UMLS:C0019204 - FACTA Search

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Query: UMLS:C0019204 (hepatocellular carcinoma)
71,386 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We have carried out parallel analyses of the regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGr) and low density lipoprotein receptor (LDLr) in two highly differentiated human hepatoma cell lines, HepG2 and Hep3B, and primary cultures of human fibroblasts. Analyses of the levels of HMGr and LDLr mRNAs under a variety of culture conditions that perturb intracellular sterol metabolism, or which differ in the levels of extracellular sterols, indicated that the hepatoma cells and fibroblasts responded similarly in terms of the repression or induction ratios of both mRNAs. However, the absolute levels of the mRNAs were severalfold higher in the hepatoma cells. The major difference between the responses of the hepatoma cells and fibroblasts involved the increase in expression of LDLr which occurred upon shifting the cells from complete to lipoprotein-depleted serum. Under these conditions, the 3-fold increase in rate of synthesis of LDLr in the hepatomas was closely matched by increases in the level of its mRNA. In the case of fibroblasts, a 10-fold increase in translational efficiency was required to explain the 30-fold change in rate of synthesis of LDLr. Polysome profiles from both hepatoma cells and fibroblasts suggest that the rate of elongation or termination on LDLr mRNA is relatively low in the presence of reconstituted complete serum, and that it increases in fibroblasts upon lipoprotein depletion, but not in the hepatoma cells. These data indicate that hepatic expression of LDLr may be relatively refractory to induction by decreased circulating levels of lipoprotein when compared with peripheral tissues.
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PMID:Differences between the regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and low density lipoprotein receptor in human hepatoma cells and fibroblasts reside primarily at the translational and post-translational levels. 165 47

Regulation of expression of the genes for the low density lipoprotein receptor (LDLR) and 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) is of central importance in the control of cholesterol metabolism and thus in influencing the concentration of low density lipoprotein in the plasma. This can be studied by investigating the effects of factors (hormones, drugs, etc.) on the levels of mRNA for these genes. An RNase protection assay is reported for measurement of the levels of mRNA for the LDLR and HMGR. Several probes have been developed for these genes, together with probes for the "housekeeping" genes, beta-actin and glyceraldehyde-3-phosphate dehydrogenase. Various conditions in the assay have been examined and optimised, e.g. conditions for solution hybridization and RNase digestion and the use of "sense" RNA standards. The assay allows accurate measurement of approximately 2 x 10(7) copies of LDLR and HMGR mRNAs, which is equivalent to the number of copies present in approximately 1 x 10(6) human dermal fibroblasts and approximately 5 x 10(5) Hep G2 liver hepatoma cells cultured in 10% fetal calf serum. The average number of copies of mRNA per cell was estimated in fibroblasts and Hep G2 cells under various conditions of regulation of the LDLR and revealed the following: [table: see text] Under the chosen conditions 10 copies per cell was the detection limit for the assay. The effect of these treatments on the number of copies of mRNA per cell for beta-actin and glyceraldehyde-3-phosphate dehydrogenase was also determined.
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PMID:A sensitive RNase protection assay for the quantitation of the mRNAs for the LDL receptor and HMG-CoA reductase in human total RNA. Effects of treatments on cells in culture designed to up- and down-regulate expression of the LDL receptor. 182 10

Regulation of squalene epoxidase in the cholesterol biosynthetic pathway was studied in a human hepatoma cell line, HepG2 cells. Since the squalene epoxidase activity in cell homogenates was found to be stimulated by the addition of Triton X-100, enzyme activity was determined in the presence of this detergent. Incubation of HepG2 cells for 18 h with L-654,969, a potent competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, increased squalene epoxidase activity dose-dependently. On the other hand, low density lipoprotein (LDL) and 25-hydroxy-cholesterol decreased the enzyme activity. These results demonstrate that squalene epoxidase is regulated by the concentrations of endogenous and exogenous sterols. The affinity of the enzyme for squalene was not changed by treatment with L-654,969. Cytosolic (S105) fractions, prepared from HepG2 cells treated with or without L-654,969, had no effect on microsomal squalene epoxidase activity of HepG2 cells, in contrast to the stimulating effect of S105 fractions from rat liver homogenate. Mevalonate, LDL, and oxysterol treatment abolished the effect of L-654,969. Simultaneous addition of cycloheximide and actinomycin D also prevented enzyme induction in HepG2 cells. From these results, the change in squalene epoxidase activity is thought to be caused by the change in the amount of enzyme protein. It is further suggested that squalene epoxidase activity is suppressed only by sterols, not by nonsterol derivative(s) of mevalonate, in contrast to the regulation of HMG-CoA reductase.
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PMID:Regulation of squalene epoxidase in HepG2 cells. 196 54

We report the isolation and nucleotide sequence of the human farnesyl pyrophosphate synthetase cDNA, an enzyme in the cholesterogenic pathway. Partial cDNAs for the human farnesyl pyrophosphate synthetase were isolated by screening human hepatoma (HepG2) and placental cDNA libraries with the rat liver cDNA for farnesyl pyrophosphate synthetase as a probe. Anchored polymerase chain reaction was used to isolate the 5'-end of the cDNA. The nucleotide sequence of the human farnesyl pyrophosphate synthetase cDNA has high identity (86%) to the rat liver cDNA. Treatment of the human monocytic leukemia cell line THP-1 with phorbol esters led to 2--7-fold increases in mRNA concentrations for the three cholesterogenic enzymes, farnesyl pyrophosphate synthetase, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, and HMG-CoA synthase within 5 h. Immunoprecipitation of radiolabeled cells demonstrated that there was a corresponding increase in the rate of synthesis of all three proteins. The addition of cycloheximide to cells also led to increases in the mRNA concentrations of the three enzymes. Treatment of cells with phorbol esters and cycloheximide resulted in superinduction of all three mRNAs; HMG-CoA synthase mRNA levels increased 35-fold, farnesyl pyrophosphate synthetase 17-fold, and HMG-CoA reductase 16-fold 5 h after treatment. The mRNA levels returned to pretreatment levels by 20 h. Cells were also preincubated in the presence of a lipoprotein-deficient fraction of serum plus mevinolin to induce the levels of the three mRNAs. Addition of phorbol esters and cycloheximide to these derepressed cells led to further increases in the mRNA levels for all three enzymes. These results are consistent with the hypothesis that THP-1 cells contain a short-lived negative transcription factor which regulates transcription of the FPP synthetase, HMG-CoA reductase, and HMG-CoA synthase genes. Phorbol esters also regulate these same genes, presumably by modifying a common negative transcription factor and/or by inducing a positive transcription factor(s).
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PMID:Isolation and sequence of the human farnesyl pyrophosphate synthetase cDNA. Coordinate regulation of the mRNAs for farnesyl pyrophosphate synthetase, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and 3-hydroxy-3-methylglutaryl coenzyme A synthase by phorbol ester. 196 62

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase mRNA is expressed in two highly differentiated human hepatoma cell lines, HepG2 and Hep3B, at exceptionally high levels relative to human fetal liver and fibroblasts. Blotting experiments revealed that the mRNA consists of three major size classes of approximately 4.7, 4.5, and 4.2 kb that responded coordinately to agents that alter HMG-CoA reductase activity. In view of the markedly elevated levels of reductase mRNA in the hepatoma cell lines, we compared the pattern of transcriptional initiation in these cells with those in normal liver and fibroblasts. These analyses revealed a complex pattern of initiation sites, all of which were suppressed by oxysterols, extending over approximately 300 nucleotides. However, all of the major sites detected in the hepatomas could also be found in human liver and fibroblasts. Heterogeneity of transcriptional initiation does not account for the three major size classes of mRNA detected by RNA blotting. RNase H mapping demonstrates that these are produced by use of three polyadenylation sites. To determine the extent to which these sites have been conserved between the human gene and the previously characterized Chinese hamster gene, we cloned and sequenced the 3' untranslated region of the longest form of the human mRNA. These studies revealed that, despite a high overall degree of sequence conservation, the spectrum of polyadenylation sites used differs qualitatively between the two species. Features of the mRNA sequence that may contribute to these differences are described.
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PMID:Characterization of three distinct size classes of human 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA: expression of the transcripts in hepatic and nonhepatic cells. 197 42

We have examined the mechanism of the inhibition of cholesterol synthesis in cells treated with exogenous sphingomyelinase. Treatment of rat intestinal epithelial cells (IEC-6), human skin fibroblasts (GM-43), and human hepatoma (HepG2) cells in culture with sphingomyelinase resulted in a concentration- and time-dependent inhibition of the activity of HMG-CoA reductase, a key regulatory enzyme in cholesterol biosynthesis. The following observations were obtained with IEC-6 cells. Free fatty acid synthesis or general cellular protein synthesis was unaffected by the addition of sphingomyelinase. Addition of sphingomyelinase to the in vitro reductase assay had no effect on activity, suggesting that an intact cell system is required for the action of sphingomyelinase. The products of sphingomyelin hydrolysis, e.g., ceramide and phosphocholine, had no effect on reductase activity. Sphingosine, a further product of ceramide metabolism, caused a stimulation of reductase activity. Examination of the incorporation of [3H]acetate into the nonsaponifiable lipid fractions in the presence of sphingomyelinase showed no changes in the percent distribution of radioactivity in the post-mevalonate intermediates of the cholesterol biosynthetic pathway, but there was increased radioactivity associated with the polar sterol fraction. Pretreatment of cells with ketoconazole, a known inhibitor of oxysterol formation, prevented the inhibition of reductase activity by sphingomyelinase and decreased the incorporation of [3H]acetate in the polar sterol fraction. Ketoconazole had no effect on exogenous sphingomyelinase activity in vitro in the presence or absence of cells. Endogenous sphingomyelinase activity was also unaffected by ketoconazole. Addition of inhibitors of endogenous sphingomyelinase activity, e.g., chlorpromazine, desipramine, and N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W-7), to the culture medium caused a dose-dependent stimulation of reductase activity. However, these agents had no effect on the inhibition of reductase activity by exogenous sphingomyelinase. Treatment of cells with small unilamellar vesicles of dioleyl phosphatidylcholine or high density lipoprotein3 resulted in increased efflux of cholesterol and stimulation of reductase activity. Under similar conditions, the inhibitory effect of exogenous sphingomyelinase on reductase activity was prevented by incubation with small unilamellar vesicles of phosphatidylcholine or high density lipoprotein. These results support the hypothesis that alteration of the ratio of sphingomyelin:cholesterol in the plasma membrane plays a modulatory role on the flow of membrane cholesterol to a site where it may be converted to a putative regulatory molecule, possibly an oxysterol.
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PMID:Plasma membrane sphingomyelin and the regulation of HMG-CoA reductase activity and cholesterol biosynthesis in cell cultures. 201 Jun 84

The level of hepatic triglyceride lipase (H-TGL) synthesis and secretion was examined in response to changes in cholesterol biosynthesis in the human hepatoma cell line HepG2. Cells were first fed a lipoprotein-deficient serum-supplemented medium to eliminate exogenous cholesterol. Mevinolin, a 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase inhibitor, was then added at a concentration (37 microM) which inhibited cholesterol biosynthesis by greater than 85% and decreased total cell cholesterol from 36.1 to 27.4 micrograms/ml of cell protein. Mevinolin treatment caused a 4.9 +/- 0.8-fold increase in the amount of H-TGL activity secreted into the medium, a 1.8 +/- 0.4-fold rise in H-TGL-specific mRNA, and a concurrent 14-fold increase in HMG-CoA reductase mRNA. Addition of 1 mM mevalonic acid to normal or mevinolin-treated cells raised the cellular cholesterol content and decreased the amount of secreted H-TGL activity to levels below control values. Mevalonic acid also prevented mevinolin-induction of H-TGL and HMG-CoA reductase mRNA, suggesting a common regulatory step for H-TGL and HMG-CoA reductase. Exposure of cells to mevinolin and 25-hydroxycholesterol together resulted in a marked repression of HMG-CoA reductase mRNA levels, whereas these conditions further enhanced the secretion of H-TGL activity and the expression of H-TGL mRNA. These results demonstrate a differential role for 25-hydroxycholesterol in the regulation of H-TGL and HMG-CoA reductase expression.
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PMID:Differential regulation of hepatic triglyceride lipase and 3-hydroxy-3-methylglutaryl-CoA reductase gene expression in a human hepatoma cell line, HepG2. 217 19

Human hepatoma HepG2 cells were used to demonstrate coordinate regulation of three enzymes of cholesterol synthesis under a variety of conditions. Addition of either delipidized serum and mevinolin or low density lipoprotein, 25-hydroxycholesterol, or mevalonic acid to HepG2 cells resulted in rapid changes both in the levels of the mRNAs and in the rates of synthesis of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase, HMG-CoA reductase, and farnesyl pyrophosphate synthetase (prenyltranferase). In all cases, the changes in mRNA levels were paralleled by changes in the rates of specific protein synthesis. Pulse-chase techniques were used to determine the half-lives of all three proteins. Addition of low density lipoprotein to the media during the chase increased the rate of degradation of HMG-CoA reductase 4.6-fold but had no affect on the half-lives of HMG-CoA synthase or prenyltransferase. Therefore, we conclude that the coordinate regulation of these three enzymes under a variety of conditions occurs at the level of enzyme synthesis and not at the level of protein stability.
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PMID:Coordinate regulation of 3-hydroxy-3-methylglutaryl-coenzyme A synthase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, and prenyltransferase synthesis but not degradation in HepG2 cells. 256 58

Cellular processes responsible for maintaining cholesterol homoeostasis are highly regulated. To determine whether two of these processes, cholesterol biosynthesis and receptor-mediated uptake of low-density lipoprotein (LDL), are co-ordinately regulated in human liver, we employed a human hepatoma cell line (HepG2) and measured the accumulation of mRNA for LDL receptor, 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and HMG-CoA synthase under a variety of conditions. Genomic Southern-blot analysis demonstrated that the integrity of these genes is maintained in the transformed cell. Treatment of HepG2 cells with mevalonate, 25-hydroxycholesterol, LDL, lovastatin or miconazole resulted in a similar effect on the accumulation of all three mRNAs at the concentrations tested. The onset of the response to drug, whether repression or induction of mRNA accumulation, occurred after approximately the same period of exposure for each mRNA. We conclude that the expression of the LDL receptor, HMG-CoA reductase and HMG-CoA synthase is co-ordinately regulated in HepG2 cells.
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PMID:Co-ordinate regulation of low-density-lipoprotein receptor and 3-hydroxy-3-methylglutaryl-CoA reductase and synthase gene expression in HepG2 cells. 256 63

Cholesterol biosynthesis was characterized in cell-free post-mitochondrial supernatant systems prepared from both normal rat liver and Morris hepatoma 3924A. The rate of cholesterol synthesis per cell was 9-fold greater in the tumour system than in that from normal liver, and the tumour systems showed the loss of rate-limiting control at the hydroxymethylglutaryl-CoA reductase (HMGR)-catalysed step. The apparent absence of rate-limiting control over cell-free tumour cholesterogenesis was traced primarily to a discoordinate and dramatic increase in the amount of HMGR in the tumour relative to the liver system. Preliminary evidence for an altered control of the post-lanosterol portion of the pathway was also obtained with the tumour system.
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PMID:A discoordinate increase in the cellular amount of 3-hydroxy-3-methylglutaryl-CoA reductase results in the loss of rate-limiting control over cholesterogenesis in a tumour cell-free system. 270 93

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