Endocytosis of formaldehyde-denatured serum albumin by nonparenchymal liver cells in vitro

Mouse liver sinusoidal endothelial cell responses to the glucocorticoid receptor agonist dexamethasone

Liver sinusoidal endothelial cells (LSECs) which make up the fenestrated wall of the hepatic sinusoids, are active scavenger cells involved in blood waste clearance and liver immune functions. Dexamethasone is a synthetic glucocorticoid commonly used in the clinic and as cell culture supplement. However, the response is dependent on tissue, cell type, and cell state. The aim of this study was to investigate the effect of dexamethasone on primary mouse LSECs (C57BL/6J); their viability (live-dead, LDH release, caspase 3/7 assays), morphology (scanning electron microscopy), release of inflammatory markers (ELISA), and scavenging functions (endocytosis assays), and associated biological processes and pathways. We have characterized and catalogued the proteome of LSECs cultured for 1, 10, or 48 h to elucidate time-dependent and dexamethasone-specific cell responses. More than 6,000 protein IDs were quantified using tandem mass tag technology and advanced mass spectrometry (synchronous precursor selection multi-notch MS3). Enrichment analysis showed a culture-induced upregulation of stress and inflammatory markers, and a significant shift in cell metabolism already at 10 h, with enhancement of glycolysis and concomitant repression of oxidative phosphorylation. At 48 h, changes in metabolic pathways were more pronounced with dexamethasone compared to time-matched controls. Dexamethasone repressed the activation of inflammatory pathways (IFN-gamma response, TNF-alpha signaling via NF-kB, Cell adhesion molecules), and culture-induced release of interleukin-6, VCAM-1, and ICAM-1, and improved cell viability partly through inhibition of apoptosis. The mouse LSECs did not proliferate in culture. Dexamethasone treated cells showed upregulation of xanthine dehydrogenase/oxidase (Xdh), and the transcription regulator Foxo1. The drug further delayed but did not block the culture-induced loss of LSEC fenestration. The LSEC capacity for endocytosis was significantly reduced at 48 h, independent of dexamethasone, which correlated with diminished expression of several scavenger receptors and C-type lectins and altered expression of proteins in the endocytic machinery. The glucocorticoid receptor (NR3C1) was suppressed by dexamethasone at 48 h, suggesting limited effect of the drug in prolonged LSEC culture. Conclusion: The study presents a detailed overview of biological processes and pathways affected by dexamethasone in mouse LSECs in vitro.

Infection of liver sinusoidal endothelial cells with Muromegalovirus muridbeta1 involves binding to neuropilin-1 and is dynamin-dependent

Liver sinusoidal endothelial cells (LSEC) are scavenger cells with a remarkably high capacity for clearance of several blood-borne macromolecules and nanoparticles, including some viruses. Endocytosis in LSEC is mainly via the clathrin-coated pit mediated route, which is dynamin-dependent. LSEC can also be a site of infection and latency of betaherpesvirus, but mode of virus entry into these cells has not yet been described. In this study we have investigated the role of dynamin in the early stage of muromegalovirus muridbeta1 (MuHV-1, murid betaherpesvirus 1, murine cytomegalovirus) infection in mouse LSECs. LSEC cultures were freshly prepared from C57Bl/6JRj mouse liver. We first examined dose- and time-dependent effects of two dynamin-inhibitors, dynasore and MitMAB, on cell viability, morphology, and endocytosis of model ligands via different LSEC scavenger receptors to establish a protocol for dynamin-inhibition studies in these primary cells. LSECs were challenged with MuHV-1 (MOI 0.2) ± dynamin inhibitors for 1h, then without inhibitors and virus for 11h, and nuclear expression of MuHV-1 immediate early antigen (IE1) measured by immune fluorescence. MuHV-1 efficiently infected LSECs in vitro. Infection was significantly and independently inhibited by dynasore and MitMAB, which block dynamin function via different mechanisms, suggesting that initial steps of MuHV-1 infection is dynamin-dependent in LSECs. Infection was also reduced in the presence of monensin which inhibits acidification of endosomes. Furthermore, competitive binding studies with a neuropilin-1 antibody blocked LSEC infection. This suggests that MuHV-1 infection in mouse LSECs involves virus binding to neuropilin-1 and occurs via endocytosis.

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.

Liver sinusoidal endothelial cells contribute to the uptake and degradation of entero bacterial viruses

The liver is constantly exposed to dietary antigens, viruses, and bacterial products with inflammatory potential. For decades cellular uptake of virus has been studied in connection with infection, while the few studies designed to look into clearance mechanisms focused mainly on the role of macrophages. In recent years, attention has been directed towards the liver sinusoidal endothelial cells (LSECs), which play a central role in liver innate immunity by their ability to scavenge pathogen- and damage-associated molecular patterns. Every day our bodies are exposed to billions of gut-derived pathogens which must be efficiently removed from the circulation to prevent inflammatory and/or immune reactions in other vascular beds. Here, we have used GFP-labelled Enterobacteria phage T4 (GFP-T4-phage) as a model virus to study the viral scavenging function and metabolism in LSECs. The uptake of GFP-T4-phages was followed in real-time using deconvolution microscopy, and LSEC identity confirmed by visualization of fenestrae using structured illumination microscopy. By combining these imaging modalities with quantitative uptake and inhibition studies of radiolabelled GFP-T4-phages, we demonstrate that the bacteriophages are effectively degraded in the lysosomal compartment. Due to their high ability to take up and degrade circulating bacteriophages the LSECs may act as a primary anti-viral defence mechanism.

Fc receptor mediated endocytosis of small soluble immunoglobulin G immune complexes in Kupffer and endothelial cells from rat liver

Soluble circulating immunoglobulin G immune complexes are mainly eliminated by the liver, predominantly by uptake in the Kupffer cells, but also the liver endothelial cells seem to be of importance. In the present study we have followed the intracellular turnover of immune complexes after Fc(gamma) receptor mediated endocytosis in cultured rat liver endothelial cells and Kupffer cells by means of isopycnic centrifugation, DAB cross-linking and morphological techniques. For the biochemical experiments the antigen, dinitrophenylated bovine serum albumin (BSA), was labeled with radioiodinated tyramine cellobiose that cannot cross biological membranes and therefore traps labeled degradation products at the site of formation. The endocytic pathway followed by immune complexes was compared with that followed by scavenger receptor ligands, such as formaldehyde treated BSA and dinitrophenylated BSA, and the mannose receptor ligand ovalbumin. Both Kupffer cells and liver endothelial cells took up and degraded the immune complexes, but there was a clear delay in the degradation of immune complexes as compared to degradation of ligands taken up via scavenger receptors. The kinetics of the endocytosis of scavenger receptor ligand was unaffected by simultaneous uptake of immune complexes. Experiments using both biochemical and morphological techniques indicated that the delayed degradation was due to a late arrival of the immune complexes at the lysosomes, which partly was explained by retroendocytosis of immune complexes. Electron microscopy studies revealed that the immune complexes were retained in the early endosomes that remained accessible to other endocytic markers such as ovalbumin. In addition, the immune complexes were seen in multivesicular compartments apparently devoid of other endocytic markers. Finally, the immune complexes were degraded in the same lysosomes as the ligands of scavenger receptors. Thus, immune complexes seem to follow an endocytic pathway that is kinetically or maybe morphologically different from that followed by scavenger and mannose receptor ligands.

Chronic ethanol consumption impairs receptor-mediated endocytosis of formaldehyde-treated albumin by isolated rat liver endothelial cells

Receptor-mediated endocytosis (RME) by a scavenger receptor on sinusoidal liver endothelial cells (LECs) for formaldehyde-treated bovine serum albumin (f-Alb) has previously been shown to be impaired following chronic ethanol consumption. These studies were initially performed by in situ perfusion, making it difficult to determine the point in the process at which RME is affected. Therefore, it was the purpose of this study to use isolated LECs to begin elucidating at what point in the process chronic ethanol consumption affects RME. Initial studies showed that degradation at the single-cell level were similarly decreased at levels that had been observed for in situ studies, suggesting that the ethanol effects can be repeated using isolated LECs, making them useful for in vitro studies. Binding studies with 125I-formaldehyde-treated bovine serum albumin (125I-f-Alb) demonstrated there was a slight, but significantly different, decrease in binding by LECs from ethanol-fed rats when compared with pair-fed or chow-fed rats. However, the affinity of these receptors was not different between these groups. In contrast, a defect in the initial stages of receptor-ligand internalization was indicated as less surface-bound ligand was internalized and subsequently degraded in cells from the ethanol-treated animals as compared with controls. Additionally, once the data were adjusted for the amount of ligand internalized, the degradation of the internalized ligand was only slightly impaired. These results indicate that chronic ethanol feeding impairs the process of RME by the liver; the major cause of this impairment appears to be caused by a decreased ability of these cells to internalize all of the surface-bound ligand, with a minimal defect in postinternalization events.

Liver cell uptake and degradation of soluble immunoglobulin G immune complexes in vivo and in vitro in rats

Immune complexes were formed between dinitrophenylated human serum albumin (DNP-HSA) and polyclonal rabbit immunoglobulin G (IgG) anti-DNP antibodies at antibody excess. The antigen was labelled with isotope (125I-tyramine-cellobiose) or fluorochrome, (6-[fluorescein-5-(and-6)-carboxamido] hexanoic-acid, succinimidyl ester). The radiolabelled antigen, native or antibody complexed, was given intravenously to rats. Radioactivity was measured in various organs at 1 hour following injection. The liver was the main site for removal of the antigen as well as of the immune complexes. Within the liver, immune complexes were mainly associated with nonparenchymal liver cells, the total recovery from Kupffer cells being about 10 times greater than from the liver endothelial cells. The uncomplexed radiolabelled antigen was readily degraded by both cells types. After IgG complexing, the degradation decreased, both in Kupffer cells and in liver endothelial cells. In vitro experiments with isolated liver cells, showed that IgG complexing increased antigen uptake to about the same extent in Kupffer cells and in liver endothelial cells. The degradation of both antigen and immune complexes was less efficient in vitro than in vivo. Immune complex uptake in vitro was shown also by confocal fluorescence microscopy in Kupffer cells and in liver endothelial cells. Also in vitro, only minor uptake was found in the hepatocytes. We conclude that both liver endothelial cells and Kupffer cells are involved in the hepatic handling of soluble IgG immune complexes, but we found no evidence for substantial uptake by hepatocytes.

Macromolecular permeability across the blood-nerve and blood-brain barriers

The permeability of insulin (Ins), nerve growth factor (NGF), albumin (Alb), transferrin (Trf), and IgG across the blood-nerve barrier (BNB) and blood-brain barrier (BBB) in normal adult rats was quantified by measuring the (permeability coefficient x surface area) product (PS) with the i.v. bolus-injection technique in the cannulated brachial vein and artery using radioiodinated proteins. The PS values of the BNB for IgG and Alb were low: 0.079 +/- 0.029 x 10(-6) and 0.101 +/- 0.088 x 10(-6) ml.g-1.s-1, (mean +/- SD, respectively). The PS values for NGF and Trf were 16.1-fold and 25.5-fold higher than for Alb. The PS for Ins across the BNB was 33.190 +/- 2.053 x 10(-6) ml.g-1.s-1--a remarkable 329-fold increase compared with Alb. The PS values of the BBB for IgG and Alb in different brain regions were all low, from 0.028 +/- 0.017 to 0.151 +/- 0.035 x 10(-6) ml.g-1.s-1 (mean +/- SD). NGF and Trf had comparable PS values from 13- to 32-fold higher than for Alb, except for the brain stem, where the PS for Trf was 66-fold higher than for Alb. The mean PS for Ins across the BBB ranged from 15.78 +/- 5.45 x 10(-6) ml.g-1.s-1 for the cortex to 22.62 +/- 7.50 x 10(-6) ml.g-1.s-1 for the brain stem--again a remarkable 105- to 390-fold increase relative to Alb. Because reliable PS measurements were obtained for all proteins tested, the BBB and BNB cannot be considered impermeable to proteins--a concept that has plagued brain- and nerve-barrier research. The low PS values for IgG and Alb indicate low rates of transfer; however, Alb, in particular, is the major protein of endoneurial and ventricular fluid, which suggests that these PS values may be significant. Ins had the highest PS values, which likely reflect the mechanism of transport across the barriers--that is, receptor-mediated transport. Because NGF and Trf had PS values 13- to 66-fold higher than for Alb, whether this reflects receptor-mediated uptake, adsorptive-mediated transcytosis, or some other mechanism is unclear. That the PS values for NGF and Trf differ from Alb and IgG clearly suggests, however, a different uptake mechanism. Finally, the remarkably high PS values for Ins across the BBB and BNB identify this protein and its putative receptor on capillary endothelial cells as a potential target for drug delivery into the central and peripheral nervous systems.

Biodistributions of air-filled albumin microspheres in rats and pigs

The air-filled microspheres of the ultrasound-contrast agent Albunex are unique in that the walls consist of human serum albumin molecules which have been made insoluble by sonication of the albumin solution. The microspheres were isolated by flotation, and the washed microspheres were labelled with 125I. The labelled material was cleared from the circulation mainly as particles, not as soluble albumin molecules. In rats, 80% of intravenously injected microspheres were cleared from the blood within 2 min. Nearly 60% of the dose was recovered in the liver, only 5% in the lungs, 9% in the spleen, and negligible quantities in kidneys, heart and brain. Of the radioactivity in the liver, more than 90% was taken up by Kupffer cells (liver macrophages). The protein in the liver was degraded apparently with first-order kinetics (half-life 40 min). In pigs, over 90% of the intravenously injected dose was recovered in the lungs. The vastly increased recovery in pig lungs, compared with that in rats, is probably due to the pulmonary intravascular macrophages of the pig; macrophages are not normally found in this location in rats (or humans). In a separate series of experiments in rats, the biodistribution of shell material from the microspheres was examined. The microspheres were made to collapse by applying external pressure on the suspension, leaving sedimentable protein material consisting of layers of insoluble albumin from the 'shells' surrounding the air bubble. The 'shells' and the microspheres were cleared from the circulation and taken up by the liver with the same kinetics. In the lungs, a higher proportion (15%) of shells than of microspheres was recovered.

Chemistry and biochemistry of 4-hydroxynonenal, malonaldehyde and related aldehydes

Lipid peroxidation often occurs in response to oxidative stress, and a great diversity of aldehydes are formed when lipid hydroperoxides break down in biological systems. Some of these aldehydes are highly reactive and may be considered as second toxic messengers which disseminate and augment initial free radical events. The aldehydes most intensively studied so far are 4-hydroxynonenal, 4-hydroxyhexenal, and malonaldehyde. The purpose of this review is to provide a comprehensive summary on the chemical properties of these aldehydes, the mechanisms of their formation and their occurrence in biological systems and methods for their determination. We will also review the reactions of 4-hydroxyalkenals and malonaldehyde with biomolecules (amino acids, proteins, nucleic acid bases), their metabolism in isolated cells and excretion in whole animals, as well as the many types of biological activities described so far, including cytotoxicity, genotoxicity, chemotactic activity, and effects on cell proliferation and gene expression. Structurally related compounds, such as acrolein, crotonaldehyde, and other 2-alkenals are also briefly discussed, since they have some properties in common with 4-hydroxyalkenals.

Intracellular transport of endocytosed proteins in rat liver endothelial cells

1. Receptor-mediated endocytosis of mannose-terminated glycoproteins in rat liver endothelial cells has been followed by means of subcellular fractionation and by immunocytochemical labelling of ultrathin cryosections after intravenous injection of ovalbumin. For subcellular-fractionation studies the ligand was labelled with 125-tyramine-cellobiose adduct, which leads to labelled degradation products being trapped intracellularly in the organelle where the degradation takes place. 2. Isopycnic centrifugation in sucrose gradients of a whole liver homogenate showed that the ligand is sequentially associated with three organelles with increasing buoyant densities. The ligand was, 1 min after injection, recovered in a light, slowly sedimenting vesicle and subsequently (6 min) in larger endosomes. After 24 min the ligand was recovered in dense organelles, where also acid-soluble degradation products accumulated. 3. Immunocytochemical labelling of ultrathin cryosections showed that the ligand appeared rapidly after internalization in coated vesicles and subsequently in two larger types of endosomes. In the 'early' endosomes (1 min after injection) the labelling was seen closely associated with the membrane of the vesicle; after 6 min the ligand was evenly distributed in the lumen. At 24 min after injection the ligand was found in the lysosomes. 4. A bimodal distribution of endothelial cell lysosomes with different buoyant densities was revealed by centrifugation in iso-osmotic Nycodenz gradients, suggesting that two types of lysosomes are involved in the degradation of mannose-terminated glycoproteins in liver endothelial cells. Two populations of lysosomes were also revealed by sucrose-density-gradient centrifugation after injection of large amounts of yeast invertase. 5. In conclusion, ovalbumin is transferred rapidly through three endosomal compartments before delivering to the lysosomes. The degradation seems to take place in two populations of lysosomes.

Scavenger functions of the liver endothelial cell

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Intracellular transport of formaldehyde-treated serum albumin in liver endothelial cells after uptake via scavenger receptors

Endocytosis of formaldehyde-treated serum albumin (FSA) mediated by the scavenger receptor was studied in rat liver endothelial cells. Suspended cells had about 8000 receptors/cell, whereas cultured cells had about 19,000 receptors/cell. Kd was 10(-8) M in both systems. Cell-surface scavenger receptors were found exclusively in coated pits by electron microscopy, by using ligand labelled with colloidal gold. Cell-surface-bound FSA could be released by decreasing the pH to 6.0; it was therefore possible to assess the rate of internalization of surface-bound ligand. This rate was very high: t1/2 for internalization of ligand prebound at 4 degrees C was 24 s. The endocytic rate constant at 37 degrees C, Ke, measured as described by Wiley & Cunningham [(1982) J. Biol. Chem. 257, 4222-4229], was 2.44 min-1, corresponding to t1/2 = 12 s. Uptake of FSA at 37 degrees C after destruction of one cell-surface pool of receptors by Pronase was decreased to 60%. This finding is compatible with a relatively large intracellular pool of receptors. The intracellular handling of 125I-tyramine-cellobiose-labelled FSA (125I-TC-FSA) was studied by subcellular fractionation in sucrose gradients, Nycodenz gradients or by differential centrifugation. The density distributions of degraded and undegraded 125I-TC-FSA after fractionation of isolated non-parenchymal cells and whole liver were similar, when studied in Nycodenz and sucrose gradients, suggesting that the subcellular distribution of the ligand was not influenced by the huge excess of non-endothelial material in a whole liver homogenate. Fractionation in sucrose gradients showed that the ligand was sequentially associated with organelles banding at 1.14, 1.17 and 1.21 g/ml. At 9-12 min after intravenous injection the ligand was in a degradative compartment, as indicated by the accumulation of acid-soluble radioactivity at 1.21 g/ml. A rapid transfer of ligand to the lysosomes was also indicated by the finding that a substantial proportion of the ligand could be degraded by incubating mitochondrial fractions prepared 12 min after intravenous injection of the ligand. The results indicate that FSA is very rapidly internalized and transferred through an endosomal compartment to the lysosomes. The endosomes are gradually converted into lysosomes between 9 and 12 min after injection of FSA. The rate-limiting step in the intracellular handling of 125I-TC-FSA is the degradation in the lysosomes.

High sensitivity towards monensin of receptor-mediated endocytosis of formaldehyde treated albumin by liver endothelial cells

Endocytosis of formaldehyde-treated serum albumin (f-albumin) in isolated liver sinusoidal endothelial cells was studied. Uptake occurs via the scavenger receptor and was found to be very sensitive to the ionophore monensin. Binding at 4 degrees C of f-albumin was reduced to 50% of control values by preincubation for 2 min with 2 microM monensin. Both uptake and degradation of f-albumin were more sensitive to monensin. No lag-phase in the inhibitory effect on uptake and degradation was detected. A concentration of 0.1 microM monensin reduced uptake of f-albumin by 50%. Degradation of internalized f-albumin was reduced by 50% in the presence of 0.2 microM monensin. Since uptake and degradation of f-albumin were very sensitive to monensin, the effect of introducing the drug during endocytosis of the ligand was tested. All processing of f-albumin stopped instantly upon addition of monensin; hence, there seems to be no step in the endocytic process beyond which monensin is ineffective. The data suggest that the scavenger receptor of liver endothelial cells is internalized and recycled very rapidly.

Endocytosis of acetylated low-density lipoprotein, endothelial cell-modified low-density lipoprotein, and formaldehyde-treated serum albumin by rat liver endothelial cells. Evidence of uptake via a common receptor

Formaldehyde-treated serum albumin (FSA) and acetylated low-density lipoprotein (Ac-LDL) are taken up in vivo and in vitro by the sinusoidal endothelial cells of the liver. It is not known whether both these ligands are removed by the scavenger receptor. We have studied the effect of increasing concentrations of unlabeled FSA, Ac-LDL, and endothelial cell-modified LDL (Ec-LDL) on the endocytosis of trace amounts of these ligands labeled with 125I. Uptake of 125I-Ac-LDL and 125I-Ec-LDL was strongly inhibited by FSA. Likewise, Ac-LDL and Ec-LDL reduced the uptake of 125I-FSA effectively. Our data indicate that these modified LDLs and FSA are bound to and internalized via the same receptor on liver endothelial cells.

Quantitative aspects of the uptake and degradation of lysozyme in the rat kidney in vivo

Uptake and degradation of lysozyme in the rat kidney were studied in vivo. The protein was labeled with 125I by way of a moiety (tyramine-cellobiose or 'TC') which remained trapped inside the cells even after proteolysis of the peptide chain (in contrast, the label from conventionally labeled proteins escapes after degradation). Following the injection of 'trapped-label' lysozyme, the radioactivity in the kidneys represented the total amount of lysozyme that was taken up during the experiment. Proteolysis could be followed by determining the amount of acid-soluble degradation products. By adding the radioactivity in the urine to that in the kidneys, a measure of the total filtered load was obtained. When only a trace dose of 125I-labeled TC lysozyme was injected into rats, the amount of radioactivity in the kidneys increased on average by 0.09% per min, after the concentration in the blood had become nearly stable. After 100 min, 30% of the injected dose was recovered in the kidneys. The labeled protein was degraded to acid-soluble molecules of Mr less than 1000. There was apparently a 'lag period' between the endocytosis in the kidneys and the start of degradation. 40 min after the injection of a trace dose, about 0.6% of the 'trapped-label' lysozyme in the kidneys was degraded per min.; subsequently, there was a decline in the fraction which was degraded per min. The amount of lysozyme in the urine increased after the injection of increasing amounts of lysozyme, showing that the capacity of the uptake mechanism was being exceeded, but truly saturating levels of lysozyme could not be reached in vivo.

Isolation of pronephros cells which endocytose chemically modified proteins in the rainbow trout

Modified serum albumin is cleared from the blood by kidney cells in salmonid fishes. The present study deals with isolation of cells from pronephros which endocytose formaldehyde-treated human serum albumin (fHSA). Radioactively labelled fHSA or dinitrophenyl-conjugated albumin (DNP-HSA) were injected intravenously into rainbow trouts. Pronephros cells, containing the endocytosed protein, were isolated and further separated by centrifugal elutriation and density-gradient centrifugation. Most of the radioactive protein was elutriated together with small cells. After centrifuging the cells through a Percoll density gradient, radioactive protein was located in cells recovered in the upper part of the gradient. In mammals, fHSA and other modified proteins are mainly taken up by sinusoidal endothelial cells in the liver via a "scavenger receptor"0. Our results suggest that a comparable function in salmonids is located in a subpopulation of relatively small cells in kidney tissue, possibly sinusoidal lining cells. The separation techniques used seemed to be suitable for isolation of different populations of pronephros cells.