Biomedical subjects

T Berg

Publications and source records attributed to T Berg.

At least 181 records · Page 10Linked to original sources

PubMed1994

[Long-term follow-up of chronic hepatitis C after treatment with recombinant interferon alpha-2a].

As part of a multicenter randomized study 40 patients with chronic hepatitis C (HCV)-infection, 28 kryptogenic and 12 posttransfusional, were treated with recombinant interferon alfa (IFN alpha-2a) for 1 year in a dosage of 3 x 3 Mio. units per week versus dosis escalation after 8 and 16 weeks in serological non-responders. 36 of the 40 patients were followed over 3 years. The rate of patients with normalization of aminotransferases was 42% after two months of therapy, 28% at the end of treatment, 28% after 1 year and 23% after 3 years of follow-up. The polymerase chain reaction (PCR) for detection of HCV-RNA became negative after two months of treatment in 73%, at the end of therapy in 63%, after 1 year follow-up in 63% and after 3 years in 35%. All patients with persisting remission maintained HCV-RNA negative. Dosis escalation was realized in 8 patients without increase of responder rate. Antibodies against IFN alpha-2a developed in 4 (10%) patients without remarkable influence on the IFN-effect. Histological improvement at the end of treatment was observed in 61% including all patients with serological remission. The data support the prognostic relevance of the course of aminotransferases. If aminotransferases are not normalized during the first two months the treatment can be terminated. Persisting normalization of aminotransferases during 1 year after therapy and negative HCV-PCR result indicate maintaining remission.

Adult↗

PubMed1994

Complications of limb lengthening. A learning curve.

Major complication rates during limb lengthening were plotted in a consecutive series to produce a learning curve. All unwanted events during and after treatment were considered complications, and graded as minor, serious, and severe. All serious and severe complications were considered major. A novel system was used to classify the preoperative severity of each deformity. One-hundred ten patients had 140 bone segments lengthened between 2.2 cm and 10.5 cm, with a mean of 4.4 cm. Three methods were used in lengthening: the Wagner method in 22 patients, the DeBastiani method in 34 patients, and the Ilizarov method in 84 patients. Ninety-eight complications categorized as serious or severe occurred, for a total major complication rate of 72%. The percentage of major complications began to drop after 30 lengthenings to a current rate of 25%. Major complications were frequent in patients with more severe deformities, particularly in those whose cases occurred early in the series. Bone healing complications were high (72%) in the Wagner segments but were also high (80%) in the first ten patients treated with the DeBastiani technique. The first ten Ilizarov patients, who were treated later in the series, had a 40% rate of bone-healing complications. The current rate of major complications is 13% for those patients treated with DeBastiani's method and 33% for those patients treated with Ilizarov's method. This difference in complication rates appears to relate to the severity of the deformity, rather than the device used. There was a significant decrease in complications as experience was gained. Directed formal study and surgical instruction should help diminish these complications.

Adolescent↗

PubMed1993

Endocytosed LDL and beta-VLDL follow different intracellular pathways in rat liver.

The intracellular transport of [125I]tyramine cellobiose low-density lipoprotein ([125ITC]LDL) and [131ITC]beta-very-low-density lipoprotein ([131ITC]beta-VLDL) in rat liver was studied by means of centrifugation in sucrose and Nycodenz gradients. At time-points up to 45 min after intravenous injection, the two ligands were found in endosomes with distinctly different buoyant densities. In the Nycodenz gradients [131ITC]beta-VLDL appeared at 1.08 g/ml partly coinciding with the distribution of the cation independent (alpha)mannose-6-phosphate receptor, whereas [125ITC]LDL was found at 1.13 mg/ml, where the degradation of [125ITC]LDL started. [131ITC]beta-VLDL, on the other hand, was transferred to denser vesicles, banding at 1.16 g/ml, and degradation started in these organelles, similar to that observed with asialoorosomucoid (ASOR) that was used as a control ligand. Since degradation products coincided with beta-N-acetylglucosaminidase we assume that these organelles are secondary lysosomes. [125ITC]LDL was subsequently also transferred to these dense secondary lysosomes, and the distribution of degraded [125ITC]LDL was therefore bimodal until [125ITC]LDL was completely cleared from the circulation. Furthermore our results show that the different intracellular pathways observed are not due to uptake in different liver cell types, since the bimodal distribution of [125ITC]LDL was also evident in purified liver parenchymal cells. The data suggest that LDL and beta-VLDL follow different endosomal pathways in the rat hepatocytes and that both pathways meet in a common final lysosome. The data also support the notion that LDL and beta-VLDL are taken up through different endocytic receptors. However, following estradiol treatment, both ligands seem to follow a common pathway. In this case the density distributions of the two ligands coincide and resemble the pathway of LDL observed in control animals. This may be due to a pronounced up-regulation of LDL receptors following estradiol treatment, and beta-VLDL may under these conditions be taken up via the LDL receptor.

Animals↗

PubMed1993

Hepatic uptake and intracellular processing of LDL in rainbow trout.

The process of receptor-mediated endocytosis in poikilothermic vertebrates such as salmonid fish have not been subjected to much research, compared to the detailed studies done in mammalian systems. We have investigated the hepatic uptake and intracellular processing of low-density lipoprotein (LDL) in rainbow trout liver. After intravenous injection of the [125I]tyramine-cellobiose ([125I]TC) -labelled lipoprotein, the liver was perfused and cells isolated or fractionated by differential centrifugation and isopycnic centrifugation in Nycodenz gradients. We found that LDL was mainly endocytosed by parenchymal cells of the liver. Cell fractionation experiments showed that LDL was localized sequentially in three groups of organelles of increasing density. Initially, LDL was localized in small, slowly sedimenting endosomes before being transferred to denser endosomes (prelysosomes) and finally to dense lysosomes. The lysosomes were identified by three lysosomal marker enzymes. Degradation products formed from [125I]TC-labelled LDL could also be detected in prelysosomal vesicles. In vitro experiments with cultured trout hepatocytes demonstrated that intracellular processing of [125I]TC-LDL in these cells could be suppressed by endocytic and lysosomal inhibitors. The catabolism of LDL in rainbow trout therefore follows the endocytic-lysosomal pathway described for many macromolecules in mammalian cells.

Animals↗

PubMed1993

Endocytosis of ricin by rat liver cells in vivo and in vitro is mainly mediated by mannose receptors on sinusoidal endothelial cells.

Upon intravenous injection into rats, the plant toxin ricin was rapidly cleared from the circulation by the liver. Among the different liver cell populations, most of the injected ricin associated with the sinusoidal endothelial cells (EC), whereas the liver parenchymal cells (PC) and Kupffer cells (KC) yielded minor contributions to the total liver uptake in vivo. Co-injection of mannan strongly inhibited ricin uptake by the EC, showing that it was mediated by mannose receptors. On the other hand, co-injection of lactose, which inhibits the galactose-specific association of ricin with cells, enhanced ricin uptake by the EC. The carbohydrate-dependency of the EC contribution to the uptake of ricin in vivo was reflected in the carbohydrate-dependency of the uptake in vivo by whole liver. In vitro, the EC also endocytosed ricin more efficiently than did the PC or KC. Whereas uptake in vitro in the EC was mainly mannose-specific, uptake in the two other cell types was mainly galactose-specific. Western blotting showed that the mannose receptors of liver non-parenchymal cells are identical with the mannose receptor previously isolated from alveolar macrophages. The mannose receptors are expressed at a higher level in EC than in KC. Ligand blotting showed that, in the presence of lactose, the mannose receptor is the only protein in the EC that binds ricin, and the binding is mannose-specific and Ca(2+)-dependent.

Animals↗

PubMed1993

Transfer of retinol-binding protein from HepG2 human hepatoma cells to cocultured rat stellate cells.

Rat liver stellate cells were cocultured with HepG2 human hepatoma cells, which are known to synthesize and secrete retinol-binding protein (RBP). Transfer of human RBP from HepG2 cells to stellate cells was studied by cryoimmunoelectron microscopy. In stellate cells, human RBP was found on the cell surface and within endosomes. The transfer of human RBP from HepG2 cells to stellate cells was blocked by addition of RBP antibodies to the culture medium. Very little uptake of RBP was observed when fibroblasts were cocultured with HepG2 cells. In a series of experiments, RBP was bound to its putative cell surface receptor at 4 degrees C, and the stellate cells were washed and then incubated at 37 degrees C in order to allow them to internalize a pulse of RBP. About 50% of the RBP was internalized after 6 min of incubation. The RBP-positive vesicles were initially (after 1-2 min) located close to the cell surface and later were found deeper in the cytoplasm. During the first 10 min, RBP was mainly observed in close association with membranes. After 2 hr, however, most RBP was localized in intracellular vesicles at a distance from the vesicular membranes, suggesting that RBP had been released from its receptor. Saturable binding of RBP to liver cells was demonstrated when cells were incubated with 125I-RBP at 4 degrees C and cell-associated radioactivity was determined. The calculated dissociation constant for the specific binding was 12.7 +/- 3.2 nM. A binding assay was also developed for determination of solubilized RBP receptor. Solubilized proteins from the nonparenchymal liver cells bound about 30 times more 125I-labeled RBP than did parenchymal cells (based on mass of cell protein). These data suggest that RBP mediates the paracrine transfer of retinol from hepatocytes to perisinusoidal stellate cells in liver and that stellate cells bind and internalize RBP by receptor-mediated endocytosis.

Animals↗

PubMed1993

Receptor-mediated endocytosis of retinol-binding protein by liver parenchymal cells: interference by radioactive iodination.

Retinol-binding protein (RBP) was iodinated directly by radio-iodine substitution on the tyrosyl residues by the sodium hypochlorite (NaOCl) or the Enzymobead (EB) methods, or indirectly by linkage of 125I-tyramine-cellobiose (TC) or 125I-N-succinimidyl-3-(4- hydroxyphenyl)propionic acid ester (SHPP) adduct on to free amino residues of RBP. Binding, uptake and degradation of iodinated RBP were studied in isolated rat and rabbit liver parenchymal cells. The amount of ligand bound to cells at 4 degrees C was dependent on the type of labelling, in that the 125I-TC ligand was bound to a lesser extent than NaClO-labelled 125I-RBP, EB-labelled 125I-RBP and 125I-SHPP-RBP. At 37 degrees C, the 125I-SHPP-RBP and the EB-labelled 125I-RBP became cell-associated more rapidly than the other two ligands. The higher cell association at 37 degrees C than at 4 degrees C suggests that internalization of the ligand occurred at the higher temperature. The degradation of the ligands was also different. The EB-labelled 125I-RBP, the 125I-TC-RBP and the 125I-SHPP-RBP showed an apparent lag phase before a steady increase in acid-soluble radioactivity was observed. Much less of EB-labelled 125I-RBP and 125I-TC-RBP were degraded (about 6%) than of the other two ligands (about 16%) after 120 min. About 50% of the acid-soluble radioactivity in these experiments could be accounted for by degradation in the medium, suggesting that about half of the degradation observed was intracellular. The present study therefore shows that the different labelling techniques yield varying estimates of the cellular handling of RBP. In addition, a rapid release of RBP was observed in experiments where cells were pulsed with radioactive RBP at 4 degrees C, washed and incubated further at 37 degrees C. Between 50% and 70% was released after 5 min of incubation. By increasing the temperature during the pulse to 37 degrees C, or by lowering the temperature during the chase to 4 degrees C, much less RBP was released from the cells. These data suggest that the release process represents recycling of internalized ligand from an early endosome.

Animals↗

PubMed1993

Characterization of two distinct pathways of endocytosis of ricin by rat liver endothelial cells.

We have studied the characteristics of internalization and intracellular transport of ricin via two distinct pathways in rat liver endothelial cells (EC), i.e., via binding to mannose receptors and surface galactosyl-residues, respectively. Treatments that inhibit endocytosis from coated pits, i.e., hyperosmolarity and acidification of the cytoplasm, decreased uptake via mannose receptors much more than uptake via galactosyl-residues, indicating that mannose receptors are largely internalized from coated pits, whereas internalization via galactosyl-residues is to a significant extent independent of coated pits. Uptake of ricin via mannose receptors was strongly inhibited by NH4Cl and monensin, and accordingly, NH4Cl protected the cells against ricin intoxication via mannose receptors. On the other hand, uptake via galactosyl-residues was not significantly inhibited by NH4Cl or monensin, and NH4Cl even sensitized the cells to intoxication via this pathway. Brefeldin A, which did not affect ricin uptake, protected the cells against ricin intoxication via either pathway. Protein synthesis in the EC was efficiently inhibited by ricin, even after very short periods of uptake at low ricin concentrations. The onset of protein synthesis inhibition was more rapid upon internalization of ricin via mannose receptors than via galactosyl-residues. Also, ricin internalized via mannose receptors was more efficiently transported from endosomes to lysosomes than ricin internalized via galactosyl-residues. Partial blocking of the galactosyl-binding sites of ricin caused a reduction in the extent of recycling of ricin from endosomes to the cell surface (retroendocytosis), indicating that binding of ricin to membrane galactosyl-residues, which is relatively stable at the slightly acidic pH of endosomes, is an important determinant of the intracellular handling of ricin. We suggest that the observed difference in the transport from endosomes to lysosomes between the two internalization pathways is related to the different stability of the two binding mechanisms at endosomal pH.

Ammonium Chloride↗

PubMed1993

Maxillary distal-extension removable partial denture abutments with reduced periodontal support.

An in vitro study that used photoelastic models compared stress distribution characteristics of three maxillary, bilateral, distal-extension removable partial denture designs when the abutments were subjected to a progressive loss of periodontal support. One design used I-bar retention, a second design used a semiprecision, spring-loaded plunger attachment, and a third design used the ERA attachment. Both attachment designs were tested with and without splinted abutments. The ERA design was also tested with and without supporting rests and included light and heavy retention elements. Periodontal support loss up to 35%, a 60/40 crown-to-root ratio, resulted in increased stress concentrations. The ERA design with supporting rests, light retention elements, and splinted abutments compared very favorably with the I-bar retained design on nonsplinted abutments.

Alveolar Bone Loss↗

PubMed1993

Vitamin A metabolism in rat liver: a kinetic model.

Vitamin A metabolism in the liver involves both hepatocytes and the nonparenchymal perisinusoidal stellate cells. To describe and quantitate the dynamic relationships between retinol in these cells and in plasma, we administered either chylomicrons labeled with [3H]retinyl esters or plasma containing [3H]retinol-retinol-binding protein-transthyretin to rats. Radioactivity and retinol masses were measured in plasma, liver, and isolated hepatocytes for 15 days; data were analyzed by model-based compartmental analysis. The resulting model predicts that: 1) approximately 20% of the total plasma turnover of retinol goes to the liver (vs. nonhepatic tissues) and approximately 20% of plasma retinol input is from liver (vs. nonhepatic tissues), 2) about one-half of the retinol recycling from plasma to liver is taken up by hepatocytes and about one-half by nonparenchymal cells, 3) retinyl esters in both cell types are derived preferentially from newly taken up retinol rather than from the main intracellular retinol pools, and 4) at least one-half of the retinol secreted by hepatocytes of rats consuming low levels of vitamin A is directly transferred to nonparenchymal cells. In addition, the data are compatible with the hypothesis that retinol-binding protein is the vehicle for transfer of retinol from hepatocytes to nonparenchymal stellate cells and between plasma and liver cells.

Animals↗

PubMed1993

Hepatic uptake of beta-VLDL in cholesterol-fed rabbits.

The hepatic uptake of intravenously injected beta-very low density lipoprotein (beta-VLDL) in rabbits fed 2% (w/w) cholesterol for 3 weeks was investigated. In vitro studies were also conducted to examine the specificity and the capacity of the uptake in isolated liver parenchymal cells. The hepatic uptake of beta-VLDL was 15.8 +/- 6.7% (n = 6) in the cholesterol-fed rabbits as compared to 26.6 +/- 7.5% (n = 6) of the injected dose in control rabbits (P < 0.05). Although this is a fractional reduction, it represents a more than 10-fold increase in absolute hepatic uptake of lipoproteins in the cholesterol-fed rabbits. In these animals the liver parenchymal, endothelial, and Kupffer cells took up 10.2 +/- 2.7%, 3.0 +/- 0.9%, and 1.8 +/- 0.4% of the injected dose, respectively, compared to 25.9 +/- 6.1%, 3.6 +/- 1.6%, and 1.5 +/- 0.8% of the injected dose in chow-fed controls. However, taking into account the high plasma lipoprotein levels in the cholesterol-fed rabbits, the absolute cellular uptake was 10-fold increased in the parenchymal liver cells and more than 20-fold increased in the nonparenchymal cells. In vitro results indicated a 40% down-regulation of the specific receptor for beta-VLDL in the parenchymal cells, and this, together with an increased competition for binding sites in the hypercholesterolemic rabbits, probably explains the reduced uptake of beta-VLDL in terms of % of injected dose observed in vivo. In vitro data suggested that the receptor involved in both hypercholesterolemic and normolipemic rabbits was the apolipoprotein (apo) B,E receptor. On a per cell basis, parenchymal cells from chow-fed control animals took up 2.4 +/- 0.8% of the injected dose per 10(9) cells; this uptake was reduced to 1.1 +/- 0.5% in hypercholesterolemic animals. No differences in uptake of beta-VLDL in nonparenchymal liver cells were observed on a per cell basis between the two feeding groups, indicating that binding sites involved in this uptake are not down-regulated by cholesterol feeding. On the contrary, the absolute uptake in the nonparenchymal liver cells is greatly increased in hypercholesterolemic rabbits as compared to controls. In cholesterol-fed rabbits the three different liver cell types took up approximately the same amount of beta-VLDL per cell. The liver nonparenchymal cells, therefore, assume a prominent role in uptake of beta-VLDL in hypercholesterolemic rabbits, accounting for more than 30% of the total hepatic uptake as compared to 16% in control animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Receptor-mediated endocytosis of ricin in rat liver endothelial cells. An immunocytochemical study.

The endocytic pathway of ricin in sinusoidal liver endothelial cells (EC) was traced by means of immunocytochemical labeling of ultrathin cryosections. Ricin, a highly mannosylated glycoprotein, is internalized mainly by receptor-mediated endocytosis via the mannose receptor in the EC. Labeling of specimens fixed at different time points after injection of ligand showed that several subcellular compartments are involved in processing of endocytosed ricin. One minute after injection ricin is seen in coated pits, coated vesicles and cisternal-shaped endosomes. After 6 min, the ligand associates with electron-dense, spherical vesicles and electron-lucent vesicles, presumably representing late endosomes. In the same time period we observed labeling in the vicinity of the Golgi stack. At later time points, ricin is increasingly localized in lysosomes. Both late endosomes and lysosomes showed labeling for Igp120, the lysosomal membrane glycoprotein. To compare uptake of ricin with another mannosylated ligand, we coinjected ricin and mannosylated colloidal gold particles (Man-Aun). Man-Au20, injected 24 h before fixation as a marker for late endocytic compartments, was found in two distinct compartments, presumably representing late endosomes and lysosomes. The distribution of ricin and Man-Au10, the latter injected 15 min before fixation, in early endosomes was strikingly different, indicating that the structure of this compartment is important in the process of sorting of ligand and receptor.

Animals↗

PubMed1992

Uptake, intracellular transport, and degradation of polyethylene glycol-modified asialofetuin in hepatocytes.

Polyethylene glycol (PEG) is attached to proteins in order to increase their half-life in the circulation and reduce their immunogenicity in vivo. For many applications involving "targeting" molecules, it is important to know how PEG modification of the molecule affects its interaction with a receptor and the subsequent internalization, intracellular transport, and lysosomal degradation. As a model system, we used asialofetuin, which binds to the galactose receptor of hepatocytes, because removal of sialic acid exposes galactose residues. We modified asialofetuin by attaching various amounts of PEG of molecular weight 1900 or 5000. The preparations were labeled with 125I so that endocytosis and degradation could be followed in suspended hepatocytes. Depending on the number of PEG molecules attached, receptor-mediated uptake was affected to varying degrees. If two-thirds of the exposed amino groups of the asialofetuin molecule were modified, the rate of uptake decreased to less than one-fourth of controls; degradation of endocytosed molecules was 12% of controls. The reduction in endocytic uptake was due to a reduced rate of formation of the receptor-ligand complex. Subcellular frationation in density gradients showed that PEG-modified asialofetuin is transported intracellularly and degraded in the same manner as the native protein, but the rate of proteolysis is reduced. This observation explains the paradoxical result of experiments with injection of modified asialofetuin into rats in vivo: even though the clearance of one preparation of PEG-asialofetuin was much slower than that of the native protein, accumulation of radioactivity in the liver from the modified protein was twice as high. The hepatocytes accounted for 85% of the hepatic accumulation of either PEG-modified or native asialofetuin in vivo.

Animals↗

PubMed1992

Characterization of retroendocytosis in rat liver parenchymal cells and sinusoidal endothelial cells.

After receptor-mediated endocytosis, internalized ligands may be recycled to the cell surface instead of being routed to lysosomes for degradation, a process termed retroendocytosis. We have investigated the kinetics and extent of retroendocytosis of neoglycoproteins after internalization via two carbohydrate-specific receptors in rat liver cells: galactose receptors in parenchymal cells (PC) and mannose receptors in sinusoidal endothelial cells (EC). Retroendocytosis in both cell types occurred with first-order kinetics, and the rate of recycling of internalized ligands was about 4 times higher in EC than in PC. As the length of the internalization pulse was increased, the extent of subsequent retroendocytosis decreased, indicating that retroendocytosis takes place from a relatively early stage in the endocytic pathway. Furthermore, as the degree of carbohydrate substitution of the neoglycoprotein ligands increased, the affinities of the receptors for the ligands and the extent of ligand retroendocytosis increased. In the EC, the relationship between degree of substitution and extent of retroendocytosis was not immediately apparent, as some of the neoglycoprotein ligands used may also bind to and be internalized by scavenger receptors on the EC, causing a decreased apparent retroendocytosis. However, when this interaction was inhibited, this relationship was restored. We conclude that retroendocytosis mainly occurs because of incomplete dissociation of ligands from receptors before receptor recycling to the cell surface and that the affinities of a receptor for its ligand at the cell surface and in the endosomal environment are major factors in determining the extent of retroendocytosis.

Animals↗

PubMed1992

Heterogeneity of carboxylesterases in rat liver cells.

Rat liver cells were separated into parenchymal cells (PC), Kupffer cells (KC) and endothelial cells (EC). The distribution of carboxylesterases (EC 3.1.1.1) between these cell types was investigated by PAGE and chromatogenic substrate staining, and compared with the results for total liver preparation and individual isoenzymes isolated by chromatofocusing. All of the liver carboxylesterase isoenzymes could be detected in the PC, whereas in both KC and EC only those with isoelectric point (pI) 6.4/6.2 could be detected. Use of carboxylesterase inhibitors like bis-(4-nitrophenyl)phosphate and paraoxon, and organophosphorus compound hydrolase inhibitors like 4-hydroxymercuribenzoate and EDTA confirmed that these esterases were of the carboxylesterase type.

Animals↗

PubMed1992

Evidence for carbohydrate-independent endocytosis of tissue-type plasminogen activator by liver cells.

In the liver, tissue-type plasminogen activator (t-PA) is endocytosed by hepatic parenchymal (PC), endothelial (EC) and Kupffer (KC) cells. Although the endocytosis is receptor-mediated, it remains a matter of discussion which receptors are involved in this catabolic process. To evaluate the role of a protein-specific receptor, as well as the possible involvement of the galactose receptor on PC and the mannose receptor on EC, we have employed different glycosylation variants of t-PA in biochemical and immunocytochemical studies. Partial or total removal of carbohydrate side-chains by endoglycosidases did not prevent clearance and hepatic endocytosis of t-PA by either of the liver cell types. Blockade of the galactose and mannose receptors by co-application of a large excess of the glycoprotein ovalbumin remained without effect on the binding and uptake of t-PA by hepatic cells. However, the contribution of different liver cell types to the hepatic clearance of t-PA was to a certain extent dependent on the type of oligosaccharide chains removed. The mannose receptor on EC is partially responsible for the clearance of t-PA by this cell type, whereas the galactose receptor does not seem to be involved in this process. The results obtained in this study further demonstrate that the major portion of the hepatic catabolism of t-PA is independent of its carbohydrate side-chains.

Amidohydrolases↗

PubMed1992

Metabolism of high-density lipoproteins in rainbow trout.

Trout high-density lipoproteins have been labelled with residualizing tracers for the lipid and protein moieties ([3H]cholesteryloleyl ether and 125I-tyramine-cellobiose, respectively). Plasma kinetics and tissue site of catabolism were determined for both tracers. The lipid tracer was cleared about twice as fast from the blood as the protein tracer (half lifes were 63.5 and 125.3 h, respectively). This selective removal of lipid from the lipoprotein was mainly accomplished by the higher liver uptake of the cholesteryl ether. The main catabolic site for HDL protein was kidney tissue. This data established the existence of differential HDL catabolism in a lower vertebrate, in which HDL is the dominant plasma lipoprotein. In addition, the findings confirm the importance of fish kidney as a major site of endocytosis of macromolecules, of both exogenous and endogenous origin.

Animals↗

PubMed1992

Endocytosis and intracellular processing of tissue-type plasminogen activator by rat liver cells in vivo.

Endocytosis of tissue-type plasminogen activator (t-PA) by different types of rat liver cells was studied in immunocytochemically labelled cryosections as well as in biochemical experiments. For morphological localization of the ligand in different endocytic compartments involved in its catabolism, rat livers were fixed at various times (1-24 min) after injection of t-PA. Late-endosomal and lysosomal compartments were identified by double-labelling the sections with antibodies to the lysosomal proteins glycoprotein Igp 120 and cathepsin D. In liver t-PA was localized in sinusoidal endothelial cells (EC), parenchymal cells (PC) and to some extent in Kupffer cells (KC), indicating that it is internalized and degraded in all three cell types. In specimens fixed 6 min after injection PC, EC and KC were found to contribute to 69, 24 and 7% respectively of total t-PA endocytosed. The transfer from late endosomes to lysosomes was found to be faster in EC than in PC. The morphological findings were supported by studies of the endocytic mechanisms employing isolated perfused livers and primary hepatocytes. The presence of monensin, an inhibitor of lysosomal protein degradation, reduced the amount of t-PA degraded to about 50% of the control values. The catalytic site seems not to be required for the catabolism of t-PA in hepatic cells. The inhibition of t-PA by D-phenylalanyl-L-prolylarginyl-chloromethane did not influence receptor recognition and catabolic processing, as determined in morphological studies using labelled cryosections, in binding studies employing liver cell membranes and primary hepatocytes, as well as in liver-perfusion experiments.

Animals↗