Scavenger function of sinusoidal liver cells. Acetylated low-density lipoprotein is endocytosed via a route distinct from formaldehyde-treated serum albumin

Molecular Imaging of Inflammation in a Mouse Model of Atherosclerosis Using a Zirconium-89-Labeled Probe

Background

Beyond clinical atherosclerosis imaging of vessel stenosis and plaque morphology, early detection of inflamed atherosclerotic lesions by molecular imaging could improve risk assessment and clinical management in high-risk patients. To identify inflamed atherosclerotic lesions by molecular imaging in vivo, we studied the specificity of our radiotracer based on maleylated (Mal) human serum albumin (HSA), which targets key features of unstable atherosclerotic lesions.

Materials and Methods

Mal-HSA was radiolabeled with a positron-emitting metal ion, zirconium-89 (⁸⁹Zr⁴⁺). The targeting potential of this probe was compared with unspecific ⁸⁹Zr-HSA and ¹⁸F-FDG in an experimental model of atherosclerosis (Apoe^–/– mice, n=22), and compared with wild-type (WT) mice (C57BL/6J, n=21) as controls.

Results

PET/MRI, gamma counter measurements, and autoradiography showed the accumulation of ⁸⁹Zr-Mal-HSA in the atherosclerotic lesions of Apoe^–/– mice. The maximum standardized uptake values (SUV_max) for ⁸⁹Zr-Mal-HSA at 16 and 20 weeks were 26% and 20% higher (P<0.05) in Apoe^–/– mice than in control WT mice, whereas no difference in SUV_max was observed for ¹⁸F-FDG in the same animals. ⁸⁹Zr-Mal-HSA uptake in the aorta, as evaluated by a gamma counter 48 h postinjection, was 32% higher (P<0.01) for Apoe^–/– mice than in WT mice, and the aorta-to-blood ratio was 8-fold higher (P<0.001) for ⁸⁹Zr-Mal-HSA compared with unspecific ⁸⁹Zr-HSA. HSA-based probes were mainly distributed to the liver, spleen, kidneys, bone, and lymph nodes. The phosphor imaging autoradiography (PI-ARG) results corroborated the PET and gamma counter measurements, showing higher accumulation of ⁸⁹Zr-Mal-HSA in the aortas of Apoe^–/– mice than in WT mice (9.4±1.4 vs 0.8±0.3%; P<0.001).

Conclusion

⁸⁹Zr radiolabeling of Mal-HSA probes resulted in detectable activity in atherosclerotic lesions in aortas of Apoe^–/– mice, as demonstrated by quantitative in vivo PET/MRI. ⁸⁹Zr-Mal-HSA appears to be a promising diagnostic tool for the early identification of macrophage-rich areas of inflammation in atherosclerosis.

Advanced Glycation End Products (AGE)-Modified Proteins and Their Potential Relevance to Atherosclerosis

Modification of proteins by long-term incubation with glucose leads, through the formation of early products such as Schiff base and Amadori rearrangement products, to the formation of advanced glycation end products (AGE). AGE-modified proteins are characterized physicochemically by fluorescence, brown coloration, and intramolecular or intermolecular cross-linking. Biologically, they are specifically recognized by the AGE receptors of the cell surface membrane. Recent studies have provided evidence for the involvement of AGE proteins in atherosclerosis. First, in vitro experiments using Chinese hamster ovary cells overexpressing the macrophage scavenger receptor (MSR) and peritoneal macrophages from MSR-knockout mice demonstrated that MSR plays a major role as the AGE receptor in the endocytotic uptake of AGE by macrophages. Second, immunohistochemical studies using anti-AGE antibody and anti-MSR antibody revealed the presence of AGE proteins in human atherosclerotic lesions in arterial walls. Because MSR is closely associated with the formation of early atherosclerotic lesions, these results suggest a potential role played by AGE proteins or their interaction with MSR in the atherosclerotic process. (Trends Cardiovasc Med 1996;6:163-168).

Review of the putative cell-surface receptors for alpha-fetoprotein: identification of a candidate receptor protein family

The identification of a receptor for alpha-fetoprotein (AFP) has long been sought in the field of medicine. The uptake and endocytosis of AFP by rat tumor cells in 1984 sparked a series of confirmatory reports and the original studies were then extended to include multiple tumor types in rats, mice, and humans. The following year, French investigators partially characterized the binding properties of the AFP receptor, but they were not able to purify the receptor. It was not until 1991-1992 that an AFP receptor was partially purified and characterized from both human monocytes and breast cancer cells. By 1993, a monoclonal antibody had been raised against the AFP receptor produced from a breast cancer extract with claims that the receptor was a widespread (universal) oncofetal biomarker for cancer. However, that receptor has yet to be cloned and/or purified due to its complex multimeric binding interactions and associations. The present report will review the literature of the multiple putative AFP receptors described to date, the cellular uptake and endocytosis of AFP, and the biochemical characterization of these putative cell-surface proteins. In addition, evidence derived from computer modeling, proteolytic degradation patterns, and amino acid sequence analysis will be presented in a proposed identification of a family of multi-ligand binding receptors; this family fits many, if not most, of the criteria required for an AFP receptor. The purposed receptor protein family is tentatively identified as the Scavenger receptors which comprise several classes of single- and double-pass integral transmembrane proteins. Present data do not support the concept that the AFP receptor is a "universal" tumor receptor and/or biomarker.

Potential role of chitotriosidase gene in nonalcoholic fatty liver disease evolution

BACKGROUND

Nonalcoholic steatohepatitis (NASH) is a liver disease characterized by steatosis and periportal and lobular inflammation. The molecular mechanisms involved in the anomalous behavior of liver cells have only partially been disclosed. Human Chitotriosidase (Chit) is a member of the chitinase family that it is mainly synthesized by activated macrophages. We investigated chitotriosidase gene expression in Kupffer cells to determine the potential implication of this enzyme in the inflammation and in the progression from uncomplicated steatosis to steatohepatitis with progressive fibrosis.

METHODS

Seventy-five liver biopsies from 40 subjects with NASH, 20 with simple steatosis, and 15 controls were used to detect CHIT expression, tumor necrosis factor-alpha (TNF-alpha), alpha-smooth muscle actin (alpha-SMA), and lipid peroxidation.

RESULTS

CHIT was expressed exclusively by Kupffer cells. The levels of CHIT expression were significantly higher in NASH patients than in simple steatosis patients and in the control group. In addition, we found that CHIT over-expression influenced hepatic stellate cells activation, as demonstrated by the significant correlation between CHIT and alpha-SMA expression in NASH patients. A significant correlation was observed also between CHIT, TNF-alpha and lipid peroxidation in both NASH and simple steatosis.

CONCLUSION

These results suggest that CHIT over-produced by Kupffer cells may contribute to the progression of hepatic fibrosis.

The structural and pharmacokinetic properties of oxidized human serum albumin, advanced oxidation protein products (AOPP)

To determine the pharmacokinetic properties of advanced oxidation protein products (AOPP), we prepared oxidized human serum albumin (oxi-HSA) using chloramine-T (a hypochlorite analogue) in vitro. The AOPP and dityrosine content of oxi-HSA (AOPP content, 244.3+/-12.3 microM; dityrosine content, 0.7+/-0.11 nmol of dityrosine/mg protein) were similar to those of uremic patients. In structural analysis, the increases in AOPP and dityrosine content of HSA induced slight decreases in its alpha-helical content. In pharmacokinetic analysis, oxi-HSA left the circulation rapidly, and organ distribution of oxi-HSA 30 min after intravenous injection was 51% for the liver, 23% for the spleen, and 9% for the kidney, suggesting that the liver and spleen were the main routes of plasma clearance of oxi-HSA. The liver and spleen uptake clearance of oxi-HSA were significantly greater than those of normal HSA (CLliver, 5058+/-341.6 vs 24+/-4.2 microL/hr [p<0.01]; CLspleen, 2118+/-322.1 vs 32+/-2.7 microL/hr [p<0.01]). However, uptake by other organs was not significantly affected by oxidation. These results suggest that the liver and spleen play important roles in elimination of AOPP.

Subtilisins of Bacillus spp. hydrolyze keratin and allow growth on feathers

Keratinase is a serine protease produced by Bacillus licheniformis PWD-1 that effectively degrades keratin and confers the ability to grow on feathers to a protease-deficient B. subtilis strain. Studies presented herein demonstrate that B. licheniformis Carlsberg strain NCIMB 6816, which produces the well-characterized serine protease subtilisin Carlsberg, also degrades and grows on feathers. The PWD-1 and Carlsberg strains showed a similar time-course of enzyme production, and the purified serine proteases have similar enzymatic properties on insoluble azokeratin and soluble FITC-casein. Kinetic analysis of both enzymes demonstrated that they have high specificity for aromatic and hydrophobic amino acids in the P1 substrate position, although keratinase discriminates more than subtilisin Carlsberg against charged residues at this site. Nucleotide sequence analysis of the serine protease genes from B. licheniformis strains PWD-1, Carlsberg NCIMB 6816, ATCC 12759, and NCIMB 10689 showed that the kerA-encoded protease of PWD-1 differs from the others only by having V222, rather than A222, near the active site serine S220. Further, high-level expression of subE-encoded subtilisin from B. subtilis (78% similar to subtilisin Carlsberg) also confers growth on feathers on a protease-deficient B. subtilis strain. While strain PWD-1 and the kerA protease efficiently degrade keratin, keratin hydrolysis and growth on feathers is a property that can be conferred by appropriate expression of the major subtilisins, including the industrially produced enzymes.

Localization of human acyl-coenzyme A: cholesterol acyltransferase-1 (ACAT-1) in macrophages and in various tissues

To investigate the distribution of acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT-1) in various human tissues, we examined tissues of autopsy cases immunohistochemically. ACAT-1 was demonstrated in macrophages, antigen-presenting cells, steroid hormone-producing cells, neurons, cardiomyocytes, smooth muscle cells, mesothelial cells, epithelial cells of the urinary tracts, thyroid follicles, renal tubules, pituitary, prostatic, and bronchial glands, alveolar and intestinal epithelial cells, pancreatic acinar cells, and hepatocytes. These findings showed that ACAT-1 is present in a variety of human tissues examined. The immunoreactivities are particularly prominent in the macrophages, steroid hormone-producing cells, followed by hepatocytes, and intestinal epithelia. In cultured human macrophages, immunoelectron microscopy revealed that ACAT-1 was located mainly in the tubular rough endoplasmic reticulum; immunoblot analysis showed that the ACAT-1 protein content did not change with or without cholesterol loading; however, on cholesterol loading, about 30 to 40% of the total immunoreactivity appeared in small-sized vesicles. These vesicles were also enriched in 78-kd glucose-regulated protein (GRP 78), a specific marker for the endoplasmic reticulum. Immunofluorescent microscopy demonstrated extensive colocalization of ACAT-1 and GRP 78 signals in both the tubular and vesicular endoplasmic reticulum before and after cholesterol loading. These results raise the possibility that foam cell formation may activate an endoplasmic reticulum vesiculation process, producing vesicles enriched in the ACAT-1 protein.

Receptors for proteins modified by advanced glycation endproducts (AGE)--their functional role in atherosclerosis

Long-term incubation of proteins with glucose leads, through the formation of early stage products such as Schiff base and Amadori rearrangement products, to the formation of advanced glycation end products (AGE). Recent studies of AGE-structures as well as the receptor for AGE-proteins (AGE-receptors) have emphasized the involvement of protein modification by AGE in aging and age-enhanced disease processes. Immunohistochemical analyses of human atherosclerotic lesions using a monoclonal anti-AGE antibody have demonstrated diffuse extracellular AGE-deposition as well as dense intracellular AGE-deposition in macrophage- and vascular smooth muscle cell (SMC)-derived foam cells. In vitro experiments using both CHO cells overexpressing macrophage scavenger receptor-A (MSR-A) and peritoneal macrophages from MSR-A-knockout mice have shown that the MSR-A plays a major role in endocytic uptake of AGE-proteins by macrophages. Furthermore, in vitro experiments with rabbit arterial SMCs demonstrated a novel AGE-receptor mediating endocytosis of AGE-proteins. These in vivo and in vitro experiments suggest that AGE-proteins formed extracellularly in atherosclerotic lesions are endocytosed by macrophages through MSR-A in the early stage, and by SMCs through the novel AGE-receptor in the advanced stage, implicating functional contribution of the AGE-receptor-mediated interaction of AGE-proteins with these cells to atherosclerotic processes in arterial walls.

Liver uptake of phosphodiester oligodeoxynucleotides is mediated by scavenger receptors

The therapeutic activity of antisense oligodeoxynucleotides (ODNs) often is impaired due to premature degradation and poor ability to reach the (intra)cellular target. In this study, we addressed the in vivo fate of ODNs and characterized the major sites responsible for the clearance of intravenously injected phosphodiester ODN. On injection into rats, 32P-ODNs (miscellaneous sequences and GT-containing ODNs with variable G content) are rapidly cleared from the bloodstream (t1/2 = 0.6-0.7 min), with the liver being the main site of elimination. The contribution of the liver to ODN clearance depended on its sequence and varied considerably. Hepatic uptake tended to be lower for G-rich ODNs as a result of increased bone marrow uptake. Within the liver, both Kupffer cells (KC) and endothelial cells (EC) were responsible for 32P-ODN uptake. To elucidate the mechanism of liver uptake, 32P-ODN binding studies using isolated EC and KC were performed. Binding to both cell types seemed to be saturable, of moderate affinity, and mediated by a membrane-bound protein. The inhibition profiles of 32P-ODN binding to EC and KC by various (poly)anions were essentially equal and corresponded closely to those of 125I-acetylated low-density lipoprotein. In summary, the results indicate that scavenger receptors on nonparenchymal liver and bone marrow cells contribute to the elimination of ODNs from the bloodstream. Minor changes in ODN sequence markedly affect receptor recognition, resulting in considerable shifts in the biodistribution of antisense ODNs.

Bafilomycin A1, a specific inhibitor of V-type H+-ATPases, inhibits the acidification of endocytic structures and inhibits horseradish peroxidase uptake in isolated rat sinusoidal endothelial cells

The role of vacuolar type H+-ATPases (v-ATPases) and pH gradient between the endocytic compartments and cytoplasm in the endocytosis of horseradish peroxidase, a mannose-terminated glycoprotein, was investigated morphologically in isolated rat sinusoidal endothelial cells. Toward this purpose, a specific inhibitor of v-ATPases, bafilomycin A1, was used to inhibit v-ATPases in the vacuolar system. Uptake of horseradish peroxidase was examined by electron microscopy. Fluorescent staining by acridine orange showed that bafilomycin A1 inhibited the acidification of the endocytic compartments. Horseradish peroxidase was taken up via mannose receptors and was distributed in the endocytic structures in the isolated sinusoidal endothelial cells. Uptake of horseradish peroxidase was significantly inhibited by bafilomycin A1, and this finding was confirmed by morphometrical analysis. These results suggest that: a) v-ATPases are necessary for acidification of the endocytic compartments in the sinusoidal endothelial cells and b) the pH gradient between the endocytic compartments and the cytoplasm that is generated by v-ATPases is necessary for the receptor-mediated endocytosis of a mannose-terminated glycoprotein, horseradish peroxidase.

Changes in receptor-mediated endocytosis in liver sinusoidal cells after partial hepatectomy in the rat

Liver sinusoidal cells play an important role in host defense by clearing particulate matter and macromolecules from the circulation. In this study, receptor-mediated endocytosis in sinusoidal cells was examined in two-thirds hepatectomized rats using 125I-labeled formaldehyde-treated bovine serum albumin (fBSA) as an endocytable macromolecule. The liver-weight to body-weight ratio in hepatectomized rats returned to the control value 10 days after hepatectomy. The endocytotic index for fBSA in sinusoidal cells decreased significantly to 0.0210 +/- 0.0017 (controls, 0.0598 +/- 0.0019) on the first day, then returned to the control level at 5 days (0.0554 +/- 0.0030). The changes in hepatic uptake for fBSA showed a similar time course of the endocytotic index. A transient increase in the uptake of fBSA per unit weight of liver of 22-39% above control occurred 2 to 3 days after hepatectomy. In contrast to fBSA, the endocytotic index in hepatocytes evaluated with 125I-labeled asialofetuin reached the minimum level on the second day, and then recovered to the control level 10 days after hepatectomy. These results suggest that endocytosis of fBSA by sinusoidal cells decreases after hepatectomy and rapidly recovers to normal before the completion of liver regeneration, whereas endocytosis of asialofetuin by hepatocytes decreases following hepatic resection and returns to normal when regeneration is substantially complete.

Receptor-mediated endocytosis of chemically modified albumins by sinusoidal endothelial cells and Kupffer cells in rat and human liver

Human serum albumin (HSA), formaldehyde-treated HSA (FHSA), and HSA polymerized with glutaraldehyde (pHSA) were conjugated with colloidal gold (15 (15G) or 50 (50G) nm in diameter). The labeled proteins were injected into the portal veins of rats and followed by electron microscopy. Both 15G-FHSA and 15G-pHSA were taken up by sinusoidal endothelial cells (Ec) and Kupffer cells (Kc). Five minutes after injection, gold particles were observed on the surface of Ec and Kc. At 10 min, most gold particles were gathered in the coated pits and vesicles of Ec. In Kc, gold particles were observed in both coated vesicles and macropinocytotic vesicles. At 15 min, the gold particles were localized mainly in the endosomes and some lysosomes of Ec and in the large vacuoles of Kc. At 30 min, the gold particles had been gathered into the secondary lysosomes and condensed. At 60 min, some gold particles were observed in the cytoplasm of Ec. The fate of 15G-pHSA was the same as that of 15G-FHSA. Simultaneous injection of 15G-pHSA and 50G-FHSA revealed that particles of both sizes were taken up together into the coated pits and vesicles of Ec. Preperfusion of livers with unlabeled FHSA, pHSA, or formaldehyde-treated bovine serum albumin (FBSA) inhibited the uptake of 15G-FHSA or 15G-pHSA by Ec. In a human liver biopsy specimen, both 15G-FHSA and 15G-pHSA were taken up by Ec and Kc through coated vesicles, as in the rat liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of murine macrophage growth by modified LDLs

We previously reported that cell membrane components and lipoproteins were able to induce the growth of murine peritoneal macrophages. The aim of the present study was to examine whether macrophage growth could also be induced by chemically modified lipoproteins, such as acetylated low density lipoprotein (acetyl-LDL) or oxidized LDL, ligands known to be endocytosed by the macrophage scavenger receptors. When murine peritoneal exudate macrophages were cultured in vitro with 25-100 micrograms/mL acetyl-LDL or oxidized LDL, significant growth was induced. On comparing the dose-response curves of these LDLs, a more potent effect was seen with oxidized LDL than acetyl-LDL, especially on resident macrophages. On the other hand, growth of these cells was not stimulated by native (unmodified) LDL or high density lipoprotein. These in vitro data revealed a new function of chemically modified LDLs as effective inducers of macrophage cell growth. This aspect may be physiologically relevant to the growth of macrophage foam cells in situ in the development of atherosclerosis.

Formaldehyde treated albumin contains monomeric and polymeric forms that are differently cleared by endothelial and Kupffer cells of the liver: evidence for scavenger receptor heterogeneity

Formaldehyde treated albumin (F-HSA) was found to consist of a monomeric and a polymeric fraction. Both fractions were primarily endocytosed by rat liver sinusoidal cells. However, immunohistochemical staining of endocytosed material showed that the relative contribution of the endothelial and Kupffer cells in uptake of the monomer and the polymer differed significantly, with the monomer mainly having an endothelial cell- and the polymer predominantly having a Kupffer cell pattern of distribution. To directly confirm these heterogeneous patterns, we injected in vivo the 125I-labeled F-HSA fractions and isolated the endothelial and Kupffer cells by centrifugal elutriation. 73.7% of the monomeric F-HSA was found in endothelial cells and only 14.9% was found in Kupffer cells. In contrast, the polymeric F-HSA (1500 kD) was mainly endocytosed by Kupffer cells (71%), whereas the endothelial cells contributed only for 24% in hepatic uptake. In vivo studies and isolated perfused rat liver experiments showed that endocytosis of both monomer and polymer was inhibited by co-administration of polyinosinic acid, a well known inhibitor for scavenger receptors, indicating that these receptors on endothelial and Kupffer cells are mainly involved in this uptake process.

Biochemical demonstration of endocytosis and subsequent resecretion of high-density lipoprotein by rat peritoneal macrophages

To investigate the post-binding events of high-density lipoprotein (HDL), rat peritoneal macrophages were enriched by collagen gel-coated plates and incubated in a cell-suspension system with fluorescein isothiocyanate-labeled HDL (FITC-HDL), followed by fluorescence spectroscopic analyses. Upon incubation with FITC-HDL at 37 degrees C for 30 min, the microenvironmental pH of the cell-associated FITC-HDL was 6.50, whereas a 0 degree C-incubation gave a corresponding pH of 7.15. Carbonyl cyanide m-chlorophenylhydrazone, a protonophore known to dissipate the proton gradient, restored the former acidic pH (pH 6.40) to pH 7.20, but had no effect on the latter. This indicates that cell surface-bound HDL is internalized and exposed to an acidic compartment. When cells were incubated with FITC-HDL at 37 degrees C for 30 min and the cell-associated FITC-HDL was chased at 37 degrees C, the fluorescence intensity at 490 nm showed a time-dependent increase. This increase was explained by a release of endocytosed FITC-HDL into the extracellular medium but not by a simple outward dissociation of the cell-associated FITC-HDL. The microenvironmental acidic pH of the cell-associated FITC-HDL changed to a less acidic pH during the chase whereas that of FITC-HDL became constant after released into the medium, indicating that endocytosed HDL was resected back into the extracellular medium. This resection process was temperature-dependent and accelerated by HDL itself. These results provide the biochemical evidence for the presence of an endocytic-exocytic pathway for HDL in rat peritoneal macrophages.

Sinusoidal endothelial cells of the liver: fine structure and function in relation to age

Liver endothelial cells form a continuous lining of the liver capillaries, or sinusoids, separating parenchymal cells and fat-storing cells from sinusoidal blood. Liver sinusoidal endothelial cells differ in fine structure from endothelial cells lining larger blood vessels and from other capillary endothelia in that they lack a distinct basement membrane and also contain open pores, or fenestrae, in the thin cytoplasmic projections which constitute the sinusoidal wall. This distinctive morphology supports the protective role played by liver endothelium, the cells forming a general barrier against pathogenic agents and serving as a selective sieve for substances passing from the blood to parenchymal and fat-storing cells, and vice versa. Sinusoidal endothelial cells, furthermore, significantly participate in the metabolic and clearance functions of the liver. They have been shown to be involved in the endocytosis and metabolism of a wide range of macromolecules, including glycoproteins, lipoproteins, extracellular matrix components, and inert colloids, establishing endothelial cells as a vital link in the complex network of cellular interactions and cooperation in the liver. Fine structural studies in combination with the development of cell isolation and culture techniques from both experimental animal and human liver have greatly contributed to the elucidation of these endothelial cell functions. Morphological and biochemical investigations have both revealed little changes with age except for an accumulation of iron ferritin and a decrease in the activities of glucose-6-phosphatase, Mg-ATPase, and in glucagon-stimulated adenylcyclase. Future studies are likely to disclose more fully the role of sinusoidal endothelial cells in the regulation of liver hemodynamics, in liver metabolism and blood clearance, in the maintenance of hepatic structure, in the pathogenesis of various liver diseases, and in the aging process in the liver.

Microquantification of cholesterol and cholesteryl esters in rat peritoneal macrophages by reverse-phase high-performance liquid chromatography

A simple and rapid method for the microquantification of cholesterol and cholesteryl esters by reverse-phase high performance liquid chromatography has been established. Comparison of elution patterns of authentic cholesterol and cholesteryl esters revealed that a mu Bondasphere reverse-phase C8 (300-A) column was more suitable than a corresponding reverse-phase C4 or C18 column in terms of rapidity and sensitivity. Recovery of cholesterol and cholesteryl esters from a C8 column was greater than 98% when determined either by radioactive cholesterol and cholesteryl oleate or by cholesteryl heptadecanoate. The sensitivity of the quantification ranged from 5 ng to 50 micrograms for both cholesterol and cholesteryl esters. This method was applied to determination of cellular cholesterol and cholesteryl esters of rat peritoneal macrophages. Lipid extracts of these cells were found to contain 38.01 +/- 2.60 micrograms of cholesterol and 3.18 +/- 0.36 micrograms of cholesteryl esters per milligram of cell protein. When the cells were loaded with cholesteryl esters by incubation for 24 h with various concentrations of acetylated low-density lipoprotein, a cellular level of cholesteryl esters showed a dose-dependent increase and reached a maximal level of 106.60 +/- 3.05 micrograms/mg cell protein. Thus, the present method is useful for the microquantification of cholesterol and cholesteryl esters from lipid extracts of biological samples.

Intracellular degradation by liver endothelial cells

Investigations were carried out on the intracellular fate of formaldehyde treated bovine serum albumin (F-BSA), in liver non-parenchymal cells. This paper reports the observations and results obtained by us. The first part of our work involved the injecting of the compound into either a) normal rats, b) rats injected with Triton WR 1339 or c) rats treated with mannan. Fractions obtained after differential and isopycnic centrifugation in sucrose gradients, were analysed by SDS-gel electrophoresis and fluorography. The degradation takes place in a two step process. The molecule is first split into radiolabeled compounds that are still acid precipitable. This is followed by the appearance of acid soluble radioactive molecules. In a sucrose gradient the first kind of degradation products exhibit a distribution totally different from that of acid soluble degradation compounds. In the second part of our experiments, fairly pure fractions of the organelles, known to be involved in the endocytic pathway i.e. endosomes, transfer lysosomes and accumulation lysosomes (marked by the presence of either Triton WR 1339 or mannan) were isolated and incubated with [125I]-F-BSA. These experiments revealed that endosomes, isolated by us, are incapable of degradation. Accumulation lysosomes arising exclusively from liver non-parenchymal cells (in which mannan had accumulated) though rich in certain hydrolases eg. arylsulfatase did not have an efficient proteolytic machinery. Our results, both from in vivo and in vitro studies, suggest that the first degradation step occurs in one type of structure (probably not endosomes), a sort of hybrid endosome-lysosome (as they are not affected by glycyl-1-phenyl-2-napthylamide) and the second step in a different type of lysosomes, what we have designated transfer lysosomes.

Scavenger receptor of human monocytic leukemia cell line (THP-1) and murine macrophages for nonenzymatically glycosylated proteins

Long-term incubation of proteins with glucose undergo a series of nonenzymatic reactions to form advanced glycosylation end product (AGE) with fluorescence and brown color. The receptor for AGE-proteins was demonstrated in murine macrophages (Vlassara et al. (1985) Proc. Natl. Acad. Sci. USA 82. 5588). Our recent study with rat macrophages revealed that the receptor also recognized proteins modified with aliphatic aldehydes such as formaldehyde or glycolaldehyde, indicating its close identity to a scavenger receptor for aldehyde-modified proteins (Takata, K. et al. (1988) J. Biol. Chem. 263. 14819). This notion was tested in the present study with human monocytic leukemia cell line (THP-1 cells), human monocyte macrophages and murine peritoneal macrophages. Endocytic uptake of AGE-proteins and aldehyde-modified proteins was inhibited in a cross-competitive fashion. The receptor activities of THP-1 cells for AGE-albumin and aldehyde-modified proteins were induced synchronously by phorbol 12-myristate 13-acetate. Furthermore, upon reduction by NaBH4 of the Schiff base formed between proteins and glucose or aldehydes, no ligand activity was generated. However, once the ligand activity was generated, NaBH4 was no longer effective for the ligand activity. Thus, a structure in common between AGE-proteins and aldehyde-modified proteins may be crucial for recognition by the human macrophage receptor.

Scavenger receptor-mediated recognition of maleylated albumin and its relation to subsequent endocytic degradation

Rat sinusoidal liver cells take up maleylated bovine serum albumin (maleyl-BSA) and its demaleylated form (demaleyl-BSA) by scavenger receptor-mediated endocytosis. Cellular binding of maleyl-BSA and demaleyl-BSA and its quantitative relation to subsequent intracellular degradation were investigated. The binding affinities of these ligands were almost equal whereas the number of binding sites for maleyl-BSA was more than twice as large than that for demaleyl-BSA. However, no difference was observed in their endocytic degradation. The amounts of maleyl-BSA degraded were proportional to those bound to the cell surface up to a certain level. However, a further increase in cell-bound ligands did not affect the degradation of maleyl-BSA. Several polyanions such as fucoidin and dextran sulfate of Mr = 5000 inhibited the binding of maleyl-BSA but did not affect its degradation. In contrast, acetylated or oxidized low density lipoprotein had virtually no effect on cellular binding of maleyl-BSA but exhibited profound effects on its intracellular degradation. Similar results were obtained with rat peritoneal macrophages. Based on these data, we would propose that two binding sites are involved in the receptor-mediated ligand recognition; one is coupled to subsequent endocytic degradation, and the other serves as a binding site for polyanionic compounds.

Regional ligand domain is involved in scavenger receptor-mediated recognition of maleyl-albumin by rat sinusoidal liver cells

Scavenger receptor-mediated endocytosis of maleyl-albumin was studied with rat sinusoidal liver cells. Upon maleylation of greater than 28 mol lysine residues per protein, bovine serum albumin became an active ligand. Further modification of up to 37 mol lysine residues per protein resulted in a sharp increase in the ligand activity, reaching a maximum level thereafter. Removal of maleyl moieties from maleyl-albumin (demaleylation) from 53 mol to 14 mol lysine residues per protein did not affect the ligand activity. However, further demaleylation to less than 5 mol lysine residues per protein led to complete loss of the ligand activity. Thus, the covalently incorporated maleyl moieties are needed for the ligand activity. The ligand activity was also generated when two peptides (Frag N and Frag C) from cyanogen bromide-cleaved albumin were maleylated, indicating that the formation of an active ligand would not require a whole albumin molecule. Maleyl Frag C was further separated into three peptides; maleyl Frag C-1 (261 amino acid residues), maleyl Frag C-2 (102 residues) and maleyl Frag C-3 (36 residues). The cellular binding and endocytic degradation of maleyl-albumin or acetylated low density lipoprotein were effectively competed for by maleyl Frag C-1 and maleyl Frag C-2 but not by maleyl Frag C-3. Thus, regional domains might be involved in the ligand recognition by the scavenger receptor.

Intracellular accumulation of cholesteryl esters suppresses production of lipopolysaccharide-induced interleukin 1 by rat peritoneal macrophages

Interleukin 1 (IL-1) is a major cytokine of macrophages secreted by several stimulants such as lipopolysaccharide (LPS). Macrophages are known to possess the scavenger receptor for acetylated low density lipoprotein (acetyl-LDL) and maleylated albumin. In the present study we determined effects of these ligands on LPS-induced IL-1 production by rat peritoneal macrophages. These ligands themselves did not induce IL-1 production. However, upon short incubation with acetyl-LDL, LPS-induced IL-1 production was significantly suppressed. The extent of the suppression was proportional to cellular cholesteryl esters. Thus, intracellular accumulation of cholesteryl esters might be responsible for suppression of LPS-induced IL-1 production.

A receptor for formaldehyde-treated serum albumin on human placental brush-border membrane

Formaldehyde-treated serum albumin (f-Alb) is known to be taken up and degraded by sinusoidal liver cells via receptor-mediated endocytosis. We report that 125I-labeled f-Alb (125I-f-Alb) binding to human placental brush-border membranes also occurs. This binding reached equilibrium within 40 min at 37 degrees C. Kinetic studies demonstrated the presence of saturable binding with an apparent Kd of 2.1 micrograms of f-Alb/ml and a maximal binding of 2.3 micrograms/mg of membrane protein at pH 7.5. Maximal binding was observed at between pH 7.5 and 8.0. 125I-f-Alb binding to the membranes was little inhibited by a 1000-fold molar excess of ovalbumin, human apo-transferrin and native bovine serum albumin. No binding was observed with membranes which had been pretreated with proteinase or trypsin. This f-Alb receptor was extremely heat-stable, since the binding was not abolished even by pretreatment of the membranes at 78 degrees C for 30 min. EDTA, Ca2+ and Mg/4 had no effect on 125I-f-Alb binding, so the binding was independent of divalent cations. These data suggest that a receptor specific for f-Alb exists on human placental brush-border membranes of syncytial trophoblasts.

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.

Interaction of natural and synthetic albumin polymers with hepatocytes

The hepatitis B virus binds avidly to albumin polymers which in turn may mediate the initial binding of viral particles to the liver cell. However, the interaction of albumin polymers with the liver remains poorly characterized, and the possibility that hepatic binding reflects an artifact of polymerization with glutaraldehyde has not been excluded. We therefore characterized the binding of 125I-labeled natural and synthetic albumin polymers to suspensions of rat hepatocytes. Saturable binding was demonstrated for all preparations of monomeric and polymeric albumin studied. Glutaraldehyde-polymerized albumin (mean polymerization number = 15) bound much more avidly than naturally occurring albumin polymers (mostly dimers and trimers) or monomeric albumin. Competition between monomer and synthetic polymer was not observed. Reduction of free aldehyde groups on the synthetic polymer decreased nonsaturable binding without affecting saturable binding. Autoradiography confirmed binding of polyalbumin to hepatic parenchymal cells. Glutaraldehyde-polymerized ovalbumin, a protein unrelated to serum albumin, also bound hepatocytes saturably. We conclude that hepatic binding of synthetic albumin polymers is not due to residual aldehyde groups on the polymer and is much more avid than for natural polymer. This difference may reflect the higher degree of polymerization or chemical modification of the synthetic polymer. The hepatic binding sites for synthetic polymer appear distinct from those previously described for monomeric albumin and may not be specific for albumin.

Acetylated low-density lipoprotein is endocytosed through coated pits by rat peritoneal macrophages

The surface distribution of the scavenger receptors for acetylated low-density lipoprotein (acetyl-LDL) and their endocytic behavior were studied by the direct immunoperoxidase method using monomeric conjugates of horseradish peroxidase with Fab' antibody raised against LDL. The receptors were demonstrated to be distributed diffusely on the surface membrane of cultured peritoneal macrophages, with preferential localization in coated pit regions. With temperature shift from 4 degrees C to 37 degrees C, acetyl-LDL bound to the surface membrane rapidly disappeared, but became detectable in coated vesicles or lysosomes. Further incubation in the presence of acetyl-LDL revealed lipid vacuoles devoid of a limiting membrane in the cytoplasm, transforming macrophages into typical foam cells. These data suggest that the binding of acetyl-LDL to its receptors triggers the clustering of the receptors into the coated pit regions through which acetyl-LDL is endocytosed by coated vesicles to be degraded in lysosomes with subsequent intracellular accumulation of cholesterol esters.

Scavenger receptor for malondialdehyde-modified high density lipoprotein on rat sinusoidal liver cells

We report here the presence of a membrane-associated receptor which mediates endocytic uptake of malondialdehyde-modified high density lipoprotein (MDA-HDL) on sinusoidal liver cells. Binding of [125I]MDA-HDL to the cells was followed by internalization and degradation in lysosomes. The binding and lysosomal degradation of [125I]MDA-HDL were effectively inhibited by unlabeled MDA-HDL and acetyl-HDL. However, formaldehyde-treated serum albumin or low density lipoprotein modified either by acetylation or malondialdehyde, ligands known to undergo receptor-mediated endocytosis by sinusoidal liver cells, did not affect the binding of [125I]MDA-HDL to the cells. These results indicate that a receptor for MDA-HDL is described as a distinct member among the scavenger receptors for chemically modified proteins.