium, 'Integrating basic and clinical hepatology', signified the inclusion of both basal and applied clinical results of importance in the field of liver sinusoidal physiology and pathophysiology. Of nearly 50 oral presentations, nine were invited tutorial lectures. The authors of the review have avoided writing a 'flat summary' of the presentations given at ISCHS2008, and instead focused on important novel information. The tutorial presentations have served as a particularly important basis in the preparation of this update. In this review, we have also included references to recent literature that may not have been covered by the ISCHS2008 programme. The sections of this review reflect the scientific programme of the symposium (http://www.ub.uit.no/munin/bitstream/10037/1654/1/book.pdf): 1. Liver sinusoidal endothelial cells. 2. Kupffer cells. 3. Hepatic stellate cells. 4. Immunology. 5. Tumor/metastasis. Symposium abstracts are referred to by a number preceded by the letter A.

Liver sinusoidal endothelial cells depend on mannose receptor-mediated recruitment of lysosomal enzymes for normal degradation capacity

Liver sinusoidal endothelial cells (LSECs) are largely responsible for the removal of circulating lysosomal enzymes (LE) via mannose receptor (MR)-mediated endocytosis. We hypothesized that LSECs rely on this uptake to maintain their extraordinarily high degradation capacity for other endocytosed material. Circulatory half-life studies of (125)I-cathepsin-D in MR knockout (MR(-/-)) and wild-type mice, and endocytosis studies in LSEC cultures, showed a total dependence on the MR for effective clearance of cathepsin-D. Radioiodinated formaldehyde-treated serum albumin, a ligand for the LSEC scavenger receptors, was used to study catabolism of endocytosed material in MR(-/-) and wild-type mice. The plasma clearance, liver uptake, and the starting point for release of degradation products to blood, were similar in both experimental groups, indicating normal endocytosis and intracellular transport of scavenger receptor ligands in MR(-/-) mice. However, the rate of formaldehyde-treated serum albumin catabolism in the liver of the MR deficient animals was reduced to approximately 50% of wild-type values. A similar reduction in intracellular degradation was recorded in LSEC cultures from MR(-/-) mice compared to wild-type controls. In accordance with this, MR(-/-) LSECs had markedly and significantly reduced enzyme activities for four out of five LE tested, i.e., cathepsin-D, alpha-mannosidase, beta-hexosaminidase and arylsulfatase, but not acid phosphatase, compared to wild-type controls. Immunoblot analysis showed that the content of pro-cathepsin-D relative to total cathepsin-D in wild-type LSECs was less than one-fifth of that in hepatocytes, indicating lower endogenous LE production in the LSECs.

CONCLUSION

We show for the first time that LSEC depend on MR-mediated recruitment of LE from their surroundings for effective catabolism of endocytosed macromolecules.

HB-EGF is a paracrine growth stimulator for early tumor prestages in inflammation-associated hepatocarcinogenesis

BACKGROUND/AIMS

We studied the impact of heparin-binding epidermal growth factor-like growth factor (HB-EGF) on inflammation-driven hepatocarcinogenesis.

METHODS

HB-EGF expression was determined by qRT-PCR and immunodetection in hepatocellular adenoma and carcinoma and in mesenchymal (MC) and parenchymal liver cells obtained from different models of inflammation. The functions of HB-EGF in early hepatocarcinogenesis were assessed in co-cultures of unaltered and initiated/premalignant hepatocytes.

RESULTS

In human and rat (pre)malignant liver lesions, HB-EGF levels were comparable to that of the surrounding tissue. In inflamed livers HB-EGF was expressed predominantly in MC and was further increased by pro-inflammatory lipopolysaccharide (LPS) or linoleic acid hydroperoxide (LOOH). In culture, DNA-replication occurred rather in initiated/premalignant than unaltered hepatocytes and was further elevated by LOOH- or LPS-stimulated MC-supernatants. The supernatant effects were abrogated by pre-incubation with HB-EGF-neutralizing antisera. HB-EGF itself induced DNA-replication and mitosis preferentially in the initiated/premalignant cells. When transducing hepatocytes with a dominant-negative ErbB1-construct, HB-EGF-induced DNA-replications were blocked completely in unaltered hepatocytes but incompletely in initiated/premalignant cells, which suggests elevated ErbB-mediated signal transduction in first stages of hepatocarcinogenesis.

CONCLUSIONS

Pro-inflammatory stimuli induce the release of HB-EGF from MC, which stimulates DNA-replication in initiated/premalignant hepatocytes. Similar mechanisms may contribute to carcinogenesis in human inflammatory liver diseases.

Old age and the hepatic sinusoid

Morphological changes in the hepatic sinusoid with old age are increasingly recognized. These include thickening and defenestration of the liver sinusoidal endothelial cell, sporadic deposition of collagen and basal lamina in the extracellular space of Disse, and increased numbers of fat engorged, nonactivated stellate cells. In addition, there is endothelial up-regulation of von Willebrand factor and ICAM-1 with reduced expression of caveolin-1. These changes have been termed age-related pseudocapillarization. The effects of old age on Kupffer cells are inconsistent, but impaired responsiveness is likely. There are functional implications of these aging changes in the hepatic sinusoid. There is reduced sinusoidal perfusion, which will impair the hepatic clearance of highly extracted substrates. Blood clearance of a variety of waste macromolecules takes place in liver sinusoidal endothelial cells (LSECs). Previous studies indicated either that aging had no effect, or reduced the endocytic capacity of LSECs. However, a recent study in mice showed reduced endocytosis in pericentral regions of the liver lobules. Reduced endocytosis may increase systemic exposure to potential harmful waste macromolecules such as advanced glycation end products Loss of fenestrations leads to impaired transfer of lipoproteins from blood to hepatocytes. This provides a mechanism for impaired chylomicron remnant clearance and postprandial hyperlipidemia associated with old age. Given the extensive range of substrates metabolized by the liver, age-related changes in the hepatic sinusoid and microcirculation have important systemic implications for aging and age-related diseases.

Liver sinusoidal endothelial cells are the principal site for elimination of unfractionated heparin from the circulation

The mechanism of elimination of blood borne heparin was studied. To this end unfractionated heparin (UFH) was tagged with FITC, which served as both a visual marker and a site of labeling with (125)I-iodine. UFH labeled in this manner did not alter the anticoagulant activity or binding specificity of the glycosaminoglycan. Labeled heparin administered intravenously to rats (0.1 IU/kg) had a circulatory t(1/2) of 1.7 min, which was increased to 16 min upon coinjection with unlabeled UFH (100 IU/kg). At 15 min after injection, 71% of recovered radioactivity was found in liver. Liver cell separation revealed the following relative uptake capacity, expressed per cell: liver sinusoidal endothelial cell (LSEC)-parenchymal cell-Kupffer cell = 15:3.6:1. Fluorescence microscopy on liver sections showed accumulation of FITC-UFH only in cells lining the liver sinusoids. No fluorescence was detected in parenchymal cells or endothelial cells lining the central vein. Fluorescence microscopy of cultured LSECs following binding of FITC-UFH at 4 degrees C and chasing at 37 degrees C, showed accumulation of the probe in vesicles located at the periphery of the cells after 10 min, with transfer to large, evenly stained vesicles in the perinuclear region after 2 h. Immunogold electron microscopy of LSECs to probe the presence of FITC following injection of FITC-UFH along with BSA-gold to mark lysosomes demonstrated colocalization of the probe with the gold particles in the lysosomal compartment. Receptor-ligand competition experiments in primary cultures of LSECs indicated the presence of a specific heparin receptor, functionally distinct from the hyaluronan/scavenger receptor (Stabilin2). The results suggest a major role for LSECs in heparin elimination.

The liver sinusoidal endothelial cell: a cell type of controversial and confusing identity

A look through the literature on liver sinusoidal endothelial cells (LSECs) reveals that there are several conflicts among different authors of what this cell type is and does. Major controversies that will be highlighted in this review include aspects of the physiological role, the characterization, and the protocols of isolation and cultivation of these cells. Many of these conflicts may be ascribed to the fact that the cell was only recently established as a distinct cell type and that researchers from different disciplines tend to define their structure and function differently. This field is in need of a common platform to obtain a sound communication and a unified understanding of how to interpret novel research results. The aim of this review is to encourage scientists not to ignore the fact that there are, indeed, different opinions in the literature on LSECs. We also hope that this review will point out to the reader that some issues that may seem well established regarding our knowledge about the LSECs, in reality, are still unresolved and, indeed, controversial.

Effects of CpG-oligonucleotides, poly I:C and LPS on Atlantic cod scavenger endothelial cells (SEC)

Carrying out a remarkably efficient clearance of colloidal and soluble macromolecular waste substances from the circulation, the scavenger endothelial cells (SECs) represent an important part of the reticuloendothelial system of vertebrates. It has been previously shown that these cells play an important role in the innate immune system by eliminating from the blood a number of molecules known to elicit inflammatory reactions. In the present study we have investigated the uptake of LPS and oligonucleotides in cultured Atlantic cod SECs, and determined if interaction with these pathogen associated molecules affect the scavenger activity and/or production of immune modulating molecules of SECs. Preincubation of cultured SECs with CpG (5 and 20 microg/ml) or Poly I:C (10 or 40 microg/ml) gave selective down-regulation of scavenger receptor-mediated endocytosis, but only marginal effects were noted on endocytosis via the mannose- and collagen alpha chain receptors. Preincubation with LPS or a non-inflammatory ligand for the scavenger receptor did not result in altered endocytosis via any of the receptors tested. Only Poly I:C (40 microg/ml) was observed to increase the production of NO. RT-PCR analyses showed IL-1 production which was not increased above control after pre-treatment with two different CpG-oligonucleotides or Poly I:C. In fact, preincubation with Poly I:C, but not CpGs, resulted in degradation of total intracellular RNA. In conclusion, our study shows that SECs respond differently to the different immunomodulators used and that their important clearance activity as scavenger cells can be regulated by the use of oligonucleotides.

Studies on uptake and intracellular processing of infectious pancreatic necrosis virus by Atlantic cod scavenger endothelial cells

Previous work in our group has identified the scavenger endothelial cells (SECs) of heart endocardium in cod, Gadus morhua L., as the major site for elimination of both physiological and foreign macromolecular waste from the circulation. The present study was undertaken to establish the role of cod SECs in the clearance of virus. We focused on infectious pancreatic necrosis virus (IPNV) as it is a well-known virus with a broad host range, and causes significant economic losses in the salmon industry. Our results showed that cod SEC cultures infected by the IPNV produce high titres of new virus. Ligand-receptor inhibition experiments suggested that the virus did not enter the cells through any of the major endocytosis receptors of cod SECs. Yet, the infection lowered the capacity of the cells to endocytose ligands via the scavenger receptor. Inhibitors of receptor recycling and vesicle acidification did not affect virus infectivity. The finding that SEC cultures prepared from 25% of the cod produced high titres of IPNV without being infected in the laboratory, suggests that SECs of cod may serve as reservoirs for IPNV in persistently infected cod.

Significant contribution of liver nonparenchymal cells to metabolism of ammonia and lactate and cocultivation augments the functions of a bioartificial liver

A bioartificial liver (BAL) will bridge patients with acute liver failure (ALF) to either spontaneous regeneration or liver transplantation. The nitrogen metabolism is important in ALF, and the metabolism of nonparenchymal liver cells (NPCs) is poorly understood. The scope of this study was to investigate whether cocultivation of hepatocytes with NPCs would augment the functions of a BAL (HN-BAL) compared with a BAL equipped with only hepatocytes (H-BAL). In addition, NPCs were similarly cultivated alone. The cells were cultivated for 8 days in simulated microgravity with serum-free growth medium. With NPCs, initial ammonia and lactate production were fivefold and over twofold higher compared with later time periods despite sufficient oxygen supply. Initial lactate production and glutamine consumption were threefold higher in HN-BAL than in H-BAL. With NPCs, initial glutamine consumption was two- to threefold higher compared with later time periods, whereas initial ornithine production and arginine consumption were over four- and eightfold higher compared with later time periods. In NPCs, the conversion of glutamine to glutamate and ammonia can be explained by the presence of glutaminase, as revealed by PCR analysis. Drug metabolism and clearance of aggregated gamma globulin, probes administered to test functions of hepatocytes and NPCs, respectively, were higher in HN-BAL than in H-BAL. In conclusion, NPCs produce ammonia by hydrolysis of amino acids and may contribute to the pathogenesis of ALF. High amounts of lactate are produced by NPCs under nonhypoxic conditions. Cocultivation augments differentiated functions such as drug metabolism and clearance of aggregated gamma-globulin.

Endocytosis and degradation of serglycin in liver sinusoidal endothelial cells

We have previously reported that liver sinusoidal endothelial cells (LSECs) are responsible for the clearance of monocyte chondroitin sulfate proteoglycan serglycin from the circulation (Øynebråten et al.(2000) J. Leukocyte Biol. 67; 183-188). The aim of the present study was to investigate the kinetics of degradation of endocytosed serglycin in primary cultures of LSECs. The final degradation products of serglycin labelled biosynthetically in the glycosaminoglycan (GAG) chains with [3H] in the acetyl groups of N-acetyl galactosamine residues, [14C] in the pyranose rings, or [35S] in the sulfate groups were identified as[3H]-acetate, [14C]-lactate and [35S]-sulfate. Comparison of the rate of release of degradation products from the cells after endocytosis of serglycin labelled chemically with 125I in the tyrosine residues, or biosynthetically with [35S] or [3H] in the sulfate or acetyl groups, respectively, showed that 125I appeared more rapidly in the medium than [35S]-sulfate and [3H]-acetate. Judging from the speed of appearance of free 125I both intracellularly and in the medium, the core protein is degraded considerably more rapidly than the GAG chains. Desulfation of the GAG chains starts after the GAG chains are released from the core protein. Generation of lactate and acetate as the final products from degradation of the carbon skeleton of the GAG chains indicates that catabolism of endocytosed macromolecules in LSECs proceeds anaerobically.

Toll-like receptor 9 (TLR9) is present in murine liver sinusoidal endothelial cells (LSECs) and mediates the effect of CpG-oligonucleotides

BACKGROUND/AIMS

Bacterial DNA and synthetic oligonucleotides containing unmethylated motifs have become the focus of many studies due to their ability to activate cells of the innate immune system through interaction with Toll-like receptor 9 (TLR9). This study was undertaken to investigate if and how CpG-oligonucleotides (CpGs) activate liver sinusoidal endothelial cells (LSECs), known to be the main site of clearance of DNA-oligonucleotides from the circulation.

METHODS

Expression of TLR9 was analyzed by RT-PCR and immunohistochemistry. Production of IL-1beta and IL-6 was measured by ELISA.

RESULTS

Here we show for the first time that mouse LSECs express TLR9 mRNA and protein. Moreover, our findings suggest that CpGs are first taken up by LSECs by scavenger receptor(s)-mediated endocytosis, and then join TLR9 in the lysosomal compartments. Furthermore, we found that uptake of CpGs in LSECs results in the activation of transcription factor NF-kappaB and secretion of IL-1beta and IL-6.

CONCLUSIONS

The presence of functional TLR9 in LSECs emphasizes the importance of these cells in the innate defense mechanisms of the liver.

Osteoprotegerin is expressed in colon carcinoma cells

BACKGROUND

Osteoprotegerin (OPG), a soluble member of the tumor necrosis factor family, is produced by various cell types and tissues and plays a key role in the physiological regulation of osteoclast differentiation and activity. Also, OPG is a soluble decoy receptor for tumor necrosis factor-related apoptosis-inducing factor (TRAIL). In the present study we investigated whether the human colon cancer cell lines HT-29 and SW-480 produce and secrete OPG in vitro.

MATERIALS AND METHODS

Expression of OPG mRNA was examined by RT-PCR. OPG protein was analysed by ELISA assay and immunostaining methods. The effect of OPG secretion on TRAIL-mediated apoptosis was also investigated.

RESULTS

By RT-PCR, it was demonstrated that mRNA transcripts for OPG were produced by both cell lines. By ELISA analysis, OPG was detected in the culture medium; and treatment of cells with proinflammatory cytokines TNF-alpha and IL-1beta increased OPG secretion significantly. Tumor xenografts in nude mice also were shown to express OPG by immunohistochemistry. When RANKL, which selectively binds OPG, was added to cell cultures along with recombinant TRAIL, apoptosis was shown to increase significantly.

CONCLUSION

These data indicate that OPG may be involved in tumorigenesis and the progression of colon cancer.

Long-term preservation of high endocytic activity in primary cultures of pig liver sinusoidal endothelial cells

Together with Kupffer cells, liver sinusoidal endothelial cells (LSECs) constitute the most powerful scavenger system in the body. However, studies on LSEC function are hampered by the fact that the cells lose their scavenger ability and start deteriorating after a few days in culture. The purpose of the present study was to improve the conditions of cultivation to prolong the survival of pig LSECs in vitro. We used the high capacity receptor-mediated endocytosis of soluble waste molecules as a marker for functionally intact cells in the cultures. Compared with two commercially-, and two other media specifically designed for use with either SECs or hepatocytes from rat, our newly developed serum-free medium, DM 110/SS, devoid of any components of animal origin, was superior in maintaining the endocytic activity. Of six growth factors studied for their effect on endocytosis, basic fibroblast, and recombinant epidermal, but not vascular endothelial growth factor, were found to be most beneficial. After 8 days in DM 110/SS, LSECs maintained endocytosis via the scavenger receptor, mannose receptor, collagen alpha-chain receptor and the Fc-gamma receptor. All endocytosed ligands, except for aggregated IgG were degraded in 8-day-old cultures. Using the new medium, the cells endocytosed ligands for up to 20 days, and survived for at least an additional 10 days, albeit without the high endocytic activity typical of intact LSECs. Importantly, DNA synthesis in prolonged cultures of LSECs was observed only when maintained in DM 110/SS medium. In conclusion, we describe a protocol for the maintenance of LSECs in culture for the longest period yet reported.

Stabilin-1 and stabilin-2 are both directed into the early endocytic pathway in hepatic sinusoidal endothelium via interactions with clathrin/AP-2, independent of ligand binding

Liver sinusoidal endothelial cells (LSECs) mediate clearance of hyaluronan (HA) and scavenger receptor ligands, for example, advanced glycation end product (AGE)-modified proteins and oxidized lipids from the circulation. We recently cloned stabilin-1 and -2, two members of a novel family of transmembrane proteins expressed in LSECs. By using primary LSECs and HEK293 cells separately expressing either stabilin, we have investigated their roles in the early events of endocytosis with respect to localization, ligand-binding properties, and associations with clathrin and adaptor protein (AP)-2. Both stabilins were present at the cell surface, although surface levels of stabilin-1 were limited. In addition, stabilins were present in early endosomal antigen (EEA)-1+ organelles colocalizing with endocytosed AGE-modified bovine serum albumin (BSA). Treating cells with monensin further pronounced this distribution. Recombinant stabilin-2, but not recombinant stabilin-1, bound HA and the scavenger receptor ligands AGE-modified BSA, formaldehyde-treated BSA, and collagen N-terminal propeptides. In LSECs, both stabilins were associated with clathrin and AP-2, but not with each other. These interactions did not change upon addition of exogenous HA, suggesting that stabilins are constitutively internalized. In conclusion, hepatic stabilins are both present in the early endocytic pathway, associating with clathrin/AP-2, but whereas stabilin-2 has a clear scavenging profile, stabilin-1 does not recognize these ligands.