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Soleymanjahi S, Blanc V, Davidson N. APOBEC1 mediated C-to-U RNA editing: target sequence and trans-acting factor contribution to 177 RNA editing events in 119 murine transcripts in-vivo. RNA (NEW YORK, N.Y.) 2021; 27:rna.078678.121. [PMID: 34083494 PMCID: PMC8284327 DOI: 10.1261/rna.078678.121] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/08/2021] [Accepted: 05/31/2021] [Indexed: 05/04/2023]
Abstract
Mammalian C-to-U RNA editing was described more than 30 years ago as a single nucleotide modification in small intestinal Apob RNA, later shown to be mediated by the RNA-specific cytidine deaminase APOBEC1. Reports of other examples of C-to-U RNA editing, coupled with the advent of genome-wide transcriptome sequencing, identified an expanded range of APOBEC1 targets. Here we analyze the cis-acting regulatory components of verified murine C-to-U RNA editing targets, including nearest neighbor as well as flanking sequence requirements and folding predictions. RNA secondary structure of the editing cassette was associated with editing frequency and exhibited minimal free energy values comparable to small nuclear RNAs. We summarize findings demonstrating the relative importance of trans-acting factors (A1CF, RBM47) acting in concert with APOBEC1. Co-factor dominance was associated with editing frequency, with RNAs targeted by both RBM47 and A1CF edited at a lower frequency than RBM47 dominant targets. Using this information, we developed a multivariable linear regression model to predict APOBEC1 dependent C-to-U RNA editing efficiency, incorporating factors independently associated with editing frequencies based on 103 Sanger-confirmed editing sites, which accounted for 84% of the observed variance. This model also predicted a composite score for available human C-to-U RNA targets, which again correlated with editing frequency.
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2
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Schaefermeier P, Heinze S. Hippocampal Characteristics and Invariant Sequence Elements Distribution of GLRA2 and GLRA3 C-to-U Editing. Mol Syndromol 2016; 8:85-92. [PMID: 28611548 DOI: 10.1159/000453300] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 10/25/2016] [Indexed: 11/19/2022] Open
Abstract
Glycine receptor α2 and α3 subunit (GLRA2/GLRA3) high-affinity variants, of which the subjacent amino acid substitutions issue from C-to-U RNA editing, are thought to influence tonic inhibition and pathophysiology. In light of the detection of GLRA3 NM_006529:r.1157C>U and GLRA2 NM_002063:r.1416C>U exchanges in hippocampus explants of temporal lobe epilepsy patients, we now examine the healthy situation and relate it to the epileptic situation by ascertaining controls in a legitimate reanalysis. The GLRA2 and GLRA3 editing events that would ultimately result in a glycine receptor with increased affinity occur in the postmortem nonepileptic hippocampus. Most notably, their relative amounts do not significantly differ from those in increased damaged hippocampus explants, whereas curbed relative amounts in epileptic explants without cell loss come out statistically significant. Local sequence alignment reveals invariant sequence stretches consistent in GLRA2/ GLRA3 and other edited transcripts that coincide with known APOB sequence elements. Concerning the essential mooring element, GLRA2/GLRA3 comply strictly only with the motif's 5' part. While this lack of canonical mooring elements and uncertain action of the famous deaminase APOBEC1 suggest a specific regulation of GLRA2/GLRA3 editing, its reduction in the less-damaged epileptic hippocampus could be attributed to anomalous epileptic neurogenesis.
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Affiliation(s)
- Philipp Schaefermeier
- Charité University Medicine Berlin, Germany,Helmholtz Group RNA Editing and Hyperexcitability Disorders, Max-Delbrück-Centre for Molecular Medicine, Berlin, Germany
| | - Sarah Heinze
- Institute of Forensic Sciences and Legal Medicine, Charité Berlin, Berlin, Germany,Klinikum Oldenburg gGmbH, Oldenburg, Germany,Institute of Forensic Medicine, Johannes Gutenberg University of Mainz, Mainz, Germany
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3
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Blanc V, Xie Y, Luo J, Kennedy S, Davidson NO. Intestine-specific expression of Apobec-1 rescues apolipoprotein B RNA editing and alters chylomicron production in Apobec1 -/- mice. J Lipid Res 2012; 53:2643-55. [PMID: 22993231 DOI: 10.1194/jlr.m030494] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Intestinal apolipoprotein B (apoB) mRNA undergoes C-to-U editing, mediated by the catalytic deaminase apobec-1, which results in translation of apoB48. Apobec1(-/-) mice produce only apoB100 and secrete larger chylomicron particles than those observed in wild-type (WT) mice. Here we show that transgenic rescue of intestinal apobec-1 expression (Apobec1(Int/O)) restores C-to-U RNA editing of apoB mRNA in vivo, including the canonical site at position 6666 and also at approximately 20 other newly identified downstream sites present in WT mice. The small intestine of Apobec1(Int/O) mice produces only apoB48, and the liver produces only apoB100. Serum chylomicron particles were smaller in Apobec1(Int/O) mice compared with those from Apobec1(-/-) mice, and the predominant fraction of serum apoB48 in Apobec1(Int/O) mice migrated in lipoproteins smaller than chylomicrons, even when these mice were fed a high-fat diet. Because apoB48 arises exclusively from the intestine in Apobec1(Int/O) mice and intestinal apoB48 synthesis and secretion rates were comparable to WT mice, we were able to infer the major sites of origin of serum apoB48 in WT mice. Our findings imply that less than 25% of serum apoB48 in WT mice arises from the intestine, with the majority originating from the liver.
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Affiliation(s)
- Valerie Blanc
- Department of Medicine, Washington University School of Medicine, St. Louis, MO 63110, USA
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4
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Smith HC, Bennett RP, Kizilyer A, McDougall WM, Prohaska KM. Functions and regulation of the APOBEC family of proteins. Semin Cell Dev Biol 2011; 23:258-68. [PMID: 22001110 DOI: 10.1016/j.semcdb.2011.10.004] [Citation(s) in RCA: 155] [Impact Index Per Article: 11.9] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2011] [Revised: 09/30/2011] [Accepted: 10/03/2011] [Indexed: 10/16/2022]
Abstract
APOBEC1 is a cytidine deaminase that edits messenger RNAs and was the first enzyme in the APOBEC family to be functionally characterized. Under appropriate conditions APOBEC1 also deaminates deoxycytidine in single-stranded DNA (ssDNA). The other ten members of the APOBEC family have not been fully characterized however several have deoxycytidine deaminase activity on ssDNAs. Despite the nucleic acid substrate preferences of different APOBEC proteins, a common feature appears to be their intrinsic ability to bind to RNA as well as to ssDNA. RNA binding to APOBEC proteins together with protein-protein interactions, post-translation modifications and subcellular localization serve as biological modulators controlling the DNA mutagenic activity of these potentially genotoxic proteins.
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Affiliation(s)
- Harold C Smith
- Department of Biochemistry and Biophysics, University of Rochester, School of Medicine and Dentistry, Rochester, NY 14642, USA.
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5
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Transcriptome-wide sequencing reveals numerous APOBEC1 mRNA-editing targets in transcript 3' UTRs. Nat Struct Mol Biol 2011; 18:230-6. [PMID: 21258325 PMCID: PMC3075553 DOI: 10.1038/nsmb.1975] [Citation(s) in RCA: 181] [Impact Index Per Article: 13.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2010] [Accepted: 11/10/2010] [Indexed: 12/23/2022]
Abstract
Apolipoprotein B-editing enzyme, catalytic polypeptide-1 (APOBEC1) is a cytidine deaminase, initially identified by its activity in converting a specific cytidine (C) to uridine (U) in apolipoprotein B (apoB) mRNA transcripts in the small intestine. Editing results in translation of a truncated apoB isoform with distinct functions in lipid transport. To address the possibility that APOBEC1 edits additional mRNAs, we developed a transcriptome-wide comparative RNA-Seq screen. We identified and validated 32 previously undescribed mRNA targets of APOBEC1 editing, all of which are located in AU-rich segments of transcript 3′ untranslated regions (3′ UTRs). Further analysis established several characteristic sequence features of editing targets, which were predictive for the identification of additional APOBEC1 substrates. The transcriptomics approach to RNA editing presented here dramatically expands the list of APOBEC1 mRNA editing targets and reveals a novel cellular mechanism for the modification of transcript 3′ UTRs.
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Chen Z, Eggerman TL, Bocharov AV, Baranova IN, Vishnyakova TG, Csako G, Patterson AP. Hypermutation induced by APOBEC-1 overexpression can be eliminated. RNA (NEW YORK, N.Y.) 2010; 16:1040-1052. [PMID: 20348446 PMCID: PMC2856876 DOI: 10.1261/rna.1863010] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/03/2009] [Accepted: 02/05/2010] [Indexed: 05/29/2023]
Abstract
APOBEC-1 overexpression in liver has been shown to effectively reduce apoB-100 levels. However, nonspecific hypermutation and liver tumor formation potentially related to hypermutation in transgenic animals compromise its potential use for gene therapy. In studying apoB mRNA editing regulation, we found that the core editing auxiliary factor ACF dose-dependently increases APOBEC-1 nonspecific hypermutation and specific editing with variable site sensitivity. Overexpression of APOBEC-1 together with ACF in human hepatic HepG2 cells hypermutated apoB mRNAs 20%-65% at sites 6639, 6648, 6655, 6762, 6802, and 6845, in addition to the normal 90% editing at 6666. The hypermutation activity of APOBEC-1 was decreased to background levels by a single point APOBEC-1 mutation of P29F or E181Q, while 50% of wild-type control editing at the normal site was retained. The hypermutations on both apoB and novel APOBEC-1 target 1 (NAT1) mRNA were also decreased to background levels with P29F and E181Q mutants in rat liver primary culture cells. The loss of hypermutation with the mutants was associated with significantly decreased APOBEC-1/ACF interaction. These data suggest that nonspecific hypermutation induced by overexpressing APOBEC-1 can be virtually eliminated by site-specific mutation, while maintaining specific editing activity at the normal site, reopening the potential use of APOBEC-1 gene therapy for hyperlipidemia.
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Affiliation(s)
- Zhigang Chen
- Department of Laboratory Medicine, Clinical Center, National Institutes of Health, Bethesda, Maryland 20892, USA
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7
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Galloway CA, Smith HC. The expression of apoB mRNA editing factors is not the sole determinant for the induction of editing in differentiating Caco-2 cells. Biochem Biophys Res Commun 2009; 391:659-63. [PMID: 19932086 DOI: 10.1016/j.bbrc.2009.11.116] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2009] [Accepted: 11/18/2009] [Indexed: 11/17/2022]
Abstract
Apolipoprotein B mRNA is edited at cytidine 6666 in the enterocytes lining the small intestine of all mammals; converting a CAA codon to a UAA stop codon. The conversion is approximately 80% efficient in this tissue and leads to the expression of the truncated protein, ApoB48, essential for secretion of dietary lipid as chylomicrons. Caco-2 cell raft cultures have been used as an in vitro model for the induction of editing activity during human small intestinal cell differentiation. This induction of apoB mRNA editing has been ascribed to the expression of APOBEC-1. In agreement our data demonstrated differentiation-dependent induction of expression of the editing enzyme APOBEC-1 and in addition we show alternative splicing of the essential auxiliary factor ACF. However, transfection of these editing factors in undifferentiated proliferating Caco-2 cells was not sufficient to induce robust apoB mRNA editing activity. Only differentiation of Caco-2 cells could induce more physiological like levels of apoB mRNA editing. The data suggested that additional regulatory mechanism(s) were induced by differentiation that controlled the functional activity of editing factors.
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Affiliation(s)
- Chad A Galloway
- Department of Biochemistry and Biophysics, University of Rochester School of Medicine, 601 Elmwood Ave, Rochester, NY 14642, USA
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8
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Chen Z, Eggerman TL, Patterson AP. ApoB mRNA editing is mediated by a coordinated modulation of multiple apoB mRNA editing enzyme components. Am J Physiol Gastrointest Liver Physiol 2007; 292:G53-65. [PMID: 16920700 DOI: 10.1152/ajpgi.00118.2006] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Apolipoprotein (apo)B mRNA editing is accomplished by a large multiprotein complex. How these proteins interact to achieve the precise single-nucleotide change induced by this complex remains unclear. We investigated the relationship between altered apoB mRNA editing and changes in editing enzyme components to evaluate their roles in editing regulation. In the mouse fetal small intestine, we found that the dramatic developmental upregulation of apoB mRNA editing from approximately 3% to 88% begins with decreased levels of inhibitory CUG binding protein 2 (CUGBP2) expression followed by increased levels of apoB mRNA editing enzyme (apobec)-1 and apobec-1 complementation factor (ACF) (4- and 8-fold) and then by decreased levels of the inhibitory components glycine-arginine-tyrosine-rich RNA binding protein (GRY-RBP) and heterogeneous nuclear ribonucleoprotein (hnRNP)-C1 (75% and 56%). In contrast, the expression of KH-type splicing regulatory protein (KSRP), apobec-1 binding protein (ABBP)1, ABBP2, and Bcl-2-associated athanogene 4 (BAG4) were unaltered. In the human intestinal cell line Caco-2, the increase of apoB mRNA editing from approximately 1.7% to approximately 23% was associated with 6- and 3.2-fold increases of apobec-1 and CUGBP2, respectively. In the mouse large intestine, the editing was 48% and had a 2.7-fold relatively greater CUGBP2 level. Caco-2 and the large intestine thus have increased instead of decreased CUGBP2 and a lower level of editing, suggesting that inhibitory CUGBP2 may play a critical role in the magnitude of editing regulation. Short interfering RNA-mediated gene-specific knockdown of CUGBP2, GRY-RBP, and hnRNP-C1 resulted in increased editing in Caco-2 cells, consistent with their known inhibitory function. These data suggest that a coordinated expression of editing components determines the magnitude and specificity of apoB mRNA editing.
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Affiliation(s)
- Zhigang Chen
- Office of Biotechnology Activities, National Institutes of Health, 6705 Rockledge Dr., Suite 750, Bethesda, MD 20892, USA
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9
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Smith HC. Measuring editing activity and identifying cytidine-to-uridine mRNA editing factors in cells and biochemical isolates. Methods Enzymol 2007; 424:389-416. [PMID: 17662851 DOI: 10.1016/s0076-6879(07)24018-2] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
Cytidine deaminases with the capacity to act on nucleic acids play a critical role in regulating the proteome through diversification of expressed sequence beyond that encoded in the genome. A family of these enzymes, known as the APOBEC family of cytidine deaminases, has been identified in mammalian cells. APOBEC-1 edits messenger RNA, whereas other family members affect mRNA coding capacity by editing single-stranded DNA in expressed regions of the genomes. Biochemical isolation and analysis of APOBEC proteins and their interacting factors have led to an understanding of the diverse cellular processes including lipoprotein metabolism, antibody production, viral infectivity, and cancer. Practical approaches will be described for the measurement of editing activity and the analysis of proteins involved in C-to-U and dC-to-dU editing.
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Affiliation(s)
- Harold C Smith
- Department of Biochemistry, University of Rochester School of Medicine and Dentistry, Rochester, New York, USA
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10
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Abstract
Chromosome 9, which is often partially or fully reduced to homozygosity in bladder cancer cells, harbors several tumor suppressor loci including deleted in bladder cancer chromosome region 1 (DBCCR1) at 9q32-33. To study DBCCR1 function, stable cell lines, inducible for DBCCR1 expression by tetracycline, were made, but the DBCCR1 protein was not expressed at detectable levels. To understand the fate of DBCCR1-expressing cells, human bladder tumor cells were transiently transfected with an expression vector containing DBCCR1 fused to enhanced green fluorescent protein (EGFP). Initially, DBCCR1-EGFP-expressing cells demonstrated diffuse cytoplasmic green fluorescence with nuclear exclusion patterns. After time, the intensity level of green fluorescence increased and a granular distribution of protein became visible in the cells. At this point, cells rounded up and detached from the tissue culture dish. Cells transfected with a control vector, containing only EGFP, and partial DBCCR1-EGFP fusion constructs did not demonstrate this behavior. DBCCR1-mediated cell death in cultured tumor cells was independent of caspase-3 activation and did not result in detectable DNA strand breaks by TUNEL staining that are hallmarks of the classical apoptotic pathway.
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Affiliation(s)
- Kate O Wright
- Department of Pathology, University of Rochester, Rochester, New York, USA
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11
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Sowden MP, Lehmann DM, Lin X, Smith CO, Smith HC. Identification of novel alternative splice variants of APOBEC-1 complementation factor with different capacities to support apolipoprotein B mRNA editing. J Biol Chem 2003; 279:197-206. [PMID: 14570923 DOI: 10.1074/jbc.m307920200] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Two novel mRNA transcripts have been identified that result from species- and tissue-specific, alternative polyadenylation and splicing of the pre-mRNA encoding the apolipoprotein B (apoB) editing catalytic subunit 1 (APOBEC-1) complementation factor (ACF) family of related proteins. The alternatively processed mRNAs encode 43- and 45-kDa proteins that are components of the previously identified p44 cluster of apoB RNA binding, editosomal proteins. Recombinant ACF45 displaced ACF64 and ACF43 in mooring sequence RNA binding but did not demonstrate strong binding to APOBEC-1. In contrast, ACF43 bound strongly to APOBEC-1 but demonstrated weak binding to mooring sequence RNA. Consequently ACF45/43 complemented APOBEC-1 in apoB mRNA editing with less efficiency than full-length ACF64. These data, together with the finding that all ACF variants were co-expressed in rat liver nuclei (the site of apoB mRNA editing), suggested that ACF variants might compete with one another for APOBEC-1 and apoB mRNA binding and thereby contribute to the regulation of apoB mRNA editing. In support for this hypothesis, the ratio of nuclear ACF65/64 to ACF45/43 decreased when hepatic editing was inhibited by fasting and increased when editing was re-stimulated by refeeding. These findings suggested a new model for the regulation of apoB mRNA editing in which the catalytic potential of editosomes is modulated at the level of their assembly by alterations in the relative abundance of multiple related RNA-binding auxiliary proteins and the expression level of APOBEC-1.
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Affiliation(s)
- Mark P Sowden
- Department of Biochemistry and Biophysics, University of Rochester, NY 14642, USA
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12
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Abstract
Apolipoprotein B is a large, amphipathic protein that plays a central role in lipoprotein metabolism. Because its overproduction and deficiency leads to metabolic and pathologic disorders, much effort has been paid to investigate the mechanisms of how its homeostasis is achieved. Earlier and recent studies have showed that apoB gene locus might reside in different chromatin domains in the hepatic and intestinal cells, and two sets of very distinct regulatory elements operate to control its transcription. Posttranscriptional modification of apoB mRNA is performed by a multicomponent enzyme complex, several possible pathways regulate the editing efficiency. Understanding of the mechanism responsible for apoB mRNA editing will provide the basis for C-to-U editing in gene therapy. In addition to apoB mRNA abundance and stability, its translation can be also regulated at the steps of elongation. The translocation of apoB into the ER is an important and complicated process that is less understood. Successful transport and correct folding of apoB may lead to its final secretion, otherwise subject to intracellular degradation, which is accomplished by proteasomal and nonproteasomal pathways at multiple levels and may differ among cell types.
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Affiliation(s)
- Ai-Bing Wang
- National Laboratory of Medical Molecular Biology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences, Beijing 10005, People's Republic of China
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13
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Hersberger M, Patarroyo-White S, Qian X, Arnold KS, Rohrer L, Balestra ME, Innerarity TL. Regulatable liver expression of the rabbit apolipoprotein B mRNA-editing enzyme catalytic polypeptide 1 (APOBEC-1) in mice lacking endogenous APOBEC-1 leads to aberrant hyperediting. Biochem J 2003; 369:255-62. [PMID: 12374571 PMCID: PMC1223090 DOI: 10.1042/bj20020694] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2002] [Revised: 09/17/2002] [Accepted: 10/09/2002] [Indexed: 11/17/2022]
Abstract
Apolipoprotein (apo) B mRNA editing is the deamination of C(6666) to uridine, which results in translation of the apoB-48 protein instead of the genomically encoded apoB-100. ApoB-48-containing lipoproteins are cleared more rapidly from plasma and are less atherogenic than apoB-100-containing low-density lipoproteins (LDLs). In humans, the intestine predominantly produces apoB-48 whereas the liver secretes apoB-100 only. To evaluate a potential therapeutic use for liver-induced apoB mRNA editing in humans, we investigated the efficiency and safety of transgenic expression of apoB mRNA-editing enzyme catalytic polypeptide 1 (APOBEC-1) in the absence of endogenous editing in the mouse model. Here we show that regulatable tetO-mediated APOBEC-1 expression in the livers of gene-targeted mice lacking endogenous APOBEC-1 results in 30% apoB mRNA editing. In a time-course experiment, the expression of tetO-APOBEC-1 mRNA was suppressed within 2 days after mice were fed doxycycline and apoB mRNA editing and apoB-48 formation were suppressed within 4 days. However, tetO-APOBEC-1 expression resulted in regulatable aberrant hyperediting of several cytidines downstream of C(6666) in apoB mRNA and in novel APOBEC-1 target 1 (NAT1) mRNA. Several of the cytidines in apoB mRNA were hyperedited to a level similar to that of C(6666), although editing at C(6666) was lower than that in wild-type mice. These results demonstrate that even moderate APOBEC-1 expression can lead to hyperediting, limiting the single-gene approach for gene therapy with APOBEC-1.
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Affiliation(s)
- Martin Hersberger
- Gladstone Institute of Cardiovascular Disease, University of California, Building 40, Third Floor, P.O. Box 419100, 2550 23rd Street, San Francisco, CA 94141-9100, U.S.A.
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14
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Sowden MP, Ballatori N, Jensen KLDM, Reed LH, Smith HC. The editosome for cytidine to uridine mRNA editing has a native complexity of 27S: identification of intracellular domains containing active and inactive editing factors. J Cell Sci 2002; 115:1027-39. [PMID: 11870221 DOI: 10.1242/jcs.115.5.1027] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Apolipoprotein B mRNA cytidine to uridine editing requires the assembly of a multiprotein editosome comprised minimally of the catalytic subunit,apolipoprotein B mRNA editing catalytic subunit 1 (APOBEC-1), and an RNA-binding protein, APOBEC-1 complementation factor (ACF). A rat homolog has been cloned with 93.5% identity to human ACF (huACF). Peptide-specific antibodies prepared against huACF immunoprecipitated a rat protein of similar mass as huACF bound to apolipoprotein B (apoB) RNA in UV cross-linking reactions, thereby providing evidence that the p66, mooring sequence-selective, RNA-binding protein identified previously in rat liver by UV cross-linking and implicated in editosome assembly is a functional homolog of huACF. The rat protein (p66/ACF) was distributed in both the nucleus and cytoplasm of rat primary hepatocytes. Within a thin section, a significant amount of total cellular p66/ACF was cytoplasmic, with a concentration at the outer surface of the endoplasmic reticulum. Native APOBEC-1 co-fractionated with p66/ACF in the cytoplasm as 60S complexes. In the nucleus, the biological site of apoB mRNA editing, native p66/ACF, was localized to heterochromatin and fractionated with APOBEC-1 as 27S editosomes. When apoB mRNA editing was stimulated in rat primary hepatocytes with ethanol or insulin, the abundance of p66/ACF in the nucleus markedly increased. It is proposed that the heterogeneity in size of complexes containing editing factors is functionally significant and reflects functionally engaged editosomes in the nucleus and an inactive cytoplasmic pool of factors.
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Affiliation(s)
- Mark P Sowden
- Department of Biochemistry and Biophysics, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642, USA
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15
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Yang Y, Ballatori N, Smith HC. Apolipoprotein B mRNA editing and the reduction in synthesis and secretion of the atherogenic risk factor, apolipoprotein B100 can be effectively targeted through TAT-mediated protein transduction. Mol Pharmacol 2002; 61:269-76. [PMID: 11809850 DOI: 10.1124/mol.61.2.269] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Hepatic very-low-density lipoprotein particles (VLDL) containing full-length apolipoprotein B100 are metabolized in the blood stream to low-density lipoprotein (LDL) particles, whose elevated levels increase the risk of atherosclerosis. Statins and bile-acid sequestrants are effective LDL-lowering therapies for many patients. Development of alternative therapies remains important for patients with adverse reactions to conventional therapy, with defects in the LDL receptor-dependent lipoprotein uptake pathway and for intervention in children. Editing of apoB mRNA by the enzyme APOBEC-1 changes a glutamine codon to a stop codon, leading to the synthesis and secretion of apoB48-containing VLDL, which are rapidly cleared before they can be metabolized to LDL. Human liver does not edit apoB mRNA because it does not express APOBEC-1. Although initially promising, enthusiasm for apobec-1 gene therapy for hypercholesterolemia was blunted by the finding that uncontrolled transgenic expression of APOBEC-1 led to nonspecific editing of mRNAs and pathology. We demonstrate that APOBEC-1 fused to TAT entered primary hepatocytes, where it induced a transient increase in mRNA editing activity and enhanced synthesis and secretion of VLDL containing apoB48. Protein transduction of APOBEC-1 transiently stimulated high levels of apoB mRNA editing in a dose-dependent manner without loss of fidelity. These results suggested that apoB mRNA editing should be re-evaluated as a LDL-lowering therapeutic target in the new context of protein transduction therapy.
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Affiliation(s)
- Yan Yang
- Department of Pathology and Laboratory Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642, USA
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16
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Giangreco A, Sowden MP, Mikityansky I, Smith HC. Ethanol stimulates apolipoprotein B mRNA editing in the absence of de novo RNA or protein synthesis. Biochem Biophys Res Commun 2001; 289:1162-7. [PMID: 11741314 DOI: 10.1006/bbrc.2001.6082] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Apolipoprotein B (apoB) mRNA editing involves a site-specific modification of cytidine to form uridine. The reaction is catalyzed in the nucleus by a multi-protein editosome. Rat hepatic editing is regulated during development, metabolically and in response to ethanol. Ethanol stimulated editing in hepatocytes within minutes of exposure. In the present study, we show that ethanol stimulated apoB mRNA synthesis and apoB mRNA editing. Significantly, the proportion of edited apoB mRNA also increased following ethanol treatment of transcription or translation arrested cells. These data suggested that ethanol could regulate editing activity using pre-existing editosomal proteins. In addition, the presence of a suppressor of apoB mRNA editing activity was suggested by the finding that inhibition of mRNA or protein synthesis alone was sufficient to increase the proportion of edited RNA. It is proposed that the level of editing activity observed in hepatocytes may be the end result of positive and negative regulatory proteins.
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Affiliation(s)
- A Giangreco
- Department of Biochemistry and Biophysics, James P. Wilmot Cancer Center, University of Rochester, School of Medicine and Dentistry, 601 Elmwood Avenue, Rochester, New York 14623, USA
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17
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Sowden MP, Smith HC. Commitment of apolipoprotein B RNA to the splicing pathway regulates cytidine-to-uridine editing-site utilization. Biochem J 2001; 359:697-705. [PMID: 11672445 PMCID: PMC1222192 DOI: 10.1042/0264-6021:3590697] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
A tripartite motif located in the centre of the 7.5 kb exon 26 of apolipoprotein B (apoB) mRNA directs editosome assembly and site-specific cytidine-to-uridine editing at nucleotide 6666. apoB mRNA editing is a post-transcriptional event, occurring primarily at the time exon 26 is spliced or at a time after splicing, but before nuclear export. We show, through reporter RNA constructs, that RNA splice sites suppress editing of precursor RNAs when placed proximal or distal to the editing site. Processed RNAs were edited more efficiently than precursor RNAs. Mutation of both the splice donor and acceptor sites was necessary for RNAs to be edited efficiently. The results suggested that commitment of pre-mRNA to the splicing and/or nuclear-export pathways may play a role in regulating editing-site utilization. The HIV-1 Rev-Rev response element ('RRE') interaction was utilized to uncouple the commitment of precursor RNAs to the spliceosome assembly pathway and associated nuclear-export pathway. Under these conditions, unspliced reporter RNAs were edited efficiently. We propose that pre-mRNA passage through the temporal or spatial restriction point where they become committed to spliceosome assembly contributes regulatory information for subsequent editosome activity.
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Affiliation(s)
- M P Sowden
- Department of Biochemistry and Biophysics, University of Rochester School of Medicine and Dentistry, 601 Elmwood Avenue, Rochester, NY 14642, USA
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Chester A, Scott J, Anant S, Navaratnam N. RNA editing: cytidine to uridine conversion in apolipoprotein B mRNA. BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1494:1-13. [PMID: 11072063 DOI: 10.1016/s0167-4781(00)00219-0] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
RNA editing is a post-transcriptional process that changes the informational capacity within the RNA. These processes include alterations made by nucleotide deletion, insertion and base conversion. A to I and C to U conversion occurs in mammals and these editing events are catalysed by RNA binding deaminases. C to U editing of apoB mRNA was the first mammalian editing event to be identified. The minimal protein complex necessary for apoB mRNA editing has been determined and consists of APOBEC-1 and ACF. Overexpression of APOBEC-1 in transgenic animals caused liver dysplasia and APOBEC-1 has been identified in neurofibromatosis type 1 tumours, suggesting that RNA editing may be another mechanism for tumourigenesis. Several APOBEC-1-like proteins have been identified, including a family of APOBEC-1-related proteins with unknown function on chromosome 22. This review summarises the different types of RNA editing and discusses the current status of C to U apoB mRNA editing. This knowledge is very important in understanding the structure and function of these related proteins and their role in biology.
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Affiliation(s)
- A Chester
- MRC Molecular Medicine, Clinical Science Centre, Imperial College School of Medicine, Hammersmith Hospital, London, UK
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19
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Yang Y, Sowden MP, Smith HC. Induction of cytidine to uridine editing on cytoplasmic apolipoprotein B mRNA by overexpressing APOBEC-1. J Biol Chem 2000; 275:22663-9. [PMID: 10833526 DOI: 10.1074/jbc.m910406199] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Post-transcriptional editing of apolipoprotein B (apoB) mRNA is regulated in hepatic cells to achieve a steady state proportion of edited and unedited RNA molecules. This activity is catalyzed by APOBEC-1 (apoB mRNA editing catalytic subunit 1) in what has been widely accepted as nuclear event occurring during or after mRNA splicing. Introns impair the efficiency of editing within an adjacent exon in a distance-dependent manner in reporter RNAs. We show here that this inhibition can be overcome by overexpressing APOBEC-1 and that the enhanced editing efficiency on these reporter RNAs occurred after splicing on cytoplasmic transcripts. Given the absolute requirement of auxiliary proteins in apoB mRNA editing, the data suggested that auxiliary proteins were distributed with APOBEC-1 in both the nucleus and cytoplasm of McArdle cells. In fact, immunolocalization of one such auxiliary protein, APOBEC-1 complementation factor (ACF) demonstrated a nuclear and cytoplasmic distribution. We also demonstrate that in the absence of alterations in APOBEC-1 expression, changes in edited apoB RNA induced by ethanol arise through the stimulation of nuclear editing activity. The finding that apoB mRNA editing can occur in the cytoplasm but normally does not suggests that under biological conditions, restricting editing activity to the nucleus must be an important step in regulating the proportion of the edited apoB mRNAs.
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Affiliation(s)
- Y Yang
- Department of Biochemistry and Biophysics, Environmental Health Sciences Center, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642, USA
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Dance GS, Sowden MP, Yang Y, Smith HC. APOBEC-1 dependent cytidine to uridine editing of apolipoprotein B RNA in yeast. Nucleic Acids Res 2000; 28:424-9. [PMID: 10606639 PMCID: PMC102520 DOI: 10.1093/nar/28.2.424] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/1999] [Revised: 11/19/1999] [Accepted: 11/19/1999] [Indexed: 11/14/2022] Open
Abstract
Cytidine to uridine editing of apolipoprotein B (apoB) mRNA requires the cytidine deaminase APOBEC-1 as well as a tripartite sequence motif flanking a target cytidine in apoB mRNA and an undefined number of auxiliary proteins that mediate RNA recognition and determine site-specific editing. Yeast engineered to express APOBEC-1 and apoB mRNA supported editing under conditions of late log phase growth and stationary phase. The cis -acting sequence requirements and the intracellular distribution of APOBEC-1 in yeast were similar to those described in mammalian cells. These findings suggest that auxiliary protein functions necessary for the assembly of editing complexes, or 'editosomes', are expressed in yeast and that the distribution of editing activity is to the cell nucleus.
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Affiliation(s)
- G S Dance
- Department of Biochemistry, University of Rochester, NY 14642, USA
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21
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Siddiqui JF, Van Mater D, Sowden MP, Smith HC. Disproportionate relationship between APOBEC-1 expression and apolipoprotein B mRNA editing activity. Exp Cell Res 1999; 252:154-64. [PMID: 10502408 DOI: 10.1006/excr.1999.4598] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
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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Affiliation(s)
- J F Siddiqui
- Department of Biochemistry and Biophysics, Health Science, University of Rochester School of Medicine and Dentistry, 601 Elmwood Avenue, Rochester, New York 14642, USA
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Steinburg MF, Schock D, Backus JW, Smith HC. Tissue-specific differences in the role of RNA 3' of the apolipoprotein B mRNA mooring sequence in editosome assembly. Biochem Biophys Res Commun 1999; 263:81-6. [PMID: 10486257 DOI: 10.1006/bbrc.1999.1329] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Site-specific editing of apolipoprotein B (apoB) mRNA by the cytidine deaminase, APOBEC-1 is proposed to require interactions of auxiliary protein(s) with an eleven nucleotide element, the mooring sequence, located 3' of the C --> U editing site. An analysis of the RNA sequence dependence for protein-RNA interactions and editosome assembly in rat liver and the small intestine demonstrated that the mooring sequence was a minimal requirement for these interactions. Sequences 3' of the mooring sequence either interacted with 66 kDa and 44 kDa proteins or enhanced the interactions of these proteins with the mooring sequence. The data also suggested tissue-specific differences in the relative importance of the 3' cis-acting 'enhancer' elements in the efficiency or stability of editosome assembly. We propose that the previously demonstrated differences in apoB mRNA editing efficiency and its regulation in liver and intestine may in part be due to differences in auxiliary protein interactions with apoB mRNA 3' of the mooring sequence.
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Affiliation(s)
- M F Steinburg
- Department of Biochemistry and Biophysics, Department of Pathology, Environmental Health Sciences Center, 601 Elmwood Avenue, Rochester, New York 14642, USA
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Sowden MP, Collins HL, Smith HC, Garrow TA, Sparks JD, Sparks CE. Apolipoprotein B mRNA and lipoprotein secretion are increased in McArdle RH-7777 cells by expression of betaine-homocysteine S-methyltransferase. Biochem J 1999; 341 ( Pt 3):639-45. [PMID: 10417327 PMCID: PMC1220401] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/13/2023]
Abstract
The cDNA encoding rat betaine-homocysteine S-methyltransferase (BHMT) was isolated through production of monoclonal antibodies against protein fractions enriched with apolipoprotein B (apo B)-mRNA-editing complexes. BHMT mRNA was expressed predominantly in liver, and also in kidney, but not in small intestine. In stable McArdle RH-7777 (McA) cell lines expressing differing levels of BHMT, the editing efficiency of apo B mRNA was unchanged. Evaluation of apo B-mRNA expression revealed that steady-state levels were increased significantly and in parallel with BHMT protein expression. The highest levels of BHMT mRNA and BHMT enzyme activity expressed in stably transfected McA cells were comparable with those found in rat hepatocytes. In contrast to the changes in apo B-mRNA abundance, levels of other apolipoprotein-encoding mRNAs and several liver-specific and ubiquitously expressed mRNAs were unchanged by BHMT expression. In the cell line expressing the highest level of BHMT, apo B-containing lipoprotein secretion was increased, indicating utilization of increased endogenous message. Results suggest that apo B-mRNA abundance in McA cells is related to the expression of BHMT, an enzyme important in homocysteine metabolism.
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Affiliation(s)
- M P Sowden
- Department of Biochemistry and Biophysics, University of Rochester School of Medicine, 601 Elmwood Avenue, Rochester, NY 14642, USA
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Van Mater D, Sowden MP, Cianci J, Sparks JD, Sparks CE, Ballatori N, Smith HC. Ethanol increases apolipoprotein B mRNA editing in rat primary hepatocytes and McArdle cells. Biochem Biophys Res Commun 1998; 252:334-9. [PMID: 9826530 DOI: 10.1006/bbrc.1998.9647] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
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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Affiliation(s)
- D Van Mater
- Departments of Biochemistry and Biophysics, University of Rochester School of Medicine and Dentistry, Rochester, New York, 14642, USA
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