Uptake of LDL in parenchymal and non-parenchymal rabbit liver cells in vivo. LDL uptake is increased in endothelial cells in cholesterol-fed rabbits

1. Hepatic uptake of low-density lipoprotein (LDL) in parenchymal cells and non-parenchymal cells was studied in control-fed and cholesterol-fed rabbits after intravenous injection of radioiodinated native LDL (125I-TC-LDL) and methylated LDL (131I-TC-MetLDL). 2. LDL was taken up by rabbit liver parenchymal cells, as well as by endothelial and Kupffer cells. Parenchymal cells, however, were responsible for 92% of the hepatic LDL uptake. 3. Of LDL in the hepatocytes, 89% was taken up via the B,E receptor, whereas 16% and 32% of the uptake of LDL in liver endothelial cells and Kupffer cells, respectively, was B,E receptor-dependent. 4. Cholesterol feeding markedly reduced B,E receptor-mediated uptake of LDL in parenchymal liver cells and in Kupffer cells, to 19% and 29% of controls, respectively. Total uptake of LDL in liver endothelial cells was increased about 2-fold. This increased uptake is probably mediated via the scavenger receptor. The B,E receptor-independent association of LDL with parenchymal cells was not affected by the cholesterol feeding. 5. It is concluded that the B,E receptor is located in parenchymal as well as in the non-parenchymal rabbit liver cells, and that this receptor is down-regulated by cholesterol feeding. Parenchymal cells are the main site of hepatic uptake of LDL, both under normal conditions and when the number of B,E receptors is down-regulated by cholesterol feeding. In addition, LDL is taken up by B,E receptor-independent mechanism(s) in rabbit liver parenchymal, endothelial and Kupffer cells. The non-parenchymal liver cells may play a quantitatively important role when the concentration of circulating LDL is maintained at a high level in plasma, being responsible for 26% of hepatic uptake of LDL in cholesterol-fed rabbits as compared with 8% in control-fed rabbits. The proportion of hepatic LDL uptake in endothelial cells was greater than 5-fold higher in the diet-induced hypercholesterolaemic rabbits than in controls.

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Hepatic lipase is localized at the parenchymal cell microvilli in rat liver

Biochemical Journal ◽

10.1042/bj3210425 ◽

1997 ◽

Vol 321 (2) ◽

pp. 425-430 ◽

Cited By ~ 25

Author(s):

Belinda BREEDVELD ◽

Kees SCHOONDERWOERD ◽

Adrie J. M. VERHOEVEN ◽

Rob WILLEMSEN ◽

Hans JANSEN

Keyword(s):

Endothelial Cells ◽

Rat Liver ◽

Kupffer Cells ◽

Binding Capacity ◽

Hepatic Lipase ◽

Parenchymal Cell ◽

Liver Cells ◽

Minor Amount ◽

Parenchymal Liver ◽

Parenchymal Cells

Hepatic lipase (HL) is thought to be located at the vascular endothelium in the liver. However, it has also been implicated in the binding and internalization of chylomicron remnants in the parenchymal cells. In view of this apparent discrepancy between localization and function, we re-investigated the localization of HL in rat liver using biochemical and immunohistochemical techniques. The binding of HL to endothelial cells was studied in primary cultures of rat liver endothelial cells. Endothelial cells bound HL in a saturable manner with high affinity. However, the binding capacity accounted for at most 1% of the total HL activity present in the whole liver. These results contrasted with earlier studies, in which non-parenchymal cell (NPC) preparations had been found to bind HL with a high capacity. To study HL binding to the different components of the NPC preparations, we separated endothelial cells, Kupffer cells and blebs by counterflow elutriation. Kupffer cells and endothelial cells showed a relatively low HL-binding capacity. In contrast, the blebs, representing parenchymal-cell-derived material, had a high HL-binding capacity (33 m-units/mg of protein) and accounted for more than 80% of the total HL binding in the NPC preparation. In contrast with endothelial and Kupffer cells, the HL-binding capacity of parenchymal cells could account for almost all the HL activity found in the whole liver. These data strongly suggest that HL binding occurs at parenchymal liver cells. To confirm this conclusion in situ, we studied HL localization by immunocytochemical techniques. Using immunofluorescence, we confirmed the sinusoidal localization of HL. Immunoelectron microscopy demonstrated that virtually all HL was located at the microvilli of parenchymal liver cells, with a minor amount at the endothelium. We conclude that, in rat liver, HL is localized at the microvilli of parenchymal cells.

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Characterization of the interaction both in vitro and in vivo of tissue-type plasminogen activator (t-PA) with rat liver cells. Effects of monoclonal antibodies to t-PA

Biochemical Journal ◽

10.1042/bj2840545 ◽

1992 ◽

Vol 284 (2) ◽

pp. 545-550 ◽

Cited By ~ 17

Author(s):

M Otter ◽

J Kuiper ◽

R Bos ◽

D C Rijken ◽

T J van Berkel

Keyword(s):

Endothelial Cells ◽

Mannose Receptor ◽

Liver Cells ◽

Tissue Type ◽

Tissue Type Plasminogen Activator ◽

Type Plasminogen Activator ◽

Liver Endothelial Cells ◽

Parenchymal Cells

The interaction of 125I-labelled tissue-type plasminogen activator (125I-t-PA) with freshly isolated rat parenchymal and endothelial liver cells was studied. Binding experiments at 4 degrees C with parenchymal cells and endothelial liver cells indicated the presence of 68,000 and 44,000 high-affinity t-PA-binding sites, with an apparent Kd of 3.5 and 4 nM respectively. Association of 125I-t-PA with parenchymal cells was Ca(2+)-dependent and was not influenced by asialofetuin, a known ligand for the galactose receptor. Association of 125I-t-PA with liver endothelial cells was Ca(2+)-dependent and mannose-specific, since ovalbumin (a mannose-terminated glycoprotein) inhibited the cell association of t-PA. Association of 125I-t-PA with liver endothelial cells was inhibited by anti-(human mannose receptor) antiserum. Anti-(galactose receptor) IgG had no effect on 125I-t-PA association with either cell type. Degradation of 125I-t-PA at 37 degrees C by both cell types was inhibited by chloroquine or NH4Cl, indicating that t-PA is degraded lysosomally. in vitro experiments with three monoclonal antibodies (MAbs) demonstrated that anti-t-PA MAb 1-3-1 specifically decreased association of 125I-t-PA with the endothelial cells, and anti-t-PA Mab 7-8-4 inhibited association with the parenchymal cells. Results of competition experiments in rats in vivo with these antibodies were in agreement with findings in vitro. Both antibodies decreased the liver uptake of 125I-t-PA, while a combination of the two antibodies was even more effective in reducing the liver association of 125I-t-PA and increasing its plasma half-life. We conclude from these data that clearance of t-PA by the liver is regulated by at least two pathways, one on parenchymal cells (not galactose/mannose-mediated) and another on liver endothelial cells (mediated by a mannose receptor). Results with the MAbs imply that two distinct sites on the t-PA molecule are involved in binding to parenchymal cells and liver endothelial cells.

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Uptake and Degradation of Tissue Plasminogen Activator in Rat Liver

Thrombosis and Haemostasis ◽

10.1055/s-0038-1647518 ◽

1988 ◽

Vol 59 (03) ◽

pp. 474-479 ◽

Cited By ~ 35

Author(s):

Monica Einarsson ◽

Bård Smedsrød ◽

Håkan Pertoft

Keyword(s):

Endothelial Cells ◽

Plasminogen Activator ◽

Tissue Plasminogen Activator ◽

Rat Liver ◽

Liver Cells ◽

Terminal Phase ◽

Tissue Plasminogen ◽

Liver Endothelial Cells ◽

Parenchymal Cells

SummaryThe mechanism of uptake of tissue plasminogen activator (tPA) in rat liver was studied. Radio-iodinated tPA was removed from the circulation after intravenous administration in a biphasic mode. The initial half life, t1/2(α), and the terminal phase, t1/2(β), were determined to be 0.5 min and 7.5 min, resp. Separation of the liver cells by collagenase perfusion and density centrifugation, revealed that the uptake per cell was two to three times higher in the non-parenchymal cells than in the parenchymal cells.Endocytosis of fluorescein isothiocyanate-labelled or 125I-labelled tPA was studied in pure cultures of liver cells in vitro. Liver endothelial cells and parenchymal cells took up and degraded tPA. Endocytosis was more efficient in liver endothelial cells than in parenchymal cells, and was almost absent in Kupffer cells.Competitivb inhibition experiments showing that excess unlabelled tPA could compete with the uptake and degradation of 125I-tPA, suggested that liver endothelial cells and parenchymal cells interact with the activator in a specific manner. Endocytosis of trace amounts of 125I-tPA in cultures of liver endothelial cells and parenchymal cells was inhibited by 50% in the presence of 19 nM unlabelled tPA. Agents that interfere with one or several steps of the endocytic machinery inhibited uptake and degradation of 125I-tPA in both cell types.These findings suggest that 1) liver endothelial cells and parenchymal cells are responsible for the rapid hepatic clearance of intravenously administered tPA; 2) the activator is taken up in these cells by specific endocytosis, and 3) endocytosed tPA is transported to the lysosomes where it is degraded.

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Evidence for reverse cholesterol transport in vivo from liver endothelial cells to parenchymal cells and bile by high-density lipoprotein

Biochemical Journal ◽

10.1042/bj2680685 ◽

1990 ◽

Vol 268 (3) ◽

pp. 685-691 ◽

Cited By ~ 32

Author(s):

H F Bakkeren ◽

F Kuipers ◽

R J Vonk ◽

T J C Van Berkel

Keyword(s):

Endothelial Cells ◽

High Density Lipoprotein ◽

Kupffer Cells ◽

Reverse Cholesterol Transport ◽

Density Lipoprotein ◽

Cholesterol Transport ◽

Cholesterol Esters ◽

Liver Endothelial Cells ◽

Parenchymal Cells

Acetylated low-density lipoprotein (acetyl-LDL), biologically labelled in the cholesterol moiety of cholesteryl oleate, was injected into control and oestrogen-treated rats. The serum clearance, the distribution among the various lipoproteins, the hepatic localization and the biliary secretion of the [3H]cholesterol moiety were determined at various times after injection. In order to monitor the intrahepatic metabolism of the cholesterol esters of acetyl-LDL in vivo, the liver was subdivided into parenchymal, endothelial and Kupffer cells by a low-temperature cell-isolation procedure. In both control and oestrogen-treated rats, acetyl-LDL is rapidly cleared from the circulation, mainly by the liver endothelial cells. Subsequently, the cholesterol esters are hydrolysed, and within 1 h after injection, about 60% of the cell- associated cholesterol is released. The [3H]cholesterol is mainly recovered in the high-density lipoprotein (HDL) range of the serum of control rats, while low levels of radioactivity are detected in serum of oestrogen-treated rats. In control rats cholesterol is transported from endothelial cells to parenchymal cells (reverse cholesterol transport), where it is converted into bile acids and secreted into bile. The data thus provide evidence that HDL can serve as acceptors for cholesterol from endothelial cells in vivo, whereby efficient delivery to the parenchymal cells and bile is assured. In oestrogen-treated rats the radioactivity from the endothelial cells is released with similar kinetics as in control rats. However, only a small percentage of radioactivity is found in the HDL fraction and an increased uptake of radioactivity in Kupffer cells is observed. The secretion of radioactivity into bile is greatly delayed in oestrogen-treated rats. It is concluded that, in the absence of extracellular lipoproteins, endothelial cells can still release cholesterol, although for efficient transport to liver parenchymal cells and bile, HDL is indispensable.

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Studies in vivo and in vitro on the uptake and degradation of soluble collagen α1(I) chains in rat liver endothelial and Kupffer cells

Biochemical Journal ◽

10.1042/bj2280415 ◽

1985 ◽

Vol 228 (2) ◽

pp. 415-424 ◽

Cited By ~ 51

Author(s):

B Smedsrød ◽

S Johansson ◽

H Pertoft

Keyword(s):

Endothelial Cells ◽

Endothelial Cell ◽

Kupffer Cell ◽

Kupffer Cells ◽

Cultured Cells ◽

Chondroitin Sulphate ◽

Liver Cells ◽

Type I ◽

Liver Endothelial Cell ◽

Liver Endothelial Cells

Intravenously administered 125I-labelled monomeric alpha 1 chains (125I-alpha 1) of collagen type I were rapidly cleared and degraded by the liver of rats. Isolation of the liver cells after injection of the label revealed that the uptake per liver endothelial cell equalled the uptake per Kupffer cell, whereas the amount taken up per hepatocyte was negligible. The uptake of 125I-alpha 1 in cultured cells was 10 times higher per liver endothelial cell than per Kupffer cell. The ligand was efficiently degraded by cultures of both cell types. However, spent medium from cultures of Kupffer cells, unlike that from cultures of other cells, contained gelatinolytic activity which degraded 125I-alpha 1. The presence of hyaluronic acid, chondroitin sulphate or mannose/N-acetylglucosamine-terminal glycoproteins, which are endocytosed by the liver endothelial cells via specific receptors, did not interfere with binding, uptake or degradation of 125I-alpha 1 by these cells. Unlabelled alpha 1 and heat-denatured collagen inhibited the binding to a much greater extent than did native collagen. The presence of fibronectin or F(ab')2 fragments of anti-fibronectin antibodies did not affect the interaction of the liver endothelial cells, or of other types of liver cells, with 125I-alpha 1. The accumulation of fluorescein-labelled heat-denatured collagen in vesicles of cultured liver endothelial cells is evidence that the protein is internalized. Moreover, chloroquine, 5-dimethylaminonaphthalene-1-sulphonylcadaverine (dansylcadaverine), monensin and cytochalasin B, which impede one or more steps of the endocytic process, inhibited the uptake of 125I-alpha 1 by the liver endothelial cells. Leupeptin, an inhibitor of cathepsin B and ‘collagenolytic cathepsins’, inhibited the intralysosomal degradation of 125I-alpha 1, but had no effect on the rate of uptake of the ligand. The current data are interpreted as follows. (1) The ability of the liver endothelial cells and the Kupffer cells to sequester circulating 125I-alpha 1 efficiently may indicate a physiological pathway for the breakdown of connective-tissue collagen. (2) The liver endothelial cells express receptors that specifically recognize and mediate the endocytosis of collagen alpha 1(I) monomers. (3) The receptors also recognize denatured collagen (gelatin). (4) Fibronectin is not involved in the binding of alpha 1 to the receptors. (5) Degradation occurs intralysosomally by leupeptin-inhibitable cathepsins.

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Quantitative role of parenchymal and non-parenchymal liver cells in the uptake of [14C]sucrose-labelled low-density lipoprotein in vivo

Biochemical Journal ◽

10.1042/bj2240021 ◽

1984 ◽

Vol 224 (1) ◽

pp. 21-27 ◽

Cited By ~ 33

Author(s):

L Harkes ◽

J C Van Berkel

Keyword(s):

Low Density Lipoprotein ◽

Density Lipoprotein ◽

Cell Types ◽

Liver Cells ◽

Low Density ◽

Apo B ◽

Parenchymal Liver ◽

Ldl Uptake ◽

Parenchymal Cells

In order to assess the relative importance of the receptor for low-density lipoprotein (LDL) (apo-B,E receptor) in the various liver cell types for the catabolism of lipoproteins in vivo, human LDL was labelled with [14C]sucrose. Up to 4.5h after intravenous injection, [14C]sucrose becomes associated with liver almost linearly with time. During this time the liver is responsible for 70-80% of the removal of LDL from blood. A comparison of the uptake of [14C]sucrose-labelled LDL and reductive-methylated [14C]sucrose-labelled LDL ([14C]sucrose-labelled Me-LDL) by the liver shows that methylation leads to a 65% decrease of the LDL uptake. This indicated that 65% of the LDL uptake by liver is mediated by a specific apo-B,E receptor. Parenchymal and non-parenchymal liver cells were isolated at various times after intravenous injection of [14C]sucrose-labelled LDL and [14C]sucrose-labelled Me-LDL. Non-parenchymal liver cells accumulate at least 60 times as much [14C]sucrose-labelled LDL than do parenchymal cells accumulate at least 60 times as much [14C]sucrose-labelled LDL than do parenchymal cells when expressed per mg of cell protein. This factor is independent of the time after injection of LDL. Taking into account the relative protein contribution of the various liver cell types to the total liver, it can be calculated that non-parenchymal cells are responsible for 71% of the total liver uptake of [14C]sucrose-labelled LDL. A comparison of the cellular uptake of [14C]sucrose-labelled LDL and [14C]sucrose-labelled Me-LDL after 4.5h circulation indicates that 79% of the uptake of LDL by non-parenchymal cells is receptor-dependent. With parenchymal cells no significant difference in uptake between [14C]sucrose-labelled LDL and [14C]sucrose-labelled Me-LDL was found. A further separation of the nonparenchymal cells into Kupffer and endothelial cells by centrifugal elutriation shows that within the non-parenchymal-cell preparation solely the Kupffer cells are responsible for the receptor-dependent uptake of LDL. It is concluded that in rats the Kupffer cell is the main cell type responsible for the receptor-dependent catabolism of lipoproteins containing only apolipoprotein B.

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New Scavenger Receptor-Like Receptors for the Binding of Lipopolysaccharide to Liver Endothelial and Kupffer Cells

Infection and Immunity ◽

10.1128/iai.66.11.5107-5112.1998 ◽

1998 ◽

Vol 66 (11) ◽

pp. 5107-5112 ◽

Cited By ~ 41

Author(s):

Marijke van Oosten ◽

Erika van de Bilt ◽

Theo J. C. van Berkel ◽

Johan Kuiper

Keyword(s):

Endothelial Cells ◽

Kupffer Cells ◽

Binding Sites ◽

Oxidized Ldl ◽

Scavenger Receptor ◽

Scavenger Receptors ◽

Liver Endothelial Cells ◽

Parenchymal Cells

ABSTRACT Lipopolysaccharide (LPS) is cleared from the blood mainly by the liver. The Kupffer cells are primarily responsible for this clearance; liver endothelial and parenchymal cells contribute to a lesser extent. Although several binding sites have been described, only CD14 is known to be involved in LPS signalling. Among the other LPS binding sites that have been identified are scavenger receptors. Scavenger receptor class A (SR-A) types I and II are expressed in the liver on endothelial cells and Kupffer cells, and a 95-kDa receptor, identified as macrosialin, is expressed on Kupffer cells. In this study, we examined the role of scavenger receptors in the binding of LPS by the liver in vivo and in vitro. Fucoidin, a scavenger receptor ligand, significantly reduced the clearance of 125I-LPS from the serum and decreased the liver uptake of 125I-LPS about 40%. Within the liver, the in vivo binding of 125I-LPS to Kupffer and liver endothelial cells was decreased 72 and 71%, respectively, while the binding of 125I-LPS to liver parenchymal cells increased 34% upon fucoidin preinjection. Poly(I) inhibited the binding of 125I-LPS to Kupffer and endothelial cells in vitro 73 and 78%, respectively, while poly(A) had no effect. LPS inhibited the binding of acetylated low-density lipoprotein (acLDL) to Kupffer and liver endothelial cells 40 and 55%, respectively, and the binding of oxidized LDL (oxLDL) to Kupffer and liver endothelial cells 65 and 61%, respectively. oxLDL and acLDL did not significantly inhibit the binding of LPS to these cells. We conclude that on both endothelial cells and Kupffer cells, LPS binds mainly to scavenger receptors, but SR-A and macrosialin contribute to a limited extent to the binding of LPS.

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Uptake and degradation of human low-density lipoprotein by human liver parenchymal and Kupffer cells in culture

Biochemical Journal ◽

10.1042/bj2760135 ◽

1991 ◽

Vol 276 (1) ◽

pp. 135-140 ◽

Cited By ~ 14

Author(s):

J A A M Kamps ◽

J K Kruijt ◽

J Kuiper ◽

T J C Van Berkel

Keyword(s):

Kupffer Cells ◽

Human Liver ◽

Low Density Lipoprotein ◽

Ldl Receptor ◽

Density Lipoprotein ◽

Cell Types ◽

Liver Cells ◽

Cell Protein ◽

Parenchymal Liver ◽

Parenchymal Cells

The association with and degradation by cultured human parenchymal liver cells and human Kupffer cells of human low-density lipoprotein (LDL) was investigated in order to define, for the human situation, the relative abilities of the various liver cell types to interact with LDL. With both human parenchymal liver cells and Kupffer cells the association of LDL with the cells followed saturation kinetics which were coupled to LDL degradation. The association of LDL (per mg of cell protein) to both cell types was comparable, but the association with human Kupffer cells was much more efficiently coupled to degradation than was the case in parenchymal cells. The capacity of human Kupffer cells to degrade LDL was consequently 18-fold higher (per mg of cell protein) than that of the human parenchymal liver cells. Competition studies showed that unlabelled LDL competed efficiently with the cell association and degradation of 125I-labelled LDL with both parenchymal and Kupffer cells, while unlabelled acetyl-LDL was ineffective. The degradation of LDL by parenchymal and Kupffer cells was blocked by chloroquine and NH4Cl, indicating that it occurs in the lysosomes. Binding and degradation of LDL by human liver parenchymal cells and human Kupffer cells appeared to be completely calcium-dependent. It is concluded that the association and degradation of LDL by human Kupffer and parenchymal liver cells proceeds through the specific LDL receptor, whereas the association of LDL to Kupffer cells is more efficiently coupled to degradation. The presence of the highly active LDL receptor on human Kupffer cells might contribute significantly to LDL catabolism by human liver, especially under conditions whereby the LDL receptor on parenchymal cells is down-regulated.

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Disposition of the Acyclic Nucleoside Phosphonate (S)-9(3-Hydroxy-2-Phosphonylmethoxypropyl)Adenine

Antimicrobial Agents and Chemotherapy ◽

10.1128/aac.42.5.1146 ◽

1998 ◽

Vol 42 (5) ◽

pp. 1146-1150 ◽

Cited By ~ 4

Author(s):

Martin K. Bijsterbosch ◽

Louis J. J. W. Smeijsters ◽

Theo J. C. van Berkel

Keyword(s):

Cell Types ◽

Hepatic Uptake ◽

Liver Cells ◽

The Body ◽

Total Uptake ◽

Parenchymal Liver ◽

Acyclic Nucleoside ◽

Parenchymal Cells ◽

Acyclic Nucleoside Phosphonate

ABSTRACT The acyclic nucleoside phosphonate (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine [(S)-HPMPA] has been shown to be active against pathogens, like hepatitis B viruses and Plasmodiumparasites, that infect parenchymal liver cells. (S)-HPMPA is therefore an interesting candidate drug for the treatment of these infections. To establish effective therapeutic protocols for (S)-HPMPA, it is essential that the kinetics of its hepatic uptake be evaluated and that the role of the various liver cell types be examined. In the present study, we investigated the disposition of (S)-HPMPA and assessed its hepatic uptake. Rats were intravenously injected with [3H](S)-HPMPA, and after an initial rapid distribution phase (360 ± 53 ml/kg of body weight), the radioactivity was cleared from the circulation with a half-life of 11.7 ± 1.4 min. The tissue distribution of [3H](S)-HPMPA was determined at 90 min after injection (when >99% of the dose cleared). Most (57.0% ± 1.1%) of the injected [3H](S)-HPMPA was excreted unchanged in the urine. The radioactivity that was retained in the body was almost completely recovered in the kidneys and the liver (68.4% ± 2.5% and 16.1% ± 0.4% of the radioactivity in the body, respectively). The uptake of [3H](S)-HPMPA by the liver occurred mainly by parenchymal cells (92.1% ± 3.4% of total uptake by the liver). Kupffer cells and endothelial cells accounted for only 6.1% ± 3.5% and 1.8% ± 0.8% of the total uptake by the liver, respectively. Preinjection with probenecid reduced the hepatic and renal uptake of [3H](S)-HPMPA by approximately 75%, which points to a major role of a probenecid-sensitive transporter in the uptake of (S)-HPMPA by both tissues. In conclusion, we show that inside the liver, (S)-HPMPA is mainly taken up by parenchymal liver cells. However, the level of uptake by the kidneys is much higher, which leads to nephrotoxicity. An approach in which (S)-HPMPA is coupled to carriers that are specifically taken up by parenchymal cells may increase the effectiveness of the drug in the liver and reduce its renal toxicity.

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Role of the scavenger receptor in the uptake of methylamine-activated α2-macroglobulin by rat liver

Biochemical Journal ◽

10.1042/bj2870447 ◽

1992 ◽

Vol 287 (2) ◽

pp. 447-455 ◽

Cited By ~ 12

Author(s):

M C M van Dijk ◽

W Boers ◽

C Linthorst ◽

T J C van Berkel

Keyword(s):

Endothelial Cells ◽

Rat Liver ◽

Kupffer Cells ◽

Proteolytic Enzymes ◽

Scavenger Receptor ◽

Liver Cells ◽

Cell Protein ◽

Liver Uptake ◽

Inosinic Acid ◽

Parenchymal Cells

Alpha 2-Macroglobulin (alpha 2M) requires activation by small nucleophiles (e.g. methylamine; giving alpha 2M-Me) or proteolytic enzymes (e.g. trypsin; giving alpha 2M-Tr) in order to be rapidly removed from the circulation by the liver. Separation of rat liver cells into parenchymal, endothelial and Kupffer cells at 10 min after injection indicates that liver uptake of alpha 2M-Me is shared between parenchymal and endothelial cells, with relative contributions of 51.3% and 48.3% respectively of total liver-associated radioactivity. In contrast, alpha 2M-Tr is almost exclusively taken up by the parenchymal cells (90.1% of liver-associated radioactivity). A preinjection of 5 mg of poly(inosinic acid) decreased liver uptake of alpha 2M-Me to 39.9% of the control value, while it had no effect on liver uptake of alpha 2M-Tr. It appears that poly(inosinic acid) specifically reduces the uptake of alpha 2M-Me in vivo by endothelial cells, leaving uptake by parenchymal cells unaffected. In vitro studies with isolated liver cells indicate that the association of alpha 2M-Me with endothelial cells is 21-fold higher per mg of cell protein than with parenchymal cells. The capacity of endothelial cells to degrade alpha 2M-Me appears to be 46 times higher than that of parenchymal cells. Competition studies show that poly(inosinic acid) or acetylated low-density lipoprotein effectively competes with the association of alpha 2M-Me with endothelial and Kupffer cells, but association with parenchymal cells is unaffected. It is suggested that activation of alpha 2M by methylamine induces a charge distribution on the protein which triggers specific uptake by the scavenger receptor on endothelial cells. It is concluded that the uptake of alpha 2M-Me by the scavenger receptor might function as an additional system for the uptake of activated alpha 2M.

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