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

The regulation of the hepatic catabolism of normal human very-low-density lipoproteins (VLDL) was studied in human-derived hepatoma cell line HepG2. Concentration-dependent binding, uptake and degradation of 125I-labeled VLDL demonstrated that the hepatic removal of these particles proceeds through both the saturable and non-saturable processes. In the presence of excess unlabeled VLDL, the specific binding of 125-labeled VLDL accounted for 72% of the total binding. The preincubation of cells with unlabeled VLDL had little effect on the expression of receptors, but reductive methylation of VLDL particles reduced their binding capacity. Chloroquine and colchicine inhibited the degradation of 125I-labeled VLDL and increased their accumulation in the cell, indicating the involvement of lysosomes and microtubuli in this process. Receptor-mediated degradation was associated with a slight (13%) reduction in de novo sterol synthesis and had no significant effect on the cellular cholesterol esterification. Competition studies demonstrated the ability of unlabeled VLDL, low-density lipoproteins (LDL) and high-density lipoproteins (HDL) to effectively compete with 125I-labeled VLDL for binding to cells. No correlation was observed between the concentrations of apolipoproteins A-I, A-II, C-I, C-II and C-III of unlabeled lipoproteins and their inhibitory effect on 125I-labeled VLDL binding. When unlabeled VLDL, LDL and HDL were added at equal contents of either apolipoprotein B or apolipoprotein E, their inhibitory effect on the binding and uptake of 125I-labeled VLDL only correlated with apolipoprotein E. Under similar conditions, the ability of unlabeled VLDL, LDL and HDL to compete with 125I-labeled LDL for binding was a direct function of only their apolipoprotein B. These results demonstrate that in HepG2 cells, apolipoprotein E is the main recognition signal for receptor-mediated binding and degradation of VLDL particles, while apolipoprotein B functions as the sole recognition signal for the catabolism of LDL. Furthermore, the lack of any substantial regulation of beta-hydroxy-beta-methylglutaryl-CoA reductase and acyl-CoA:cholesterol acyltransferase activities subsequent to VLDL degradation, in contrast to that observed for LDL catabolism, suggests that, in HepG2 cells, the receptor-mediated removal of VLDL proceeds through processes independent of those involved in LDL catabolism.
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PMID:Studies on the binding and degradation of human very-low-density lipoproteins by human hepatoma cell line HepG2. 300 89

The human apolipoproteins are secretory proteins some of which have been shown to undergo proteolytic processing and post-translational addition of carbohydrate. Apolipoprotein A-I (apo-A-I), the predominant protein associated with high density lipoproteins, undergoes co-translational proteolytic processing as well as post-translational conversion of proapo-A-I to mature apo-A-I following cellular secretion. Utilizing the human hepatoma cell line HEP-G2, we have established that, in addition to proteolytic processing, secreted nascent apo-A-I is acylated with palmitate. Uniformly labeled [14C]palmitate and [1-14C]palmitate were each incorporated into apo-A-I when analyzed by sodium dodecyl sulfate gel electrophoresis and autoradiography. The acylation of apo-A-I with palmitate was confirmed by immunoprecipitation and gas chromatography/mass spectrometry. Hydroxylamine treatment resulted in the deacylation of apo-A-I. Although three of the apo-A-I isoforms analyzed by two-dimensional gel electrophoresis were shown to contain radio-labeled palmitate, 80% of acylated apo-A-I was in the proapolipoprotein A-I isoform. [14C]Oleate was not incorporated in secreted apo-A-I, indicating the specificity of the acylation of apo-A-I. Incubation of [14C] palmitate-acylated apo-A-I in serum and plasma under conditions in which proapo-A-I is proteolytically cleaved to mature apo-A-I did not result in deacylation. These data establish that fatty acid acylation occurs in human secretory proteins in addition to the previously reported acylation of cellular membrane proteins. These results suggest that the covalent linkage of lipids to apolipoproteins may play a critical role in apolipoprotein and lipoprotein metabolism.
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PMID:Human apolipoprotein A-I. Post-translational modification by fatty acid acylation. 300 8

Apolipoprotein (apo) A-I is the principal protein component of high density lipoproteins and the major in vivo activator of lecithin-cholesterol acyltransferase. We have used the human hepatoma cell line, HepG2, as a model to examine the ability of estrogen to modulate hepatic synthesis of apo-A-I. Primer extension studies have demonstrated that the major transcriptional initiation site of the apo-A-I gene utilized in the hepatoma cells is the same as that used in human liver. The kinetics of induction of high- and low-affinity estrogen-binding sites, rates of secretion of apolipoproteins, and apo-A-I mRNA levels were examined following treatment of the cells with estrogen. Initial concentrations of 20 nM 17 beta-estradiol resulted in a 14-15-fold increase in the levels of high-affinity nuclear estrogen-binding sites within 8 h, while the level of low-affinity sites increased by only 10%. During the same period, the levels of apo-A-I mRNA and the rate of accumulation of the secreted protein increased by 55 and 50%, respectively. New steady state levels of apo-A-I mRNA and rates of accumulation of protein, approximately twice those in control cultures, were established within 24-48 h of exposure to hormone. Experiments with a 50-fold higher concentration of estrogen resulted in only an additional 10% increase in mRNA levels. The increase in mRNA levels following estrogen treatment was adequate to account for 85-90% of the elevation observed in the rate of accumulation of secreted apo-A-I. Comparison of the apo-A-I mRNA levels in HepG2 cells with those present in human liver revealed that the concentration of the mRNA was approximately 3-fold lower than that found in vivo.
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PMID:Kinetics of estrogen-dependent modulation of apolipoprotein A-I synthesis in human hepatoma cells. 300 89

Apolipoprotein A-I is a major secretory product of the human hepatoma cell line, Hep G2; approx. 70% of apolipoprotein A-I was separated from the medium as lipid-poor apolipoprotein A-I in the d greater than 1.21 g/ml fraction while 30% was associated with high-density lipoproteins (HDL) of d 1.063-1.21 g/ml. The lipid-poor apolipoprotein A-I contains 50% proapolipoprotein A-I which is similar to the isoform distribution in Hep G2 preformed HDL. We tested the ability of lipid-poor apolipoprotein A-I from Hep G2 to form complexes with dimyristoylphosphatidylcholine (DMPC) vesicles at DMPC/apolipoprotein A-I molar ratios of 100:1 and 300:1. Lipid-poor apolipoprotein A-I was recovered in complex form while at a 300:1 ratio, 68.8 +/- 6.3% was recovered. On electron microscopy, the former complexes were small discs 16.9 nm +/- 4.5 S.D. in diameter while the latter were larger discs 21.4 +/- 4.4 nm diameter. Non-denaturing gradient gel electrophoresis of complexes formed at a 100:1 ratio had a peak in the region corresponding to 9.64 +/- 0.08 nm; these particles possessed two apolipoprotein A-I molecules. At the higher ratio, 300:1, two distinct complexes were identifiable, one which banded in the 9.7 nm region and the other in the 16.9-18.7 nm region. The former particles contained two molecules of apolipoprotein A-I and the latter, three molecules. This study demonstrates that lipid-poor apolipoprotein A-I which is rich in more basic isoforms forms discrete lipoprotein complexes similar to those formed by mature apolipoprotein A-I. It is further suggested that, under the appropriate conditions, precursor or nascent HDL may be assembled extracellularly.
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PMID:Lipid-poor apolipoprotein A-I in Hep G2 cells: formation of lipid-rich particles by incubation with dimyristoylphosphatidylcholine. 303 92

Apo-A-I, the major protein component of high density lipoproteins, appears intracellularly as an intermediate precursor (pro-apo-A-I) with a hexapeptide extension (RHFWQQ) at its amino terminus. Proteolytic processing of pro-apo-A-I to apo-A-I has been shown to occur extracellularly in cell and organ cultures from rat and human tissues. Recently, however, intracellular conversion has been detected in chickens. To determine what distinguishes and regulates these two processing methods, the proteolytic processing and secretion of apo-A-I was studied by metabolic labeling in chick hepatocytes and in Hep-G2 cells (derived from a human hepatocellular carcinoma). The proportions of intracellular and secreted pro-apo-A-I and apo-A-I were measured by sequencing NH2-terminal portions of the proteins and determining the location of radio-labeled amino acids. Chick hepatocytes cultured in the absence of hormones or fetal bovine serum secreted primarily processed apo-A-I (83%). In the presence of serum these cells secreted only pro-apo-A-I, whereas incubation with a combination of hormones (insulin, triiodothyronine, dexamethasone) resulted in secretion of a nearly equal mixture of the pro- and processed forms of the protein. In contrast, Hep-G2 cells, maintained in the absence of serum, secreted only pro-apo-A-I; when grown in the presence of serum these cells secreted a mixture of pro- and processed apo-A-I. Under conditions in which chick hepatocytes and Hep-G2 cells secreted both forms of the protein, a mixture of pro- and processed apo-A-I was also found intracellularly; when only the pro-form was secreted, the cells likewise contained only pro-apo-A-I. Under all the above conditions, the secreted apo-A-I exhibited similar isoform patterns in two-dimensional gel electrophoresis. These data show that both chick hepatocytes and human hepatoma cells are capable of intracellularly processing pro-apo-A-I to apo-A-I, and that the extent of intracellular processing is controlled by the cell's hormonal environment.
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PMID:Regulation of apo-A-I processing in cultured hepatocytes. 309 26

The binding of 125I-HDL obtained from nephrotic rats (HDLne) containing only apo A-I and apo C, to rat hepatoma cells (Fu5AH) grown to confluency was studied under conditions which increased the free cholesterol or the cholesteryl ester content. The high affinity binding (Kd = 5 nM) measured at 4 degrees C was unchanged. This transformed cell line also exhibited greater specificity for rat HDL compared to human HDL than has been reported for other types of cultured cells. When the cells were allowed to internalize and degrade HDLne at 37 degrees C, the acid-soluble products were derived almost entirely from the breakdown of apo A-I. Competition experiments with an LDL fraction from nephrotic rat plasma (LDLne) which contained 20% of apo A-I indicated that it was as effective as other rat HDL preparations in competing for the binding of HDLne at 4 degrees C, based on its content of apo A-I. Control experiments indicated that labeled apo A-I in HDLne exchanged less than 1% when incubated with a 50-fold excess of unlabeled LDLne for 2 h at 4 degrees C. These results point to a critical role of cell type in HDL binding. They support the view that apo A-I is a ligand. The up-regulation of high affinity HDL binding by cholesterol which has been reported with cultured human fibroblasts and Hep G2 cells does not occur in the Fu5AH rat hepatoma cell line.
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PMID:High density lipoprotein binding by rat Fu5AH hepatoma cells is not related to cholesterol content. 311 94

We have isolated and characterized a 2.5-kilobase pairs genomic DNA fragment which includes the 5'-flanking region and the first and second exons of the human apolipoprotein (apo) A-I gene. The major transcriptional start site was determined by primer extension analysis and is 235 base pairs (bp) upstream from the AUG translational start codon in liver and 234 bp upstream in the intestine. TATA box-like and CAT box-like sequences and two GC box sequences are present in the intestine 30, 108, 220, and 440 bp upstream, respectively, from the transcriptional start site. Fragments of 570 bp (-487 to +71) and 2.15 kilobase pairs (-2067 to +99) containing the 5'-flanking region of the apoA-I gene were fused upstream to the bacterial chloramphenicol acetyltransferase (CAT) gene. These constructs, designated pA-I(0.6)CAT and pA-I(2.2)CAT, respectively, were introduced into human oral epithelial cells (KB), mouse NIH 3T3 cells, Chinese hamster ovary (CHO) cells, human hepatoma cells (Hep G2), human duodenal epithelial cells (Hutu 80), and human colonic epithelial cells (Caco-2) by calcium phosphate coprecipitation. When compared with control vectors, highly efficient CAT expression of both the pA-I(0.6)CAT and pA-I(2.2)CAT constructs were observed only in cells derived from the liver (Hep G2) and intestine (Caco-2), which is consistent with the tissue specificity of expression of the native gene. Analysis of deletion mutants of the human apoA-I 5'-flanking region revealed that: 1) the region from -250 to -199 bp, from -487 to -413 bp, and -1021 to -691 bp upstream from the transcriptional start site contain sequences required for maximum gene expression; and 2) the regions from -2067 to -1476 bp and -199 to -80 bp contain the sequences required for tissue-specific repression of apoA-I gene expression in non-apoA-I producing cells.
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PMID:Tissue-specific expression of apolipoprotein A-I (ApoA-I) is regulated by the 5'-flanking region of the human ApoA-I gene. 314 80

In order to study the regulation of expression of the two arginase genes in mammalian tissues, we undertook to clone cDNA specific for rat liver arginase. mRNA was isolated from rat liver polysomes enriched for the arginase message by immunopurification and was used to produce an 800-member cDNA library carried in pBR322. Four arginase clones were identified by hybrid selection, and one was used to find two others following colony hybridization. Clonal identity was verified by its enrichment in the cDNA made from immunopurified mRNA; by hybrid selection, immunoprecipitation, and competition by purified arginase; hybridization on Northern analysis with liver-derived RNA (high in arginase) and its absence with mRNA from tissues low in arginase; and independent identification by hybrid selection and colony hybridization. Northern analysis of mRNA from H4-II-E-C3 (H4) rat hepatoma cells in which arginase activity was induced by hydrocortisone demonstrated equal, eightfold augmentation of both arginase activity and arginase mRNA levels. Southern blot analysis of DNA from these cells indicated that no change in arrangement or copy number accompanied induction. Southern analysis also suggested that the gene for rat liver arginase is present in a single copy, without pseudogenes, and that a high degree of homology exists between it and its mouse counterpart.
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PMID:Cloning of rat liver arginase cDNA and elucidation of regulation of arginase gene expression in H4 rat hepatoma cells. 346 68

The early events in high-density lipoprotein biogenesis involve the extracellular action of two converting enzymes affecting the cleavage of the prosegment of either proapolipoprotein A-I or proapolipoprotein A-II and the generation of mature apolipoprotein (apo) A-I and apo A-II, the main apolipoprotein of high-density lipoproteins. These two converting enzymes differ from each other in mechanism of action and specificity. The observation that they can be secreted by human hepatocarcinoma G2 cells in culture provides an experimental basis for examining the possible coordination between the synthesis and secretion of these two converting enzymes and the events attending the production and cellular export of apo A-I and apo A-II.
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PMID:Proapolipoprotein-converting enzymes and high-density lipoprotein early events in biogenesis. 354 66

We have confirmed that arginine-deficient diets increase the liver activities (units per 100 g) of the first four arginine biosynthetic enzymes of the urea cycle in Wistar rats, but not the activity of arginase. In contrast, rat liver cells cultured in monolayers for 48, 72 or 96 h in arginine-free L-15 or minimum essential medium showed no changes in carbamoyl-phosphate synthase (EC 6.3.4.16), ornithine transcarbamylase (EC 2.1.3.3), argininosuccinate synthase (EC 6.3.4.5), argininosuccinase (EC 4.3.2.1) or arginase (EC 3.5.3.1) activities. The arginine content of the cells grown on deficient medium was 36% of that of cells grown on 2.9 mM arginine-sufficient L-15, yet the urea excretion rate into the medium was reduced to 7% of the rate in control cells and the excretion of orotic acid was 400% of that in control cells. A Morris rat hepatoma cell line, 7800C1, which maintains activities of all five urea cycle enzymes, showed no consistent increases in the activities of the first four enzymes when the arginine in the medium was varied between 0 and 2 mM. Thus, in spite of severe arginine deficiency, cultured rat liver cells and hepatoma cells do not show the derepression-like response seen by other investigators when nonliver cells were cultured in arginine-deficient media. The difference between in vivo and in vitro effects of arginine deficiency on urea cycle activities remains unexplained.
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PMID:Differing effects of arginine deficiency on the urea cycle enzymes of rat liver, cultured hepatocytes and hepatoma cells. 368 73


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