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)

Hepatic expression of apolipoprotein (apo) B mRNA-editing enzyme catalytic polypeptide 1 (APOBEC-1) has been proposed as a gene therapy approach for lowering plasma low density lipoprotein (LDL) levels. However, high-level expression of APOBEC-1 in transgenic mouse and rabbit livers causes liver dysplasia and hepatocellular carcinoma. To determine the physiological and pathological effects of low-level hepatic expression of APOBEC-1, we used a 52-kb rat APOBEC-1 genomic clone (RE4) to generate transgenic mice expressing low levels of APOBEC-1 (2 to 5 times those in nontransgenic mice). Liver function, liver histology, editing of apoB mRNA at the normal editing site (C6666), and abnormal editing at multiple sites (hyperediting) in these mice were compared with those in transgenic mice expressing intermediate (I-20) or high (I-28) levels of APOBEC-1 in the liver. Hyperediting of mRNA coding for the novel APOBEC-1 target 1 (NAT1) was also examined. In the high-expressing I-28 line, 50% of the mice had palpable tumors at 15 weeks of age, whereas in the intermediate-expressing I-20 line, 50% of the mice had evidence of liver tumors after 1 year. In contrast, low-expressing RE4 mice had normal liver function and histology and did not develop liver tumors when examined at 3 to 17 months of age. Moreover, hyperediting of apoB and NAT1 mRNA in the liver was robust in the I-20 mice but barely detectable in the RE4 mice. The low-level expression resulted in sufficient APOBEC-1 to edit essentially all apoB mRNA at the normal editing site, virtually eliminating apoB-100 and LDL in the plasma of RE4 mice. When RE4 mice were crossed with human apoB transgenic mice, which possess high plasma LDL concentrations, plasma LDL levels in the offspring were reduced to very low levels. These results indicates that long-term hepatic expression of APOBEC-1 at low levels sufficient to eliminate LDL does not cause apparent liver damage or liver tumors in transgenic mice. RE4 APOBEC-1 transgenic mice should prove valuable for studying the roles of apoB-containing lipoproteins in lipid metabolism and atherosclerosis.
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PMID:Low expression of the apolipoprotein B mRNA-editing transgene in mice reduces LDL levels but does not cause liver dysplasia or tumors. 963 45

The expression of apolipoprotein (apo) B can be modulated by mRNA editing, a unique posttranscriptional base change in the apo B mRNA. Apo B-48, the translation product of edited apo B mRNA, is not a precursor of the atherogenic low density lipoproteins and lipoprotein(a). In humans and various other mammals, the apo B mRNA is edited in the intestine but not in the liver, which exclusively secretes apo B-100-containing lipoproteins as precursors for low density lipoprotein formation. In species such as the rat, mouse, dog, and horse, apo B mRNA is also edited in the liver, resulting in low plasma levels of low density lipoprotein. Editing of the apo B mRNA is mediated by the apo B mRNA-editing enzyme complex, of which the catalytic subunit APOBEC-1 is not expressed in the liver of species without hepatic editing. To understand the molecular basis for liver-specific expression of APOBEC-1 and the editing of hepatic apo B mRNA, the expression pattern and genomic organization of the rat APOBEC-1 gene have been characterized. The rat APOBEC-1 gene contains 6 exons and 2 promoters with distinct activities. The expression of APOBEC-1 in the rat liver is the result of a promoter located upstream, with tissue-specific exon use and alternate splicing within the 5'-untranslated region of APOBEC-1 mRNA encoded by exon 2. In addition to the liver, this promoter also induces APOBEC-1 expression in the spleen, lung, kidney, heart, and skeletal muscle. The promoter located downstream belongs to a new class of TATA-less promoters and is responsible for the abundant expression of APOBEC-1 in the intestine. Mapping of the transcriptional start sites and deletion analysis of the promoter regions by using luciferase as the reporter gene have defined the regulatory elements of both promoters. The downstream, intestine-specific promoter contains a negative regulatory element between -1100 and -500, which appears to restrict its activity to the intestine. The upstream, liver-specific promoter of the rat APOBEC-1 gene induces APOBEC-1 expression and editing of apo B mRNA in human hepatoma HuH-7 and Hep G2 cells. Understanding the molecular basis for the liver-specific expression of APOBEC-1 in the rat promises new strategies to induce APOBEC-1 expression in the human liver for the reduction of atherogenic lipoprotein levels by hepatic apo B mRNA editing.
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PMID:Distinct promoters induce APOBEC-1 expression in rat liver and intestine. 967 68

Homocysteinemia and hypercholesterolemia are important risk factors associated with the occurrence of arteriosclerotic vascular diseases. A positive correlation between plasma levels of homocysteine and cholesterol was found in homocysteinemic patients as well as in experimental animals. In the present study, the effect of homocysteine on the production and secretion of cholesterol in human hepatoma cell line HepG2 cells was investigated. When cells were incubated with 4 mM homocysteine, the amounts of total cholesterol produced as well as the cholesterol secreted by these cells were significantly increased (from 32 +/- 5 to 74 +/- 5 nmol/mg cellular protein). Further biochemical analyses revealed that the increase in cholesterol was resulted from an enhancement in the production and secretion of the unesterified cholesterol with no concomitant change in the level of cholesteryl esters. The activity of intracellular 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase was markedly elevated by 131% and 190% after cells were incubated with homocysteine for 24 and 48 h. Homocysteine also stimulated the secretion of apo B100 by HepG2 cells (from 0.84 +/- 0.11 to 1.37 +/- 0.12 micrograms apolipoprotein B/mg cellular protein). Our results demonstrate that homocysteine stimulates the production and secretion of cholesterol and apolipoprotein B100 in HepG2 cells. The increase in the production of cholesterol induced by homocysteine may contribute to the pathogenesis of arteriosclerosis.
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PMID:Homocysteine stimulates the production and secretion of cholesterol in hepatic cells. 974 42

Apolipoprotein B (apoB) mRNA editing involves a site-specific cytidine to uridine transition catalyzed by the cytidine deaminase, APOBEC-1, in the context of and regulated by a multi-protein-containing editosome. ApoB mRNA editing in vivo is subject to tissue specific, developmental and metabolic regulation. We demonstrate for the first time that the amount of edited apoB mRNA in rat primary hepatocytes is markedly increased subsequent to transient treatment with ethanol in vitro. The apparent change in editing efficiency was dose-dependent (from 0.1%-2.4% initial ethanol dose) and occurred with rapid onset. The proportion of edited apoB mRNA was also markedly enhanced in a rat hepatoma cell line, McArdle RH7777 cells and in a stable McArdle cell line over-expressing APOBEC-1 by transient treatment with 2.5 % ethanol. In contrast, the apoB mRNA editing in a human hepatoma cell line, HepG2 cells and a stable HepG2 cell line over-expressing APOBEC-1 did not respond to ethanol treatment. The data support the possibility that editing activity is ethanol-responsive but suggest that this change is cell type-specific.
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PMID:Ethanol increases apolipoprotein B mRNA editing in rat primary hepatocytes and McArdle cells. 982 30

We studied the effect of NTE-122 (trans-1,4-bis[[1-cyclohexyl-3-(4-dimethylamino phenyl) ureido]methyl]cyclohexane), a novel acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, on intracellular cholesterol esterification and the secretion of apolipoprotein B100 (apoB)-containing lipoprotein and bile acids in the human hepatoma cell line HepG2. NTE-122 markably inhibited [3H]oleate incorporation into cholesteryl esters in HepG2 cells incubated with 5 microg/ml 25-hydroxycholesterol as a stimulus for ACAT (IC50=6.0 nM). On the other hand, NTE-122 did not affect [3H]oleate incorporation into triglycerides and phospholipids and [14C]acetate incorporation into cholesterol. The stimulation of ACAT by 25-hydroxycholesterol caused significant increases in the secretion of radiolabeled cholesteryl esters, radiolabeled triglycerides and apoB mass. NTE-122 pronouncedly inhibited the secretion of radiolabeled cholesteryl esters in proportion to the inhibition of cellular cholesterol esterification, and it significantly reduced the secretion of radiolabeled triglycerides and apoB mass in HepG2 cells incubated with 25-hydroxycholesterol. Furthermore, NTE-122 increased the secretion of bile acids synthesized from [14C]-cholesterol. These results suggest that NTE-122 is capable of exhibiting anti-hyperlipidemic effects by reducing both the cholesterol content and the amount of secreted very low-density lipoprotein and enhancing the excretion of bile acid from the liver.
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PMID:Effects of NTE-122, a novel acyl-CoA:cholesterol acyltransferase inhibitor, on cholesterol esterification and secretions of apolipoprotein B-containing lipoprotein and bile acids in HepG2. 1020 50

Intracellular cholesterol biosynthesis may play a key role in supplying cholesterol (as cholesteryl ester) for the neutral core of very low density lipoprotein (VLDL), thus modulating the secretion of apolipoprotein B-100 (apo B-100) from hepatocytes. The effect of compound NK-104 was studied, a new competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA-reductase), on apo B-100 synthesis and secretion from the human hepatoma cell line Hep G2. Cells were preincubated with NK-104 (0.01-5 microM) in the presence or absence of oleate (0.8 mM). Apo B-100 in the medium was determined by an enzyme-linked immunosorbent assay (ELISA). Incubation of Hep G2 with NK-104 resulted in a marked inhibition of cholesterogenesis (up to 95%), determined as incorporation of [14C]acetate into sterols, and decreased in a dose-dependent manner apo B-100 secretion, both in basal conditions (from 110 to 82 ng/mg cell protein, P < 0.01) and after incubation with oleate (from 227 to 165 ng/mg cell protein, P < 0.01). Density gradient for distribution of apo B-100 secreted, showed that this decrease was essentially due to a reduction of apo B-100 associated with lipoproteins in the density range of low density lipoproteins (LDL). Pulse chase experiment demonstrated that NK-104 did not affect the synthetic rate of apo B-100 but increased intracellular degradation of newly synthesized protein. The compound had only marginal effect on the mass of intracellular triglyceride but significantly decreased intracellular mass of free cholesterol and cholesteryl ester (P < 0.01). It is speculated that the ability of compound NK-104 to decrease apo B-100 secretion from Hep G2 cells is due to a decreased intracellular cholesterol availability.
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PMID:NK-104, a potent 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, decreases apolipoprotein B-100 secretion from Hep G2 cells. 1042 99

A microsomal triacylglycerol hydrolase (TGH) was recently purified from porcine liver [Lehner and Verger (1997) Biochemistry 36, 1861-1868]. To gain further insight into the function of TGH, we have cloned a cDNA encoding TGH from a rat liver cDNA library and generated McArdle RH7777 rat hepatoma cell lines that stably express the rat TGH. The putative protein derived from the cDNA sequence contains a cleavable signal sequence and a catalytic site serine residue present within a pentapeptide motif (GXSXG) that is conserved in all known lipases. TGH-transfected cells showed a 2-fold increase, compared with control cells, in the rate of depletion of prelabelled triacylglycerol stores. Thus, TGH is capable of hydrolysis of stored triacylglycerol. In contrast, the rate of turnover of labelled phosphatidylcholine was similar in both the vector- and TGH-transfected cells. Studies in TGH-transfected cells demonstrated that utilization of intracellular triacylglycerol pools for secretion was approx. 30% higher than in vector-transfected cells. Whereas phosphatidylcholine secretion was essentially the same in control and TGH-transfected cells, TGH-transfected cells also secreted an approx. 25% greater mass of triacylglycerol into the medium and had increased levels of apolipoprotein B100 in the very-low-density lipoprotein density range compared with control cells. The results suggest that the microsomal TGH actively participates in the mobilization of cytoplasmic triacylglycerol stores, some of which can be used for lipoprotein assembly.
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PMID:Cloning and expression of a cDNA encoding a hepatic microsomal lipase that mobilizes stored triacylglycerol. 1049 5

Apolipoprotein B (apoB) mRNA editing is a site-specific (nucleotide 6666) cytidine to uridine transition catalyzed by a cytidine deaminase, APOBEC-1, in the context of a multiprotein complex referred to as the C/U editosome. This report quantifies for the first time the effect of altering APOBEC-1 protein abundance on the proportion of edited apoB mRNAs using transfected McArdle rat hepatoma cells which had been sorted by flow cytometry into populations expressing different levels of green fluorescent protein-APOBEC-1 chimera, GFP-APOBEC. A correlation was observed in which increased expression of GFP-APOBEC protein resulted in a higher proportion of edited apoB mRNA. The number of enzyme molecules required to increase the proportion of edited apoB RNAs was disproportionately high relative to that which might have been predicted from a typical catalytic relationship. Moreover, editing of apoB mRNA at inappropriate sites (promiscuous editing) occurred in response to overexpressing GFP-APOBEC. The data suggest that experimental manipulation of APOBEC-1 abundance in the absence of other regulatory considerations will always result in some level of promiscuous editing. Coordinate expression of APOBEC-1 and the auxiliary proteins and/or regulation of their interactions may be required to increase editing activity without losing editing-site fidelity.
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PMID:Disproportionate relationship between APOBEC-1 expression and apolipoprotein B mRNA editing activity. 1050 8

Studies of truncated apoB peptides in human subjects with familial hypobetalipoproteinemia, as well as of puromycin-generated spectra of nascent apoB peptides in rat and hamster liver, suggest that a minimum size is required for N-terminal fragments of apoB to be efficiently assembled into full-sized VLDL. We report here results of experiments undertaken to examine this phenomenon in greater detail by expressing individual carboxyl-truncated human apoB constructs in McArdle cells. Thus, apoB-29, -32, -37, -42, -47, -53, -70 and full length apoB-100 were transiently expressed in rat McA-RH7777 hepatoma cells, or human apoB-31 and apoB-53 were stably expressed in the same cells, and the secreted VLDL particles were characterized by kinetic gradient ultracentrifugal flotation. Calibration with rat plasma VLDL subfractions showed that about 90 and 50%, respectively, of lipoprotein particles containing endogenous rat B-100 and B-48 floated between fractions 2;-8 of the 11-fraction gradient. This corresponds to the normal VLDL diameter range of about 47 to 28 nm, with the remaining half of rat B-48 recovered as HDL particles in the 1.1 g/ml range. In contrast, regardless of their size, only 2;-5% of any of the truncated human apoB peptides expressed in these cells was recovered in the VLDL region of the gradient. The remaining 95+% of the lipoproteins were found as high density particles; as previously found in other systems the densities of the latter were inversely related to their peptide chain-length. Furthermore, transiently expressed full-length human apoB-100 was inefficiently secreted as VLDL by these cells, with the remainder appearing as LDL-sized particles. Thus, although we showed that McA-RH7777 cells secreted endogenous rat apoB as normal-sized VLDL, we found them unsuitable for our original purpose of using human apoB fragments to further define effects of apoB size on VLDL assembly. These cells appeared unable to efficiently use any size of human apoB for that process. Pulse-labeled untransfected McA-RH7777 cells chased in the presence of puromycin did, however, show a sharp decline in VLDL assembly efficiency for endogenous nascent rat apoB peptides shorter than B-48, similar to that originally found in normal rat liver.
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PMID:Rat McA-RH7777 cells efficiently assemble rat apolipoprotein B-48 or larger fragments into VLDL but not human apolipoprotein B of any size. 1062 9

We used wild-type (WT) mice and mice engineered to express either apoB-100 only (B100 mice) or apoB-48 only (B48 mice) to examine the effects of streptozotocin-induced diabetes (DM) on apoB-100- and apoB-48-containing lipoproteins. Plasma lipids increased with DM in WT mice, and fat tolerance was markedly impaired. Lipoprotein profiles showed increased levels and cholesterol enrichment of VLDL in diabetic B48 mice but not in B100 mice. C apolipoproteins, in particular apoC-I in VLDL, were increased. To investigate the basis of the increase in apoB-48 lipoproteins in streptozotocin-treated animals, we characterized several parameters of lipoprotein metabolism. Triglyceride and apoB production rates were normal, as were plasma lipase activity, VLDL glycosaminoglycan binding, and VLDL lipolysis. However, beta-VLDL clearance decreased due to decreased trapping by the liver. Whereas LRP activity was normal, livers from treated mice incorporated significantly less sulfate into heparan sulfate proteoglycans (HSPG) than did controls. Hepatoma (HepG2) cells and endothelial cells cultured in high glucose also showed decreased sulfate and glucosamine incorporation into HSPG. Western blots of livers from diabetic mice showed a decrease in the HSPG core protein, perlecan. Delayed clearance of postprandial apoB-48-containing lipoproteins in DM appears to be due to decreased hepatic perlecan HSPG.
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PMID:Delayed catabolism of apoB-48 lipoproteins due to decreased heparan sulfate proteoglycan production in diabetic mice. 1086 96


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