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)

Amino acid sequence information was obtained for the NH2 terminus, and for endogenous peptides generated by trypsin digestion, of a purified, truncated form of rat hepatic squalene synthase (RSS, EC 2.5.1.21) (Shechter, I., Klinger, E., Rucker, M. L., Engstrom, R. G., Spirito, J. A., Islam, M. A., Boettcher, B. R., and Weinstein, D. B. (1992) J. Biol. Chem. 267, 8628-8635). Degenerate primers, based on the amino acid sequences, were synthesized and used for the amplification and sequencing of a 1708-base pair (bp) cDNA for RSS from the rat hepatoma cell line H35. An open reading frame of 1248 bp encoding 416 amino acids (M(r) = 48,103) was detected for RSS. We have constructed a pRSS1327 expression vector by molecular cloning of a 1327-bp cDNA, which includes sequences of the entire coding region for RSS, into pBluescript. Expression in Escherichia coli of a functional, full-length RSS was confirmed by immunoblot analysis and enzymatic activity. We present and evaluate a model for the secondary structure of RSS and its possible membrane orientation. The model predicts a 315-residue domain at the center of the protein that contains the catalytic site and is released in a soluble form by partial proteolysis. The 33-residue NH2-terminal and 98-residue COOH-terminal sections are not involved in catalysis. Sequence analysis of the catalytic domain of RSS indicate three regions with high homology to sequences in a number of functionally distinct proteins that utilize polyprenyl diphosphate substrates.
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PMID:Molecular cloning, expression, and characterization of the cDNA for the rat hepatic squalene synthase. 140 Apr 48

We have cloned and characterized the 5'-flanking region of the gene encoding human squalene synthase. We report here the promoter activity of successively 5'-truncated sections of a 1 kilobase of this region by fusing it to the coding region of a luciferase reporter gene. DNA segments of 200 base pairs (bp) 5' to the transcription start site, as determined by primer extension analysis, show a strong promoter effect on the expression of the luciferase chimeric gene and a high response to the presence of sterols when transiently transfected into the human hepatoma cell line HepG2 or to the hamster-derived CHO-K1 cells. An approximately 50-fold induction of luciferase activity, in the absence of sterols, was observed in transiently transfected HepG2 cells for fusion constructs containing sections of 200, 459, and 934 bp of the putative human squalene synthase promoter. Loss of promoter activity and response to sterols was localized to a 69-bp section located 131 nucleotides 5' to the transcription start site. Sequence analysis of this region showed that it contained a sterol regulatory element 1 (SRE-1) previously identified in other sterol regulated genes (Smith, J. R., Osborne, T. F., Brown, M. S., Goldstein, J. L., and Gil, G. (1988). J. Biol. Chem. 263, 18480-18487) and two potential NF-1 binding sites. Additional CCAAT box, SRE-1 element, and two Sp1 sites were identified 3' to this section. Sequences within this 69-bp DNA, including the SRE-1 cis-acting element, show strong binding to the purified nuclear transcription factor ADD1 (Tonzonoz, P., Kim, J. B., Graves, R. A., and Spiegelman B. M. (1993) Mol. Cell Biol. 13, 4753-4759) by mobility shift assay and footprinting analyses.
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PMID:Molecular cloning and functional analysis of the promoter of the human squalene synthase gene. 766 18

Primers, based on the cDNA nucleotide sequences for rat hepatic squalene synthase (EC 2.5.1.21) (McKenzie, T.L., Jiang, G., Straubhaar, J.R., Conrad, D., and Shechter, I. (1992) J. Biol. Chem. 267, 21368-21374), were synthesized and used for the amplification and sequencing of a 1672-base pair (bp) cDNA for the human hepatic squalene synthase (HSS) from human hepatic RNA. An open reading frame of 1251 bp encoding 417 amino acids (M(r) = 48,200) was detected for HSS. We have constructed a pHSS 1286 expression vector by molecular cloning of a 1286-bp cDNA, that includes sequences of the entire coding region for HSS, into pBluescript. Expression in Escherichia coli of a functional, full-length HSS was confirmed by immunoblot analysis and enzymatic activity. Northern blot analyses of poly(A+) RNA obtained from the human hepatoma cell line HepG2 show three distinct size classes of mRNA for HSS. 1.4-, 1.6- and 2.1-kilobase mRNA were observed. The relative abundance is in the order 1.6 > 1.4 > 2.1 and did not change when the cells were grown in the presence of 25-hydroxycholesterol or lovastatin. The ratio between the level of HSS mRNA in cells grown in the absence and presence of 5 micrograms/ml 25-hydroxycholesterol varies between 8- and 16-fold. This lowering of the mRNA level was observed when the cells were grown in 10% of either full serum or lipid-depleted serum. A 2.7- and 4.0-fold increase of HSS mRNA was observed when HepG2 cells were grown in the presence of 5 micrograms/ml lovastatin in lipid-depleted or full serum, respectively. These studies show that HSS exhibit a relatively high level of transcriptional regulation in response to 25-hydroxycholesterol regardless of the presence of cholesterol in the growth media.
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PMID:Transcriptional regulation by lovastatin and 25-hydroxycholesterol in HepG2 cells and molecular cloning and expression of the cDNA for the human hepatic squalene synthase. 768 52

The possible difference between lovastatin (mevinolin, MK-803), simvastatin (MK-733) and pravastatin (CS-514), all chemically-related competitive inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, were tested in the human hepatoma cell line Hep G2, which is often used as a model for the human hepatocyte. After an 18-hr incubation of the cells with the drugs, pravastatin (IC50 = 1900 nM) was less potent than simvastatin and lovastatin (IC50 = 34 and 24 nM, respectively) in inhibiting the sterol synthesis. As a consequence of this inhibition, the HMG-CoA reductase mRNA levels and squalene synthase activity, both negatively-regulated by sterols, were increased equally by simvastatin and lovastatin, whereas the induction by pravastatin was much less. In contrast, there were fewer differences between the compounds in inhibiting HMG-CoA reductase activity, when assayed directly in Hep G2 cell homogenates (IC50 values = 18, 61 and 95 nM for simvastatin, lovastatin and pravastatin, respectively). Moreover, in experiments with human hepatocytes in primary culture the IC50 values for inhibition of the cholesterol synthesis by simvastatin and pravastatin were of the same order of magnitude (23 and 105 nM, respectively). The results are therefore explained as follows: the three drugs act in the same way within the Hep G2 cell in terms of inhibiting HMG-CoA reductase and their subsequent effect on the feedback regulation of the cholesterol synthesis, i.e. increasing squalene synthase and HMG-CoA reductase mRNA. However, pravastatin seems to be less able to enter the cells compared with simvastatin and lovastatin, possibly because of the higher hydrophobicity of the latter compounds. The observation with human hepatocytes suggests that in Hep G2 cells a specific hepatic transporter is missing. On one hand the human hepatoma cell line Hep G2 has proved to be a good model for the study of the feedback regulation of enzymes of the cholesterol biosynthetic pathway such as HMG-CoA reductase and squalene synthase, but, on the other hand seems to be less suitable as a model for the study of specific uptake of drugs, e.g. the vastatins, in human hepatocytes.
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PMID:Pravastatin inhibited the cholesterol synthesis in human hepatoma cell line Hep G2 less than simvastatin and lovastatin, which is reflected in the upregulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase and squalene synthase. 851 61

Sterol biosynthesis requires the removal of the 14 alpha-methyl group from lanosterol in animals and fungi and from obtusifoliol in plants. This reaction is catalyzed by a microsomal cytochrome P450, the sterol 14 alpha-demethylase (P450(14DM), which is the only P450 described so far to be expressed in different phyla. A cDNA encoding human P450(14DM) was isolated from a liver cDNA library using a partial rat lanosterol 14 alpha-demethylase cDNA probe. The deduced amino acid sequence is 93% and 38--42% identical to rat and fungal P450(14DM), respectively. Expression of the human CYP51 cDNA in Escherichia coli showed that the cDNA encodes an enzyme having lanosterol 14 alpha-demethylase activity. Northern blot analysis showed that CYP51 mRNA is ubiquitously expressed with highest levels in testis, ovary, adrenal, prostate, liver, kidney, and lung. Many genes involved in cholesterol homeostasis are regulated by cholesterol or its metabolites. In the case of CYP51, cholesterol deprivation led to a 2.6- to 3.8-fold induction of mRNA levels in human adrenocortical H295R cells and this effect was suppressed by the addition of 25-hydroxycholesterol. In human hepatoma HepG2 cells, no effect of cholesterol deprivation was observed; however, the levels of CYP51 mRNA were reduced 4- to 6-fold by the addition of 25-hydroxycholesterol. Thus, like several other genes in the cholesterol biosynthetic pathway, including the 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) synthase, HMG CoA reductase, squalene synthase, and farnesyl diphosphate synthase, the expression of the human CYP51 is suppressed by oxysterols.
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PMID:The ubiquitously expressed human CYP51 encodes lanosterol 14 alpha-demethylase, a cytochrome P450 whose expression is regulated by oxysterols. 861 37

The signaling pathway involved in low density lipoprotein (LDL) receptor gene expression induced by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) was investigated in the human hepatoma HepG2 cell line. Treatment of HepG2 cells with 100 nM TPA resulted in an approximately 20-fold increase in LDL receptor mRNA level, as determined by RT-PCR, which peaked at 2-4 h of treatment and subsequently declined. The protein kinase C (PKC) inhibitors calphostin C and staurosporine prevented TPA-mediated LDL receptor mRNA induction. In contrast, TPA did not affect squalene synthase mRNA expression. Immunoblotting of cell extracts with isozyme-specific PKC antibodies revealed that HepG2 cells expressed PKC alpha, which was mainly cytosolic, and PKC beta, PK epsilon, and PKC zeta, all of which were present in both the cytosolic and particulate fractions. Treatment of HepG2 cells with 100 nM TPA resulted in translocation of cytosolic PKC alpha to the particulate fraction, with a maximum at 30 min-2 h of treatment, but was without effect on the subcellular distribution of the other isozymes. TPA treatment also led to activation of the mitogen-activated protein kinase (MAPK) ERK cascade. The specific MAPK pathway inhibitor PD98059 blocked TPA-induced ERK activation. Furthermore, pretreatment of cells with PD98059 inhibited TPA-induced LDL receptor mRNA induction. Moreover, pretreatment of cells with calphostin C inhibited TPA-mediated ERK activation and LDL receptor mRNA induction in a dose-dependent fashion. Based on a close kinetic correlation between PKC alpha translocation and ERK activation, and the effects of specific inhibitors, these findings suggest that translocation/activation of PKC alpha, and subsequent activation of the Raf-1/MEK/ERK MAPK cascade, represent key events in the transcriptional induction of LDL receptor gene by TPA in HepG2 cells.
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PMID:Phorbol ester-induced low density lipoprotein receptor gene expression in HepG2 cells involves protein kinase C-mediated p42/44 MAP kinase activation. 939 22

YM-16638 ([[5-[[3-(4-acetyl-3-hydroxy-2-propylphenoxy)propyl]thio]-1,3,4-++ +thiadiazol-2-yl] thio] acetic acid) showed a strong hypocholesterolemic effect in humans and monkeys. To clarify the mechanism of this hypocholesterolemic effect, the action of YM-16638 on cholesterol biosynthesis in the cultured human hepatoma cell line HepG2 and cynomolgus monkey liver was examined. Cholesterol biosynthesis activity derived from [14C]acetic acid, [3H/14C]mevalonic acid or [14C]isopentenyl pyrophosphate substrates was significantly decreased, but not that from [3H]farnesyl pyrophosphate or [3H]squalene substrates in HepG2 cells treated with YM-16638. Simultaneously, treatment of these cells with YM-16638 changed neither the rate of apolipoprotein B synthesis from [35S]methionine nor its secretion. In addition, the activities of hepatic cholesterol biosynthesis enzymes HMG-CoA reductase, mevalonate kinase (MK), isopentenyl pyrophosphate isomerase (IPPI), farnesyl pyrophosphate synthase (FPPS), squalene synthase and squalene epoxidase were measured in monkeys fed a diet supplemented with YM-16638. Among these enzymes, MK, IPPI and FPPS activities in the YM-16638-treated group significantly decreased by 38%, 56% and 30%, respectively, when compared to those from control animals receiving no drug treatment. These results indicate that YM-16638 has the characteristics of a cholesterol biosynthesis inhibitor.
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PMID:Effect of the hypocholesterolemic agent YM-16638 on cholesterol biosynthesis activity and apolipoprotein B secretion in HepG2 and monkey liver. 1008 20

The finding that expression of a cholesterol 7alpha-hydroxylase (CYP7A1) transgene in cultured rat hepatoma cells caused a coordinate increase in lipogenesis and secretion of apoB-containing lipoproteins led to the hypothesis that hepatic production of apoB-containing lipoproteins may be linked to the expression of CYP7A1 (Wang, S.-L., Du, E., Martin, T. D., and Davis, R. A. (1997) J. Biol. Chem. 272, 19351-19358). To examine this hypothesis in vivo, a transgene encoding CYP7A1 driven by the constitutive liver-specific enhancer of the human apoE gene was expressed in C56BL/6 mice. The expression of CYP7A1 mRNA (20-fold), protein ( approximately 10-fold), and enzyme activity (5-fold) was markedly increased in transgenic mice compared with non-transgenic littermates. The bile acid pool of CYP7A1 transgenic mice was doubled mainly due to increased hydrophobic dihydroxy bile acids. In CYP7A1 transgenic mice, livers contained approximately 3-fold more sterol response element-binding protein-2 mRNA. Hepatic expression of mRNAs encoding lipogenic enzymes (i.e. fatty-acid synthase, acetyl-CoA carboxylase, stearoyl-CoA desaturase, squalene synthase, farnesyl-pyrophosphate synthase, 3-hydroxy-3-methylglutaryl-CoA reductase, and low density lipoprotein receptor) as well as microsomal triglyceride transfer protein were elevated approximately 3-5-fold in transgenic mice. CYP7A1 transgenic mice also displayed a >2-fold increase in hepatic production and secretion of triglyceride-rich apoB-containing lipoproteins. Despite the increased hepatic secretion of apoB-containing lipoproteins in CYP7A1 mice, plasma levels of triglycerides and cholesterol were not significantly increased. These data suggest that the 5-fold increased expression of the low density lipoprotein receptor displayed by the livers of CYP7A1 transgenic mice was sufficient to compensate for the 2-fold increase production of apoB-containing lipoproteins. These findings emphasize the important homeostatic role that CYP7A1 plays in balancing the anabolic lipoprotein assembly/secretion pathway with the cholesterol catabolic bile acid synthetic pathway.
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PMID:Increased production of apolipoprotein B-containing lipoproteins in the absence of hyperlipidemia in transgenic mice expressing cholesterol 7alpha-hydroxylase. 1132 27

Squalestatin1 (SQ1), a potent inhibitor of squalene synthase produced a dose-dependent induction of cytochromes P450 CYP2H1 and CYP3A37 mRNAs in chicken hepatoma cells. The effect of SQ1 was completely reversed by 25-hydroxycholesterol. Bile acids elicited an induction of CYP3A37 and CYP2H1 mRNA. Bile acids also reduced the phenobarbital induction of CYP2H1 but not of CYP3A37 mRNA. The effects of SQ1 and its reversal by 25-hydroxycholesterol and the effects of bile acids were reproduced in reporter gene assays with a phenobarbital-responsive enhancer unit of CYP2H1. These data suggest that an endogenous molecule related to cholesterol homeostasis regulates induction of drug-inducible CYPs.
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PMID:A Link between cholesterol levels and phenobarbital induction of cytochromes P450. 1184 16

Clofibrate is a hypolipidemic drug belonging to the peroxisome proliferator (PP) family. PPs are well-recognized hepatocarcinogens, though only for rodents and not for humans. Their oncogenicity is usually ascribed to mitogenic or antiapoptotic action. However, we have reported that clofibrate can trigger fast and extensive apoptosis in rodent and human tumor cell lines. The present study examines the possible mechanisms involved in clofibrate-induced apoptosis in AH-130 hepatoma cells. The results show that the apoptogenic effect of clofibrate does not depend on induction of peroxisome proliferator activated receptors (PPARs), but on interference with HMG-CoA reductase (HMGR), a key enzyme that regulates cholesterol biosynthesis and production of isoprenoid units for protein farnesylation. The level and activity of HMGR mRNA are reduced in clofibrate-treated AH-130 cells and apoptosis can be partially prevented by addition of mevalonate. Moreover, cholesterol and cholesterol ester content decreases early in mitochondria, and cytocrome c is released in the cytosol. On the contrary, perturbations at the level of protein farnesylation are not important in determining the fast apoptogenic effect, since treatment of AH-130 cells with an inhibitor of farnesyltransferase induces apoptosis only after 4 h. In conclusion, inhibition of HMGR and decreased cholesterol content are crucial events in clofibrate-induced apoptosis in AH-130 hepatoma cells.
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PMID:Apoptosis induced by clofibrate in Yoshida AH-130 hepatoma cells: role of HMG-CoA reductase. 1251 23

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