Recognition of lysosomal glycosidases in vivo inhibited by modified glycoproteins

Prominent Receptors of Liver Sinusoidal Endothelial Cells in Liver Homeostasis and Disease

Liver sinusoidal endothelial cells (LSECs) are the most abundant non-parenchymal cells lining the sinusoidal capillaries of the hepatic system. LSECs are characterized with numerous fenestrae and lack basement membrane as well as a diaphragm. These unique morphological characteristics of LSECs makes them the most permeable endothelial cells of the mammalian vasculature and aid in regulating flow of macromolecules and small lipid-based structures between sinusoidal blood and parenchymal cells. LSECs have a very high endocytic capacity aided by scavenger receptors (SR), such as SR-A, SR-B (SR-B1 and CD-36), SR-E (Lox-1 and mannose receptors), and SR-H (Stabilins). Other high-affinity receptors for mediating endocytosis include the FcγRIIb, which assist in the antibody-mediated removal of immune complexes. Complemented with intense lysosomal activity, LSECs play a vital role in the uptake and degradation of many blood borne waste macromolecules and small (<280 nm) colloids. Currently, seven Toll-like receptors have been investigated in LSECs, which are involved in the recognition and clearance of pathogen-associated molecular pattern (PAMPs) as well as damage associated molecular pattern (DAMP). Along with other SRs, LSECs play an essential role in maintaining lipid homeostasis with the low-density lipoprotein receptor-related protein-1 (LRP-1), in juxtaposition with hepatocytes. LSECs co-express two surface lectins called L-Specific Intercellular adhesion molecule-3 Grabbing Non-integrin Receptor (L-SIGN) and liver sinusoidal endothelial cell lectin (LSECtin). LSECs also express several adhesion molecules which are involved in the recruitment of leukocytes at the site of inflammation. Here, we review these cell surface receptors as well as other components expressed by LSECs and their functions in the maintenance of liver homeostasis. We further discuss receptor expression and activity and dysregulation associated with the initiation and progression of many liver diseases, such as hepatocellular carcinoma, liver fibrosis, and cirrhosis, alcoholic and non-alcoholic fatty liver diseases and pseudocapillarization with aging.

The Emerging Role of Rab5 in Membrane Receptor Trafficking and Signaling Pathways

Ras analog in brain (Rab) proteins are small guanosine triphosphatases (GTPases) that belong to the Ras-like GTPase superfamily, and they can regulate vesicle trafficking. Rab proteins alternate between an activated (GTP-bound) state and an inactivated (GDP-bound) state. Early endosome marker Rab5 GTPase, a key member of the Rab family, plays a crucial role in endocytosis and membrane transport. The activated-state Rab5 recruits its effectors and regulates the internalization and trafficking of membrane receptors by regulating vesicle fusion and receptor sorting in the early endosomes. In this review, we summarize the role of small Rab GTPases Rab5 in membrane receptor trafficking and the activation of signaling pathways, such as Ras/MAPK and PI3K/Akt, which ultimately affect cell growth, apoptosis, tumorigenesis, and tumor development. This review may provide some insights for our future research and novel therapeutic targets for diseases.

Mannose Receptor 1 Restricts HIV Particle Release from Infected Macrophages

Human mannose receptor 1 (hMRC1) is expressed on the surface of most tissue macrophages, dendritic cells, and select lymphatic or liver endothelial cells. HMRC1 contributes to the binding of HIV-1 to monocyte-derived macrophages (MDMs) and is involved in the endocytic uptake of HIV-1 into these cells. Here, we identify hMRC1 as an antiviral factor that inhibits virus release through a bone marrow stromal antigen 2 (BST-2)-like mechanism. Virions produced in the presence of hMRC1 accumulated in clusters at the cell surface but were fully infectious. HIV-1 counteracted the effect by transcriptional silencing of hMRC1. The effect of hMRC1 was not virus isolate specific. Surprisingly, deletion of the Env protein, which is known to interact with hMRC1, did not relieve the hMRC1 antiviral activity, suggesting the involvement of additional cellular factor(s) in the process. Our data reveal an antiviral mechanism that is active in primary human macrophages and is counteracted by HIV-1 through downregulation of hMRC1.

The Ligands of C-Type Lectins

In this chapter, a comprehensive overview of the known ligands for the C-type lectins (CTLs) is provided. Emphasis has been placed on the chemical structure of the glycans that bind to the different CTLs and the amount of structural variation (or overlap) that each CTL can tolerate. In this way, both the synthetic carbohydrate chemist and the immunologist can more readily gain insight into the existing structure-activity space for the CTL ligands and, ideally, see areas of synergy that will help identify and refine the ligands for these receptors.

Effects of di-isopropyl phosphorofluoridate on rat liver microsomal and lysosomal beta-glucuronidase

iPr(2)P-F (di-isopropyl phosphorofluoridate) administration to rats produces a liver-dependent specific elevation of plasma beta-glucuronidase activity. The response is unaffected by puromycin pretreatment. By using subcellular-fractionation techniques, the rise in plasma beta-glucuronidase activity was correlated temporally with a fall in liver microsomal beta-glucuronidase activity. After iPr(2)P-F treatment, liver microsomal membranes are depleted of beta-glucuronidase but slowly return to normal over 1 week. On the other hand, liver lysosomal beta-glucuronidase activity is high at early time points (less than 60min) after iPr(2)P-F administration but decreases to below control values; this lasts for a few days. The response to iPr(2)P-F was demonstrated in isolated hepatocytes prepared from iPr(2)P-F-treated rats. In such preparations, microsomal beta-glucuronidase is lost rapidly, followed by a specific decrease in hepatocyte lysosomal beta-glucuronidase. The results suggest that a pool of microsomal beta-glucuronidase serves as precursor to plasma beta-glucuronidase in iPr(2)P-F-treated rats, and further, that microsomal beta-glucuronidase may serve as precursor to lysosomal beta-glucuronidase.

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.

IL4/PGE2 induction of an enlarged early endosomal compartment in mouse macrophages is Rab5-dependent

The endosomal compartment and the plasma membrane form a complex partnership that controls signal transduction and trafficking of different molecules. The specificity and functionality of the early endocytic pathway are regulated by a growing number of Rab GTPases, particularly Rab5. In this study, we demonstrate that IL4 (a Th-2 cytokine) and prostaglandin E2 (PGE2) synergistically induce Rab5 and several Rab effector proteins, including Rin1 and EEA1, and promote the formation of an enlarged early endocytic (EEE) compartment. Endosome enlargement is linked to a substantial induction of the mannose receptor (MR), a well-characterized macrophage endocytic receptor. Both MR levels and MR-mediated endocytosis are enhanced approximately 7-fold. Fluid-phase endocytosis is also elevated in treated cells. Light microscopy and fractionation studies reveal that MR colocalizes predominantly with Rab5a and partially with Rab11, an endosomal recycling pathway marker. Using retroviral expression of Rab5a:S34N, a dominant negative mutant, and siRNA Rab5a silencing, we demonstrate that Rab5a is essential for the large endosome phenotype and for localization of MR in these structures. We speculate that the EEE is maintained by activated Rab5, and that the EEE phenotype is part of some macrophage developmental program such as cell fusion, a characteristic of IL4-stimulated cells.

Enhanced expression of the mannose receptor by endothelial cells of the liver and spleen microvascular beds in the macrophage-deficient PU.1 null mouse

Mice null for the haematopoietic lineage-specific transcription factor PU.1 lack mature Mphi and are compromised in their ability to clear cellular debris from the blood circulation. We investigated the possibility that non-professional phagocytes may partially compensate for the lack of Mphi in clearance functions. In the absence of Kupffer cells (resident liver Mphi) in the PU.1 null mice, electron microscopy revealed ingested debris in sinusoidal endothelial cells and hepatocytes although debris was also seen free in blood vessels. To investigate whether an increased clearance function of non-professional phagocytes might be linked to expression of Mphi-associated phagocytic and pinocytic receptors by other cells in PU.1 null mouse, we examined expression of several candidate proteins by immunocytochemistry and Western blotting. We found mannose receptor (MR) comparably expressed in PU.1 null and PU.1+ mice liver and spleen whereas class A scavenger receptor was substantially reduced and complement receptor 3 was absent in PU.1 null animals. By morphometric analysis, liver and spleen sinusoidal endothelial cells were seen to express significantly more MR in the PU.1 null mouse. This study provides the first evidence of apparently compensatory alterations in the microvasculature of the Mphi-deficient PU.1 null mouse.

The expression of mannose receptors in skin fibroblast and their involvement in Leishmania (L.) amazonensis invasion

Leishmania are protozoa that invade mononuclear phagocytes with the involvement of different ligand-receptor systems, including mannose receptors. Until now, scant data are available concerning the mechanisms that govern the infection of Leishmania in other host cell types such as fibroblasts. Our aim was to analyze the expression of mannose receptors in primary cultures of skin fibroblasts (SF) further characterizing their role during the invasion of promastigotes of Leishmania (L.) amazonensis. Both fluorescent, light, and electron microscopy assays revealed that SF have mannose receptors since they bound and internalized mannosylated ligands in addition to being positively labeled by fuc-BSA-FITC probes. d-mannose competition assays revealed the participation of mannose receptors during the parasite association with SF presenting upregulated receptor expression during the initial steps of the infection. After longer periods of Leishmania:fibroblasts contact, the modulation noted in the host mannose receptors was reverted concomitantly to the infection control, suggesting that the parasites were required for the alteration maintenance and providing evidences that the SF may display microbicidal mechanisms to control the Leishmania infection.

Mannose receptor-mediated regulation of serum glycoprotein homeostasis

Carbohydrates are thought to function as tags that mark circulatory glycoproteins for rapid clearance. To examine the role of the mannose receptor (MR) in glycoprotein clearance, we generated mice genetically deficient in MR. MR-/- mice were defective in clearing proteins bearing accessible mannose and N-acetylglucosamine residues and had elevated levels of eight different lysosomal hydrolases. Proteomic analysis of MR-/- and control mouse sera showed that an additional 4 out of 52 proteins identified were elevated in MR-/- serum. Each of these is up-regulated during inflammation and wound healing. Thus, MR appears to operate as an essential regulator of serum glycoprotein homeostasis.

Vascularization in the murine allantois occurs by vasculogenesis without accompanying erythropoiesis

The aim of this study was to determine whether the blood vessels of the murine allantois are formed by vasculogenesis or angiogenesis. Morphological analysis revealed that differentiation of allantoic mesoderm into an outer layer of mesothelium and an inner vascular network begins in the distal region of the allantois, which is most remote from other tissues, as early as the late neural plate stage (approximately 7.75 days postcoitum). Nascent blood vessels were not found in the base of the allantois until 4-somite pairs had formed in the fetus (approximately 8.25 days postcoitum), and vascular continuity with the yolk sac and fetus was not present until the 6-somite-pair stage (approximately 8.5 days postcoitum). Immunohistochemical analysis demonstrated that flk-1, a molecular marker of early endothelial cells, is expressed in significantly more distal than basal core cells in the early allantois and never in mesothelium. Furthermore, synchronous grafting of donor yolk sac containing blood islands into blood islands of headfold-stage host conceptuses provided no evidence that the yolk sac contributes endothelial cells to the allantois. Finally, when removed from conceptuses and cultured in isolation, neural plate and headfold-stage allantoises formed a conspicuous vascular network that was positive for Flk-1. Hence, the vasculature of the allantois is formed intrinsically by vasculogenesis rather than extrinsically via angiogenesis from the adjacent yolk sac or fetus. Whether allantoic vasculogenesis is associated with erythropoiesis was also investigated. Benzidine-staining in situ revealed that primitive erythroid cells were not identified in the allantois until 6-somite pairs when continuity between its vasculature and that of the yolk sac was first evident. Nevertheless, a small number of allantoises removed from conceptuses at a considerably earlier stage were found to contain erythroid precursor cells following culture in isolation. To determine whether such erythroid cells could be of allantoic origin, host allantoises were made chimeric with lacZ-expressing donor allantoises that were additionally labeled with [3H]methyl thymidine. Following culture and autoradiography, many lacZ-expressing benzidine-stained cells were observed in donor allantoises, but none contained silver grains above background. Moreover, no cells of donor allantoic origin were found in the fetus or yolk sac. Hence, vasculogenesis seems to be independent of erythropoiesis in the allantois and to involve a distal-to-proximal gradient in differentiation of allantoic mesoderm into the endothelial cell lineage. Furthermore, this gradient is established earlier than reported previously, being present at the neural plate stage.

Excessive in vitro bacterial lipopolysaccharide-induced production of monokines in cirrhosis

The objective of this study was to analyze monokine production by peripheral blood mononuclear cells from patients with alcoholic cirrhosis. The capacity of peripheral blood mononuclear cells and purified monocytes from these patients to produce tumor necrosis factor alpha, interleukin 1 beta, and interleukin 6 was investigated. Spontaneous production of tumor necrosis factor alpha, interleukin 6 and interleukin 1 beta was similar in cirrhotic and healthy subjects, but serum levels of interleukin 6 (less than 2 U/ml vs. 9.5 +/- 3 U/ml) and tumor necrosis factor alpha (3.1 +/- 1.2 pg/ml vs. 12.0 +/- 1.2 pg/ml) were significantly higher in cirrhotic patients. However, peripheral blood mononuclear cells or purified monocytes from patients with alcoholic liver cirrhosis, stimulated in vitro with Escherichia coli lipopolysaccharide, displayed a marked increase of tumor necrosis factor alpha, interleukin 1 beta and interleukin 6 secretions compared with healthy controls. A striking feature of this overproduction was its reversibility as assessed by allowing cells to rest in vitro without lipopolysaccharide for 1 to 7 days before stimulation. In such conditions, tumor necrosis factor alpha and interleukin 6 secretions declined to levels present in healthy subjects in whom production remained stable, whereas interleukin 1 beta secretion markedly decreased in both groups to the point where no difference could be seen. This reversible oversecretion of cytokines after lipopolysaccharide stimulation, along with the lack of abnormality of spontaneous cytokine secretion, suggests that monocytes in these patients may have undergone an in vivo activation process analogous to a priming phenomenon. The in vitro activation with lipopolysaccharide may represent the correlate of in vivo endotoxemia observed during acute events such as sepsis.(ABSTRACT TRUNCATED AT 250 WORDS)

Endotoxin-induced tumor necrosis factor (TNF): selective triggering of TNF and interleukin-1 production by distinct glucosamine-derived lipids

The isolated lipid A of Bordetella pertussis endotoxin (LipA) has been found to induce in vitro release of tumor necrosis factor (TNF) by murine macrophages, albeit much less efficiently than does the intact lipopolysaccharide. Synthetic analogs (monosaccharides M4 and M6) of both glucosamine units present in the LipA backbone induced production of TNF by peritoneal macrophages of Swiss mice. Macrophages from A/J mice gave higher responses than those from Swiss mice, while those of C3H/HeJ mice were unresponsive. Enhancement of TNF secretion was observed for all cells if they were pretreated with a calcium ionophore, and no otherwise inactive substance became active with cells thus treated. For synthetic monosaccharide derivatives, a phosphate group on O-4 was not required for, and a phosphate group on O-1 abolished, the TNF-inducing activity. Synthetic monosaccharides, chemically closely related to substructures recognized to be present in isolated lipid A preparations, could induce either TNF or interleukin-1 (IL-1) production, but not both simultaneously: the monosaccharides M4 and M6 were active TNF inducers, but did not initiate IL-1 production, while the monosaccharides M9 and lipid X efficiently elicited IL-1 production, but did not trigger TNF secretion. It should be noted, however, that the active synthetic compounds are considerably less efficient TNF inducers as is the intact B. pertussis endotoxin.

Isolation and immunochemical study of a soluble cerebellar lectin delineating its structure and function

A procedure of sequential extractions of cerebellar tissue was set up, which allowed specific solubilization of endogenous lectins by mannose. Two cerebellar soluble lectins, CSL1 (Mr = 33,000) and CSL2 (Mr = 31,500), were isolated. They appeared to consist of structurally and immunologically related polypeptides chains. By immunoaffinity, another minor component (Mr = 45,000) was isolated. Immunological studies suggested that the minor component is the precursor of the two other, i.e., CSL1 and CSL2, subunits. CSL1 (mainly lysosomal) possesses an additional peptide compared with CSL2 (mainly cytoplasmic and extracellular), which seems to be implicated in the signal for secretion and release.

In vivo depletion of mannose receptor bearing cells from rat liver by ricin A chain: effects on clearance of beta-glucuronidase

Ricin A chain has previously been shown to intoxicate macrophages in vitro following binding and endocytosis by the macrophage mannose receptor. In this report it is demonstrated that the intravenous injection of ricin A chain in nephrectomized rats leads to a prolonged plasma half-life for [125I]beta-glucuronidase, a ligand for the mannose receptor. Clearance of [125I]asialofetuin, a ligand for the galactose receptor of hepatocytes, was unaffected by injection of A chain. Microscopic examination of the livers of A chain-treated animals revealed a loss of phagocytic cells from the liver sinusoids. These results suggest that ricin A chain may be useful as a toxin specific for mannose receptor bearing cells of the reticuloendothelial system.

Rat liver endothelial cells have a greater capacity than Kupffer cells to endocytose N-acetylglucosamine- and mannose-terminated glycoproteins

The capacity of rat liver Kupffer and endothelial cells to endocytose glycoproteins with N-acetylglucosamine- or mannose-terminated oligosaccharide chains was studied. For this purpose, agalactoorosomucoid, ahexosaminoorosomucoid and horseradish peroxidase were used as ligands. A reliable determination of the amount of ligand endocytosed in vivo or in vitro was made possible by using the recently developed cold pronase method for the isolation and purification of Kupffer and endothelial cells. Both cell types participated in the uptake of the ligands in vivo as well as in vitro, but their endocytic capacity was several times greater in vivo than in vitro. Under both conditions, endothelial cells possessed a greater capacity to endocytose the ligands than did Kupffer cells. Since the total number of endothelial cells in the liver is at least twice the number of Kupffer cells, the contribution of endothelial cells to the liver uptake of N-acetylglucosamine-terminated glycoproteins in vivo was estimated to be 3 to 7 times higher than that of the Kupffer cells. In vitro experiments showed that the uptake of the glycoproteins followed saturation kinetics and was strongly inhibited at 4 degrees C and in the presence of mannan. Ultrastructural investigations revealed that horseradish peroxidase was taken up by all Kupffer and endothelial cells. These results emphasize the important role liver endothelial cells play in the clearance of specific glycoproteins from the circulation.

Intracellular pathway followed by invertase endocytosed by rat liver

Yeast invertase, when injected into rats, is endocytosed by the liver, mainly by sinusoidal cells. The work reported here aims at investigating the organelles involved in the intracellular journey of this protein. Experiments were performed on rats injected with 125I-invertase (25 micrograms/100 g body wt) and killed at various times after injection. Homogenates were fractioned by differential centrifugation, according to de Duve, Pressman, Gianetto, Wattiaux and Appelmans [(1955) Biochem. J. 63, 604-617]. Early after injection the radioactivity was recovered mainly in the microsomal fraction P; later it was found in the mitochondrial fractions (ML). At all times a peak of relative specific activity was observed in the light mitochondrial fraction L. After isopycnic centrifugation in a sucrose gradient, structures bearing 125I-invertase, present in P, exhibited a relatively flattened distribution with a density of around 1.17 g/ml, relatively similar to that of alkaline phosphodiesterase a plasma membrane marker. The organelles located in ML were endowed with a more homogeneous distribution, their median equilibrium density increasing up to 30 min after injection (1.20 g/ml----1.23 g/ml); with time the radioactivity distribution became more closely related to the distribution of arylsulfatase, a lysosomal enzyme. ML fractions, isolated 10 min and 180 min after 125I-invertase injection, were subjected to isopycnic centrifugation in Percoll gradient with, as solvent, 0.25 M, 0.5 M and 0.75 M sucrose. The change of density of the particles bearing 125I-invertase, as a function of the sucrose concentration, paralleled the change of density of the lysosomes as ascertained by the behaviour of arylsulfatase. The distribution of radioactivity and arylsulfatase in a sucrose gradient was established after isopycnic centrifugation of the ML fraction of rats injected with 125I-invertase, the animals having received or not an injection of 900 micrograms/100 g body weight of unlabelled invertase 15 h before killing. In agreement with our previous results, a shift towards higher densities of about 25% or arylsulfatase takes place in rats pretreated with unlabelled invertase. At 10 min, invertase preinjection did not change the radioactivity distribution curve. Later, it caused a progressive shift of the distribution towards higher-density regions of the gradient where the arylsulfatase, which had been shifted, was located.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect on lysosomes of invertase endocytosed by rat-liver

The intracellular localization of invertase endocytosed by rat liver was investigated by analytical centrifugation in sucrose and Percoll gradients of mitochondrial fractions originating from rats killed 15 h after injection. After isopycnic centrifugation in a sucrose gradient, invertase is located in higher density zones than acid hydrolases. The difference between the distribution of invertase and that of acid hydrolases increases with the amount of invertase injected. When the invertase dose is sufficiently high, a change of lysosomal enzyme distribution is clearly visible. It consists in the shift of a proportion of these enzymes to higher density regions where invertase is located. The proportion of hydrolase activity affected by invertase is different for each enzyme measured; it is the least pronounced for acid phosphatase, and most for acid deoxyribonuclease and arylsulfatase. A pretreatment of the rat with Triton WR 1339 considerably decreases the equilibrium density of structures bearing invertase. Nevertheless invertase distribution is quite distinct from that of the bulk of lysosomal enzymes that are recovered in lower density zones of the gradient; on the other hand the invertase injection to rats treated with Triton WR 1339 causes a spreading of the acid hydrolase distribution towards higher density zones. The distribution of acid hydrolases and invertase in a Percoll gradient depends on the sucrose concentration of the solvent. It is shifted towards higher densities when the sucrose concentration increases. The phenomenon is more important for invertase. These results are best explained by supposing that invertase accumulates in a distinct population of lysosomes that can be individualized as a result of the density increase they are subjected to by the invertase they accumulate. It is proposed that these lysosomes mainly originate from non-parenchymal cells of the liver.

Effect of glycyl-L-phenylalanine 2-naphthylamide on invertase endocytosed by rat liver

The release by glycyl-L-phenylalanine 2-naphthylamide (Gly-L-Phe-2-NNap) of endocytosed invertase associated with the MLP fraction (sum of the M, L and P fractions [de Duve, Pressman, Gianetto, Wattiaux & Appelmans (1955) Biochem. J. 63, 604-617]) of rat liver was investigated and compared with the release of cathepsin C. The percentage of invertase released increases with time after the enzyme injection, whereas the release of cathepsin C is not influenced by this treatment and corresponds to 85-90% of the total activity of the enzyme. It takes about 2h to attain a similar release of both enzymes. The quantity of invertase releasable or not by Gly-L-Phe-2-NNap was plotted against the time after the injection. Results agree well with the hypothesis that unreleasable invertase is associated with a pre-lysosomal compartment, whereas releasable invertase is present in lysosomes. A kinetic analysis indicates that invertase enters the pre-lysosomal compartment with a zero-order rate constant of 0.48 unit/min per g fresh wt., and leaves this compartment with a first-order rate constant of 0.042 min-1.

The role of the liver in clearance of glycoproteins from the general circulation, with special reference to intestinal alkaline phosphatase

Glycoproteins represent a wide variety of macromolecules with important physiological functions. Characteristic variations in carbohydrate composition and plasma concentration of these proteins may occur during pathological conditions. Steady-state plasma concentrations are determined by release from normal or diseased tissues and simultaneous clearance from the general circulation. The liver occupies a central position in the production but also clearance and catabolism of such glycoproteins. A number of specialized receptor-mediated transport processes for different types of glycoproteins in this organ is reviewed. Membrane recognition is generally followed by absorptive endocytosis and vesicle transport to lysosomes, Golgi system and/or bile canaliculis. The charge of the protein, the nature of the terminal sugar residue or complex formation with other glycoproteins may determine the extent of uptake in the various cell types of the liver. By means of these transport processes the liver is able to remove potentially dangerous macromolecules such as denatured proteins, aggressive enzymes and immunocomplexes from the general circulation. Drugs can bind to some of these proteins or may interact with the hepatic transport or catabolism processes. Special attention is paid to the hepatic clearance of asialoglycoproteins with terminal galactose groups. Intestinal alkaline phosphatase is used as a model compound to characterize the pharmacokinetic profiles of hepatic uptake and biliary excretion in the rat in vivo and isolated perfused rat livers. Histochemical and electron-microscopic studies demonstrated a galactose-specific, receptor-mediated endocytotic process, mainly but not exclusively localized in centrolobular hepatocytes. Drug interactions with these processes will be the subject of further investigations.

Active-site-dependent, thrombin-induced release of adenine nucleotides from cultured human endothelial cells

The effect of thrombin on release of adenine nucleotides is studied in cultured cell monolayers from the human umbilical vein. Thrombin-induced release of radioactivity from endothelial cells is dose-dependent and saturable with maximal response seen at 1 x 10(-8) M thrombin. The products are identified by thin-layer chromatography as adenine nucleotides. Diisopropylphosphoryl thrombin, which is enzymatically inactive, does not cause release of tritium. A 50-fold excess of diisopropylphosphoryl-thrombin, despite causing 98 percent inhibition of binding of 125I-thrombin to its high-affinity binding sites, does not inhibit thrombin-induced release. We conclude that (1) thrombin causes release from endothelial cells of adenine nucleotides and that (2) high-affinity, active-site-independent binding of thrombin is not involved in this process.

Uptake and distribution of placental glucocerebrosidase in rat hepatic cells and effects of sequential deglycosylation

The clearance of native human placental glucocerebrosidase by rat liver shows the presence of two distinct enzyme forms with different recognition characteristics. The clearance and uptake of native enzyme by liver cells was compared to that of glucocerebrosidase sequentially treated with neuraminidase, beta-galactosidase and beta-N-acetylglucosaminidase. The initial rate of clearance of infused enzyme was increased greater than 10-fold for the asialo-, agalacto- and ahexoenzymes over that of native glucocerebrosidase. Incorporation of asialo enzyme was increased in hepatocytes over that of native enzyme, while the distribution of agalacto- and ahexoenzyme preparations was increased in non-parenchymal cells. This observation is consistent with the identification of a galactose receptor on hepatocytes and N-acetylglucosamine/mannose receptors on Kupffer cells. These data and inhibition studies by specified monosaccharide-terminal glycoprotein derivatives demonstrate the importance of these sugars in the uptake of this lysosomal enzyme by receptor-mediated endocytosis. Modification of the enzyme to expose certain monosaccharide moieties results in increased delivery to specific cell types. Therefore, naturally occurring receptors can be utilized for targeting glucocerebrosidase to the non-parenchymal cell in liver.

The role of extra-hepatic tissues in the receptor-mediated plasma clearance of glycoproteins terminated by mannose or N-acetylglucosamine

The mannose- and N-acetylglucosamine-specific pathway for the clearance of mammalian glycoproteins has been characterized by using 125I-labelled neoglycoproteins, glycosidase-treated orosomucoid and lysosomal glycosidases (beta-glucuronidase and beta-N-acetylglucosaminidase) as probes. There are two components to this pathway in vivo; one liver-dependent and the other extrahepatic or liver-independent. Cells that mediate clearance by the latter component of the pathway are present in spleen, bone and in elements of the reticuloendothelial system, but not in the kidney. Glycoproteins that possess terminal mannose, glucose or N-acetylglucosamine residues, including various lysosomal enzymes, are rapidly cleared from plasma via this pathway. Glucose-terminated glycoproteins are recognized by two pathways in the intact animal; the hepatic galactose-specific pathway and the mannose/N-acetylglycosamine-specific pathway, which is present in liver and in peripheral tissues. Following removal of the liver by surgical evisceration, glucose-terminated glycoproteins are cleared whereas glycoproteins bearing galactose are not cleared. Uptake of 125I-labelled neoglycoproteins and agalacto-orosomucoid by isolated alveolar macrophages closely mimics clearance in vivo by the mannose/N-acetylglucosamine pathway. Neoglycoproteins terminated by mannose, glucose or N-acetylglucosamine all compete with 125I-labelled agalacto-orosomucoid for uptake by receptor-mediated pinocytosis. The extent of substitution of the neoglycoproteins is a critical determinant of their inhibitory potency. It is proposed that mononuclear phagocytes are in important component of the clearance pathway in vivo. The mannose/N-acetylglucosamine pathway may be important in the regulation of extracellular levels of various glycosylated macromolecules, including lysosomal hydrolases.

Chick embryo fibroblasts produce two forms of hyaluronidase

Cultured chick embryo fibroblasts derived from skin and skeletal muscle exhibit hyaluronidase activity both associated with the cell layer and secreted into the medium. Although both forms of the enzyme have a number of similar characteristics (R.W. Orkin and B.P. Toole, 1980, J. Biol. CHem. 255), they differ in thermal stability at neutral pH and in behavior on ion-exchange chromatography. Both forms of the enzyme are equally stable at acidic pH for long intervals, but the cell-associated hyaluronidase is significantly less stable than the secreted froms at neutral pH and at temperatures more than or equal to 30 degrees C. Neither the presence of proteases nor inhibitors of hyaluronidase appear to be involved in the cell-asspcoated enzyme. Chromatography of the two forms of hyaluronidase on carboxymethyl cellulose reveals that most (60-90 percent) of the secreted form of the enzyme elutes at a lower ionic strength than the cell- associated enzyme. Treatment of the secreted form of hyaluronidase with neuraminidase shifts its elution profile on carboxymethyl cellulose toward that of the cell-associated form, and also decreases its thermal stability at neutral pH. In contrast, treatment of the secreted form of hyaluronidase with alkaline phosphatase has no detectable effect. These data suggest that the secreted hyaluronidase differs from the cellular form in possessing additional sialic acid residues which endow the former with increased stability in the extracellular milieu.

Receptor-mediated pinocytosis of mannose glycoconjugates by macrophages: characterization and evidence for receptor recycling

125I-Mannose--BSA is taken up by alveolar macrophages by receptor-mediated endocytosis. Uptake is macrophage-specific and does not occur in polymorphonuclear leukocytes. Binding (4 degrees C) and uptake (37 degrees C) of 125I--Man--BSA are time- and ligand concentration-dependent [Kuptake = 40 nM; Kd (4 degrees C) = 10 nM]. When adjusted for ligand degradation, ligand uptake is linear with time. Binding saturates at 60 min and requires Ca++. Following binding, ligand remains on the cell surface where it can be released by EGTA and trypsin. Internalization of prebound ligand occurs very rapidly (t 1/2 less than 5 min) when cells are warmed to 37 degrees C. Following internalization of prebound ligand, binding activity is rapidly recovered (t 1/2 less than 5 min). Trypsin treatment (4 degrees C) substantially reduces binding activity (greater than 70% per 30 min). However, binding activity is rapidly recovered in cells treated with trypsin at 4 degrees C by warming to 37 degrees C in the absence of added ligand. Trypsin treatment at 37 degrees C rapidly destroys binding and uptake. On the contrary, 4 degrees C trypsin treatment produces only a modest reduction in subsequent ligand uptake. These results, taken together with the observation that cycloheximide has no effect on ligand uptake, suggest that receptors must be spared from degradation and that reutilization of receptors probably occurs.

Rapid removal to the liver of intravenously injected lipoprotein lipase

Lipoprotein lipase was purified from bovine milk and labeled with 125I. After intravenous injection to rats the labeled lipase rapidly disappeared from the blood. The initial half-life was about 1 min and more than 70% of the radioactivity was found in the liver at 10 min. 30 min after the injection about 10% of the injected radioactivity was present in acid-soluble form in blood, indicating that the enzyme had been rapidly degraded. Injection of asialofetuin, ribonuclease B or mannan in amounts known to block the hepatic receptors for glycoproteins with exposed galactose, N-acetylglucosamine or mannose residues did not retard the removal of the lipoprotein lipase. Thus, some other, as yet undefined, receptor is implicated. Lipoprotein lipase is known to bind to heparin and some related polysacchrides. Heparin injected before the enzyme delayed its removal and heparin injected after the enzyme caused an immediate increase in blood radioactivity, signifying return from tissues to blood of labeled enzyme. Lipoprotein lipase is present at the endothelium in several extrahepatic tissues and is rapidly turned over. Its presence in blood in appreciable amounts would cause a derangement of lipid transport. The efficient hepatic removal of the enzyme may thus serve an important physiological purpose in keeping the blood levels of this enzyme low.

Plasma clearance of glycoproteins with terminal mannose and N-acetylglucosamine by liver non-parenchymal cells. Studies with beta-glucuronidase, N-acetyl-beta-D-glucosaminidase, ribonuclease B and agalacto-orosomucoid

Glycoproteins having mannose and/or N-acetylglucosamine in the terminal non-reducing position [Stockert, Morell & Scheinberg (1976) Biochem. Biophys. Res. Commun. 68, 988--993], and various lysosomal enzymes [Stahl, Schlesinger, Rodman & Doebber (1976) Nature (London) 264, 86--8] are rapidly cleared from plasma by the liver after intravenous administration. A liver cell-separation technique was used to determine the cellular localization of 125I-labelled beta-glucuronidase, ribonuclease B, agalacto-orosomucoid and asialo-orosomucoid. On a specific readioactivity basis, all ligands except 125I-labelled asialo-orosomucoid were enriched in the non-parenchymal cell fraction. Isolated cells, fixed and stained for beta-glucuronidase or N-acetyl-beta-D-glucosaminidase activity after intravenous injection of the enzymes, showed enrichment in the non-parenchymal cell fraction (probably Kupffer cells). After uptake by the non-parenchymal cells, liver lysosomal beta-glucuronidase and N-acetyl-beta-D-glucosaminidase showed degradation half-times of 2.2 and 0.4 days respectively.