Ontogenetic regulation of apolipoprotein B mRNA editing during human and rat development in vivo

Characterization of chylomicron in preterm infants

BACKGROUND

The aim of this study was to investigate cholesterol and triglyceride levels in the chylomicron fraction of preterm infants at birth and during the early postnatal period.

METHODS

The subjects consisted of 133 infants (81 boys and 52 girls): 74 were term infants born at 37-41 weeks of gestation and 59 were preterm infants born at 29-36 weeks of gestation. Cholesterol and triglyceride in the chylomicron fraction were measured using high-performance liquid chromatography.

RESULTS

Compared with term infants, preterm infants had higher cholesterol and lower triglyceride in the chylomicron fraction, both in cord blood and at 1 month after birth. Thus, the chylomicron triglyceride/cholesterol ratio was significantly lower in preterm infants than in term infants in cord blood and at 1 month of age. On single regression analysis the chylomicron triglyceride/cholesterol ratio correlated positively with gestational age at birth (r = 0.331, P = 0.0003) and at 1 month (r = 0.221, P = 0.0119).

CONCLUSIONS

Preterm infants have a less-lipidated chylomicron composition at birth and at 1 month of age. Some prenatal factors may persist to influence chylomicron lipidation during the early postnatal period.

Insights from human congenital disorders of intestinal lipid metabolism

The intestine must challenge the profuse daily flux of dietary fat that serves as a vital source of energy and as an essential component of cell membranes. The fat absorption process takes place in a series of orderly and interrelated steps, including the uptake and translocation of lipolytic products from the brush border membrane to the endoplasmic reticulum, lipid esterification, Apo synthesis, and ultimately the packaging of lipid and Apo components into chylomicrons (CMs). Deciphering inherited disorders of intracellular CM elaboration afforded new insight into the key functions of crucial intracellular proteins, such as Apo B, microsomal TG transfer protein, and Sar1b GTPase, the defects of which lead to hypobetalipoproteinemia, abetalipoproteinemia, and CM retention disease, respectively. These "experiments of nature" are characterized by fat malabsorption, steatorrhea, failure to thrive, low plasma levels of TGs and cholesterol, and deficiency of liposoluble vitamins and essential FAs. After summarizing and discussing the functions and regulation of these proteins for reader's comprehension, the current review focuses on their specific roles in malabsorptions and dyslipidemia-related intestinal fat hyperabsorption while dissecting the spectrum of clinical manifestations and managements. The influence of newly discovered proteins (proprotein convertase subtilisin/kexin type 9 and angiopoietin-like 3 protein) on fat absorption has also been provided. Finally, it is stressed how the overexpression or polymorphism status of the critical intracellular proteins promotes dyslipidemia and cardiometabolic disorders.

ApoB mRNA editing is mediated by a coordinated modulation of multiple apoB mRNA editing enzyme components

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.

Functional characterization of APOBEC-1 complementation factor phosphorylation sites

ApoB mRNA editing involves site-specific deamination of cytidine 6666 producing an in-frame translation stop codon. Editing minimally requires APOBEC-1 and APOBEC-1 complementation factor (ACF). Metabolic stimulation of apoB mRNA editing in hepatocytes is associated with serine phosphorylation of ACF localized to editing competent, nuclear 27S editosomes. We demonstrate that activation of protein kinase C (PKC) stimulated editing and enhanced ACF phosphorylation in rat primary hepatocytes. Conversely, activation of protein kinase A (PKA) had no effect on editing. Recombinant PKC efficiently phosphorylated purified ACF64 protein in vitro, whereas PKA did not. Mutagenesis of predicted PKC phosphorylation sites S154 and S368 to alanine inhibited ethanol-stimulated induction of editing suggesting that these sites function in the metabolic regulation of editing. Consistent with this interpretation, substitution of S154 and S368 with aspartic acid stimulated editing to levels comparable to ethanol treatment in control McArdle RH7777 cells. These data suggest that phosphorylation of ACF by PKC may be a key regulatory mechanism of apoB mRNA editing in rat hepatocytes.

Inhibition of the synthesis of apolipoprotein B-containing lipoproteins

Increased serum concentrations of low density lipoproteins represent a major cardiovascular risk factor. Low-density lipoproteins are derived from very low density lipoproteins secreted by the liver. Apolipoprotein (apo)B that constitutes the essential structural protein of these lipoproteins exists in two forms, the full length form apoB-100 and the carboxy-terminal truncated apoB-48. The generation of apoB-48 is due to editing of the apoB mRNA which generates a premature stop translation codon. The editing of apoB mRNA is an important regulatory event because apoB-48-containing lipoproteins cannot be converted into the atherogenic low density lipoproteins. The apoB gene is constitutively expressed in liver and intestine, and the rate of apoB secretion is regulated post-transcriptionally. The translocation of apoB into the endoplasmic reticulum is complicated by the hydrophobicity of the nascent polypeptide. The assembly and secretion of apoB-containing lipoproteins within the endoplasmic reticulum is strictly dependent on the microsomal tricylceride transfer protein which shuttles triglycerides onto the nascent lipoprotein particle. The overall synthesis of apoB lipoproteins is regulated by proteosomal and nonproteosomal degradation and is dependent on triglyceride availability. Noninsulin dependent diabetes mellitus, obesity and the metabolic syndrome are characterized by an increased hepatic synthesis of apoB-containing lipoproteins. Interventions aimed to reduce the hepatic secretion of apoB-containing lipoproteins are therefore of great clinical importance. Lead targets in these pathways are discussed.

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.

Isolation, characterization and developmental regulation of the human apobec-1 complementation factor (ACF) gene

Mammalian apolipoprotein B (apo B) mRNA undergoes site-specific C to U deamination which is mediated by a multicomponent enzyme complex containing a minimal core composed of apobec-1 and a complementation factor, ACF. We have isolated and characterized the human ACF gene and examined its tissue-specific and developmental expression. The ACF gene spans approximately 80 kb and contains 15 exons, three of which are non-coding. Multiple alternative splice acceptor sites were found, generating at least nine different transcripts. Of these, the majority (approximately 75-89%) encode functional protein. In order to examine the role of ACF mRNA expression in the regulation of apo B mRNA editing, we examined a panel of fetal intestinal and hepatic mRNAs as well as RNA from an intestinal cell line. A developmental increase in C to U RNA editing has been previously noted in the human intestine. In both instances, the pattern of alternative splicing and overall abundance of ACF mRNA was relatively constant during development in both liver and small intestine. Taken together, the data demonstrate a complex pattern of differential, tissue-specific splicing of ACF mRNA, but suggest that other mechanisms are responsible for the developmental increase noted in intestinal apo B mRNA editing in humans.

Phosphorylation is a regulatory mechanism in apolipoprotein B mRNA editing

The editing of apolipoprotein B (apoB) mRNA is under tissue-specific, developmental and metabolic regulation. We found that multiple protein kinase inhibitors or activators increased apoB mRNA editing up to 2.5-fold in Caco-2 cells and 3-8-fold in McA7777 and FAO rat cells respectively. The phosphorylation-agent-induced modulation is independent of the apolipoprotein B editing catalytic subunit 1 (APOBEC-1) and of apoB mRNA expression levels, indicating the involvement of a protein modification, such as phosphorylation, regulating the cellular editing of apoB mRNA. Transient expression of protein kinase C-&theta; more than doubled apoB mRNA editing in FAO cells. Chronic exposure to ethanol, a treatment known to increase the expression of protein kinases and to change protein phosphorylation status, increased apoB mRNA editing in FAO cells up to 2.5-fold without increasing the mRNA abundance of APOBEC-1. The elimination of potential phosphorylation sites 47 and 72 of human APOBEC-1 decreased its activity to approx. one-eighth of control levels by a Ser(47)-->Ala mutation, but more than doubled the activity by a Ser(72)-->Ala mutation. The activity modulation was reversed by a Ser-->Asp mutation at sites 47 and 72, which introduced a phosphorylation-like carbonic acid group. Both human APOBEC-1 dephosphorylated by alkaline phosphase and the Ser(47,72)-to-alanine double mutant protein demonstrated a shifted isoelectric focusing pattern compared with the wild type, indicating phosphorylation at these sites. Taken together, these results suggest that phosphorylation might be an important mechanism in the regulation of apoB mRNA editing.

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.

Detection of apolipoprotein B mRNA editing by peptide nucleic acid mediated PCR clamping

Apolipoprotein B (apoB) mRNA editing leads to a single base change in its mRNA and the production of apoB-48. Currently, the degree of apoB mRNA editing is analyzed by the RT-PCR primer extension method. While this method is quantitative, it is labor intensive, utilizes radioactivity for labeling and may not be sensitive enough to discriminate between low levels of editing and inherent assay background levels. Peptide nucleic acid (PNA) oligonucletides have been used in single point mutation detection through PCR clamping. In the present work, we developed a PCR based assay which can detect the single base change responsible for the apoB-48 production. We found that as low as 0.5% of the edited form can be clearly detected by PNA mediated PCR clamping. When combined with the primer extension assay, an approximately 180-fold enrichment of the edited percentage is observed, reflecting selected PCR amplification of templates containing the edited base.

Adenoviral delivery of low-density lipoprotein receptors to hyperlipidemic rabbits: receptor expression modulates high-density lipoproteins

Plasma concentrations of low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs) are inversely related in several dyslipoproteinemias. To elucidate the interactions between these lipoproteins, we used a recombinant adenovirus (hLDLR-rAdV) to express human LDL receptors (hLDLRs) in LDL receptor-deficient rabbits. hLDLR-rAdV administration resulted in hepatocyte expression and a reduction of total, intermediate-density lipoprotein (IDL), and LDL cholesterol. In addition, we found that hLDLR-rAdV treatment induced (1) increased very-low-density lipoprotein (VLDL) cholesterol, (2) increased VLDL, IDL and LDL triglycerides, (3) decreased alpha- and pre-beta-migrating apolipoprotein E (apo E) and decreased pre-beta-migrating apo A-I at 2 to 4 days posttreatment, and (4) increased total plasma apo A-I and pre-beta-migrating apo A-I beginning 8 to 10 days posttreatment. Virtually all plasma apo A-I was present on alpha- and pre-beta-HDL. Pre-beta-HDL particles with size and electrophoretic properties consistent with nascent HDL demonstrated the greatest relative apo A-I enrichment following hLDLR-rAdV treatment. In summary, enhanced expression of hepatocyte LDLRs by hLDLR-rAdV treatment markedly altered apo A-I-containing lipoproteins and IDL and LDL. The use of recombinant viruses to express physiologically relevant genes in intact animals, analogous to transfection of cells in culture, provides a new strategy for the evaluation of effects of specific gene products on metabolic systems in vivo.

Insulin modulation of newly synthesized apolipoproteins B-100 and B-48 in human fetal intestine: gene expression and mRNA editing are not involved

We investigated insulin's effect on intestinal lipid, transport and, particularly, the biogenesis of apolipoproteins crucial to lipoprotein secretion. Adding insulin (3 mU) to the serum-free medium of cultured jejunal explants from human fetuses (17-20 weeks) reduced triglyceride and chylomicron production and inhibited apo B-48 and apo B-100 secretion. When apo B mRNA was assayed by RT-PCR and its editing by primer extension, no change was detectable following the addition of insulin. HDL lipid content, apo A-1 synthesis and RNA level were unaffected by insulin. Collectively, these results suggest that the insulin-stimulated decline in intestinal chylomicron output may involve apo B co- or post-translational modifications.

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.

Quantitative and qualitative characterization of apolipoprotein B containing lipoproteins produced by the visceral rat yolk sac in two different in vitro systems: organ culture and isolated epithelial cells in suspension culture

Tissue pieces as well as isolated epithelial cells taken from visceral rat yolk sacs at the 18th day of gestation were able to synthesize and to secrete apo B containing lipoproteins floating in the density ranges of VLDL, IDL and LDL. In all three density classes only the high molecular weight apo B was detectable. VLDL secreted from yolk sac tissue or isolated epithelial cells lacked apo E and apo C. Studies with cycloheximide revealed that about 77% of the particles was delivered from a pool of preformed lipoproteins. Evidence was given that also de novo-synthesis of apo B containing lipoproteins took place in the tissue segments and the isolated epithelial cells. Most of apo B mass (90%) and of apo B radioactivity (60%) secreted by the tissue pieces of the visceral yolk sac floated in the density range 1.020-1.064 g/ml, the remainder being found in the d < or = 1.020 g/ml fraction. In contrast, only 40% of apo B mass and 45% of apo B radioactivity delivered from isolated epithelial cells belonged to the d = 1.020-1.064 g/ml fraction. LDL released from yolk sacs and isolated epithelial cells contained more triacylglycerols (41% vs. 25%) and had a larger mean diameter (24 nm vs. 21.8 nm) than those obtained from fetal rat serum, whereas the comparatively small VLDL produced in vitro (mean diameter = 34 nm) contained less triacylglycerols (46% vs. 60.5%) and more protein (20% vs. 10.2%) in comparison with fetal serum VLDL (mean diameter = 42.3 nm). Incubation experiments with [125I]VLDL led to the conclusion that the lipase secreted by yolk sac tissue into the medium could not be responsible for the conversion of VLDL into LDL, thus supporting our view of a direct LDL secretion by visceral rat yolk sacs.

Effect of fat feeding on human intestinal apolipoprotein B mRNA levels and editing

Our purpose was to assess the effect of a fat-rich meal on intestinal apolipoprotein B (apoB) mRNA levels and editing. We obtained jejunal biopsies from eight healthy adults in the fasting state and 3 h after a meal containing 1 g/kg of fat. In the fasting state, 93% of the apoB mRNA contained the editing sequence resulting in apoB-48 production. Feeding induced no significant changes in apoB mRNA levels or editing. Our data are consistent with the concept that the significant increase in apoB-48 within triglyceride-rich lipoproteins in postprandial plasma is not due to alterations in apoB gene expression or editing.