Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UMLS:C0019204 (hepatocellular carcinoma)
 71,386 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Intracellular movement of cell surface 5'-nucleotidase was studied in H4S cells, a rat hepatoma cell line. Surface labelled cells were incubated for various periods at 37 degrees C and treated with neuraminidase at 0 degrees C. Removal of sialic acid residues from glycoproteins results in a change of their isoelectric points. Analysis with isoelectric focusing was then used to distinguish between cell surface and intracellular 5'-nucleotidase. Incubation of 125I-surface-labelled cells at 37 degrees C resulted in a gradual decrease of labelled 5'-nucleotidase at the plasma membrane until, at 60 to 90 min, a steady state was reached with 52% of the label on the cell surface and 48% intracellular. Pretreatment of the cells with the weak base primaquine had no influence on this distribution while at the same time uptake of iron via the transferrin receptor was inhibited. Using immunoelectron microscopy 5'-nucleotidase was found on the cell surface, in multivesicular endosomes and the Golgi complex. Preincubation of the cells in the presence of cycloheximide caused a reduction of labelling in the Golgi complex, whereas the label in the other compartments was retained. These results lead to the conclusion that 5'-nucleotidase does not recycle through the Golgi complex and that in contrast to the transferrin receptor the recycling of 5'-nucleotidase is not inhibited by primaquine.
 ...
PMID:Recycling of 5'-nucleotidase in a rat hepatoma cell line. 285 Jan 62

We have investigated the simultaneous regulation of cell surface distribution and ligand binding of the asialoglycoprotein (ASGP) receptor and the transferrin receptor in a hepatoma cell line by phorbol esters. One hour exposure to phorbol esters causes a redistribution of both receptors to the cell interior as shown by radioligand binding at 4 degrees C and selective immunoprecipitation from the plasma membrane. This effect is temperature- and dose-dependent and is not seen with 4-alpha-phorbol, an inactive tumor promoter. The mechanism and kinetics of the ASGP receptor response to phorbol esters appears to differ from that of the transferrin receptor in this cell line. Within the first 10 min there is a decrease in binding of iodinated ligands for both receptors to the HepG2 cell surface. For the transferrin receptor this results from a net internalization of receptor molecules from the plasma membrane pool, while for the ASGP receptor this decrease is accounted for by a 3.5-fold reduction in ligand binding affinity (6.6 X 10(-8) M to 24.0 X 10(-8) M), with essentially no change in the number of ASGP receptors recoverable from the plasma membrane pool by immunoprecipitation. The altered affinity of the ASGP-R is transient; the Kd returns to control levels by 20 min of continued exposure to the agent. The transferrin receptor shows no change in binding affinity during the course of exposure to phorbol esters. ASGP receptors in cells exposed to phorbol esters for 1 h maintain their competence to deliver exogenous ligand to intracellular sites of degradation and to participate in the recycling pathway of receptor-mediated endocytosis, although at a lower rate than in control cells. We conclude that under identical conditions phorbol esters modulate the binding capacity of two receptors at the cell surface by separate mechanisms. Furthermore, the transient nature of the altered ASGP-R binding affinity suggests that at least two mechanisms, receptor redistribution as well as decreased binding affinity, are operative in the modulation of ASGP-R cell surface binding during the first hour of exposure to the phorbol esters.
 ...
PMID:Regulation by phorbol esters of asialoglycoprotein and transferrin receptor distribution and ligand affinity in a hepatoma cell line. 302 65

We used a conjugate of transferrin and horseradish peroxidase (Tf/HRP) to label the intracellular transferrin receptor route in the human hepatoma cell line HepG2. The recycling kinetics of [125I]Tf/HRP were similar to those of unmodified [125I]Tf, implying identical routes for both ligands. 3,3'Diaminobenzidine (DAB)-cytochemistry was performed on post-nuclear supernatants of homogenates of cells which were incubated with both Tf/HRP and [125I]Tf, and caused two different effects: (a) the equilibrium density of [125I]Tf containing microsomes in a Percoll density gradient was increased, and (b) the amount of immunoprecipitable [125I]Tf from density-shifted lysed microsomes was only 20% of that of nonDAB treated microsomes. The whole biosynthetic route of alpha 1-antitrypsin (AT), a typical secretory glycoprotein in HepG2 cells, was labeled during a 60-min incubation with [35S]methionine. DAB cytochemistry was performed on post-nuclear supernatants of homogenates of cells which were also incubated with Tf/HRP. DAB cytochemistry caused approximately 40% of microsome-associated "complex" glycosylated [35S]alpha 1-antitrypsin ([35S]c-AT) to shift in a Percoll density gradient. Only part of the density shifted [35S]c-AT could be recovered by immunoprecipitation. A maximum effect was measured already after 10 min of Tf/HRP uptake. The density distribution of the "high mannose" glycosylated form of 35S-alpha 1-anti-trypsin [( 35S]hm-AT) was not affected by Tf/HRP. If in addition to Tf/HRP also an excess of non-conjugated transferrin was present in the medium, [35S]c-AT was not accessible for Tf/HRP, showing the involvement of the transferrin receptor (TfR) in the process. Furthermore, we show that if Tf/HRP and [35S]c-AT were located in different vesicles, the density distribution of [35S]c-AT was not affected by DAB-cytochemistry. Pulse-labeling with [35S]methionine was used to show that [35S]c-AT became accessible to endocytosed Tf/HRP minutes after acquirement of the complex configuration. A common intracellular localization of endocytosed Tf/HRP and secretory protein could be confirmed by immuno-electron microscopy: cryosections labeled with anti-albumin (protein A-colloidal gold) as well as DAB reaction product showed double-labeling in the trans-Golgi reticulum.
 ...
PMID:The pathways of endocytosed transferrin and secretory protein are connected in the trans-Golgi reticulum. 326 Feb 38

With few exceptions, receptor-mediated endocytosis of specific ligands is mediated through clustering of receptor-ligand complexes in coated pits on the cell surface, followed by internalization of the complex into endocytic vesicles. During this process, ligand-receptor dissociation occurs, most probably in a low pH prelysosomal compartment. In most cases the ligand is ultimately directed to the lysosomes, wherein it is degraded, while the receptor recycles to the cell surface. We have studied the kinetics of internalization and recycling of both the asialoglycoprotein receptor and the transferrin receptor in a human hepatoma cell line. By employing both biochemical and morphological/immunocytochemical approaches, we have gained some insight into the complex mechanisms which govern receptor recycling as well as ligand sorting and targeting. We can, in particular, explain why transferrin is exocytosed intact from the cells, while asialoglycoproteins are degraded in lysosomes. We have also localized the intracellular site at which endocytosed receptor and ligand dissociate.
 ...
PMID:Sorting and recycling of cell surface receptors and endocytosed ligands: the asialoglycoprotein and transferrin receptors. 632 36

Regulated endocytosis by growth factor receptors requires intact receptor-associated tyrosine kinase activity. To determine whether a similar requirement exists for the asialoglycoprotein (ASGP) receptor which lacks intrinsic tyrosine kinase activity and participates in constitutive endocytosis, we examined the effect of three tyrosine kinase inhibitors, tyrphostin, genistein, and staurosporine, on receptor-mediated endocytosis in the human hepatoma line HepG2. These compounds inhibited early receptor internalization from the plasma membrane to internal protease-resistant sites in a concentration-dependent manner. This effect correlated with their inhibition of tyrosine phosphorylation of the ASGP receptor in vitro. Receptor trafficking subsequent to receptor internalization was unaffected. Endocytosis of another constitutively internalized protein, the transferrin receptor, was also inhibited by these compounds. In contrast, pinocytosis of the fluid-phase marker Lucifer yellow was not inhibited. The tyrosine kinase inhibitors also decreased the endocytic rate of transfected ASGP receptor H1 subunit in SK-Hep-1 cells. Therefore an intact ASGP receptor heterooligomeric complex is not required for this effect. Mutation of the single cytoplasmic tyrosine at position 5 of the H1 subunit to phenylalanine produced an ASGP receptor which was endocytosed regardless of treatment with the tyrosine kinase inhibitors. We conclude that tyrosine kinase activity modulates the rate of receptor endocytosis at a point early in the internalization process.
 ...
PMID:Defective asialoglycoprotein receptor endocytosis mediated by tyrosine kinase inhibitors. Requirement for a tyrosine in the receptor internalization signal. 751 56

Synaptophysin is a major transmembrane glycoprotein of a type of small vesicle with an electron-translucent content (SET vesicles), including the approximately 50-nm presynaptic vesicles in neuronal cells, and of similar, somewhat larger (< or = approximately 90 nm) vesicles (SLMV) in neuroendocrine (NE) cells. When certain epithelial non-NE cells, such as human hepatocellular carcinoma PLC cells, were cDNA transfected to synthesize synaptophysin, the new molecules appeared in specific SET vesicles. As this was in contrast to other reports that only NE cells were able to sort synaptophysin away from other plasma membrane proteins into presynaptic- or SLMV-type vesicles, we have further characterized the vesicles containing synaptophysin in transfected PLC cells. Using fractionation and immunoisolation techniques, we have separated different kinds of vesicles, and we have identified a distinct type of synaptophysin-rich, small (30-90-nm) vesicle that contains little, if any, protein of the constitutive secretory pathway marker hepatitis B surface antigen, of the fluid phase endocytosis marker HRP, and of the plasma membrane recycling endosomal marker transferrin receptor. In addition, we have found variously sized vesicles that contained both synaptophysin and transferrin receptor. A corresponding result was also obtained by direct visualization, using double-label immunofluorescence microscopy for the endocytotic markers and synaptophysin in confocal laser scan microscopy and in double-immunogold label electron microscopy. We conclude that diverse non-NE cells of epithelial nature are able to enrich the "foreign" molecule synaptophysin in a category of SET vesicles that are morphologically indistinguishable from SLMV of NE cells, including one type of vesicle in which synaptophysin is sorted away from endosomal marker proteins. Possible mechanisms of this sorting are discussed.
 ...
PMID:Sorting of synaptophysin into special vesicles in nonneuroendocrine epithelial cells. 779 14

The role of the transferrin receptor in capturing and conveying transferrin through the cell during the iron-donating cycle of receptor-mediated endocytosis has been studied extensively. Nevertheless, almost nothing is known of how human transferrin binds to its receptor. In an initial approach toward delineating the receptor-recognition site(s) of human transferrin, we have studied the interactions of proteolytically-cleaved, single-sited fragments of transferrin, representing the N- and C-lobes of the molecule respectively, with cells expressing the transferrin receptor on their plasma membranes. Only the C-fragment was found capable of donating iron to hepatoma-derived HuH-7 cells or of binding to surface receptors of HuH-7 and leukemic K562 cells. Although no association of N- and C-fragments could be demonstrated by gel chromatography, the presence of excess N-fragment strengthened the binding of C-fragment by an order of magnitude. An explanation of these observations is that the primary receptor recognition site of human transferrin is on the C-lobe of the protein, but that prior binding of this lobe to receptor enables the N-lobe to respond to receptor as well, either directly or by interaction with the bound C-lobe.
 ...
PMID:Primary receptor-recognition site of human transferrin is in the C-terminal lobe. 812 19

A recombinant plasmid carrying human transferrin receptor cDNA in reverse orientation downstream from the human cytomegalovirus immediate early promoter/enhancer element was introduced into the HuH-7 human hepatoma cell line by lipofection. Cell surface transferrin binding and iron uptake from transferrin each decreased by about 50% in stable transfectants bearing integrated antisense DNA expression vector. Northern blot analysis indicated that the abundance of target transferrin receptor message was not altered by antisense RNA. These results suggest that the antisense transcript interferes with expression of the endogenous transferrin receptor gene at the level of translation.
 ...
PMID:Antisense suppression of transferrin receptor gene expression in a human hepatoma cell (HuH-7) line. 838 64

Iron chelating agents have been demonstrated to inhibit tumour cell growth. However, in vitro and in vivo results using desferrioxamine a hexadentate iron chelating agent, for anti-cancer treatment are not always in agreement. Therefore, we have studied the response of three human tumour cell lines (HL-60 promyelocytic leukaemia, MCF-7 breast cancer and HepG2 hepatoma), grown in culture medium supplemented with either human pooled (HPS) or fetal bovine serum (FBS), to desferrioxamine. Desferrioxamine, at micromolar concentrations, induced severe cytotoxicity in all tumour cell lines grown in FBS medium. When grown in HPS medium, comparable desferrioxamine cytotoxicity was observed in the millimolar range. The addition of 50% saturated human transferrin to FBS medium resulted in protection against desferrioxamine cytotoxicity. HL-60 cells were further studied for iron metabolism characteristics. HL-60 cells, grown in medium with FBS, were found to have an 8.4 fold increase in surface transferrin receptor (TfR) expression (P < 0.001) as compared with HL-60 cells grown in medium with HPS. However, iron uptake of HPS cultured HL-60 cells, after incubation with saturated human transferrin, was higher, resulting in a higher concentration of iron in HPS cultured HL-60 cells as compared with FBS cultured cells (1.72 +/- 0.02 mumol/g protein v. 1.32 +/- 0.14 mumol/g protein; P < 0.001). Using desferrioxamine it was shown that TfR expression is dependent on the biological availability of iron in the cell. Consistent with the lower iron content in FBS cultured cells, we conclude that the cytotoxicity of desferrioxamine is dependent on the ability of cells to replenish cellular iron stores from the culture medium. Cells grown in FBS medium lack this ability and are therefore more susceptible to desferrioxamine.
 ...
PMID:The in vitro response of human tumour cells to desferrioxamine is growth medium dependent. 843 91

Iron-regulatory proteins (IRP1 and IRP2) are RNA-binding proteins that bind to stem-loop structures known as iron-responsive elements (IREs). IREs are located in the 5'- or 3'-untranslated regions (UTRs) of specific mRNAs that encode proteins involved in iron homeostasis. The binding of IRPs to 5' IREs represses translation of the mRNA, whereas the binding of IRPs to 3' IREs stabilizes the mRNA. IRP1 and IRP2 binding activities are regulated by intracellular iron levels. In addition, nitric oxide (NO.) increases the affinity of IRP1 for IREs. The role of NO. in the regulation of IRP1 and IRP2 in rat hepatoma cells was investigated by using the NO.-generating compound S-nitroso-N-acetylpenicillamine (SNAP), or by stimulating cells with multiple cytokines and lipopolysaccharide (LPS) to induce NO. production. Mitochondrial and IRP1 aconitase activities were decreased in cells producing NO(.). NO. increased IRE binding activity of IRP1, but had no effect on IRE binding activity of IRP2. The increase in IRE binding activity of IRP1 was coincident with the translational repression of ferritin synthesis. Transferrin receptor (TfR) mRNA levels were increased in cells treated with NO.-generating compounds, but not in cytokine- and LPS-treated cells. Our data indicate that IRP1 and IRP2 are differentially regulated by NO. in rat hepatoma cells, suggesting a role for IRP1 in the regulation of iron homeostasis in vivo during hepatic inflammation.
 ...
PMID:Differential regulation of IRP1 and IRP2 by nitric oxide in rat hepatoma cells. 863 20


<< Previous 1 2 3 4 5 6 7 8 Next >>