monensin and Carcinoma--Hepatocellular

Pharmacological treatments for advanced hepatocellular carcinoma (HCC) have a partial efficacy. Augmented Na

Topics: Animals; Carcinoma, Hepatocellular; Cell Line; Humans; Liver Neoplasms; Mice; Monensin; Sodium; Water

Cells expressing connexin43 are able to upregulate gap junction (GJ) communication by enhancing the assembly of new GJs, apparently through increased connexin trafficking. Because G proteins are known to regulate different aspects of protein trafficking, we examined the effects of pertussis toxin (PTX; a specific inhibitor of certain G proteins) on GJ assembly. Dissociated Novikoff hepatoma cells were reaggregated for 60 min to form nascent junctions. PTX inhibited GJ assembly, as indicated by a reduction in dye transfer. Electron microscopy also revealed a 60% decrease in the number of GJ channels per cell interface. Importantly, PTX blocked the twofold enhancement in GJ assembly found in the presence of low-density lipoprotein. Two G(i alpha) proteins (G(i alpha 2) and G(i alpha 3)), which have been implicated in the control of membrane trafficking, reacted with PTX in ADP-ribosylation studies. PTX and/or the trafficking inhibitors, brefeldin A and monensin, inhibited GJ assembly to comparable degrees. In addition, assays for GJ hemichannels demonstrated reduced plasma membrane levels of connexin43 following PTX treatment. These results suggest that PTX-sensitive G proteins regulate connexin43 trafficking, and, as a result of inhibition with PTX, the number of plasma membrane hemichannels available for GJ assembly is reduced.

Topics: Adenosine Diphosphate Ribose; Animals; Brefeldin A; Carcinoma, Hepatocellular; Cell Communication; Cholesterol, LDL; Colforsin; Connexin 43; Freeze Fracturing; Gap Junctions; Gene Expression; GTP-Binding Proteins; Ionophores; Monensin; Pertussis Toxin; Phosphorylation; Protein Synthesis Inhibitors; Protein Transport; RNA, Messenger; Tumor Cells, Cultured; Virulence Factors, Bordetella

The major alpha1,3fucosyltransferase activity in plasma, liver, and kidney is related to fucosyltransferase VI which is encoded by the FUT6 gene. Here we demonstrate the presence of alpha1, 3fucosyltransferase VI (alpha3-FucT VI) in the human HepG2 hepatoma cell line by specific activity assays, detection of transcripts, and the use of specific antibodies. First, FucT activity in HepG2 cell lysates was shown to prefer sialyl-N-acetyllactosamine as acceptor substrate indicating expression of alpha3-FucT VI. RT-PCR analysis further confirmed the exclusive presence of the alpha3-FucT VI transcripts among the five human alpha3-FucTs cloned to date. alpha3-FucT VI was colocalized with beta1,4galactosyltransferase I (beta4-GalT I) to the Golgi apparatus by dual confocal immunostaining. Pulse/chase analysis of metabolically labeled alpha3-FucT VI showed maturation of alpha3-FucT VI from the early 43 kDa form to the mature, endoglycosidase H-resistant form of 47 kDa which was detected after 2 h of chase. alpha3-FucT VI was released to the medium and accounted for 50% of overall cell-associated and released enzyme activity. Release occurred by proteolytical cleavage which produced a soluble form of 43 kDa. Monensin treatment segregated alpha3-FucT VI from the Golgi apparatus to swollen peripheral vesicles where it was colocalized with beta4-GalT I while alpha2,6(N)sialyltransferase remained associated with the Golgi apparatus. Both constitutive secretion of alpha3-FucT VI and its monensin-induced relocation to vesicles analogous to beta4-GalT I suggest a similar post-Golgi pathway of both alpha3-FucT VI and beta4-GalT I.

Topics: Animals; Carcinoma, Hepatocellular; CHO Cells; Cricetinae; Cricetulus; Extracellular Space; Fucosyltransferases; Galactosyltransferases; Golgi Apparatus; Humans; Liver Neoplasms; Microscopy, Confocal; Microscopy, Fluorescence; Molecular Weight; Monensin; Neoplasm Proteins; Recombinant Fusion Proteins; Reverse Transcriptase Polymerase Chain Reaction; Transfection; Tumor Cells, Cultured

alpha-Tocopherol (alphaTocH) is transported in association with lipoproteins in the aqueous milieu of the plasma. Although up to 50% of circulating alphaTocH is transported by high-density lipoproteins (HDLs), little is known about the mechanisms of uptake of HDL-associated alphaTocH. During the current study, human apolipoprotein (apo)E-free HDL subclass 3 (HDL3) labelled with [14C]alphaTocH was used to investigate uptake mechanisms of HDL3-associated alphaTocH by a permanent hepatoblastoma cell line (HepG2). HDL3-associated alphaTocH was taken up independently of HDL3 holoparticles in excess of apoA-I comparable with the non-endocytotic delivery of cholesteryl esters to cells termed the 'selective' cholesteryl ester uptake pathway. Experiments with unlabelled HDL3 demonstrated net mass transfer of alphaTocH to HepG2 cells. Time-dependent studies with [14C]alphaTocH-labelled HDL3 revealed tracer uptake in 80-fold excess of apoA-I and in 4-fold excess of cholesteryl linoleate. In addition to HLDs, low-density lipoprotein (LDL)-associated alphaTocH was also taken up in excess of holoparticles, although to a lesser extent. These findings were confirmed with unlabelled lipoprotein preparations, in which HDL3 displayed a 2- to 3-fold higher alphaTocH donor efficiency than LDLs (lipoproteins adjusted for equal amounts of alphaTocH). An important factor affecting particle-independent uptake of alphaTocH was the cellular cholesterol content (a 2-fold increase in cellular cholesterol levels resulted in a 2.3-fold decrease in uptake). Pulse-chase studies demonstrated that some of the HDL3-associated alphaTocH taken up independently of holoparticle uptake was resecreted along with a newly synthesized apoB-containing lipoprotein fraction.

Topics: Acrylamide; Acrylamides; Adenosine Triphosphatases; Apolipoproteins B; Carcinoma, Hepatocellular; Cholesterol; Colchicine; Enzyme Inhibitors; Humans; Lipoproteins, HDL; Lipoproteins, LDL; Monensin; Tumor Cells, Cultured; Vitamin E

Selective uptake of high-density lipoprotein- (HDL-) associated cholesteryl esters (CE), i.e. lipid uptake independent from particle uptake, delivers CE to the liver and steroidogenic tissues in vivo. In vitro, besides hepatocytes and steroidogenic cells many other cell types selectively take up HDL CE. Hepatic lipase (HL) stimulates the internalisation of apoprotein (apo) B-containing lipoproteins by hepatocytes independent from lipolysis. In this study the role of HL in the hepatic metabolism of apo A-I-containing lipoproteins, i.e. HDL, was investigated. HDL3 (d = 1.125-1.21 g/ml) was radiolabeled in its protein (125I) and in its CE moiety ([3H]cholesteryl oleyl ether, ([3H]CEt)). HL originated from tissue culture media of hepatoma cells and from post-heparin plasma. Human Hep 3B hepatoma cells incubated in medium containing radiolabeled HDL3. In the absence of HL, the rate of apparent HDL3 particle uptake according to the lipid tracer ([3H]CEt) was in most cases in approximately 10-fold excess on that due to the protein label (125I), indicating selective CE uptake from HDL3. Addition of HL to these incubations increased the cellular uptake of [3H]CEt and of 125I from HDL3 and quantitatively the most prominent effect was an up to approximately 2.5-fold stimulation of apparent selective CE uptake ([3H]CEt-125I). This increase in selective CE uptake was observed in the presence of tetrahydrolipstatin, an inhibitor of the catalytically active site of HL, suggesting that this HL effect is independent from lipolysis. HL binds to cell surface heparan sulfate proteoglycans. To explore the role of these molecules for the HL effect on selective CE uptake, hepatoma cells were depleted of proteoglycans or Chinese hamster ovary (CHO) cells deficient in proteoglycan synthesis were used. Proteoglycan-deficiency reduced the HL-mediated increase in selective uptake by more than 80%. To investigate if low-density lipoprotein (LDL) receptors or the LDL receptor-related protein (LRP) are involved in the HL effect on selective CE uptake, murine embryonic fibroblasts (MEF) were used which are deficient in these receptors; alternatively, monensin, an inhibitor of endocytosis was present in the medium of Hep 3B cells during the uptake assay for labeled HDL3. These experiments yielded no evidence for a role of LDL receptors or LRP in the HL-mediated increase in selective CE uptake. In summary, HL mediates an increase in HDL3 selective CE uptake by human Hep 3B hepatoma cells.

Topics: Animals; Carcinoma, Hepatocellular; CHO Cells; Cholesterol Esters; Cricetinae; Enzyme Inhibitors; Fibroblasts; Heparan Sulfate Proteoglycans; Heparin Lyase; Humans; Lactones; Lipase; Lipoproteins, HDL; Lipoproteins, HDL3; Liver; Mice; Monensin; Orlistat; Receptors, LDL; Tumor Cells, Cultured

HepG2 cells are highly differentiated hepatoma cells that have retained an apical, bile canalicular (BC) plasma membrane polarity. We investigated the dynamics of two BC-associated sphingolipids, glucosylceramide (GlcCer) and sphingomyelin (SM). For this, the cells were labeled with fluorescent acyl chain-labeled 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-amino]hexanoic acid (C6-NBD) derivatives of either GlcCer (C6-NBD-GlcCer) or SM (C6-NBD-SM). The pool of the fluorescent lipid analogues present in the basolateral plasma membrane domain was subsequently depleted and the apically located C6-NBD-lipid was chased at 37 degrees C. By using fluorescence microscopical analysis and a new assay that allows an accurate estimation of the fluorescent lipid pool in the apical membrane, qualitative and quantitative insight was obtained concerning kinetics, extent and (intra)cellular sites of the redistribution of apically located C6-NBD-GlcCer and C6-NBD-SM. It is demonstrated that both lipids display a preferential localization, C6-NBD-GlcCer in the apical and C6-NBD-SM in the basolateral area. Such a preference is expressed during transcytosis of both sphingolipids from the apical to the basolateral plasma membrane domain, a novel lipid trafficking route in HepG2 cells. Whereas the vast majority of the apically derived C6-NBD-SM was rapidly transcytosed to the basolateral surface, most of the apically internalized C6-NBD-GlcCer was efficiently redirected to the BC. The redirection of C6-NBD-GlcCer did not involve trafficking via the Golgi apparatus. Evidence is provided which suggests the involvement of vesicular compartments, located subjacent to the apical plasma membrane. Interestingly, the observed difference in preferential localization of C6-NBD-GlcCer and C6-NBD-SM was perturbed by treatment of the cells with dibutyryl cAMP, a stable cAMP analogue. While the preferential apical localization of C6-NBD-GlcCer was amplified, dibutyryl cAMP-treatment caused apically retrieved C6-NBD-SM to be processed via a similar pathway as that of C6-NBD-GlcCer. The data unambiguously demonstrate that segregation of GlcCer and SM occurs in the reverse transcytotic route, i.e., during apical to basolateral transport, which results in the preferential localization of GlcCer and SM in the apical and basolateral region of the cells, respectively. A role for non-Golgi-related, sub-apical vesicular compartments in the sorting of GlcCer and SM is proposed.

Topics: 4-Chloro-7-nitrobenzofurazan; Biological Transport; Bucladesine; Carcinoma, Hepatocellular; Cell Membrane; Cell Polarity; Dithionite; Fluorescent Dyes; Glucosylceramides; Golgi Apparatus; Humans; Liver; Liver Neoplasms; Monensin; Oxadiazoles; Sphingomyelins; Tumor Cells, Cultured

In human hepatoma HepG2 cells, the serum inter-alpha-trypsin inhibitor (ITI)-like protein is synthesized from two protein precursors, the heavy chain (H) H2 and the light chain (L). Both of them carry sulphate groups involved in the chondroitin sulphate glycosaminoglycan (GAG) linkage, as demonstrated by [35S]sulphate labelling, chondroitinase digestion and inhibition with beta-D-xyloside, an artificial GAG acceptor. While inhibition of N-glycosylation prevented neither the maturation nor the secretion of the ITI-related entities, brefeldin A induced the accumulation of H and L precursors in the cells, therefore blocking subsequent association and maturation of the precursors before their secretion. The enzyme system involved in the ester linkage between H and L chains is localized in the trans-Golgi network since no ITI-like protein could be obtained in the presence of monensin; instead free heavy-chain protein forms and bikunin were secreted in culture supernatants. The ITI-like protein synthesized by HepG2 cells is therefore composed of two heavy chains HC2 linked to two bikunin chains by chondroitin sulphate bridges, although the GAG linkage between HC2 chains is presumably different. Further, a different maturation route leading to restricted heavy-chain forms, Hm and Hd, could be shown.

Topics: Alpha-Globulins; Brefeldin A; Carcinoma, Hepatocellular; Chondroitinases and Chondroitin Lyases; Cyclopentanes; Electrophoresis, Polyacrylamide Gel; Glycosides; Glycosylation; Humans; Liver Neoplasms; Monensin; Precipitin Tests; Protein Precursors; Protein Processing, Post-Translational; Protein Synthesis Inhibitors; Receptors, Cell Surface; Sulfates; Trypsin Inhibitors; Tumor Cells, Cultured

Hep G2 cells were used to study the relationship between apolipoprotein synthesis and secretion, as revealed by their interaction with agents modulating these processes. Cycloheximide inhibited the secretion of both apolipoproteins (apo) AI and B, but the reduction in apo AI secretion was evident at earlier times. Monensin also inhibited secretion of apo AI and apo B, but only apo AI accumulated intracellularly. Pulse-chase studies showed that, at concentrations of monensin that had no effect on total protein synthesis, apo B synthesis was specifically inhibited. Triacylglycerol synthesis was inhibited to the same extent as apo B synthesis, but this preceded the latter inhibition and unlike apo B there was an accumulation of intracellular triglyceride. These results suggest that distinctive mechanisms modulate the synthesis and secretion of apo AI and apo B, and that apo B synthesis can be specifically inhibited by mechanisms that initially block triglyceride production.

Topics: Apolipoprotein A-I; Apolipoproteins; Apolipoproteins B; Carcinoma, Hepatocellular; Cycloheximide; Humans; Lipoproteins; Liver; Liver Neoplasms; Monensin; Triglycerides; Tumor Cells, Cultured

Cellular processing of 125I-labeled transforming growth factor-beta 1 was investigated in the human hepatoma cell lines Hep G2 and Hep 3B. Binding of 125I-transforming growth factor-beta 1 to cell surface receptors was specific, saturable and calcium-independent. Both cell lines exhibited a single class of high-affinity (Kd = 2.2 x 10(-10) mol/L) binding sites (4.5 x 10(3) for the Hep G2 cell; 1.5 x 10(3) for the Hep 3B cell) for both human and porcine transforming growth factor-beta 1. Binding was temperature dependent, time dependent and pH dependent. Cell-bound 125I-transforming growth factor-beta 1 was removed by brief exposure to acidic medium (pH less than 4) but was converted into an acid-resistant state rapidly after shifting the cells to 37 degrees C. Spontaneous dissociation of bound ligand over a 6 hr period at 4 degrees C was less than 10%. Disuccinimidyl suberate was used to covalently label 125I-transforming growth factor-beta 1 to cell-surface binding sites. Labeling of the ligand/receptor complexes was inhibited by unlabeled transforming growth factor-beta 1 but was unaffected by other growth factors. The radiolabeled complexes showed approximate molecular weights of 280,000, 85,000 and 65,000 when run on reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Cell-bound 125I-transforming growth factor-beta 1 was internalized and degraded at 37 degrees C, and the products were released into the medium as trichloroacetic acid-nonprecipitable radioactivity. The lysosomotropic base chloroquine and the carboxylic ionphore monensin inhibited degradation and release of 125I-labeled products from the cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Binding, Competitive; Carcinoma, Hepatocellular; Chloroquine; Cycloheximide; Humans; Hydrogen-Ion Concentration; Liver Neoplasms; Monensin; Receptors, Cell Surface; Receptors, Transforming Growth Factor beta; Time Factors; Transforming Growth Factor beta; Tumor Cells, Cultured

The mechanism controlling insulin accumulation in nuclei of H35 hepatoma cells was investigated by incubating intact cells with 125I-labeled insulin in the presence or absence of agents that perturb different intracellular sites involved in the processing of ligand-receptor complexes. Purified nuclei were isolated, and nuclear-associated 125I-insulin was determined. Insulin accumulation in the nuclei was time and temperature dependent. Nuclear accumulation was linear and insulin-concentration dependent between 5 and 50 ng insulin/ml. However, pharmacological concentrations of insulin increased the amount of insulin translocated to the nucleus to a far greater extent than it increased total cell-associated insulin. Chloroquine, an acidotrophic agent, increased total cell-associated and intracellular insulin but had no effect on nuclear accumulation. The monovalent ionophores monensin and nigericin inhibited nuclear accumulation of insulin at low concentrations (0.5-5.0 microM) without affecting total insulin binding or intracellular accumulation. At 10 or 25 microM, monensin and nigericin also acted as acidotrophic agents and increased total insulin binding and intracellular accumulation but inhibited nuclear accumulation by a maximum of 50%. Low concentrations of monensin and nigericin were additive; maximal concentrations were not. A 23187 and valinomycin did not affect insulin binding or intracellular and nuclear accumulation of insulin. Neither depletion of ATP by sodium azide, 2,4-dinitrophenol, sodium cyanide, or oligomycin nor disruption of cytoskeletal elements by cytochalasin D or colchicine had any effect on nuclear accumulation of insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Adenosine Triphosphate; Biological Transport; Carcinoma, Hepatocellular; Cell Nucleus; Chloroquine; Insulin; Liver Neoplasms; Monensin; Nigericin; Osmolar Concentration; Temperature; Tumor Cells, Cultured

A study of insulin-receptor internalization and recycling was undertaken in primary cultures of rat hepatocytes and a human hepatoma cell line (HepG2). Receptors were quantitated by measuring 125I-insulin binding to partially purified soluble receptor preparations from untreated cells (total receptors) and trypsinized cells (intracellular receptors). In resting HepG2 cells, exposure to insulin results in internalization of insulin receptors, the rate and extent of which is dependent on the insulin concentration. However, receptors do not accumulate inside the cell in proportion to the higher rates of internalization at high concentrations of insulin. This lack of accumulation is explained by much higher recycling rates after exposure to high concentrations of insulin. Similar results were noted for primary cultures of rat hepatocytes. These results imply qualitatively different fates for receptors internalized after exposure to different concentrations of insulin. To further investigate the possibility of different pathways for insulin-receptor internalization and processing, cells in low (1 ng/ml) or high (100 ng/ml) concentrations of insulin were exposed to drugs or treatments known to affect receptor metabolism. Hypotonic shock and hypokalemia, which arrest coated-pit formation, blocked internalization of insulin and insulin receptors at low concentrations of insulin but allowed internalization in response to high concentrations of insulin. The lysosomotropic drugs monensin and chloroquine caused intracellular accumulation of insulin and its receptors internalized at low concentrations of insulin but had a relatively smaller effect on receptors internalized at high concentrations of insulin. All internalization is blocked by 2,4-dinitrophenol. We conclude that high doses of insulin lead to insulin-receptor internalization and recycling through a pathway that is functionally distinct from the pathway taken by receptors internalized by low (physiologic) concentrations of insulin. The pharmacologic experiments raise the possibility that the high-dose pathway, unlike the low-dose pathway, may proceed independently of coated pits and endosomal acidification.

Topics: 2,4-Dinitrophenol; Animals; Biological Transport; Biotransformation; Carcinoma, Hepatocellular; Cells, Cultured; Chloroquine; Coated Pits, Cell-Membrane; Dinitrophenols; Humans; Insulin; Leupeptins; Liver; Liver Neoplasms; Monensin; Osmolar Concentration; Potassium; Rats; Receptor, Insulin; Tumor Cells, Cultured

This study investigated the relation between the production of natural killer cytotoxic factors (NKCF) and the phenomenon of natural killing (NK) activity against target K562 cells. Two different models of defective NK cell activity were employed. In the first instance, cytotoxic activity of mononuclear cells (MN) derived from patients with hepatocellular carcinoma was compared to the ability of these cells to produce NKCF. Although direct cytotoxicity was considerably impaired in these patients, the ability of their MN to produce NKCF when stimulated with K562 cells was found to be normal. In the second model, MN treated with the lysosomotropic drug monensin showed considerably reduced direct cytotoxic activity, although they were capable of producing normal amounts of NKCF when activated by K562 cells. These results therefore indicate that there is no correlation between NK activity and corresponding NKCF release, and suggest that NKCF production and activity is independent of direct NK cytotoxic activity.

Topics: Adult; Carcinoma, Hepatocellular; Cytotoxicity, Immunologic; Humans; Killer Cells, Natural; Killer Factors, Yeast; Leukocytes, Mononuclear; Liver Neoplasms; Lysosomes; Middle Aged; Monensin; Protein Biosynthesis; Proteins

The weak bases chloroquine, primaquine, NH4Cl and the ionophore monensin exert similar but not identical effects on sorting, transport and processing of cathepsin D in several human cell lines (fibroblasts, HepG2 cells, U937, monocytes). The drugs inhibit the segregation of newly synthesized cathepsin D from the secretory route. The kinetics of transport of nonsegregated cathepsin D precursor along the secretory route is retarded resulting in a delayed hypersecretion. Higher concentrations of the drugs can arrest the intracellular transport completely. The extent of inhibition of segregation varies among the different human cell types tested. Thus, in fibroblasts the secretion can be stimulated to exceed 80%, while in U937 cells the secretion cannot be enhanced above 50% although both cell types have the same basal rate of secretion (approximately 10% of the synthesized cathepsin D). We suggest that pH-independent sorting mechanisms contribute to the targeting of cathepsin D in U937 cells. Processing of the cathepsin D remaining in cells is characteristically changed depending on the drug. The proteolytic processing is strongly inhibited by chloroquine and is rather insensitive to monensin. Unlike the other drugs, monensin blocks the formation of complex oligosaccharides in cathepsin D and allows for extensive secretion solely of molecules that are sensitive to endo H.

Topics: Ammonium Chloride; Carcinoma, Hepatocellular; Cathepsin D; Cell Line; Chloroquine; Fibroblasts; Humans; Kinetics; Liver Neoplasms; Lysosomes; Monensin; Primaquine; Protein Processing, Post-Translational; Skin

We have used pulse-chase techniques to study the synthesis, intracellular modification, and secretion of human apolipoprotein E by cultures of HepG2 cells and peripheral blood human monocyte macrophages. We have found that modified apoE isoproteins are detectable intracellularly after 14 min of pulse, and their relative concentration increases linearly over a 2-h pulse-chase period. At the same time, the relative concentration of unmodified apoE decreases. All the major modified apoE isoproteins appear simultaneously, and they correspond to the sialo apoE forms apooEs2, apoEs4, and apoEs6. ApoE secretion is first detected after a 30-min pulse. Secreted apoE consists of 92% of the same sialated apoE forms observed intracellularly. ApoE sialation and secretion were not affected by treatment of the HepG2 cultures with tunicamycin, whereas monensin inhibited both the intracellular sialation and secretion of this protein. These findings suggest that apoE is secreted in the form of three major isoproteins generated by intracellular modification of this protein with one or more O-linked oligosaccharide chains containing sialic acid.

Topics: Apolipoproteins E; Carcinoma, Hepatocellular; Cell Line; Humans; Kinetics; Liver Neoplasms; Macrophages; Methionine; Monensin; Monocytes; Sulfur Radioisotopes

We have reported in the preceding paper that human hepatoma (Hep G2) cells synthesize thyroxine-binding globulin (TBG). In this paper, we evaluated the kinetics of secretion of the protein and the effects produced by the ionophore monensin and the glycosylation inhibitor tunicamycin. Cells were pulse labeled with [35S]methionine and then chased after addition of excess unlabeled methionine. TBG appeared in the medium after 10 min, and 50% of the protein was secreted after 45 min. After 2 h, more than 85% of TBG had been released. The rate of secretion of TBG was much slower than that of albumin, 50% of which was secreted after 20 min. Monensin, 1 microM, caused a marked delay in TBG secretion, with 50% released after 80 min. After 2 h, less than 60% had been released and a plateau was approached. Endoglycosidase H (endo H) treatment of intracellular and secreted TBG showed no alteration in the rate of conversion of TBG oligosaccharide units from high-mannose type (endo H-sensitive) to complex type (endo H-resistant), thus suggesting that monensin impeded the exit of TBG from the Golgi apparatus without affecting the terminal glycosylation of the protein. Tunicamycin, 5 micrograms/ml, completely blocked glycosylation and markedly affected TBG secretion, almost doubling the time required for the secretion of 50% of the protein. The effect was specific for TBG, since it was not observed in the case of albumin. After 2 h, only 56% of the protein had been released. Analysis of intracellular and extracellular immunoprecipitated products revealed the presence of aggregates (Mr greater than 100,000). The lack of carbohydrates, although not preventing TBG secretion, had marked quantitative effects, and increased the susceptibility to aggregation.

Topics: Carcinoma, Hepatocellular; Cell Line; Furans; Glucosamine; Humans; Kinetics; Liver Neoplasms; Monensin; Serum Albumin; Thyroxine-Binding Proteins; Tunicamycin

The human hepatoma cell line Hep G2 can be maintained in continuous culture and secretes numerous plasma proteins and lipoproteins into the medium. To better characterize cholesterol homeostasis in these cells we have examined the binding, internalization and degradation of [125I]LDL by cultured Hep G2 cells. Hep G2 cells express high-affinity low-density lipoprotein (LDL) receptors which facilitate the binding, internalization and degradation of [125I]LDL; these receptors can be induced by growth in LDL-depleted medium and repressed by further incubation in medium supplemented with LDL. The degradation of [125I]LDL by derepressed Hep G2 cells was inhibited by greater than 90% by monensin. Incubation of Hep G2 cells in the presence of increasing concentrations of LDL also inhibited cholesterol biosynthesis. Our results indicate that Hep G2 cells possess high affinity LDL receptors which are subject to metabolic regulation and suggest that this cell line affords a valuable model to further examine cholesterol and lipoprotein metabolism in human liver cells.

Topics: Acetates; Acetic Acid; Carcinoma, Hepatocellular; Cell Line; Cholesterol; Humans; Kinetics; Lipoproteins, LDL; Liver Neoplasms; Monensin; Receptors, LDL

Acidification of endocytic vesicles has been implicated as a necessary step in various processes including receptor recycling, virus penetration, and the entry of diphtheria toxin into cells. However, there have been few accurate pH measurements in morphologically and biochemically defined endocytic compartments. In this paper, we show that prelysosomal endocytic vesicles in HepG2 human hepatoma cells have an internal pH of approximately 5.4. (We previously reported that similar vesicles in mouse fibroblasts have a pH of 5.0.) The pH values were obtained from the fluorescence excitation profile after internalization of fluorescein labeled asialo-orosomucoid (ASOR). To make fluorescence measurements against the high autofluorescence background, we developed digital image analysis methods for estimating the pH within individual endocytic vesicles or lysosomes. Ultrastructural localization with colloidal gold ASOR demonstrated that the pH measurements were made when ligand was in tubulovesicular structures lacking acid phosphatase activity. Biochemical studies with 125I-ASOR demonstrated that acidification precedes degradation by more than 30 min at 37 degrees C. At 23 degrees C ligand degradation ceases almost entirely, but endocytic vesicle acidification and receptor recycling continue. These results demonstrate that acidification of endocytic vesicles, which causes ligand dissociation, occurs without fusion of endocytic vesicles with lysosomes. Methylamine and monensin raise the pH of endocytic vesicles and cause a ligand-independent loss of receptors. The effects on endocytic vesicle pH are rapidly reversible upon removal of the perturbant, but the effects on cell surface receptors are slowly reversible with methylamine and essentially irreversible with monensin. This suggests that monensin can block receptor recycling at a highly sensitive step beyond the acidification of endocytic vesicles. Taken together with other direct and indirect estimates of endocytic vesicle pH, these studies indicate that endocytic vesicles in many cell types rapidly acidify below pH 5.5, a pH sufficiently acidic to allow receptor-ligand dissociation and the penetration of some toxin chains and enveloped virus nucleocapsids into the cytoplasm.

Topics: Asialoglycoproteins; Carcinoma, Hepatocellular; Cell Line; Endocytosis; Fluorescein-5-isothiocyanate; Fluoresceins; Fluorescent Antibody Technique; Fluorescent Dyes; Glycoproteins; Humans; Hydrogen-Ion Concentration; Kinetics; Liver Neoplasms; Monensin; Thiocyanates