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)

Three inhibitors of squalene 2,3-oxide-lanosterol cyclase (AMO 1618, 4,4,10 beta-trimethyl-trans-decal-3 beta-ol (TMD) and 2,3-iminosqualene (ISq] were used to study effects on 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, and on sterol and polyprenyl synthesis from [14C]acetate and [14C]mevalonate in cultured rat hepatoma (H4) cells. After a 4 h exposure of cultures to AMO 1618 or TMD, followed by removal of the inhibitors, the utilization of [14C]acetate for synthesis of digitonin-precipitable sterols increased about twofold, an increase parallelled by the rise in HMG-CoA reductase. Mevalonate at 2.3 mM counteracted the effects of these inhibitors on the reductase. When (R)-[2-14C]mevalonate at 2.3 mM was included with the two inhibitors in the culture media, the cells were still able to synthesize cholesterol although in lesser amounts than the controls. In the presence of TMD the H4 cells also accumulated [14C]squalene 2,3-oxide and [14C]squalene 2,3-22,23-dioxide. ISq added to cells kept in full-growth medium (10 micrograms ml-1) caused an almost complete and irreversible inactivation of the squalene oxide-lanosterol cyclase but did not inhibit polyprenyl synthesis, as the amount of [14C]mevalonate converted into squalene, squalene 2,3-oxide, squalene 2,3-22,23-dioxide plus a little cholesterol was equal to the amount converted by control cells into cholesterol plus squalene. After a 24 h exposure of cells kept in full-growth medium to ISq (10 micrograms ml-1), the levels of HMG-CoA reductase rose about twofold. ISq completely abolished the suppressive effect of 2.3 mM (R)-mevalonate on the reductase. Chromatin isolated from cell nuclei contains cholesterol, which is renewed biosynthetically. It is argued that the suppressor of HMG-CoA reductase, derived from mevalonate, is a sterol and not a non-steroidal product of mevalonate metabolism.
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PMID:Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase: search for the enzyme's repressor derived from mevalonate. 289

Transcription of the low density lipoprotein receptor (LDL-R) and 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase genes was rapidly and transiently induced (8.5- and 2.3-fold, respectively) early during phorbol 12-myristate 13-acetate (PMA)-induced macrophage differentiation of the human monocytic leukemia cell line THP-1. The levels of mRNA coding for LDL-R and HMG-CoA reductase increased soon after induction, reached a maximum (12- and 7-fold increase, respectively) in 2-3 hr, and then rapidly returned to the low constitutive levels observed before induction. The stability of LDL-R mRNA did not change significantly during differentiation, whereas that of HMG-CoA reductase mRNA decreased by about 5-fold 6 hr after the addition of PMA. Transcriptional induction of both LDL-R and HMG-CoA reductase genes (5.6- and 2-fold, respectively) was also observed when undifferentiated cells were treated with cycloheximide (CHX), resulting in a transient increase in steady-state mRNA (7- and 3-fold, respectively). These results suggest that expression of the two genes is maintained at low constitutive levels in uninduced THP-1 cells by a protein with a short half-life. Superinduction of both genes occurred when PMA and CHX were added simultaneously. The induction of LDL-R and HMG-CoA reductase mRNAs during early macrophage differentiation is mediated by protein kinase C. It is hypothesized that protein kinase C acts directly or indirectly to inactivate the labile negative regulatory protein. Induction of LDL-R mRNA was also observed when the human hepatocarcinoma cell line Hep G2 was treated with PMA and CHX, suggesting that this mechanism of regulation may exist in several cell types.
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PMID:Regulation of the low density lipoprotein receptor and hydroxymethylglutaryl coenzyme A reductase genes by protein kinase C and a putative negative regulatory protein. 291 64

Ketoconazole, an imidazole derivative, is a member of a class of metabolic inhibitors acting specifically at cytochrome-P450 mediated reactions. We studied the effects of this compound on cholesterol synthesis, and on HMG-CoA reductase and LDL receptor activities, in cultures of human hepatoma cell line Hep G2. Ketoconazole, added in concentrations of 2-100 microM, inhibited cholesterol synthesis, and caused accumulation of lanosterol and dihydrolanosterol. Total mass formation of sterols was depressed. After 20 hr preincubation of the cells with the drug in these concentrations, activity of HMG-CoA reductase was markedly decreased, while the receptor-mediated binding, uptake and degradation of human LDL were increased. This increase is at least partly due to a higher affinity of LDL for its receptor. Ketoconazole prevented the fall in LDL-receptor activity caused by preincubation with LDL, whereas it did not affect the suppression caused by preincubation with exogenous mevalonate. These findings are discussed with respect to the involvement of endogenous sterol and non-sterol effectors of reductase and receptor activities.
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PMID:Effect of ketoconazole on cholesterol synthesis and on HMG-CoA reductase and LDL-receptor activities in Hep G2 cells. 303 62

A procedure is described for the assay of 3-hydroxy-3-methylglutaryl CoA-reductase (HMG-CoA reductase) in a large number of samples with minimal benchwork and within a 24-hr period. The Michaelis constants for HMG-CoA reductase were determined for microsomal enzyme from the liver of normal and cholesterol-fed rats and Morris hepatoma 5123C. The apparent Km D-HMG-CoA was ca. 3.5 microM and was not affected by assay temperature or cholesterol feeding. The apparent Km NADPH for microsomal HMG-CoA reductase was 10-15 microM and similarly was not affected by assay temperature. The Arrhenius plot parameters (activation energy and transition temperatures) were the same whether determined using the reaction velocity from fixed substrate concentrations or V from subtraction curves. This confirmed that values obtained using fixed saturating substrate concentrations are valid and not affected by a temperature-dependent alteration in the affinity of the enzyme for its substrates.
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PMID:Effect of assay temperature on the kinetics of 3-hydroxy-3-methylglutaryl-coenzyme A reductase in rat liver and Morris hepatoma 5123C. 402 89

Compactin, an inhibitor of HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) reductase, decreased cholesterol synthesis in intact Hep G2 cells. However, after the inhibitor was washed away, the HMG-CoA-reductase activity determined in the cell homogenate was found to be increased. Also the high-affinity association of LDL (low-density lipoprotein) to Hep G2 cells was elevated after incubation with compactin. Lipoprotein-depleted serum, present in the incubation medium, potentiated the compactin effect compared with incubation in the presence of human serum albumin. Addition of either mevalonate or LDL prevented the compactin-induced rise in activities of both HMG-CoA reductase and LDL receptor in a comparable manner. It is concluded that in this human hepatoma cell line, as in non-transformed cells, both endogenous mevalonate or mevalonate-derived products and exogenous cholesterol are able to modulate the HMG-CoA reductase activity as well as the LDL-receptor activity.
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PMID:Effects of compactin, mevalonate and low-density lipoprotein on 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity and low-density-lipoprotein-receptor activity in the human hepatoma cell line Hep G2. 608 62

3-hydroxy-3-methylglutaryl-coenzyme A reductase (EC 1.1.1.3.4.) activity and cell membrane fluidity measured by fluorescence polarization using 1,6 diphenyl, 1,3,5-hexatriene as fluorescent probe have been concomitantly examined in HTC hepatoma cells, both in relation to growth rate and in response to treatment with hydroxylated sterols. A high level of HMG-CoA reductase activity was observed in cells at log phase of growth which progressively decreased to reach a sustained low level at stationary phase. Similarly, membrane fluidity markedly decreased in relation to growth rate. Hydroxylated sterols such as 7 beta-hydroxycholesterol or 25-hydroxycholesterol strongly inhibited HMG-CoA reductase activity whereas a water-soluble derivative of 7 beta-hydroxycholesterol sodium 3,7-bishemisuccinate had no effect. Within the same range of concentrations 7 beta-hydroxycholesterol and 25-hydroxycholesterol strongly decreased membrane fluidity when the water-soluble derivative was ineffective. Thus, the present results provide evidence for a correlation between the two tested parameters and suggest a dependency of HMG-CoA reductase activity on cell membrane fluidity.
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PMID:Growth-rate-related and hydroxysterol-induced changes in membrane fluidity of cultured hepatoma cells: correlation with 3-hydroxy-3-methyl glutaryl CoA reductase activity. 653 90

Extensive studies have demonstrated that the normal inhibition of cholesterol synthesis by cholesterol feeding is decreased in all hepatomas studied in vivo. This loss of the normal feedback regulation of cholesterol synthesis has been shown to be due to the failure of cholesterol ingestion to inhibit the activity of hydroxymethylglutaryl (HMG)-CoA reductase. The basis for this absence of feedback control of cholesterogenesis is unknown. Studies to date have not demonstrated structural or kinetic differences between the HMG-CoA reductase of normal liver and hepatoma. The present study, however, demonstrates significant differences in the activation state of HMG-CoA reductase from normal liver and hepatoma. In normal liver only approximately 10-20% of the microsomal HMG-CoA reductase is in the dephosphorylated, active form while 80-90% is in the phosphorylated, inactive state. In contrast, in three different Morris hepatomas in vivo, from 53 to 73% of the HMG-CoA reductase is in the active state. That the increased activation state in hepatomas is a property of tumor tissue and is not solely due to rapid growth is demonstrated by the fact that in both fetal and regenerating liver an enhanced activation state of HMG-CoA reductase is not observed. Additionally, preincubation with magnesium and ATP results in the inhibition of HMG-CoA reductase both in tumor and in liver. Presumably, this decrease in HMG-CoA reductase activity is due to the phosphorylation of the enzyme. Similarly, the preincubation of tumor and liver microsomes with phosphatase results in an increase in HMG-CoA reductase activity presumably by the dephosphorylation of the enzyme to its active form. The relationship between the altered activation state of HMG-CoA reductase in hepatomas and the reduction in the feedback regulation of this enzyme in liver tumors remains to be explored.
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PMID:Altered activation state of hydroxymethylglutaryl-coenzyme A reductase in liver tumors. 660 22

A plasmid expression system has been developed which allows sequence-specific effects on mRNA degradation rates to be determined. This system uses stable, nonintegrating vectors that provide consistent levels of mRNA expression without the position effects common to integrating vectors. cDNAs encoding putative instability elements may be subcloned into the 5' untranslated region (5'UTR), the coding region, or the proximal 3'UTR of a beta-globin cDNA reporter. The effects of these sequences on mRNA stability may then be determined by actinomycin time course analyses of the fusion mRNAs and recombinant beta-globin mRNA in human cell lines. To demonstrate the utility of the vector system we fused an 820-bp fragment of the cDNA encoding the proximal 3'UTR of human 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase to the 3'UTR of the beta-globin reporter and introduced the vector into the human hepatocarcinoma cell line, HepG2. The fusion mRNA was degraded at a rate 2- to 2.5-fold greater than that of beta-globin alone, at a rate similar to that reported for HMG CoA reductase mRNA in normal rat liver. Similar to a number of other relatively unstable mRNAs, the rate of fusion mRNA degradation was greatly decreased by treatment with cycloheximide.
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PMID:An episomal expression vector system for monitoring sequence-specific effects on mRNA stability in human cell lines. 756 67

N-[(1,5,9)-trimethyldecyl]-4 alpha,10-dimethyl-8-aza-trans-decal-3 beta-ol (8-azadecalin 1), a high-energy intermediate analogue for the 2,3-oxidosqualene-lanosterol cyclase, was found to be a powerful (IC50 approximately 0.1 microM) inhibitor of cholesterol biosynthesis in human hepatoma HepG2 cells. In analogy with other mammalian cells grown in the presence of cyclase inhibitors, the decrease in C27-sterol formation was accompanied by an accumulation of 2,3-oxidosqualene, 2,3:22, 23-dioxidosqualene, and by the formation of a compound characterized as 24,25-epoxycholesterol, a repressor of HMG-CoA (3-hydroxy-3-methylglutaryl coenzyme A) reductase activity. In order to assess the cyclase as a potential pharmacological target for the design of hypocholesterolemic drugs, it is important to test whether inhibitors of this enzyme are able to act synergistically on the biosynthesis of cholesterol, i.e. by decreasing the amount of lanosterol formed and by repressing the regulatory HMG-CoA reductase via the formation of regulatory oxysterols. The accumulation of 24,25-epoxycholesterol in relationship to the decrease of C27-sterol biosynthesis and of HMG-CoA reductase activity showed only a partial correlation: e.g. at [1] = 100 x IC50 only a 50% reduction in enzyme activity could be attained. In contrast, when HepG2 cells were treated with 2,3:22,23-dioxidosqualene or 24,25-epoxycholesterol, excellent correlations were found between the inhibition of C27-sterol biosynthesis and the repression of HMG-CoA reductase activity, which was almost complete at the highest concentrations of these epoxides (10(-5) M). Altogether, our results suggest that treatment of HepG2 cells with a cyclase inhibitor such as 8-azadecalin (1) does not lead to an intracellular accumulation of repressor molecules high enough to fully trigger a regulatory pathway resulting in a complete down-regulation of HMG-CoA reductase. At intermediary concentrations of cyclase inhibitors (IC50), however, a synergistic mode of action of these inhibitors seems plausible.
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PMID:Effects of a 2,3-oxidosqualene-lanosterol cyclase inhibitor 2,3:22,23-dioxidosqualene and 24,25-epoxycholesterol on the regulation of cholesterol biosynthesis in human hepatoma cell line HepG2. 804 30

Modulation of cell growth by a combination of pravastatin [a 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor] and d-limonene (an inhibitor of protein isoprenylation) was studied using Hep G2, a human hepatoma-derived cell line. Pravastatin, at 0.1 mM, produced 85% inhibition of cholesterol biosynthesis in Hep G2 cells. The combination of 0.1 mM pravastatin and 1.0 mM d-limonene had no further effect on the reduction seen with pravastatin alone. Addition of 0.1 mM pravastatin or 1.0 mM d-limonene did not significantly suppress DNA synthesis by the cells, whereas the combination suppressed it to 50% of the control level. Production of m-p21ras was markedly decreased to 35% of the control level by the combination of these two inhibitors. Both the reduction by pravastatin of farnesylpyrophosphate as substrate for protein:farnesyl transferase and inhibition of protein farnesylation by d-limonene seem to be responsible for the profound suppression of m-p21ras formation in the cells. However, dolichol synthesis was not suppressed by the combination of these inhibitors. In human fibroblasts, the combination suppressed m-p21ras production but not DNA synthesis. These findings suggest that the combination of pravastatin and d-limonene acts on cancer cell growth through inhibition of the post-translational processing of cellular proteins including p21ras, rather than through the suppression of cholesterol and dolichol biosynthesis. Thus, the combination of an HMG-CoA reductase inhibitor and an inhibitor of protein isoprenylation offers potential as a new approach for cancer therapy.
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PMID:Modulation of the mevalonate pathway and cell growth by pravastatin and d-limonene in a human hepatoma cell line (Hep G2). 819 62


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