Familial hypercholesterolemia (FH) is a genetic disorder characterized by extremely high levels of plasma low-density lipoprotein (LDL), due to defective LDL receptor-Apolipoprotein B (APOB) binding. Current therapies such as statins or LDL apheresis for homozygous FH are insufficiently efficacious at lowering LDL cholesterol or are expensive. Treatments that target APOB100, the structural protein of LDL particles, are potential therapies for FH. We have developed a series of APOB-directed splice-switching oligonucleotides (SSOs) that cause the expression of APOB87, a truncated isoform of APOB100. APOB87, like similarly truncated isoforms expressed in patients with a different condition, familial hypobetalipoproteinemia, lowers LDL cholesterol by inhibiting VLDL assembly and increasing LDL clearance. We demonstrate that these “APO-skip” SSOs induce high levels of exon skipping and expression of the APOB87 isoform, but do not substantially inhibit APOB48 expression in cell lines. A single injection of an optimized APO-skip SSO into mice transgenic for human APOB resulted in abundant exon skipping that persists for more than 6 d. Weekly treatments generated a sustained reduction in LDL cholesterol levels of 34-51% in these mice, superior to Pravastatin in a head-to-head comparison. These results validate APO-skip SSOs as a candidate therapy for FH.
apolipoprotein B; exon skipping; familial hypercholesterolemia; oligoribonucleotides; quantitative reverse-transcription polymerase chain reaction; splice-switching oligonucleotides
Familial hypobetalipoproteinaemia (FHBL) is a codominant disorder characterised by fatty liver and reduced plasma levels of low‐density lipoprotein (LDL) and its protein constituent apolipoprotein B (apoB). FHBL is linked to the APOB gene in some but not all known cases. In a group of 59 patients with FHBL genotyped for APOB gene mutations, we found three novel splice‐site mutations: c.904+4A→G in intron 8, c.3843−2A→G in intron 24 and c.4217−1G→T in intron 25.
To assess the effects of these mutations on apoB pre‐mRNA splicing.
ApoB mRNA was analysed in the liver of one proband and in cells expressing APOB minigenes harbouring the mutations found in the other probands.
In the liver of the c.3843−2A→G carrier, an apoB mRNA devoid of exon 25 was identified, predicted to encode a truncated peptide of 1260 amino acids. The analysis of minigene transcripts in COS‐1 cells showed that the c.904+4A→G mutation caused the formation of an mRNA devoid of exon 8, predicted to encode a short apoB of 247 amino acids. The minigene harbouring the c.4217−1G→T mutation in intron 25 generated an mRNA in which exon 25 joined to a partially deleted exon 26, resulting from the activation of an acceptor site in exon 26; this mRNA is predicted to encode a truncated protein of 1380 amino acids. All these truncated apoBs were not secreted as constituents of plasma lipoproteins.
These findings demonstrate the pathogenic effect of rare splice‐site mutations of the APOB gene found in FHBL.
Apolipoprotein B (APOB) is an integral component of the chylomicron and the atherogenic lipoproteins LDL and Lp(a). Exon 26 of the APOB pre-mRNA is unusually long at 7,572 nt and is constitutively spliced. It is also subject to RNA editing in the intestine, which generates a shortened isoform, APOB48, assembled exclusively into chylomicrons. Due to its length, exon 26 contains multiple pseudo splice sites which are not spliced, but which conform to the degenerate splice site consensus.
We demonstrate that these pseudo splice sites are repressed by multiple, tandem splicing silencers distributed along the length of exon 26. The distribution of these elements appears to be heterogeneous, with a greater frequency in the middle 4,800 nt of the exon.
Repression of these splice sites is key to maintaining the integrity of exon 26 during RNA splicing and therefore the correct expression of both isoforms of APOB.
Apolipoprotein B; RNA splicing; Splicing regulation; Splice sites; Splicing silencers
We have previously described a disorder, normotriglyceridemic abetalipoproteinemia, that is characterized by the virtual absence of plasma low density lipoproteins and complete absence of apoB-100, but with apparently normal secretion of triglyceride-rich lipoproteins containing apoB-48. The patient's plasma lipoproteins were shown on polyacrylamide gels and by antibody mapping to have a new truncated apoB variant, apoB-50, circulating along with her apoB-48. We have found this individual to be homozygous for a single C-to-T nucleotide substitution at apoB codon 2252, which produces a premature in-frame stop codon. Thus, this is a rare example of homozygous hypobetalipoproteinemia. Electron photomicrographs revealed that the diameters of particles in the d less than 1.006 g/ml lipoprotein fraction, in both the postprandial and postabsorptive state, are bimodally distributed. The molar ratio of apoE to apoB in these particles is 3.5:1, similar to normal VLDL. The plasma LDL interval contains both spherical and cuboidal particles. Autologous reinfusion of labeled d less than 1.006 g/ml lipoproteins showed exponential disappearance from plasma, with an apparent half-removal time of 50 min, somewhat slower than for normal chylomicrons but within the normal range for VLDL. The calculated production rate for apoB was within the normal range in this subject. A very small amount of label was found briefly in the IDL fraction, but none at any time in LDL or HDL. Therefore, because LDL particles that contain apoB-50 lack the putative ligand domain of the LDL receptor, we conclude that the very low level of LDL is due to the rapid removal of the abnormal VLDL particles before their conversion to LDL can take place.
Apolipoprotein B (apoB) containing lipoproteins, i.e. VLDL, LDL and Lp(a), are consequently lowered by ACTH treatment in humans. This is also seen as reduced plasma apoB by 20-30% and total cholesterol by 30-40%, mostly accounted for by a decrease in LDL-cholesterol. Studies in hepatic cell line (HepG2) cells showed that apoB mRNA expression is reduced in response to ACTH incubation and is followed by a reduced apoB secretion, which may hypothesize that ACTH lowering apoB containing lipoproteins in humans may be mediated by the inhibition of hepatic apoB synthesis. This was recently confirmed in vivo in a human postprandial study, where ACTH reduced transient apoB48 elevation from the small intestine, however, the exogenic lipid turnover seemed unimpaired. In the present study we investigated if lipid synthesis and/or secretion in HepG2 cells were also affected by pharmacological levels of ACTH to accompany the reduced apoB output. HepG2 cells were incubated with radiolabelled precursors ([14C]acetate and [3H]glycerol) either before or during ACTH stimuli. Cellular and secreted lipids were extracted with chloroform:methanol and separated by the thin layer chromatography (TLC), and [14C]labelled cholesterol and cholesteryl ester and [3H]labelled triglycerides and phospholipids were quantitated by the liquid scintillation counting. It demonstrated that ACTH administration did not result in any significant change in neither synthesis nor secretion of the studied lipids, this regardless of presence or absence of oleic acid, which is known to stabilize apoB and enhance apoB production. The present study suggests that ACTH lowers plasma lipids in humans mainly mediated by the inhibition of apoB synthesis and did not via the reduced lipid synthesis.
Familial hypercholesterolemia (FH) is an autosomal dominant condition with a population prevalence of one in 300–500 (heterozygous) that is characterized by high levels of low-density lipoprotein (LDL) cholesterol, tendon xanthomata, and premature atherosclerosis and coronary heart disease (CHD). FH is caused mainly by mutations in the LDLR gene. However, mutations in other genes including APOB and PCSK9, can give rise to a similar phenotype. Homozygous FH with an estimated prevalence of one in a million is associated with severe hypercholesterolemia with accelerated atherosclerotic CHD in childhood and without treatment, death usually occurs before the age of 30 years. Current approaches for the treatment of homozygous FH include statin-based lipid-lowering therapies and LDL apheresis. Mipomersen is a second-generation antisense oligonucleotide (ASO) targeted to human apolipoprotein B (apoB)-100. This review provides an overview of the pathophysiology and current treatment options for familial hypercholesterolemia and describes novel therapeutic strategies focusing on mipomersen, an antisense apoB synthesis inhibitor. Mipomersen is distributed mainly to the liver where it silences apoB mRNA, thereby reducing hepatic apoB-100 and giving rise to reductions in plasma total cholesterol, LDL-cholesterol, and apoB concentrations in a dose-and time-dependent manner. Mipomersen has been shown to decrease apoB, LDL-cholesterol and lipoprotein(a) in patients with heterozygous and homozygous FH on maximally tolerated lipid-lowering therapy. The short-term efficacy and safety of mipomersen has been established, however, injection site reactions are common and concern exists regarding the long-term potential for hepatic steatosis with this ASO. In summary, mipomersen given alone or in combination with standard lipid-lowering medications shows promise as an adjunct therapy in patients with homozygous or refractory heterozygous FH at high risk of atherosclerotic CHD, who are not at target or are intolerant of statins.
antisense oligonucleotide; apolipoprotein B; familial hypercholesterolemia; LDL-cholesterol; metabolism; mipomersen
We aimed to identify mechanisms by which apolipoprotein B-48 (apoB-48) could have an atherogenic role by simultaneously studying the metabolism of postprandial apoB-48 and apoB-100 lipoproteins. The kinetics of apoB-48 and apoB-100, each in four density subfractions of VLDL and intermediate density lipoprotein (IDL), were studied by stable isotope labeling in a constantly fed state with half-hourly administration of almond oil in five postmenopausal women. A non-steady-state, multicompartmental model was used. Despite a much lower production rate, VLDL and IDL apoB-48 shared a similar secretion pattern with apoB-100: both were directly secreted into all fractions with similar percentage mass distributions. Fractional catabolic rates (FCRs) of apoB-48 and apoB-100 were similar in VLDL and IDL. We identified a fast turnover compartment of light VLDL that had a residence time of <30 min for apoB-48 and apoB-100. Finally, a high secretion rate of apoB-48 was associated with a slow FCR of VLDL and IDL apoB-100. In conclusion, the intestine secretes a spectrum of apoB lipoproteins, similar to what the liver secretes, albeit with a much lower secretion rate. Once in plasma, intestinal and hepatic triglyceride-rich lipoproteins have similar rates of clearance and participate interactively in similar metabolic pathways, with high apoB-48 production inhibiting the clearance of apoB-100.
kinetics; stable isotopes; triglyceride-rich lipoproteins; apolipoprotein B-48; apolipoprotein B-100
Several strategies have been pursued to increase the extent of exon 7 inclusion during splicing of SMN2 (survival of motor neuron 2) transcripts, for eventual therapeutic use in spinal muscular atrophy (SMA), a genetic neuromuscular disease. Antisense oligonucleotides (ASOs) that target an exon or its flanking splice sites usually promote exon skipping. Here we systematically tested a large number of ASOs with a 2′-O-methoxy-ethyl ribose (MOE) backbone that hybridize to different positions of SMN2 exon 7, and identified several that promote greater exon inclusion, others that promote exon skipping, and still others with complex effects on the accumulation of the two alternatively spliced products. This approach provides positional information about presumptive exonic elements or secondary structures with positive or negative effects on exon inclusion. The ASOs are effective not only in cell-free splicing assays, but also when transfected into cultured cells, where they affect splicing of endogenous SMN transcripts. The ASOs that promote exon 7 inclusion increase full-length SMN protein levels, demonstrating that they do not interfere with mRNA export or translation, despite hybridizing to an exon. Some of the ASOs we identified are sufficiently active to proceed with experiments in SMA mouse models.
Spinal muscular atrophy (SMA) is a severe genetic disease that causes motor-neuron degeneration. SMA patients lack a functional SMN1 (survival of motor neuron 1) gene, but they possess an intact SMN2 gene, which though nearly identical to SMN1, is only partially functional. The defect in SMN2 gene expression is at the level of pre-mRNA splicing (skipping of exon 7), and the presence of this gene in all SMA patients makes it an attractive target for potential therapy. Here we have surveyed a large number of antisense oligonucleotides (ASOs) that are complementary to different regions of exon 7 in the SMN2 mRNA. A few of these ASOs are able to correct the pre-mRNA splicing defect, presumably because they bind to regions of exon 7 that form RNA structures, or provide protein-binding sites, that normally weaken the recognition of this exon by the splicing machinery in the cell nucleus. We describe optimal ASOs that promote correct expression of SMN2 mRNA and, therefore, normal SMN protein, in cultured cells from SMA patients. These ASOs can now be tested in mouse models of SMA, and may be useful for SMA therapy.
Mutations inSMN1 cause spinal muscular atrophy; a nearly identical gene is not functional, but becomes functional in vitro and in vivo after addition of antisense oligos.
3-hydroxy-3-methylglutaryl-Coenzyme A reductase (HMGCR) encodes the rate-limiting enzyme in the cholesterol biosynthesis pathway and is inhibited by statins, a class of cholesterol-lowering drugs. Expression of an alternatively spliced HMGCR transcript lacking exon 13, HMGCR13(−), has been implicated in the variation of plasma LDL-cholesterol (LDL-C) and is the single most informative molecular marker of LDL-C response to statins. Given the physiological importance of this transcript, our goal was to identify molecules that regulate HMGCR alternative splicing. We recently reported gene expression changes in 480 lymphoblastoid cell lines (LCLs) after in vitro simvastatin treatment, and identified a number of statin-responsive genes involved in mRNA splicing. Heterogeneous nuclear ribonucleoprotein A1 (HNRNPA1) was chosen for follow-up since rs3846662, an HMGCR SNP that regulates exon 13 skipping, was predicted to alter an HNRNPA1 binding motif. Here, we not only demonstrate that rs3846662 modulates HNRNPA1 binding, but also that sterol depletion of human hepatoma cell lines reduced HNRNPA1 mRNA levels, an effect that was reversed with sterol add-back. Overexpression of HNRNPA1 increased the ratio of HMGCR13(−) to total HMGCR transcripts by both directly increasing exon 13 skipping in an allele-related manner and specifically stabilizing the HMGCR13(−) transcript. Importantly, HNRNPA1 overexpression also diminished HMGCR enzyme activity, enhanced LDL-C uptake and increased cellular apolipoprotein B (APOB). rs1920045, an SNP associated with HNRNPA1 exon 8 alternative splicing, was also associated with smaller statin-induced reduction in total cholesterol from two independent clinical trials. These results suggest that HNRNPA1 plays a role in the variation of cardiovascular disease risk and statin response.
APOBEC-1 Complementation Factor (ACF) is an RNA-binding protein that interacts with apoB mRNA to support RNA editing. ACF traffics between the cytoplasm and nucleus. It is retained in the nucleus in response to elevated serum insulin levels where it supports enhanced apoB mRNA editing. In this report we tested whether ACF may have the ability to regulate nuclear export of apoB mRNA to the sites of translation in the cytoplasm. Using mouse models of obesity-induced, insulin resistance and primary hepatocyte cultures we demonstrated that both nuclear retention of ACF and apoB mRNA editing were reduced in the livers of hyperinsulinemic obese mice relative to lean controls. Coincident with an increase in the recovery of ACF in the cytoplasm was an increase in the proportion of total cellular apoB mRNA recovered in cytoplasmic extracts. Cytoplasmic ACF from both lean controls and obese mouse livers was enriched in endosomal fractions associated with apoB mRNA translation and ApoB lipoprotein assembly. Inhibition of ACF export to the cytoplasm resulted in nuclear retention of apoB mRNA and reduced both intracellular and secreted ApoB protein in primary hepatocytes. The importance of ACF for modulating ApoB was supported by the finding that RNAi knockdown of ACF reduced ApoB secretion. An additional discovery from this study was the finding that leptin is a suppressor ACF expression. Dyslipidemia is a common pathology associated with insulin resistance that is in part due to the loss of insulin controlled secretion of lipid in ApoB-containing very low density lipoproteins. The data from animal models suggested that loss of insulin regulated ACF trafficking and leptin regulated ACF expression may make an early contribution to the overall pathology associated with very low-density lipoprotein secretion from the liver in obese individuals.
diabetes; insulin-resistance; APOBEC-1 Complementation Factor; apolipoprotein B; mRNA editing
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 ~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.
ACF; alternative splicing; apolipoprotein B; APOBEC-1; differentiation; Caco-2 cells; mRNA editing
Apolipoprotein (apo) B-48 mRNA is the product of RNA editing which consists of a C----U conversion changing a CAA codon encoding Gln-2153 in apoB-100 mRNA to a UAA stop codon in apoB-48 mRNA. In the adult rat, RNA editing occurs both in the small intestine and the liver. We have studied the ability of rat liver nuclear extracts to bind to synthetic apoB mRNA segments spanning the editing site. Using an RNA gel mobility shift assay, we found the sequence-specific binding of a protein(s) to a 65-nucleotide apoB-100 mRNA. UV crosslinking followed by T1 ribonuclease digestion and SDS-polyacrylamide gel electrophoresis demonstrated the formation of a 40 kDa protein-RNA complex when 32P-labeled apoB-100 mRNA was incubated with a rat liver nuclear extract but not with HeLa nuclear extract. Binding was specific for the sense strand of apoB mRNA, and was not demonstrated with single-stranded apoB DNA, or antisense apoB RNA. The complex also failed to form if SDS was present during the UV light exposure. Binding experiments using synthetic apoB mRNAs indicate that the 40 kDa protein would also bind to apoB-48 mRNA but not apoA-I, apoA-IV, apoC-II or apoE mRNA. Experiments using deletion mutants of apoB-100 mRNA indicate efficient binding of wildtype 65-nucleotide (W65), 40-nucleotide (W40) and 26-nucleotide (W26) apoB-100 mRNA segments, but not 10-nucleotide (or smaller) segments of apoB-100 mRNA to the 40 kDa protein. In contrast, two other regions of apoB-100 mRNA, B-5' (bases 1128-3003) and B-3' (bases 11310-11390), failed to bind to the protein. The 40 kDa sequence-specific binding protein in rat liver nuclear extract may play a role in apoB-100 mRNA editing.
In the search of new strategies to fight against obesity, we targeted a gene pathway involved in energy uptake. We have thus investigated the APOB mRNA editing protein (APOBEC1) gene pathway that is involved in fat absorption in the intestine. The APOB gene encodes two proteins, APOB100 and APOB48, via the editing of a single nucleotide in the APOB mRNA by the APOBEC1 enzyme. The APOB48 protein is mandatory for the synthesis of chylomicrons by intestinal cells to transport dietary lipids and cholesterol. We produced transgenic rabbits expressing permanently and ubiquitously a small hairpin RNA targeting the rabbit APOBEC1 mRNA. These rabbits exhibited a moderately but significantly reduced level of APOBEC1 gene expression in the intestine, a reduced level of editing of the APOB mRNA, a reduced level of synthesis of chylomicrons after a food challenge, a reduced total mass of body lipids and finally presented a sustained lean phenotype without any obvious physiological disorder. Interestingly, no compensatory mechanism opposed to the phenotype. These lean transgenic rabbits were crossed with transgenic rabbits expressing in the intestine the human APOBEC1 gene. Double transgenic animals did not present any lean phenotype, thus proving that the intestinal expression of the human APOBEC1 transgene was able to counterbalance the reduction of the rabbit APOBEC1 gene expression. Thus, a moderate reduction of the APOBEC1 dependent editing induces a lean phenotype at least in the rabbit species. This suggests that the APOBEC1 gene might be a novel target for obesity treatment.
Although statin drugs are generally efficacious for lowering plasma LDL-cholesterol levels, there is considerable variability in response. To identify candidate genes that may contribute to this variation, we used an unbiased genome-wide filter approach that was applied to 10,149 genes expressed in immortalized lymphoblastoid cell lines (LCLs) derived from 480 participants of the Cholesterol and Pharmacogenomics (CAP) clinical trial of simvastatin. The criteria for identification of candidates included genes whose statin-induced changes in expression were correlated with change in expression of HMGCR, a key regulator of cellular cholesterol metabolism and the target of statin inhibition. This analysis yielded 45 genes, from which RHOA was selected for follow-up because it has been found to participate in mediating the pleiotropic but not the lipid-lowering effects of statin treatment. RHOA knock-down in hepatoma cell lines reduced HMGCR, LDLR, and SREBF2 mRNA expression and increased intracellular cholesterol ester content as well as apolipoprotein B (APOB) concentrations in the conditioned media. Furthermore, inter-individual variation in statin-induced RHOA mRNA expression measured in vitro in CAP LCLs was correlated with the changes in plasma total cholesterol, LDL-cholesterol, and APOB induced by simvastatin treatment (40 mg/d for 6 wk) of the individuals from whom these cell lines were derived. Moreover, the minor allele of rs11716445, a SNP located in a novel cryptic RHOA exon, dramatically increased inclusion of the exon in RHOA transcripts during splicing and was associated with a smaller LDL-cholesterol reduction in response to statin treatment in 1,886 participants from the CAP and Pravastatin Inflamation and CRP Evaluation (PRINCE; pravastatin 40 mg/d) statin clinical trials. Thus, an unbiased filter approach based on transcriptome-wide profiling identified RHOA as a gene contributing to variation in LDL-cholesterol response to statin, illustrating the power of this approach for identifying candidate genes involved in drug response phenotypes.
Statins, or HMG CoA reductase inhibitors, are widely used to lower plasma LDL-cholesterol levels as a means of reducing risk for cardiovascular disease. We performed an unbiased genome-wide survey to identify novel candidate genes that may be involved in statin response using genome-wide mRNA expression analysis in a sequential filtering strategy to identify those most likely to be relevant to cholesterol metabolism based on their gene expression characteristics. Among these, RHOA was selected for further functional study. A role for this gene in the maintenance of intracellular cholesterol homeostasis was confirmed by knock-down in hepatoma cell lines. In addition, statin-induced RHOA transcript levels measured in a panel of lymphoblastoid cell lines was correlated with statin-induced change in plasma LDL-cholesterol measured in individuals from whom the cell lines were derived. Lastly, a cis-acting splicing QTL associated with expression of a rare cryptic RHOA exon was also associated with statin-induced changes in plasma LDLC levels. This result exemplifies the power of applying biological information of well understood molecular pathways with genome-wide expression data for the identification of candidate genes that influence drug response.
Very low density lipoproteins (VLDL) are a major secretory product of the liver. They serve to transport endogenously synthesized lipids, mainly triglycerides (but also some cholesterol and cholesteryl esters) to peripheral tissues. VLDL is also the precursor of LDL. ApoB100 is absolutely required for VLDL assembly and secretion. The amount of VLDL triglycerides secreted by the liver depends on the amount loaded onto each lipoprotein particle, as well as the number of particles. Each VLDL has one apoB100 molecule, making apoB100 availability a key determinant of the number of VLDL particles, and hence, triglycerides, that can be secreted by hepatic cells. Surprisingly, the pool of apoB100 in the liver is typically regulated not by its level of synthesis, which is relatively constant, but by its level of degradation. It is now recognized that there are multiple opportunities for the hepatic cell to intercept apoB100 molecules and to direct them to distinct degradative processes. This mini-review will summarize progress in understanding these processes, with an emphasis on autophagy, the most recently described pathway of apoB100 degradation, and the one with possibly the most physiologic relevance to common metabolic perturbations affecting VLDL production.
Recent genome-wide association studies (GWAS) identified a variant rs7575840 in the apolipoprotein B (APOB) gene region to be associated with LDL-C. However, the underlying functional mechanism of this variant that resides 6.5 kb upstream of APOB has remained unknown. Our objective was to investigate rs7575840 for association with refined apoB containing lipid particles; for replication in a non-Caucasian Mexican population; and for underlying functional mechanism.
Methods and Results
Our data show that rs7575840 is associated with serum apoB levels (P=4.85×10−10) and apoB containing lipid particles, very small VLDL, IDL and LDL particles (P=2×10−5 - 9×10−7) in the Finnish METSIM study sample (n=7,710). Fine mapping of the APOB region using 43 SNPs replicated the association of rs7575840 with apoB in a Mexican study sample (n=2,666, P=3.33×10−05). Furthermore, our transcript analyses of adipose RNA samples from 175 Finnish METSIM subjects indicate that rs7575840 alters expression of APOB (P=1.13×10−10) and a regional non-coding RNA (BU630349) (P=7.86×10−6) in adipose tissue.
It has been difficult to convert GWAS associations into mechanistic insights. Our data show that rs7575840 is associated with serum apoB levels and apoB containing lipid particles as well as influences expression of APOB and a regional transcript BU630349 in adipose tissue. We thus provide evidence how a common genome-wide significant SNP rs7575840 may affect serum apoB, LDL-C, and TC levels.
Apolipoprotein B; association analysis; gene expression; adipose tissue; Mexicans
Recent GWAS have identified SNPs at a human chromosom1 locus associated with coronary artery disease risk and LDL cholesterol levels. The SNPs are also associated with altered expression of hepatic sortilin-1 (SORT1), which encodes a protein thought to be involved in apoB trafficking and degradation. Here, we investigated the regulation of Sort1 expression in mouse models of obesity. Sort1 expression was markedly repressed in both genetic (ob/ob) and high-fat diet models of obesity; restoration of hepatic sortilin-1 levels resulted in reduced triglyceride and apoB secretion. Mouse models of obesity also exhibit increased hepatic activity of mammalian target of rapamycin complex 1 (mTORC1) and ER stress, and we found that administration of the mTOR inhibitor rapamycin to ob/ob mice reduced ER stress and increased hepatic sortilin-1 levels. Conversely, genetically increased hepatic mTORC1 activity was associated with repressed Sort1 and increased apoB secretion. Treating WT mice with the ER stressor tunicamycin led to marked repression of hepatic sortilin-1 expression, while administration of the chemical chaperone PBA to ob/ob mice led to amelioration of ER stress, increased sortilin-1 expression, and reduced apoB and triglyceride secretion. Moreover, the ER stress target Atf3 acted at the SORT1 promoter region as a transcriptional repressor, whereas knockdown of Atf3 mRNA in ob/ob mice led to increased hepatic sortilin-1 levels and decreased apoB and triglyceride secretion. Thus, in mouse models of obesity, induction of mTORC1 and ER stress led to repression of hepatic Sort1 and increased VLDL secretion via Atf3. This pathway may contribute to dyslipidemia in metabolic disease.
Apolipoprotein B (apoB) contains ligand-binding domain for the binding of LDL to LDL-R site, which enables the removal of LDL from circulation. Our recent data showed that selenium (Se) is involved in the lipid metabolism. The present study was aimed to understand the effect of Se deficiency (0.02 ppm) and selenium supplementation (1 ppm) on apoB expression in liver during hypercholesterolemia in male Sprague Dawley rats. Animals were fed with control and high cholesterol diet (2%) for 1 and 2 months. ApoB levels by ELISA and protein expression by western blot was done. Hepatic LDL receptor (LDL-R) activity (in vivo) and mRNA expression by RT-PCR was monitored.
In selenium deficiency and on high cholesterol diet (HCD) feeding apoB levels increased and LDL-R expression decreased significantly after 2 months. On 1 ppm selenium supplementation apoB expression significantly decreased and LDL-R expression increased after 2 months. But after one month of treatment there was no significant change observed in apoB and LDL-R expression.
So the present study demonstrates that Se deficiency leads to up regulation of apoB expression during experimental hypercholesterolemia. Selenium supplementation upto 1 ppm leads to downregulation of apoB expression. Further, this study will highlight the nutritional value of Se supplementation in lipid metabolism.
The mechanisms by which high-carbohydrate, low-saturated-fat diets lower LDL cholesterol (LDLC) concentrations are unknown. In this study, kinetics of VLDL, intermediate density lipoprotein (IDL), and LDL apoprotein B and VLDL triglyceride were determined in seven nondiabetic (ND) and seven non-insulin-dependent diabetic (NIDDM) Pima Indian subjects on high-fat and high-carbohydrate (HICHO) diets. Metabolic changes were similar in ND and NIDDM. On the HICHO diet, LDLC decreased (131 +/- 8 vs. 110 +/- 7 mg/dl, P less than 0.0001) in all subjects. Mean fasting and 24-h triglyceride (TG) concentrations were unchanged, as were mean production rates and fractional clearance rates (FCR) of VLDL apoB and VLDL TG. The mean VLDL apoB pool size (303 +/- 20 vs. 371 +/- 38 mg, P = 0.01) increased owing to a decrease in the mean transport rate (10.7 +/- 1.1 vs. 8.4 +/- 0.9 mg/kg fat-free mass (ffm) per day, P less than 0.0001) and the mean rate constant (2.3 +/- 0.2 vs. 1.5 +/- 0.2, P less than 0.001) for the VLDL apoB to IDL apoB conversion pathway. The mean transport rate of VLDL apoB to LDL apoB via IDL (10.2 +/- 0.9 vs. 8.0 +/- 0.8 mg/kg ffm per day, P less than 0.001) decreased. Mean LDL apoB concentrations decreased (70 +/- 5 vs. 61 +/- 5 mg/dl, P less than 0.001) on the HICHO diet. Means for total LDL apoB transport rate, LDL apoB FCR, and LDLC/apoB ratios were unchanged. In summary, the HICHO diet decreased the activity of mechanisms that convert VLDL to LDL, which contributed to the decrease in LDLC in all subjects. There was also evidence in some subjects for increased activity of LDL apoB clearance mechanisms, and a decrease in the LDLC to apoB ratio.
Apolipoprotein (apo) E and the two B apolipoproteins, apoB48 and apoB100, are important proteins in human lipoprotein metabolism. Commonly occurring polymorphisms in the genes for apoE and apoB result in amino acid substitutions that produce readily detectable phenotypic differences in these proteins. We studied changes in apoE and apoB phenotypes before and after liver transplantation to gain new insights into apolipoprotein physiology. In all 29 patients that we studied, the postoperative serum apoE phenotype of the recipient, as assessed by isoelectric focusing, converted virtually completely to that of the donor, providing evidence that greater than 90% of the apoE in the plasma is synthesized by the liver. In contrast, the cerebrospinal fluid apoE phenotype did not change to the donor's phenotype after liver transplantation, indicating that most of the apoE in CSF cannot be derived from the plasma pool and therefore must be synthesized locally. The apoB100 phenotype (assessed with immunoassays using monoclonal antibody MB19, an antibody that detects a two-allele polymorphism in apoB) invariably converted to the phenotype of the donor. In four normolipidemic patients, we determined the MB19 phenotype of both the apoB100 and apoB48 in the "chylomicron fraction" isolated from plasma 3 h after a fat-rich meal. Interestingly, the apoB100 in the chylomicron fraction invariably had the phenotype of the donor, indicating that the vast majority of the large, triglyceride-rich apoB100-containing lipoproteins that appear in the plasma after a fat-rich meal are actually VLDL of hepatic origin. The MB19 phenotype of the apoB48 in the plasma chylomicron fraction did not change after liver transplantation, indicating that almost all of the apoB48 in plasma chylomicrons is derived from the intestine. These results were consistent with our immunocytochemical studies on intestinal biopsy specimens of organ donors; using apoB-specific monoclonal antibodies, we found evidence for apoB48, but not apoB100, in donor intestinal biopsy specimens.
The apolipoprotein B-100 (apoB-100) gene in leukocytes and the apoB-100 messenger RNA (mRNA) and translated apolipoprotein in the livers from normal and abetalipoproteinemic individuals were evaluated. Four complementary DNA probes for apoB-100 covering the 5', middle, and 3' regions of the apoB-100 mRNA were utilized and Southern blot analysis indicated that the apoB-100 gene is present in abetalipoproteinemia without major insertions or deletions. Polyadenylated hepatic apoB-100 mRNA from two abetalipoproteinemic patients was normal in size, and the concentration of apoB-100 mRNA was increased sixfold compared with control hepatic apoB-100 mRNA levels. ApoB-100 was detected in hepatocytes of abetalipoproteinemic patients by immunohistochemical techniques. These results indicate that the biochemical defect in abetalipoproteinemic patients studied is most consistent with a posttranslational defect in apoB-100 processing or secretion with an up-regulation of the apoB-100 mRNA.
BACKGROUND & AIMS
The intestine efficiently incorporates and rapidly secretes dietary fat as chylomicrons (lipoprotein particles comprising triglycerides, phospholipids, cholesterol, and proteins) that contain the apolipoprotein isoform apoB-48. The gut can store lipids for many hours after their ingestion, and release them in chylomicrons in response to oral glucose, sham feeding, or unidentified stimuli. The gut hormone glucagon-like peptide-2 (GLP-2) facilitates intestinal absorption of lipids, but its role in chylomicron secretion in human beings is unknown.
We performed a randomized, single-blind, cross-over study, with 2 study visits 4 weeks apart, to assess the effects of GLP-2 administration on triglyceride-rich lipoprotein (TRL) apoB-48 in 6 healthy men compared with placebo. Subjects underwent constant intraduodenal feeding, with a pancreatic clamp and primed constant infusion of deuterated leucine. In a separate randomized, single-blind, cross-over validation study, 6 additional healthy men ingested a high-fat meal containing retinyl palmitate and were given either GLP-2 or placebo 7 hours later with measurement of TRL triglyceride, TRL retinyl palmitate, and TRL apoB-48 levels.
GLP-2 administration resulted in a rapid (within 30 minutes) and transient increase in the concentration of TRL apoB-48, compared with placebo (P = .03). Mathematic modeling of stable isotope enrichment and the mass of the TRL apoB-48 suggested that the increase resulted from the release of stored, presynthesized apoB-48 from the gut. In the validation study, administration of GLP-2 at 7 hours after the meal, in the absence of additional food intake, robustly increased levels of TRL triglycerides (P = .007), TRL retinyl palmitate (P = .002), and TRL apoB-48 (P = .04) compared with placebo.
Administration of GLP-2 to men causes the release of chylomicrons that comprise previously synthesized and stored apoB-48 and lipids. This transiently increases TRL apoB-48 levels compared with placebo.
Enterocyte; Plasma Lipid; Fatty Acid; Human Trial
Whereas hepatitis C virus (HCV) in cell culture has a density compatible with that of the Flaviviridae family, in vivo infectious particles are partly found in low density fractions, associated with triacylglycerol (TG)-rich lipoproteins (TRL). In the blood of infected patients, HCV circulates as heterogeneous particles among which are Lipo-Viro-Particles (LVP), globular particles rich in TG and containing viral capsid and RNA. The dual viral and lipoprotein nature of LVP was further addressed with respect to apolipoprotein composition and post-prandial dynamic lipid changes. TRL exchangeable apoE, CII, CIII, but not the HDL apoA-II, were present on LVP as well as the viral envelope proteins. ApoB100 and B48, the two isoforms of the non-exchangeable apoB, were equally represented on LVP, despite the fact that apoB48 was barely detectable in the plasma of these fasting patients. This indicates that a significant fraction of plasma HCV was associated with apoB48-containing LVP. Furthermore, LVP were dramatically and rapidly enriched in triglycerides after a fat meal. As apoB48 is exclusively synthesized by the intestine, our data highlight the preferential association of HCV with chylomicrons, the intestine-derived TRL. These data raise the question of the contribution of the intestine to the viral load, and suggest that the virus could take advantage of TRL assembly and secretion for its own production and of TRL fate to be delivered to the liver.
Apolipoprotein B-48; Apolipoproteins B; metabolism; Diet; Hepacivirus; metabolism; Hepatitis C, Chronic; metabolism; virology; Humans; RNA, Viral; blood; Viral Load
Familial hypercholesterolemia is caused by mutations in the LDL receptor gene (Ldlr). Elevated plasma LDL levels result from slower LDL catabolism and a paradoxical lipoprotein overproduction. We explored the relationship between the presence of the LDL receptor and lipoprotein secretion in hepatocytes from both wild-type and LDL receptor–deficient mice. Ldlr–/– hepatocytes secreted apoB100 at a 3.5-fold higher rate than did wild-type hepatocytes. ApoB mRNA abundance, initial apoB synthetic rate, and abundance of the microsomal triglyceride transfer protein 97-kDa subunit did not differ between wild-type and Ldlr–/– cells. Pulse-chase analysis and multicompartmental modeling revealed that in wild-type hepatocytes, approximately 55% of newly synthesized apoB100 was degraded. However, in Ldlr–/– cells, less than 20% of apoB was degraded. In wild-type hepatocytes, approximately equal amounts of LDL receptor–dependent apoB100 degradation occured via reuptake and presecretory mechanisms. Adenovirus-mediated overexpression of the LDL receptor in Ldlr–/– cells resulted in degradation of approximately 90% of newly synthesized apoB100. These studies show that the LDL receptor alters the proportion of apoB that escapes co- or post-translational presecretory degradation and mediates the reuptake of newly secreted apoB-containing lipoprotein particles.
Cholesteryl ester rich apolipoprotein B100 (apoB100) lipoproteins accumulate in Bruch’s membrane before the development of age-related macular degeneration. It is not known if these lipoproteins come from the circulation or local ocular tissue. Emerging, but incomplete evidence suggests that the retinal pigmented epithelium (RPE) can secrete lipoproteins. The purpose of this investigation was to determine 1) whether human RPE cells synthesize and secrete apoB100, and 2) whether this secretion is driven by cellular cholesterol, and if so, 3) whether statins inhibit this response. The established, human derived ARPE-19 cells challenged with 0–0.8mM oleic acid accumulated cellular cholesterol, but not triglycerides. Oleic acid increased the amount of apoB100 protein recovered from the medium by both Western blot analysis and 35S-radiolabeled immunoprecipitation while negative stain electron microscopy showed lipoprotein-like particles. Of nine statins evaluated, lipophilic statins induced HMG-CoA reductase mRNA expression the most. The lipophilic Cerivastatin (5µM) reduced cellular cholesterol by 39% and abrogated apoB100 secretion by 3-fold. In contrast, the hydrophilic statin Pravastatin had minimal effect on apoB100 secretion. These data suggest that ARPE-19 cells synthesize and secrete apoB100 lipoproteins, that this secretion is driven by cellular cholesterol, and that statins can inhibit apoB100 secretion by reducing cellular cholesterol.
age-related macular degeneration; apolipoprotein B100; basal deposits; Bruch’s membrane; drusen; retinal pigmented epithelium