Elimination of asialofetuin and asialoorosomucoid by the intact rat. Quantitative aspects of the hepatic clearance mechanism

Human diamine oxidase cellular binding and internalization in vitro and rapid clearance in vivo are not mediated by N-glycans but by heparan sulfate proteoglycan interactions

Human diamine oxidase (hDAO) rapidly inactivates histamine by deamination. No pharmacokinetic data are available to better understand its potential as a new therapeutic modality for diseases with excess local and systemic histamine, like anaphylaxis, urticaria or mastocytosis. After intravenous administration of recombinant hDAO to rats and mice, more than 90% of the dose disappeared from the plasma pool within 10 min. Human DAO did not only bind to various endothelial and epithelial cell lines in vitro, but was also unexpectedly internalized and visible in granule-like structures. The uptake of rhDAO into cells was dependent on neither the asialoglycoprotein-receptor (ASGP-R) nor the mannose receptor (MR) recognizing terminal galactose or mannose residues, respectively. Competition experiments with ASGP-R and MR ligands did not block internalization in vitro or rapid clearance in vivo. The lack of involvement of N-glycans was confirmed by testing various glycosylation mutants. High but not low molecular weight heparin strongly reduced the internalization of rhDAO in HepG2 cells and HUVECs. Human DAO was readily internalized by CHO-K1 cells, but not by the glycosaminoglycan- and heparan sulfate-deficient CHO cell lines pgsA-745 and pgsD-677, respectively. A docked heparin hexasaccharide interacted well with the predicted heparin binding site 568RFKRKLPK575. These results strongly imply that rhDAO clearance in vivo and cellular uptake in vitro is independent of N-glycan interactions with the classical clearance receptors ASGP-R and MR, but is mediated by binding to heparan sulfate proteoglycans followed by internalization via an unknown receptor.

Targeting microtubules sensitizes drug resistant lung cancer cells to lysosomal pathway inhibitors

Oncogene-addicted cancers are predominantly driven by specific oncogenic pathways and display initial exquisite sensitivity to designer therapies, but eventually become refractory to treatments. Clear understanding of lung tumorigenic mechanisms is essential for improved therapies.

Methods: Lysosomes were analyzed in EGFR-WT and mutant cells and corresponding patient samples using immunofluorescence or immunohistochemistry and immunoblotting. Microtubule organization and dynamics were studied using immunofluorescence analyses. Also, we have validated our findings in a transgenic mouse model that contain EGFR-TKI resistant mutations.

Results: We herein describe a novel mechanism that a mutated kinase disrupts the microtubule organization and results in a defective endosomal/lysosomal pathway. This prevents the efficient degradation of phosphorylated proteins that become trapped within the endosomes and continue to signal, therefore amplifying downstream proliferative and survival pathways. Phenotypically, a distinctive subcellular appearance of LAMP1 secondary to microtubule dysfunction in cells expressing EGFR kinase mutants is seen, and this may have potential diagnostic applications for the detection of such mutants. We demonstrate that lysosomal-inhibitors re-sensitize resistant cells to EGFR tyrosine-kinase inhibitors (TKIs). Identifying the endosome-lysosome pathway and microtubule dysfunction as a mechanism of resistance allows to pharmacologically intervene on this pathway.

Conclusions: We find that the combination of microtubule stabilizing agent and lysosome inhibitor could reduce the tumor progression in EGFR TKI resistant mouse models of lung cancer.

The Immune Functions of α1 Acid Glycoprotein

α1-acid glycoprotein (orosomucoid, AGP) is an Acute Phase Protein produced by liver and peripheral tissues in response to systemic reaction to inflammation. AGP functions have been studied mostly in human, cattle and fish, although the protein has been also found in many mammalian species and birds. AGP fulfils at least two set of functions, which are apparently different from each other but in fact intimately linked. On one hand, AGP is an immunomodulatory protein. On the other hand, AGP is one of the most important binding proteins in plasma and, beside modulating pharmacokinetics and pharmacodynamics of many drugs, it is also able to bind and transport several endogen ligands related to inflammation. The focus of this review is the immunomodulatory activity of AGP. This protein regulates every single event related to inflammation, including binding of pathogens and modulating white blood cells activity throughout the entire leukocyte attacking sequence. The regulation of AGP activity is complex: the inflammation induces not only an increase in AGP serum concentration, but also a qualitative change in its carbohydrate moiety, generating a multitude of glycoforms, each of them with different, and sometimes opposite and contradictory, activities. We also present the most recent findings about the relationship between AGP and adipose tissue: AGP interacts with leptin receptor and, given its immunomodulatory function, it may be included among the potential players in the field of immunometabolism.

In vivo clearance of alpha-1 acid glycoprotein is influenced by the extent of its N-linked glycosylation and by its interaction with the vessel wall

Alpha-1 acid glycoprotein (AGP) is a highly glycosylated plasma protein that exerts vasoprotective effects. We hypothesized that AGP's N-linked glycans govern its rate of clearance from the circulation, and followed the disappearance of different forms of radiolabeled human AGP from the plasma of rabbits and mice. Enzymatic deglycosylation of human plasma-derived AGP (pdAGP) by Peptide: N-Glycosidase F yielded a mixture of differentially deglycosylated forms (PNGase-AGP), while the introduction of five Asn to Gln mutations in recombinant Pichia pastoris-derived AGP (rAGP-N(5)Q) eliminated N-linked glycosylation. PNGase-AGP was cleared from the rabbit circulation 9-fold, and rAGP-N(5)Q, 46-fold more rapidly than pdAGP, primarily via a renal route. Pichia pastoris-derived wild-type rAGP differed from pdAGP in expressing mannose-terminated glycans, and, like neuraminidase-treated pdAGP, was more rapidly removed from the rabbit circulation than rAGP-N(5)Q. Systemic hyaluronidase treatment of mice transiently decreased pdAGP clearance. AGP administration to mice reduced vascular binding of hyaluronic acid binding protein in the liver microcirculation and increased its plasma levels. Our results support a critical role of N-linked glycosylation of AGP in regulating its in vivo clearance and an influence of a hyaluronidase-sensitive component of the vessel wall on its transendothelial passage.

Conjugates of nucleoside analogs with lactosaminated human albumin to selectively increase the drug levels in liver blood: requirements for a regional chemotherapy

Nucleoside analogs (NAs) conjugated with galactosyl terminating peptides selectively enter hepatocytes via the asialoglycoprotein receptor and, after intracellular release from the carrier, partly exit from these cells into the bloodstream, resulting in higher concentrations in liver blood than in systemic circulation. Therefore, conjugates of anticancer NAs can be exploited to accomplish a loco-regional noninvasive treatment of liver micrometastases. In the present experiments we studied whether the enhancement of drug levels in liver blood achieved when NAs are given in the coupled form depends on the rate of drug elimination from the bloodstream. Three NAs, adenine arabinoside (ara-A), 5-fluoro-2'-deoxyuridine (FUdR), and 2',2'-difluorodeoxycytidine, were coupled with lactosaminated human albumin, a galactosyl terminating carrier. In rats that received an intravenous bolus injection of these conjugates, we compared the drug concentrations in liver blood to those in the systemic circulation. We found that enhanced levels of NAs in liver blood were only achieved by administering the conjugates of the drugs (ara-A and FUdR), which are rapidly cleared from the bloodstream. Increased drug levels also were obtained when ara-A and FUdR conjugates were slowly infused (a way of administration often used for anticancer drugs). The experiments also showed that galactosyl terminating conjugates of NAs might have the potential to produce a therapeutic effect only when the coupled drugs are active at low blood concentrations, since the amounts of drugs introduced into hepatocytes and released by these cells in the bloodstream cannot be increased when the receptor for the hepatic uptake of galactosyl terminating peptides is saturated.

Biological characteristics of lactosaminated N-succinyl-chitosan as a liver-specific drug carrier in mice

Lactosaminated N-succinyl-chitosan (Lac-Suc) was prepared by reductive amination of N-succinyl-chitosan (Suc) and lactose using sodium cyanoborohydride. Six-day reaction using lactose (12.8-fold (w/w)) yielded Lac-Suc with lactosamination degree of 30% (mol/sugar unit). Fluorescein thiocarbamyl-Lac-Suc (Lac-Suc-FTC) was prepared by labeling Lac-Suc with fluorescein isothiocyanate. Lac-Suc-FTC was injected intravenously at a dose of either 1 (high dose) or 0.2 (low dose) mg/mouse. At both doses, Lac-Suc-FTC initially underwent fast hepatic clearance, showed maximum liver localization at 8 h, and the amounts localized there were maintained even at 48 h post-injection. Very slow excretion into feces and urine was observed. The ratio of liver AUC(0--48 h) to plasma AUC(0--48 h) at low dose was three times higher than that at high dose. On the other hand, the Suc derivative, Gal-Suc, obtained by reductive amination of Suc/galactose showed very little distribution to the liver similarly to Suc itself. Further, since the liver uptake of Lac-Suc-FTC was inhibited by asialofetuin, it was suggested that the liver distribution of Lac-Suc should be concerned with asialoglycoprotein receptor. Thus, Lac-Suc was found available as a carrier exhibiting a high affinity to and long retention in the liver.

Measurement of apolipoprotein A1 in cholesterol gallstones and gallbladder bile of patients with gallstones

Biliary apolipoprotein A1 in bile inhibits the nucleation of cholesterol crystals from bile super-saturated with cholesterol. In the present study, using an enzyme-linked immunosorbent assay of apolipoprotein A1, we determined the content of apolipoprotein A1 in cholesterol gallstones and samples of gallbladder bile collected simultaneously from 23 patients during cholecystectomy. Protein content in cholesterol gallstones ranged from 50 to 5700 micrograms/g, with median, quartile, and three quartile values being 250, 111, and 740; apolipoprotein A1 content ranged from 9 to 9000 ng/g (200, 41, 647). The gallbladder bile samples contained protein at concentrations of 0.4-9.0 mg/ml (2.0, 1.1, 3.2), while apolipoprotein A1 was present at concentrations of 2.0-136.0 micrograms/ml (30.0, 10.0, 90.0). A notable finding was that the A1/total protein (TP) values for gallbladder bile, which ranged from 0.13% to 6.80% (1.62, 0.89, 3.34), were several times higher than those determined for gallstone samples, which ranged from 0.01% to 1.2%, 2% (0.06, 0.02, 0.25). The results of sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that the protein profile in cholesterol gallstones was similar to that in gallbladder bile. It was concluded that: (1) the protein contained in gallstones may originate from bile, (2) the content of apolipoprotein A1 in cholesterol gallstones is only a trace amount, compared with that in gallbladder bile, and (3) biliary apolipoprotein A1 may be retained in a soluble phase in gallbladder bile, with minimal precipitation onto the surfaces of gallstones.

An evaluation of receptor-mediated gene transfer using synthetic DNA-ligand complexes

Receptor-mediated gene transfer is an attractive method for therapeutically correcting human genetic diseases since it permits the targeting of DNA to cellular receptors in specific tissues of adult animals. Genes introduced by this technique have been shown to be expressed in the target tissue for varying periods. However, to be useful for gene therapy, it is critical that both the chemical properties and physical interactions of the reagents involved in the design of the DNA delivery vehicle be rigorously characterized. In this review, we discuss the critical steps in the preparation of the DNA-ligand complex and the factors involved in the delivery and regulated expression of a transgene in animal tissues. The feasibility of using this technique for the therapeutic delivery of genes to mammalian tissues will also be evaluated.

MION-ASF: biokinetics of an MR receptor agent

Receptor-directed MR contrast agents are currently being designed to improve sensitivity and specificity of MR imaging and to provide for functional MR imaging. In the current study we have synthesized a conjugate of asialofetuin (ASF), a bovine plasma protein with a known, high affinity for the hepatic asialoglycoprotein receptor, and a well defined, single crystal superparamagnetic label (monocrystalline iron oxide nanoparticle, MION). MION-ASF is cleared from the circulation more than 300 times faster than MION, has a 3.7 times higher hepatic accumulation, increases liver R2 relaxivity 2.8-fold compared to MION, and accumulates in hepatocytes unlike MION, which accumulates only in macrophages. Competition assays indicate that receptor-mediated hepatocyte uptake can be competitively blocked and that this effect can be demonstrated by imaging. These studies indicate that sensitive iron oxide based probes can be developed for functional MR imaging.

Proteins and peptides bound to long-circulating liposomes

Liposome formulations with prolonged circulation time have recently been developed as a potential sustained-release drug delivery system. Data shown in this report indicate that such formulations can also be used to prolong the circulation time of proteins and peptides by conjugating them to the surface of liposomes. Increase of the circulation halflife ranged from 2- to 150-fold depending on the protein/lipid ratio of the liposomal formulation, liposome size, and the lipid composition of liposomes. Since the proteins/peptides localize on the liposome surface, instead of being entrapped inside the liposomes, they are directly available for binding to its receptor molecules and express the biological activity. This strategy has been successfully applied to two proteins with known fast clearance rate, i.e. asialofetuin and ricin A-chain. The biological activities of both proteins are preserved when they are formulated in liposomes. Incorporation of a peptide, i.e. a-factor of the yeast Saccharomyces cerevisiae, into the liposome membrane also significantly enhanced the circulation time of the peptide.

Prothrombin plasma clearance is not mediated by hepatic asialoglycoprotein receptors

The hepatic asialoglycoprotein receptor (AGPR) system can efficiently internalize and degrade circulating glycoproteins which lack terminal sialic acids on their carbohydrate chains. Since pro-thrombin is a glycosylated plasma protein, possible involvement of AGPR system in its clearance from circulation was evaluated. The half lives of bovine 125I-prothrombin and 125I-asialoprothrombin, injected intravenously into rats, were 192 and 1.8 minutes, respectively. Asialoprothrombin appeared to be cleared by the hepatic AGPRs since 33% of it accumulated in the liver at 30 minutes and its clearance was competitively blocked by simultaneous administration of increasing amounts of asialofetuin. Only 5% of prothrombin accumulated in the liver at 3 hours and injections asialofetuin in amounts capable of saturating the AGPR for the duration of four asialoprothrombin half lives had no effect on the disappearance of prothrombin. Our observations indicate that, although asialoprothrombin is readily cleared from plasma by the AGPR system, prothrombin is not. Thus these receptors do not appear to be involved in physiological processes that control prothrombin half life.

Relationship between pinocytic rate and uptake of transferrin by suspended rat hepatocytes

The aim of this study was to compare quantitatively the capacity to transcytose (i.e. to uptake and release) transferrin (Tf) with the pinocytic activity of suspended adult rat hepatocytes. An oligodisperse preparation of 131I-polyvinylpyrrolidone (PVP; Mr 36,000) was used to measure the inward and outward aspects of the pinocytic process in separate experiments. Cell association of rat 125I-Tf was measured at Tf concentrations approaching physiological, where 59Fe uptake obeyed first-order kinetics. Release studies with both PVP and Tf were carried out under conditions which minimized the probability of de novo endocytosis of a molecule already released. Sets of experimental points representing cell-associated radioactivities were converted into continuous algebraic functions by fitting with two-term (release studies) or three-term (uptake studies) exponential equations. Transport of PVP and Tf through the cells was computed from these equations by deconvolution. This analysis showed that, under the present experimental conditions, the fractional transcytosis rates of Tf and PVP by hepatocytes were in the ratio of 1:0.77. These values imply that, in the physiological range of Tf concentrations, about 75% of the Fe taken up by hepatocytes may be due to a pinocytic mechanism (fluid-phase or mixed). Inclusion of chloroquine (1 mM) in the suspending medium, both in uptake and release experiments, resulted in more PVP and Tf passing through the cells, while Fe uptake was reduced. It is suggested that the base probably exerted its enhancing effect on transcytosis by shunting the subcellular transport of PVP and Tf to the outward leg through a shorter circuit.

Preparation of asialofetuin-labeled liposomes with encapsulated human interferon-gamma and their uptake by isolated rat hepatocytes

The selective delivery of human recombinant interferon (IFN)-gamma to isolated rat hepatocytes was studied with asialofetuin (AF)-labeled liposomes. AF-liposomes containing buffer solution were initially prepared by the detergent removal method, and IFN-gamma was subsequently encapsulated by the freeze-thawing method without loss of activity. Virtually no free [32P]IFN-gamma was internalized into isolated rat hepatocytes, whereas AF-liposomes containing [32P]IFN-gamma were taken up to a significant degree. Liposomal binding to the hepatocytes (estimated at 4 degrees C) was one-fifth of the uptake (estimated at 37 degrees C). Since the uptake was inhibited by the addition of free AF, AF-liposomes may be taken up by the action of galactose-binding protein on the hepatocytic cell surface. The liposome preparation method reported in this paper provides a useful means for the encapsulation of unstable macromolecules into AF-liposomes. AF-liposomes were found effectively to carry IFN-gamma into hepatocytes in vitro.

Diacytosis of asialoglycoprotein in isolated hepatocytes is dependent on the structure of ligand and cellular distribution of the receptors

Diacytosis, degradation and retention of 125I-labeled asialoorosomucoid (ASOR), its reduced and carboxymethylated N-terminal cyanogen bromide-cleaved fragment (RC-ASCNBr-I) and asialofetuin preloaded into isolated rat or rabbit hepatocytes for various periods of time were compared. In rat hepatocytes preloaded with a saturating concentration (3.10(-8) M) of the ligands, the proportion of the preloaded ligands distributed to degradation and diacytosis was fairly constant during 4 h of preincubation. In addition, a small portion of the preloaded ligands was neither diacytosed nor degraded, but was retained intracellularly. Diacytosis of 125I-ASOR (29%) was greater than that of either 125I-RC-ASCNBr-I (23%) or 125I-asialofetuin (15%). Diacytosis of 125I-ASOR, when preloaded in the presence of 5 microM colchicine, was significantly enhanced by 79% (increasing from 29% to 52%), whereas those of 125I-RC-ASCNBr-I and 125I-asialofetuin were not significantly altered (with average increases of 14% and 19%, respectively). The fraction of the preloaded 125I-asialofetuin (69%) and 125I-RC-ASCNBr-I (68.6%) that was degraded was slightly higher than that of 125I-ASOR (64%) and all was decreased by colchicine. The fraction of all three ligands retained by the cells was increased 2- to 4-fold by colchicine. The extents of retention of 125I-asialofetuin and 125I-ASCNBr-I were greater than that of 125I-ASOR, particularly after preloaded for more than 2 h. Preloading of the cells with ligands at a non-saturating concentration (6.5.10(-10) M) did not change these patterns of ligand distribution. Conjugation of diphtheria toxin fragment A (DTA) to ASOR or RC-ASCNBr-I also did not significantly alter the pattern of ligand distribution. In rabbit hepatocytes containing more asialoglycoprotein receptors than rat cells, 125I-ASOR was diacytosed to a greater extent (50%, -colchicine; 61%, + cholchicine) but degraded to a lesser extent (33%, -colchicine; 13%, + colchicine) than was observed in rat cells. The extent of retention of 125I-ASOR in rabbit cells was also greater than that in rat cells. A similar pattern of differences between rabbit and rat hepatocytes was observed for 125I-DTA-ASOR. These results indicate that intracellular sorting of internalized asialglycoproteins between diacytosis and degradation is dependent on both the structure of the ligand and the distribution of the cellular receptors.

The cyanogen bromide fragment I of asialoorosomucoid is transported more efficiently than asialoorosomucoid in rat hepatocytes

Cultured rat hepatocytes internalized and degraded 125I-labeled asialoorosomucoid (125I-ASOR) through asialoglycoprotein receptor at rates about half that of its cyanogen bromide fragment I (125I-ASCNBr-I). Reduction and carboxymethylation of the fragment resulted in decreased rates of internalization and degradation which were still greater than those of 125I-ASOR. In the presence of 5 microM colchicine, degradation of all three ligands was inhibited. However, the intracellular level of 125I-ASOR at steady state remained unchanged, while those of the fragments increased continuously. Study of the binding of these ligands to hepatocytes at 4 degrees C indicated that there was no significant difference in binding parameters between ASOR, ASCNBr-I and RC-ASCNBr-I (reduced and carboxymet ASCNBr-I). Studies of the fate of these ligands preloaded in the cell at 37 degrees C indicated that a higher fraction of the internalized ASOR than of the fragments was released by diacytosis. In contrast to ASOR, diacytosis of the fragments was not enhanced by colchicine. Studies of the distribution of intracellular ligands by Percoll density gradient centrifugation indicated that they were internalized initially into two early endosomal compartments of d = 1.037 g/ml and d = 1.045 g/ml. In the presence of colchicine, accumulation of the ligands in a third endosomal compartment of d = 1.08-1.095 g/ml was revealed, while in the presence of leupeptin accumulation of the ligands in lysosomes was observed. The results of a kinetic analysis indicated that both cyanogen bromide fragments were transported to all these compartments more rapidly than was ASOR. It appears that they are internalized and degraded more rapidly than ASOR due to a more efficient sorting of the internalized ligand into the pathway of lysosomal degradation.

The hepatic galactosyl receptor system: two different ligand dissociation pathways are mediated by distinct receptor populations

After internalization of 125I-asialo-orosomucoid (ASOR) by isolated rat hepatocytes, ligand dissociates by two kinetically distinct pathways (Oka and Weigel, J. Biol. Chem. 257, 10,253, 1983). These slow and fast dissociation pathways correspond to two functionally different subpopulations of cell surface galactosyl receptors designated, respectively, State 1 and State 2 receptors. Freshly isolated cells or cells equilibrated below 24 degrees C express only State 1 receptors. Cells equilibrated at 37 degrees C express both State 1 and State 2 receptors. Ligand dissociation after internalization of surface-bound 125I-ASOR was measured using the permeabilizing detergent, digitonin. The slow dissociation pathway was mediated by State 1 receptors and was the only pathway expressed by cells which were freshly isolated or had been equilibrated at 24 degrees C. State 2 receptors are expressed at temperatures above about 20 degrees C, and both the fast and slow dissociation pathways occurred in cells equilibrated at 37 degrees C. State 2 receptors therefore mediate the rapid dissociation pathway. Dissociation and subsequent degradation of specifically bound ligand routed in either pathway were complete, respectively, within 3 and 6 hrs.

In vivo quantification of removal of asialo-orosomucoid from the circulation in anaesthetized streptozotocin-diabetic rats

The in vivo kinetic of removal of 3H asialo-orosomucoid from plasma was investigated in control and streptozotocin-diabetic rats after intravenous injection of 1 mg of asialo-orosomucoid/100 g body wt. Michaelis-Menten kinetics of disappearance were observed. In diabetic rats the maximal rate (Vmax) of disappearance of 3H asialo-orosomucoid was decreased by 30% with no modification of Michaelis constant. Since no accumulation of desialylated orosomucoid in the circulation was observed, the slower rate of removal of 3H asialo-orosomucoid was attributed to a decrease in the number of hepatic asialoglycoprotein receptors which are largely involved in the catabolism of asialoglycoproteins. Our estimate on in vivo maximal rates was 10- to 20-fold greater than our previous in vitro estimate of the maximal rate of endocytosis. In contrast, the values of the Michaelis constant obtained in vivo and in vitro were very similar.

Receptor-mediated endocytosis of asialoglycoproteins by rat hepatocytes: receptor-positive and receptor-negative endosomes

We have used combinations of subcellular fractionation, specific cytochemical tracers, and quantitative immunoadsorption to determine when, where, and in which intracellular structure internalized asialoglycoproteins (ASGPs) are segregated from their receptor. All membrane vesicles containing the receptor (R+ vesicles) were quantitatively immunoadsorbed from crude microsomes with Staphylococcus aureus cells and affinity-purified anti-ASGP receptor. Using this assay, we varied the time and temperature of exposure of perfused livers to 125I-asialoorosomucoid (125I-ASOR) and followed the movement of ligand from R+ to R- vesicles. After 2.5 min at 37 degrees C, 98% of the internalized ligand could be immunoadsorbed and thus was in R+ vesicles. Over the next 12 min of continuous 37 degrees C perfusion with 125I-ASOR, an increasing fraction of the ligand was not immunoadsorbed and therefore was present in R- vesicles. A maximum of 30% of the ligand could be found in R- vesicles (14-44 min). When livers were maintained at 16 degrees C, ligand was internalized but remained in R+ vesicles. Furthermore, ligand accumulating in R- vesicles at 37 degrees C remained there when livers were cooled to 16 degrees C. R- endosomes could be separated from R+ endosomes by flotation on sucrose density gradients and visualized by the presence of sequestered ASOR-horseradish peroxidase (ASOR-HRP). These structures resembled those labeled by ASOR-HRP in situ: R+ vesicles were relatively dense (1.12 g/cc), frequently tubular or spherical and small (100-nm diam), corresponding to the peripheral and internal tubular endosomes; R- structures were of lower density (1.09 g/cc), large (400-nm diam), and resembled internal multivesicular endosomes (MVEs). Endocytosed ASOR-HRP was found in both the peripheral and internal tubular endosomes in situ under conditions where 95% of the ligand was present in R+ vesicles by immunoadsorption, whereas MVEs containing ASOR-HRP were predominant in situ when ligand was found in R- vesicles and were often in continuity with the tubular internal endosomes. All of these results suggest that complete segregation of ligand and receptor occurs after arrival in the Golgi-lysosome region of the hepatocyte and that MVEs are R- and represent the final prelysosomal compartment.

Diacytosis of 125I-asialoorosomucoid by rat hepatocytes. A non-lysosomal pathway insensitive to inhibition by inhibitors of ligand degradation

Diacytosis of 125I-asialoorosomucoid by rat hepatocytes was studied by preincubating the cells with the labelled ligand at 37 degrees C for 30 min or 18 degrees C for 2 h, washing free of cell surface receptor-bound tracer at 4 degrees C and then reincubating at 37 degrees C. The cells preloaded at 37 degrees C released a maximum of 18% of the total intracellular ligand as undegraded molecules after 1 h of incubation with an apparent first-order rate constant of 0.018 min-1 (t1/2 = 39 min). When the preloaded cells were incubated in the presence of 100 micrograms/ml unlabelled asialoorosomucoid or 5 mM ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, the amount of the released ligand increased to 32 and 37%, respectively, without apparent change in kinetics, indicating that these agents prevented rebinding of the released ligand. In the presence of 5 microM colchicine, 20 microM cytochalasin B, 20 microM chloroquine, 10 mM NH4Cl, 10 microM monensin or 20 microM leupeptin, degradation of the preloaded ligand was inhibited, whereas the release of the ligand was either slightly increased or unchanged. Similar effects of leupeptin, colchicine and asialoorosomucoid were observed with cells preloaded at 18 degrees C. These results indicate that diacytosis of 125I-asialoorosomucoid occurs from a prelysosomal compartment via a route insensitive to inhibition by the inhibitors of ligand degradation.

Microtubule-depolymerizing agents inhibit asialo-orosomucoid delivery to lysosomes but not its endocytosis or degradation in isolated rat hepatocytes

Microtubule-depolymerizing drugs, such as colchicine, vinblastine sulfate, colcemide and podophyllotoxin, cause an apparent inhibition of the ability of rat hepatocytes to degrade asialo-orosomucoid. However, the binding of asialo-orosomucoid to the cell surface at 0 degrees C, the endocytosis of pre-bound glycoprotein at 37 degrees C, and the dissociation of internal receptor-glycoprotein complexes are unaffected by these microtubule drugs. Receptor recycling is slowed but still occurs, although degradation is blocked. The rate of degradation is decreased by low concentrations of drugs. (For example, 0.25 microM vinblastine sulfate, colchicine and colcemide inhibited 93%, 79% and 26%, respectively.) Neither beta- nor gamma-lumicolchicine affected any of the processes examined. The degree of inhibition with colchicine could be enhanced by a brief treatment of the cells at low temperature to depolymerize microtubules. However, if cells were allowed to endocytose asialo-orosomucoid at 37 degrees C prior to addition of the microtubule drug, then the inhibition of protein degradation was greatly reduced. The decrease in the inhibition of degradation was proportional to the amount of time that cells were exposed to asialoglycoprotein before addition of the drug. The results indicate that the segregation of protein from receptor after they dissociate and/or the subsequent translocation of internalized asialoglycoprotein from the cell perimeter to the lysosomal region requires intact microtubules.

Quantitation of intracellular membrane-bound enzymes and receptors in digitonin-permeabilized cells

The distribution of membrane-bound receptors and enzymes between the cell surface and the cell interior can be determined without solubilization or gross disruption of cell organelles in the presence of the nonionic detergent digitonin. This steroid glycoside permeabilizes cells, releases cytoplasmic proteins with subunit molecular weights up to 200,000, and allows exogenous molecules to gain access to intracellular receptors. All cell types examined were affected similarly by digitonin. Permeabilization was complete within 2 min at 0 degree C and did not require the continued presence of digitonin. A characteristic amount of protein (approximately 50%) was lost between 0.02 and 0.08% (w/v) digitonin. Three independent systems were examined: the insulin receptor in 3T3 fibroblasts and the asialoglycoprotein receptor and the Na+/K+-ATPase in rat hepatocytes. In each case an increase in the specific activity of enzyme/receptor occurred over a range of detergent concentration in which the retention of cell protein was constant and virtually no solubilization of membrane-bound activity occurred. The binding of 125I-asialo-orosomucoid to rat hepatocytes at 0 degree C in the presence of digitonin was linear with cell number and kinetically indistinguishable from binding to intact cells. Receptors exposed by digitonin were shown to be intracellular by light microscopic examination of permeabilized cells first treated with antiserum to the receptor and then with a second antibody horseradish peroxidase conjugate. The use of digitonin has many advantages over procedures which require total cell disruption or solubilization to assess intracellular receptors. The technique has already been valuable in studies on recycling and endocytosis mediated by the asialoglycoprotein receptor (P.H. Weigel and J.A. Oka (1983) J. Biol. Chem. 258, 5095-5102) and should also be useful in studies with other membrane-bound receptors and enzymes in other cell types.

Tissue locations for the turnover of radioactively labeled rat orosomucoid in vivo

Tissues involved in the turnover of rat serum orosomucoid were identified by methods designed to cause lysosomal trapping of radiolabel at the sites of glycoprotein degradation. 125I-, [3H]Raffinose-, and [1-14C]glucosamine-labeled orosomucoid exhibited serum half-lives of 20, 20, and 27 h when injected intravenously into rats. As expected, the asialo derivative of [3H]raffinose-labeled rat orosomucoid was lost very rapidly from the circulation and recovered quantitatively in the liver within 30 min. At 50 h after injection of [3H]raffinose-asialo-orosomucoid the liver retained 38% of the radioactivity while the remainder was found in the gastrointestinal tract and urine. Chromatography of the urine on Bio-Gel P-4 revealed a single radioactive product that eluted similar to raffinose-lysine. The same material was found in the liver. This ability of the [3H]raffinose label to resist metabolic disposal was used to evaluate tissue catabolism of native rat orosomucoid. Comparison of the tissue radioactivity in experiments using 125I- and [3H]raffinose-labeled derivatives of the nondesialylated glycoprotein showed kidney, liver, and muscle to be most active in 3H accumulation. However, the [3H]raffinose metabolites excreted in the urine was markedly different from those produced from asialo-orosomucoid and in contrast there was minimal loss of label to the gastrointestinal tract from the native substrate. Leupeptin, an inhibitor of lysosomal thiol cathespins, was administered continuously to rats by a subcutaneous osmotic pump. At 24 h after injection of 125I-orosomucoid, leupeptin-treated rats showed a net 16% increase in tissue radioactivity above sham-operated animals and a corresponding decrease occurred in the radioactivity associated with the gastrointestinal tract and urine. Tissues that exhibited increases in radioactivity were kidney, muscle, liver, and hide. The different behavior of labeled native and asialo-orosomucoids suggests that the hepatic galactose receptor system plays, at most, a limited role in maintaining homeostasis of the native glycoprotein.

Diagnostic Value of Intestinal Alkaline Phosphatase in Horse Serum

Antiserum directed against equine intestinal Alkaline Phosphatase (ALP) was produced in rabbits and used to develop a sensitive and quantitative assay for the detection of intestinal ALP in equine serum. This assay was then used to measure the half-life of intravenously injected intestinal ALP and to determine if the intestinal ALP was present in normal horse sera, sera from horses presented for lesions not involving the gastrointestinal tract and sera from horses presented with lesions involving the gastrointestinal tract. The results suggest that intestinal ALP is not likely to appear in equine serum even when gastrointestinal disease is present and, therefore, appears to be of no diagnostic value.

A kinetic study of hepatic uptake of canine intestinal alkaline phosphatase in the rat

This study describes the "plasma" disappearance and biliary excretion of injected purified canine intestinal alkaline phosphatase in the isolated perfused rat liver and in the intact rat. The plasma disappearance curve of the enzyme showed, apart from an initial rapid phase, a secondary phase, which may result from "exocytosis" of interiorized canine intestinal alkaline phosphatase into the circulation or from exhaustion of receptors involved in uptake of the enzyme during the process of endocytosis. A small fraction of the injected enzyme was found to be excreted into the bile and it was shown that there is a "kinetic" relationship between the biliary excretion pattern and the "plasma" disappearance of the enzyme. In addition, the effect of various sugars, sugar polymers and glycoproteins on plasma disappearance and biliary excretion of the enzyme was studied. Intravenous administration of galactose, lactose, galactan and especially asialofetuin prior to injection of canine intestinal alkaline phosphatase, resulted in inhibition of plasma disappearance of the enzyme, while the biliary excretion pattern of the enzyme was also changed. It is concluded that the rapid removal of injected canine intestinal alkaline phosphatase is predominantly, or even exclusively, achieved by the liver and that the hepatic uptake of the enzyme is most likely mediated by the "galactose-specific lectin" present on the plasma membrane of the hepatocytes.

Asialoglycoprotein receptor mediates the toxic effects of an asialofetuin-diphtheria toxin fragment A conjugate on cultured rat hepatocytes

We have constructed a toxic hybrid protein that is recognized by asialoglycoprotein (ASGP) receptors of cultured rat hepatocytes. The conjugate consists of fragment A of diphtheria toxin (DTA) linked by a disulfide bond to asialofetuin (ASF). This conjugate is highly toxic, inhibiting protein synthesis in primary rat hepatocytes at concentrations as low as 10 pM. The ASF-DTA conjugate was 600 and 1800 times as toxic as diphtheria toxin and DTA, respectively, on primary rat hepatocytes. The ASGP receptor recognizes galactose-terminated proteins. We tested a series of glycoproteins for their ability to block the action of the ASF-DTA conjugate. Fetuin and orosomucoid, two glycoproteins with terminal sialic acid on their oligosaccharide chains, did not block the action of the conjugate. Their galactose-terminated asialo derivatives, ASF and asialoorosomucoid, as expected, did block the action of the conjugate. The N-acetylglucosaminyl-terminated derivative (asialogalactoorsomucoid) had no appreciable effect on the activity of the conjugate. We tested the ASF-DTA conjugate on six cell types; except for primary rat hepatocytes, none of them were affected by a high concentration (10 nM) of ASF-DTA conjugate. A fetuin-DTA conjugate was less toxic by a factor of 300 than the ASF-DTA conjugate and exerted its effects primarily through non-receptor-mediated mechanisms. The highly toxic ASF-DTA conjugate is cell-type specific, and its action is mediated by a well-characterized receptor, whose mechanism of receptor-ligand internalization has been extensively investigated.