REPR and complementation factor(s) interact to modulate rat apolipoprotein B mRNA editing in response to alterations in cellular cholesterol flux

Apobec1 complementation factor (A1CF) and RBM47 interact in tissue-specific regulation of C to U RNA editing in mouse intestine and liver

Mammalian C to U RNA is mediated by APOBEC1, the catalytic deaminase, together with RNA binding cofactors (including A1CF and RBM47) whose relative physiological requirements are unresolved. Although A1CF complements APOBEC1 for in vitro RNA editing, A1cf^-/- mice exhibited no change in apolipoproteinB (apoB) RNA editing, while Rbm47 mutant mice exhibited impaired intestinal RNA editing of apoB as well as other targets. Here we examined the role of A1CF and RBM47 in adult mouse liver and intestine, following deletion of either one or both gene products and also following forced (liver or intestinal) transgenic A1CF expression. There were minimal changes in hepatic and intestinal apoB RNA editing in A1cf^-/- mice and no changes in either liver- or intestine-specific A1CF transgenic mice. Rbm47 liver-specific knockout (Rbm47^LKO ) mice demonstrated reduced editing in a subset (11 of 20) of RNA targets, including apoB. By contrast, apoB RNA editing was virtually eliminated (<6% activity) in intestine-specific (Rbm47^IKO ) mice with only five of 53 targets exhibiting C-to-U RNA editing. Double knockout of A1cf and Rbm47 in liver (AR^LKO ) eliminated apoB RNA editing and reduced editing in the majority of other targets, with no changes following adenoviral APOBEC1 administration. Intestinal double knockout mice (AR^IKO ) demonstrated further reduced editing (<10% activity) in four of five of the residual APOBEC1 targets identified in AR^IKO mice. These data suggest that A1CF and RBM47 each function independently, yet interact in a tissue-specific manner, to regulate the activity and site selection of APOBEC1 dependent C-to-U RNA editing.

Mouse models as tools to explore cytidine-to-uridine RNA editing

RNA editing is a process through which the nucleotide sequence specified in the genomic template is modified to produce a different nucleotide sequence in the transcript. RNA editing is an important mechanism of genetic regulation that amplifies genetic plasticity by allowing the production of alternative protein products from a single gene. There are two generic classes of RNA editing in nuclei, involving enzymatic deamination of either C-to-U or A-to-I nucleotides. The best characterized example of C-to-U RNA editing is that of apolipoprotein B (apoB), which is mediated by a holoenzyme that contains a minimal core composed of an RNA-specific cytidine deaminase apobec-1, and its cofactor apobec-1 complementation factor (ACF). C-to-U editing of apoB RNA generates two different isoforms--apoB100 and apoB48--from a single transcript. Both are important regulators of lipid transport and metabolism, and are functionally distinct. C-to-U apoB RNA editing is regulated by a range of factors including developmental, nutritional, environmental, and metabolic stimuli. Rodent models have provided a tractable system in which to study the effects of such stimuli on lipid metabolism. In addition, both transgenic and gene knockout experiments have provided important insights into gain and loss of function approaches for studying C-to-U RNA editing in a murine background.

Developmental regulation of apolipoprotein B mRNA editing is an autonomous function of small intestine involving homeobox gene Cdx1

Apolipoprotein B mRNA editing is developmentally regulated in the human and rodent small intestine, changing from <1% at day 14 to approximately 90% by day 20 in the rat fetus. This regulation is coincident with the developmental formation of the crypt-to-villus axis functional unit, a continuous and rapidly renewing system involving cell generation, migration, and differentiation. Utilizing small intestine isografts implanted into the subcutaneous tissue of adult recipients, apolipoprotein B mRNA editing was developmentally up-regulated, parallel to that seen with an intact control. In contrast, apoB mRNA expression remains nearly constant in the isograft, unlike the normal intact small intestine. Immunohistochemical analyses demonstrated that apoB-48 protein existed predominantly in well differentiated enterocytes along the villus surface whereas apoB-100 was in the lamina propria and crypts. ApoB mRNA editing levels were very low in the crypt-like rat intestinal cell line, IEC-6 ( approximately 0.3%), but very high in well differentiated enterocytes ( approximately 91.5%). The expression of homeobox gene Cdx1 increased 18-fold in small intestine in vivo during the same time course when apoB mRNA editing increased from approximately 2 to approximately 90%. The overexpression of Cdx1 in IEC-6 cells increased apoB mRNA editing over 10-fold compared with the vector control. This increase was associated with a significant increase of activating factor ACF, a component of the apoB mRNA editing complex. Taken together, these data suggest that the developmental regulation of apoB mRNA editing is an autonomous cytodifferentiation function of small intestine for which homeobox gene Cdx1 may play an important role.

Molecular regulation, evolutionary, and functional adaptations associated with C to U editing of mammalian apolipoproteinB mRNA

RNA editing encompasses an important class of co- or posttranscriptional nucleic acid modification that has expanded our understanding of the range of mechanisms that facilitate genetic plasticity. Since the initial description of RNA editing in trypanosome mitochondria, a model of gene regulation has emerged that now encompasses a diverse range of biochemical and genetic mechanisms by which nuclear, mitochondrial, and t-RNA sequences are modified from templated versions encoded in the genome. RNA editing is genetically and biochemically distinct from other RNA modifications such as splicing, capping, and polyadenylation although, as discussed in Section I, these modifications may have relevance to the regulation of certain types of mammalian RNA editing. This review will focus on C to U RNA editing, in particular, the biochemical and genetic mechanisms that regulate this process in mammals. These mechanisms will be examined in the context of the prototype model of C to U RNA editing, namely the posttranscriptional cytidine deamination targeting a single nucleotide in mammalian apolipoproteinB (apoB). Other examples of C to U RNA editing will be discussed and the molecular mechanisms--where known--contrasted with those regulating apoB RNA editing.

Fructose intake increases hyperlipidemia and modifies apolipoprotein expression in apolipoprotein AI-CIII-AIV transgenic mice

Fructose intake has increased steadily during the past two decades. The objective of this study was to determine the effect of fructose intake on lipid metabolism in apolipoprotein (apo) AI-CIII-AIV transgenic (Tg) mice that have severe hypertriglyceridemia and moderate hypercholesterolemia. Tg and control mice were fed for 9 mo a commercial nonpurified diet and had free access to water or 250 g/L fructose solution. In Tg mice, fructose intake increased triglycerides and cholesterol but did not induce insulin resistance. There were no differences in human hepatic apo AI and apo CIII mRNA levels in fructose-fed mice compared with untreated mice, but apo AIV mRNA was greater, indicating a differential expression of the apo AI and apo AIV genes in response to dietary perturbations. Interestingly, the plasma concentration of the three human apolipoproteins was enhanced in fructose-fed Tg mice compared with untreated Tg mice. Our data suggest that long-term fructose consumption had strong adverse effects in this hyperlipidemic mouse model.

APOLIPOPROTEIN B: mRNA editing, lipoprotein assembly, and presecretory degradation

Apolipoprotein (apo)B circulates in two distinct forms, apoB100 and apoB48. Human liver secretes apoB100, the product of a large mRNA encoding 4536 residues. The small intestine of all mammals secretes apoB48, which arises following C-to-U deamination of a single cytidine base in the nuclear apoB transcript, introducing a translational stop codon. This process, referred to as apoB RNA editing, operates through a multicomponent enzyme complex that contains a single catalytic subunit, apobec-1, in addition to other protein factors that have yet to be cloned. ApoB RNA editing also exhibits stringent cis-acting requirements that include both structural and sequence-specific elements-specifically efficiency elements that flank the minimal cassette, an AU-rich RNA context, and an 11-nucleotide mooring sequence-located in proximity to a suitably positioned (usually upstream) cytidine. C-to-U RNA editing may become unconstrained under circumstances where apobec-1 is overexpressed, in which case multiple cytidines in apoB RNA, as well as in other transcripts, undergo C-to-U editing. ApoB RNA editing is eliminated following targeting of apobec-1, establishing that there is no genetic redundancy in this function. Under physiological circumstances, apoB RNA editing exhibits developmental, hormonal, and nutritional regulation, in some cases related to transcriptional regulation of apobec-1 mRNA. ApoB and the microsomal triglyceride transfer protein (MTP) are essential for the assembly and secretion of apoB-containing lipoproteins. MTP functions by transferring lipid to apoB during its translation and by transporting triglycerides into the endoplasmic reticulum to form apoB-free lipid droplets. These droplets fuse with nascent apoB-containing particles to form mature, very low-density lipoproteins or chylomicrons. In cultured hepatic cells, lipid availability dictates the rate of apoB production. Unlipidated or underlipidated forms of apoB are subjected to presecretory degradation, a process mediated by retrograde transport from the lumen of the endoplasmic reticulum to the cytosol, coupled with multiubquitination and proteasomal degradation. Although control of lipid secretion in vivo is primarily achieved at the level of lipoprotein particle size, regulation of apoB production by presecretory degradation may be relevant in some dyslipidemic states.

Calcium increases apolipoprotein B mRNA editing

ApoB-100 and apoB-48 are major components of chylomicrons, very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL). The two proteins are generated from a single apoB mRNA by apoB mRNA editing which induces an in-frame stop codon in apoB mRNA. Apolipoprotein B (apoB) mRNA editing is an important determinant of the proportion of full-length (apoB-100) and truncated (apoB-48) proteins in total apoB metabolism. Calcium is involved in the regulation of secretion and synthesis of VLDL and apoB. In this paper, we demonstrate for the first time that the amount of edited apoB mRNA in the cultured cells Caco-2 and McA7777 is markedly increased by calcium. Increasing extracellular calcium concentration, calcium ionophore (A23187 and ionomycin) treatment, and depleting calcium stores and raising cytoplasmic calcium concentration by thapsigargin increase apoB mRNA editing up to threefold in a dose dependent manner. Calcium has no direct stimulative effect on apoB mRNA editing in an in vitro editing system. The editing increase by extracellular calcium is not related to alterations of APOBEC-1 mRNA expression. These data suggest that calcium is not only involved in the regulation of apolipoprotein metabolism but also apoB mRNA editing.

Induction of cytidine to uridine editing on cytoplasmic apolipoprotein B mRNA by overexpressing APOBEC-1

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.

Disproportionate relationship between APOBEC-1 expression and apolipoprotein B mRNA editing activity

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.

Plasma apolipoprotein B-48, hepatic apolipoprotein B mRNA editing and apolipoprotein B mRNA editing catalytic subunit-1 mRNA levels are altered in zinc-deficient rats

Apolipoprotein B (apoB) exists as two major isoforms and serves as an obligatory component of lipid-rich plasma lipoprotein particles. Apolipoprotein B mRNA editing is a zinc-dependent, site-specific cytidine deamination that determines whether the apoB-100 or apoB-48 isoform is synthesized. The objective of this work was to examine whether dietary zinc levels affect apoB mRNA editing in vivo. Adult male Sprague-Dawley rats were randomly assigned to zinc-deficient (ZD, <0.5 mg Zn/kg diet), zinc-adequate (ZA, 30 mg Zn/kg diet) or zinc-replenished (ZDA, ZD rats fed the ZA diet for last 2 d) dietary groups for 18 d. The ratio of plasma apolipoprotein B-48 (apoB-48) to total apoB was significantly lower in zinc-deficient compared with zinc-adequate rats. Primer extension analysis indicated a modest but significant reduction in hepatic apoB mRNA editing in ZD rats compared with that of the ZA group. In ZDA rats, hepatic apoB mRNA editing and the percentage of plasma apoB-48 to total apoB were not different from ZA rats. The mRNA abundance of hepatic apobec-1 (apoB mRNA editing catalytic subunit 1) was significantly lower in ZD and ZDA rats than in ZA rats. In summary, the plasma ratio of apoB-48 to total apoB protein as well as hepatic apoB mRNA editing and hepatic apobec-1 mRNA levels were reduced in rats consuming a zinc-deficient diet. These data suggest that one or more components of apoB metabolism may be influenced by dietary zinc status.

Apolipoprotein B mRNA editing and apolipoprotein gene expression in the liver of hyperinsulinemic fatty Zucker rats: relationship to very low density lipoprotein composition

We previously demonstrated increased apolipoprotein B (apoB) mRNA editing, elevated levels of mRNA for the catalytic component of the apoB mRNA editing complex, apobec-1, and increased secretion of the product of the edited mRNA, apoB48, in very low density lipoproteins (VLDL) in primary cultures of Sprague-Dawley rat hepatocytes following insulin treatment. In order to determine the effect of in vivo hyperinsulinemia on these processes, we determined apoB mRNA editing, apobec-1 expression, hepatic expression of mRNA for apoB and other VLDL apoproteins, and the quantity and composition of plasma VLDL in the hyperinsulinemic fatty Zucker rat. Total apoB mRNA content of the livers of the fatty rats and lean littermates did not differ; however, edited apoB message coding for hepatic apo B48, and abundance of mRNA for the catalytic subunit of the apoB mRNA editing complex, apobec-1, was increased by 1.7- and 3.3-fold, respectively, in fatty rats. ApoCIII mRNA abundance was increased in livers of fatty rats as well, but the abundance of hepatic apoE mRNA in the fatty animal was not different from that of the lean rat. Hepatic apoAI mRNA abundance was also increased in the fatty rats. Associated with increased apoB mRNA editing, was the 1.7-fold increase in the fraction of apoB in plasma as apoB48 in fatty rats. VLDL-triglyceride and -apoB in plasma were 15- and 3-fold higher, respectively, in fatty Zucker rats compared to lean littermates, indicating both enrichment of VLDL with triglycerides and increased accumulation of VLDL particles. Increased hepatic expression of mRNA for apoCIII and apoAI was associated with increased content of apoC (and relative depletion of apoE) in VLDL of fatty rats, and plasma apoAI was increased in fatty Zucker rats, primarily in the HDL fraction. The current study provides further evidence that chronic exposure to high levels of insulin influences both the quantity of and lipid/apoprotein composition of VLDL in plasma. The increased apoC and decreased apoE (as well as increased triglyceride) content of VLDL in the fatty Zucker rat observed in the current study may affect VLDL clearance and therefore may be a factor in the observed accumulation of VLDL in the plasma of the fatty hyperinsulinemic Zucker rats.

Insulin increases expression of apobec-1, the catalytic subunit of the apolipoprotein B mRNA editing complex in rat hepatocytes

We have previously shown that chronic insulin treatment of rat hepatocytes increases the fraction of edited apolipoprotein B (apoB) mRNA from approximately 50% to as much as 90%. We have now examined the effect of insulin on apobec-1 mRNA abundance and demonstrate that increased editing of apoB mRNA following insulin treatment is accompanied by elevated apobec-1 mRNA levels in primary rat hepatocytes. Time-course measurements of the effects of insulin on apoB mRNA editing and apobec-1 mRNA abundance showed that both were elevated almost maximally within 48 hours and sustained for at least 5 days of insulin treatment.

Analysis of protein complexes assembled on apolipoprotein B mRNA for mooring sequence-dependent RNA editing

Apolipoprotein B (apoB) mRNA editing involves a cytidine-to-uridine transition at a select site catalyzed by a cytidine deaminase known as APOBEC-1. This enzyme cannot edit RNA alone but acquires site-specific editing capacity in the context of additional protein factors (auxiliary proteins). These proteins are currently hypothesized to assemble with APBEC-1 as a holoenzyme complex or editosome. Auxiliary proteins are the focus of ongoing research as they presumably serve important structural and regulatory roles in the editosome. The abilities of auxiliary proteins to interact with APOBEC-1 and apoB RNA and to promote RNA editing activity are important endpoints used as proof that proteins are functionally involved in apoB RNA editing. This article reviews the discovery of the editosome and provides detailed protocols for its isolation and subfractionation.

Apolipoprotein B RNA sequence 3' of the mooring sequence and cellular sources of auxiliary factors determine the location and extent of promiscuous editing

Apolipoprotein B (apoB) RNA editing involves a cytidine to uridine transition at nucleotide 6666 (C6666) 5' of an essential cis -acting 11 nucleotide motif known as the mooring sequence. APOBEC-1 (apoB editing catalytic sub-unit 1) serves as the site-specific cytidine deaminase in the context of a multiprotein assembly, the editosome. Experimental over-expression of APOBEC-1 resulted in an increased proportion of apoB mRNAs edited at C6666, as well as editing of sites that would otherwise not be recognized (promiscuous editing). In the rat hepatoma McArdle cell line, these sites occurred predominantly 5' of the mooring sequence on either rat or human apoB mRNA expressed from transfected cDNA. In comparison, over-expression of APOBEC-1 in HepG2 (HepG2-APOBEC) human hepatoma cells, induced promiscuous editing primarily 5' of the mooring sequence, but sites 3' of the C6666 were also used more efficiently. The capacity for promiscuous editing was common to rat, rabbit and human sources of APOBEC-1. The data suggested that differences in the distribution of promiscuous editing sites and in the efficiency of their utilization may reflect cell-type-specific differences in auxiliary proteins. Deletion of the mooring sequence abolished editing at the wild type site and markedly reduced, but did not eliminate, promiscuous editing. In contrast, deletion of a pair of tandem UGAU motifs 3' of the mooring sequence in human apoB mRNA selectively reduced promiscuous editing, leaving the efficiency of editing at the wild type site essentially unaffected. ApoB RNA constructs and naturally occurring mRNAs such as NAT-1 (novel APOBEC-1 target-1) that lack this downstream element were not promiscuously edited in McArdle or HepG2 cells. These findings underscore the importance of RNA sequences and the cellular context of auxiliary factors in regulating editing site utilization.

Multiple protein domains determine the cell type-specific nuclear distribution of the catalytic subunit required for apolipoprotein B mRNA editing

Apolipoprotein B (apoB) mRNA editing catalyzed by apoB mRNA editing catalytic subunit 1 (APOBEC-1) has been proposed to be a nuclear process. To test this hypothesis, the subcellular distribution of hemagglutinin- (HA) tagged APOBEC-1 expressed in transiently transfected hepatoma cells was determined by indirect immunofluorescence microscopy. HA-APOBEC-1 was detected in both the nucleus and cytoplasm of rat and human hepatoma cells. Mutagenesis of APOBEC-1 demonstrated that the N-terminal 56 amino acids (1-56) were necessary for the nuclear distribution of APOBEC-1, but this region did not contain a functional nuclear localization signal (NLS). However, we identified a 24-amino acid domain in the C terminus of APOBEC-1 with characteristics of a cytoplasmic retention signal (CRS) or a nuclear export signal (NES). These data suggest, therefore, that the nuclear import of APOBEC-1 may not be mediated by a positive NLS; rather, it may be achieved by overcoming the effect of a CRS/NES. We also demonstrated that the nuclear distribution of APOBEC-1 occurred only in cell lines that were capable of editing apoB RNA. We propose that the cellular distribution of APOBEC-1 is determined by multiple domains within this protein, and a nuclear localization of the enzyme may be regulated by cell type-specific factors that render these cells uniquely editing competent.

Partial characterization of the auxiliary factors involved in apolipoprotein B mRNA editing through APOBEC-1 affinity chromatography

APOBEC-1-catalyzed apolipoprotein B (apoB) mRNA editing requires auxiliary factors, but the number and functions of these factors are unknown. We have partially purified the editing activity from extracts of a McArdle cell line overexpressing His6-hemagglutinin-tagged, rat APOBEC-1 using metal-chelating affinity chromatography. The 1,200-fold purification achieved by this approach was partially dependent on exogenously added RNA containing a mooring sequence for editosome assembly. Affinity-purified editing activity could be separated by 300 mM NaCl extraction into two fractions, a salt-resistant fraction (editing fraction 1; EF1) and a salt-soluble fraction (EF2). Neither EF1 nor EF2 alone could edit apoB RNA, but when added together they reconstituted full editing activity. Previously identified candidate auxiliary factors including the p66/p44 apoB RNA binding proteins and the presumptive editosome assembly factor p240 were all present in the affinity-purified editing complex. Moreover, virtually all of p66, p240, and APOBEC-1 were present in EF1, whereas p44 was quantitatively recovered in EF2. This is the first demonstration that p66 and p44 can bind to apoB RNA independently of one another. In addition, 100- and 55-kDa apoB RNA cross-linking proteins have been identified in the APOBEC-1 affinity-purified material. RNA competition studies demonstrated that p100, p66, and p55 bound selectively to apoB RNA, whereas p44 had general RNA cross-linking characteristics. The data underscore the multiplicity of auxiliary factors potentially involved in apoB RNA editing and suggest an editosome far more complicated than may have been previously appreciated.

Two distinct TATA-less promoters direct tissue-specific expression of the rat apo-B editing catalytic polypeptide 1 gene

The species and tissue specificity of apolipoprotein (apo) B mRNA editing is determined by the expression of apoB editing catalytic polypeptide 1 (APOBEC-1), the cytidine deaminase that catalyzes apoB mRNA editing. To understand the molecular mechanisms that regulate the transcription of APOBEC-1, we characterized rat APOBEC-1 cDNA and genomic DNA. cDNA cloning and RNase protection analysis showed two alternative promoters for the tissue-specific expression of APOBEC-1 in the liver and intestine, Pliv and Pint. Both promoters lack a TATA box, and Pint belongs to the MED-1 class of promoters, which initiate transcription at multiple sites. We also identified two allelic forms of the APOBEC-1 gene from the characterization of two rat APOBEC-1 P1 genomic clones, RE4 and RE5. The RE4 allele is 18 kilobases long and contains six exons and five introns, whereas the RE5 allele contains an additional approximately 8 kilobases of intron sequences and an extra exon encoding a 5'-untranslated region; however, the APOBEC-1 transcripts from the two alleles appear to have similar, if not identical, functions. Transgenic mouse studies showed that Pliv was preferentially used in the liver, kidney, brain, and adipose tissues, whereas Pint was preferentially used in the small intestine, stomach, and lung. Our results suggest that the tissue-specific expression of APOBEC-1 is governed by multiple regulatory elements exerting control over a single coding sequence. The presence or absence of these regulatory elements may determine the tissue-specific expression of APOBEC-1 in other mammalian species.

Dexamethasone enhanced by insulin, but not by thyroid hormones stimulates apolipoprotein B mRNA editing in cultured rat hepatocytes depending on the developmental stage

The increase of hepatic apolipoprotein B (apoB) mRNA editing during rat development was not affected by hypothyroidism. Furthermore, the addition of 3,3',5'-triiodothyronine (T3) to cultured hepatocytes taken from fetal, neonatal and adult rats had no effect on apoB mRNA editing. In contrast, dexamethasone markedly stimulated apoB mRNA editing in hepatocytes taken from neonates. This effect was enhanced by the addition of insulin. For the first time our data provide evidence that glucocorticoids together with insulin are important for the regulation of apoB mRNA editing during postnatal development, whereas thyroid hormones are not critical for this process.

Selective recruitment of ApoB-48 for the assembly of VLDL in rat triacylglycerol-enriched hepatocytes

Primary rat hepatocyte cultures were enriched in cellular triacylglycerol (TAG) by exposure to extracellular oleate for 3 days. Control cells were cultured for the same time without oleate. The large increase in TAG secretion into the medium of TAG-enriched cells during the final 24 hours (225 +/- 30 versus 40 +/- 10 micrograms/mg cell protein [control cells], P < .01) was not accompanied by a similar change in apolipoprotein B (apoB) secretion (4.22 +/- 0.94 versus 3.72 +/- 0.75 micrograms/mg per 24 hours, respectively). Instead, TAG-enriched cells recruited a larger proportion of apoB for the synthesis of very low density lipoprotein (VLDL), the secretion of which was substantially higher under these circumstances (1.46 +/- 0.39 versus 0.34 +/- 0.06 microgram apoB per milligram cell protein per 24 hours, P < .05). The increase in VLDL assembly was accompanied by a selective 2.5-fold increase (P < .05) in the specific recruitment of apoB-48. There was no significant increase in the amount of apoB-100, which appeared in the VLDL fraction when cells were enriched with TAG. Under these circumstances there was an increase in net cellular synthesis of apoB-48 (5524 +/- 667 versus 2505 +/- 598 disintegrations per minute per milligram protein per hour, P < .05). The net cellular synthesis of apoB-100 was unchanged compared with that observed in control cell cultures (1548 +/- 237 versus 2000 +/- 897 dpm/ mg per hour, respectively). A large proportion of the total secreted apoB was associated with small particles of density higher than VLDL, even when VLDL output was maximally stimulated, suggesting that apoB was oversecreted and in excess of the cells' requirement to transport TAG.

The neurofibromatosis type I messenger RNA undergoes base-modification RNA editing

A functional mooring sequence, known to be required for apolipoprotein B (apoB) mRNA editing, exists in the mRNA encoding the neurofibromatosis type I (NF1) tumor suppressor. Editing of NF1 mRNA modifies cytidine in an arginine codon (CGA) at nucleotide 2914 to a uridine (UGA), creating an in frame translation stop codon. NF1 editing occurs in normal tissue but was several-fold higher in tumors. In vitro editing and transfection assays demonstrated that apoB and NF1 RNA editing will take place in both neural tumor and hepatoma cells. Unlike apoB, NF1 editing did not demonstrate dependence on rate-limiting quantities of APOBEC-1 (the apoB editing catalytic subunit) suggesting that different trans-acting factors may be involved in the two editing processes.