Transgenic mice expressing human ApoB95 and ApoB97. Evidence that sequences within the carboxyl-terminal portion of human apoB100 are important for the assembly of lipoprotein

Monogenic Hypocholesterolaemic Lipid Disorders and Apolipoprotein B Metabolism

The study of apolipoprotein (apo) B metabolism is central to our understanding of human lipoprotein metabolism. Moreover, the assembly and secretion of apoB-containing lipoproteins is a complex process. Increased plasma concentrations of apoB-containing lipoproteins are an important risk factor for the development of atherosclerotic coronary heart disease. In contrast, decreased levels of, but not the absence of, these apoB-containing lipoproteins is associated with resistance to atherosclerosis and potential long life. The study of inherited monogenic dyslipidaemias has been an effective means to elucidate key metabolic steps and biologically relevant mechanisms. Naturally occurring gene mutations in affected families have been useful in identifying important domains of apoB and microsomal triglyceride transfer protein (MTP) governing the metabolism of apoB-containing lipoproteins. Truncation-causing mutations in the APOB gene cause familial hypobetalipoproteinaemia, whereas mutations in MTP result in abetalipoproteinaemia; both rare conditions are characterised by marked hypocholesterolaemia. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the key biochemical, clinical, metabolic and genetic features of the monogenic hypocholesterolaemic lipid disorders affecting apoB metabolism.

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.

Endoplasmic reticulum stress and apoptosis

Cell death is an essential event in normal life and development, as well as in the pathophysiological processes that lead to disease. It has become clear that each of the main cellular organelles can participate in cell death signalling pathways, and recent advances have highlighted the importance of the endoplasmic reticulum (ER) in cell death processes. In cells, the ER functions as the organelle where proteins mature, and as such, is very responsive to extracellular-intracellular changes of environment. This short overview focuses on the known pathways of programmed cell death triggering from or involving the ER.

cDNA microarray analysis of HBV transgenic mouse liver identifies genes in lipid biosynthetic and growth control pathways affected by HBV

Hepatitis B virus (HBV) transgenic mice that replicate HBV in the liver generally do not exhibit gross liver pathology, while maintaining a high level (10(7) or greater) of viral titer in the blood. We have used this model to determine the minimum effects of HBV replication in the liver on cellular gene transcription, using cDNA microarrays. cDNA microarray data from sets of HBV versus control cDNA microarrays revealed a very small impact of HBV on the cellular transcriptome. After deletion of genes that were variable in control cDNA microarrays and applying significance analysis of microarrays (SAM), an application to detect statistically significantly regulated genes, we identified 18 upregulated genes and 14 downregulated genes. Most of the regulated genes show a change in expression with respect to control of less than 40% in either direction, demonstrating small effects of HBV. The largest functional category for upregulated genes was lipid biosynthesis, in which ATP citrate lyase, fatty acid synthase, sterol regulatory element binding factor 2, and retinol binding protein 1 were all upregulated. The most strongly downregulated genes were in the cytochrome p450 group, particularly p450, 4a14. Several growth regulatory genes including cyclin D1, IGF binding protein 3, and PCNA were moderately upregulated. These data are the first to specifically identify enzymes involved in fatty acid and NADPH-electron transport pathways that are altered by the presence of HBV. The data also demonstrates that HBV is well adapted to non-cytopathic replication in hepatocytes. Cellular genes expected to be affected by viral secretion from membranes are clearly upregulated, and upregulation of growth regulatory genes may facilitate replacement of dying hepatocytes during persistent infection.

Mutation of lysine residues in apolipoprotein B-100 causes defective lipoprotein[a] formation

Lipoprotein[a] (Lp[a]) is assembled by a two-step process involving an initial lysine-dependent binding between apolipoprotein B-100 (apoB-100) and apolipoprotein[a] (apo[a]) that facilitates the formation of a disulphide bond between apoB-100Cys4,326 and apo[a]Cys4,057. Previous studies of transgenic mice expressing apoB-95 (4,330 amino acids) and apoB-97 (4,397 amino acids) have shown that apoB-100 amino acids 4,330-4,397 are important for the initial binding to apo[a]. Furthermore, a lysine-rich peptide spanning apoB-100 amino acids 4,372-4,392 has recently been shown to bind apo[a] and inhibit Lp[a] assembly in vitro. This suggests that a putative apo[a] binding site exists in the apoB-4,372-4,392 region. The aim of our study was to establish whether the apoB-4,372-4,392 sequence was important for Lp[a] assembly in the context of the full-length apoB-100. Transgenic mice were created that expressed a mutant human apoB-100, apoB-100K4-->S4, in which all four lysine residues in the 4,372-4,392 sequence were mutated to serines. The apoB-100K4-->S4 mutant showed a reduced capacity to form Lp[a] in vitro compared with wild-type human apoB-100. Double transgenic mice expressing both apoB-100K4-->S4 and apo[a] contained significant amounts of free apo[a] in the plasma, indicating a less-efficient assembly of Lp[a] in vivo. Taken together, these results clearly show that the apoB-4,372-4,392 sequence plays a role in Lp[a] assembly.

Quantitative Evaluation of the Contribution of Weak Lysine-binding Sites Present within Apolipoprotein(a) Kringle IV Types 6–8 to Lipoprotein(a) Assembly

During lipoprotein(a) (Lp(a)) assembly, non-covalent interactions between apolipoprotein(a) (apo(a)) and low density lipoprotein precede specific disulfide bond formation. Studies have shown that the non-covalent step involves an interaction between the weak lysine-binding sites (WLBS) present within each of apo(a) kringle IV types 6, 7, and 8 (KIV(6-8)), and two lysine residues (Lys(680) and Lys(690)) within the NH(2) terminus of the apolipoprotein B-100 (apoB) component of low density lipoprotein. In the present study, we introduced single point mutations (E56G) into each of the WLBS present in apo(a) KIV(6-8) and expressed these mutations in the context of a 17-kringle (17K) recombinant apo(a) variant. Single mutations that disrupt the WLBS in KIV(6), KIV(7), and KIV(8), as well as mutants that disrupt the WLBS in both KIV(6) and KIV(7), or both KIV(7) and KIV(8), were assessed for their ability to form non-covalent and covalent Lp(a) complexes. Our results demonstrate that both apo(a) KIV(7) and KIV(8), but not KIV(6), are required for maximally efficient non-covalent and covalent Lp(a) assembly. Single mutations in the WLBS of KIV(7) or KIV(8) resulted in a 3-fold decrease in the affinity of 17K recombinant apo(a) for apoB, and a 20% reduction in the rate of covalent Lp(a) formation. Tandem mutations in the WLBS in both KIV(7) and KIV(8) resulted in a 13-fold reduction in the binding affinity between apo(a) and apoB, and a 75% reduction in the rate of the covalent step of Lp(a) formation. We also showed that KIV(7) and KIV(8) specifically bind with high affinity to apoB-derived peptides containing Lys(690) or Lys(680), respectively. Taken together, our data demonstrate that specific interactions between apo(a) KIV(7) and KIV(8) and Lys(680) and Lys(690) in apoB mediate a high affinity non-covalent interaction between apo(a) and low density lipoprotein, which dictates the efficiency of covalent Lp(a) formation.

GRP94 (94 kDa glucose-regulated protein) suppresses ischemic neuronal cell death against ischemia/reperfusion injury

The 94 kDa glucose-regulated protein (GRP94), the endoplasmic reticulum (ER) resident molecular chaperone, has a role in cell death due to endoplasmic reticulum stress (ER stress). Here, we report that expression of GRP94 was increased in human neuroblastoma cells (SH-SY5Y (SY5Y) cells) exposed to hypoxia/reoxygenation (H/R). H/R mediated death of SY5Y cells was associated with the activation of major cysteine proteases, caspase-3 and calpain, along with an elevated intracellular calcium concentration. Pretreatment with adenovirus-mediated antisense GRP94 (AdGRP94AS) led to reduced viability of SY5Y cells after being subjected to H/R compared with wild-type cells or cells with adenovirus-mediated overexpression of GRP94 (AdGRP94S). These results indicate that suppression of GRP94 is associated with accelerated apoptosis and that expression of GRP94 (as a stress protein) suppresses oxidative stress-mediated neuronal death and stabilizes calcium homeostasis in the ER. We also used gerbils with transient forebrain ischemia to study the role of GRP94 in vivo. Neurons with adenovirus-mediated overexpression of GRP94 were resistant to ischemic damage. These results confirmed that GRP94 could suppress ischemic injury to neurons, suggesting that gene transfer of GRP94 into the brain may have therapeutic potential in the treatment of cerebrovascular disease.

A synthetic peptide that inhibits lipoprotein(a) assembly

OBJECTIVE

We previously reported that human apolipoprotein B100 (apoB) amino acids 4330-4397 were important for the initial noncovalent binding to apolipoprotein(a) [apo(a)] that facilitates lipoprotein(a) [Lp(a)] assembly. In this study, we aimed to further define the apoB sequences within the 4330-4397 region that were important for the noncovalent binding to apo(a).

METHODS AND RESULTS

Alignment of the human apoB4330-4397 sequence with mouse apoB, which also noncovalently binds apo(a), revealed stretches of similar sequence, including a lysine-rich sequence spanning apoB amino acids 4372-4392. Structural analysis of the apoB4372-4392 sequence using the WHEEL program predicted an amphipathic alpha-helix. Circular dichroism studies of a synthetic peptide spanning human apoB amino acids 4372-4392, both in the absence and presence of dimyristoylphosphatidylcholine, confirmed the alpha-helical nature of the sequence. We tested the ability of the apoB4372-4392 peptide to bind to apo(a) and found that the peptide bound to apo(a) with high affinity but not to Lp(a). The apoB4372-4392 peptide inhibited Lp(a) assembly in Lp(a) formation assays far more effectively than the lysine analogue, epsilon-amino-n-caproic acid (IC50=40 micromol/L versus 10 mmol/L, respectively). Incorporation of the apoB4372-4392 peptide onto dimyristoylphosphatidylcholine vesicles yielded an even more effective inhibitor (IC50=4 micromol/L).

CONCLUSIONS

Our study shows that the apoB4372-4392 sequence mediates the initial noncovalent binding to apo(a) and has demonstrated that the apoB4372-4392 peptide is a novel and effective inhibitor of Lp(a) assembly.

Mitochondrial calcium-mediated reactive oxygen species are essential for the rapid induction of the grp78 gene in 9L rat brain tumour cells

The glucose-regulated protein grp78 gene is rapidly transactivated in 9L rat brain tumour (RBT) cells treated with okadaic acid (OA) followed by heat shock (HS) (termed OA-->HS treatment). By Northern blotting analyses and transient transfection assays, we herein show that transactivation of grp78 by OA-->HS is abolished by an intracellular calcium chelator, bis(aminophenoxy)ethane N,N'-tetraacetic acid (BAPTA), and an inhibitor of mitochondrial Ca(2+) uniporter, ruthenium red (RR), while unaffected by cyclosporin A (CsA), an inhibitor of mitochondrial permeability transition pore (MTP). The inhibitory effects of BAPTA and RR also present in OA-->HS induction of transient elevation of intracellular hydrogen peroxide. The requirement of reactive oxygen intermediates (ROIs) is confirmed by substitutional addition of antioxidants, N-acetyl cysteine (NAC) and pyrrolidinedithiocarbamate (PDTC) during OA-->HS treatment, mimicking these inhibitory effects of BAPTA and RR. Western blotting analyses show that phosphorylation of transcription factor CREB is diminished only by BAPTA but not by RR, while phosphorylation of ATF-2 is unaffected by either agent. Conclusively, we present that both the disturbances of mitochondrial calcium homeostasis and reactive oxygen intermediates are essential for rapid transactivation of grp78, and this pathway is separate from protein kinase A (PKA)-dependent CREB activation or p38 mitogen-activated protein kinase (p38(MAPK))-dependent ATF-2 activation and signalling.

Identification of the proteoglycan binding site in apolipoprotein B48

An initial event in atherosclerosis is the retention of lipoproteins within the intima of the vessel wall. Previously we identified Site B (residues 3359-3369) in apolipoprotein (apo) B100 as the proteoglycan binding sequence in low density lipoproteins (LDLs) and showed that the atherogenicity of apoB-containing lipoproteins is linked to their affinity for artery wall proteoglycans. However, both apoB100- and apoB48-containing lipoproteins are equally atherogenic even though Site B lies in the carboxyl-terminal half of apoB100 and is absent in apoB48. If binding to proteoglycans is a key step in atherogenesis, apoB48-containing lipoproteins must bind to proteoglycans via other proteoglycan binding sites in the amino-terminal 48% of apoB. In vitro studies have identified five clusters of basic amino acids in delipidated apoB48 that bind negatively charged glycosaminoglycans. To determine which of these sites is functional on LDL particles, we analyzed the proteoglycan binding activity of recombinant human LDLs from transgenic mice or rat hepatoma cells. Substitution of neutral amino acids for the basic amino acids in Site B-Ib (residues 84-94) abolished the proteoglycan binding activity of recombinant apoB53. Carboxyl-truncated apoB80 bound biglycan with higher affinity than apoB100 and apoB48. ApoB80 in which Site B was mutated had the same affinity for proteoglycans as apoB48. These data support the hypothesis that the carboxyl terminus of apoB100 "masks" Site B-Ib, the amino-terminal proteoglycan binding site, and that this site is exposed in carboxyl-truncated forms of apoB. The presence of a proteoglycan binding site in the amino-terminal region of apoB may explain why apoB48- and apoB100-containing lipoproteins are equally atherogenic.

Size matters: use of YACs, BACs and PACs in transgenic animals

In 1993, several groups, working independently, reported the successful generation of transgenic mice with yeast artificial chromosomes (YACs) using standard techniques. The transfer of these large fragments of cloned genomic DNA correlated with optimal expression levels of the transgenes, irrespective of their location in the host genome. Thereafter, other groups confirmed the advantages of YAC transgenesis and position-independent and copy number-dependent transgene expression were demonstrated in most cases. The transfer of YACs to the germ line of mice has become popular in many transgenic facilities to guarantee faithful expression of transgenes. This technique was rapidly exported to livestock and soon transgenic rabbits, pigs and other mammals were produced with YACs. Transgenic animals were also produced with bacterial or P1-derived artificial chromosomes (BACs/PACs) with similar success. The use of YACs, BACs and PACs in transgenesis has allowed the discovery of new genes by complementation of mutations, the identification of key regulatory sequences within genomic loci that are crucial for the proper expression of genes and the design of improved animal models of human genetic diseases. Transgenesis with artificial chromosomes has proven useful in a variety of biological, medical and biotechnological applications and is considered a major breakthrough in the generation of transgenic animals. In this report, we will review the recent history of YAC/BAC/PAC-transgenic animals indicating their benefits and the potential problems associated with them. In this new era of genomics, the generation and analysis of transgenic animals carrying artificial chromosome-type transgenes will be fundamental to functionally identify and understand the role of new genes, included within large pieces of genomes, by direct complementation of mutations or by observation of their phenotypic consequences.

The molecular mechanism for the genetic disorder familial defective apolipoprotein B100

Familial defective apolipoprotein B100 (FDB) is a genetic disorder in which low density lipoproteins (LDL) bind defectively to the LDL receptor, resulting in hypercholesterolemia and premature atherosclerosis. FDB is caused by a mutation (R3500Q) that changes the conformation of apolipoprotein (apo) B100 near the receptor-binding site. We previously showed that arginine, not simply a positive charge, at residue 3500 is essential for normal receptor binding and that the carboxyl terminus of apoB100 is necessary for mutations affecting arginine 3500 to disrupt LDL receptor binding. Thus, normal receptor binding involves an interaction between arginine 3500 and tryptophan 4369 in the carboxyl tail of apoB100. W4369Y LDL and R3500Q LDL isolated from transgenic mice had identically defective LDL binding and a higher affinity for the monoclonal antibody MB47, which has an epitope flanking residue 3500. We conclude that arginine 3500 interacts with tryptophan 4369 and facilitates the conformation of apoB100 required for normal receptor binding of LDL. From our findings, we developed a model that explains how the carboxyl terminus of apoB100 interacts with the backbone of apoB100 that enwraps the LDL particle. Our model also explains how all known ligand-defective mutations in apoB100, including a newly discovered R3480W mutation in apoB100, cause defective receptor binding.

An analysis of the interaction between mouse apolipoprotein B100 and apolipoprotein(a)

The assembly of lipoprotein(a) (Lp(a)) involves an initial noncovalent interaction between apolipoprotein (apo) B100 and apo(a), followed by the formation of a disulfide bond between apoB100 cysteine 4326 and apo(a) cysteine 4057. The structural features of apoB100 that are required for its noncovalent interaction with apo(a) have not been fully defined. To analyze that initial interaction, we tested whether apo(a) could bind noncovalently to two apoB proteins that lack cysteine 4326: mouse apoB100 and human apoB100-C4326G. Our experiments demonstrated that both mouse apoB and the human apoB100-C4326G bind noncovalently to apo(a). We next sought to gain insights into the apoB amino acid sequences required for the interaction between apoB100 and apo(a). Previous studies of truncated human apoB proteins indicated that the carboxyl terminus of human apoB100 (amino acids 4330-4397) is important for Lp(a) assembly. To determine whether the carboxyl terminus of mouse apoB100 can interact with apo(a), transgenic mice were produced with a mutant human apoB gene construct in which human apoB100 amino acids 4279-4536 were replaced with the corresponding mouse apoB100 sequences and tyrosine 4326 was changed to a cysteine. The mutant apoB100 bound to apo(a) and formed bona fide disulfide-linked Lp(a), but Lp(a) assembly was less efficient than with wild-type human apoB100. The fact that Lp(a) assembly was less efficient with the mouse apoB sequences provides additional support for the notion that sequences in the carboxyl terminus of apoB100 are important for Lp(a) assembly.

adapt78 protects cells against stress damage and suppresses cell growth

We have previously identified several genes whose RNA products are induced in HA-1 hamster cells under conditions where a cytoprotective adaptive response is observed. One of these genes, designated adapt78, was found to have a human homolog with some homology to glucose-regulated protein 78 (Grp78). We subsequently determined that adapt78 and grp78 mRNAs are induced by the same stress agents and conclude that adapt78 is a stress-response gene and putative new member of the grp stress gene family. Here we extend these studies to assess the effect of overexpressing adapt78 on stress protection and growth arrest. HA-1 cells stably transfected with adapt78 cDNA were found to exhibit significantly reduced calcium- and hydrogen peroxide-mediated cytotoxicity as compared with control transfectants. In addition, adapt78 stable overexpressors exhibited significantly reduced cell growth. Both cytoprotection and growth arrest accompanied only modest overexpression of adapt78. Flow cytometry revealed that the growth arrest occurred in G(1)-phase. Immunoflourescent analysis revealed that Adapt78 protein exhibits significant perinuclear staining suggestive of endoplasmic reticulum localization in addition to cytoplasmic localization. These data indicate that adapt78 is both cytoprotective and growth suppressive and that these effects may be mediated by Adapt78 protein at the endoplasmic reticulum.

The seventh myth of lipoprotein(a): where and how is it assembled?

Our understanding of the genetics, metabolism and pathophysiology of the atherogenic plasma lipoprotein Lp(a) has considerably increased over past years. Nevertheless, the precise mechanisms regulating the biosynthesis and assembly of Lp(a) are poorly understood and controversially discussed. Lp(a) plasma concentrations are determined by synthesis and not by degradation. Transcriptional and post-translational mechanisms have been identified as regulating Lp(a) production in primary hepatocytes and transfected cell lines. Assembly of Lp(a) occurs extracellularly from newly synthesized apolipoprotein(a) and circulating LDL. This view has recently been challenged by in-vivo kinetic studies in humans which are compatible with an intracellular assembly event. Lp(a) assembly is a complex two-step process of multiple non-covalent interactions between apolipoprotein(a) and apolipoprotein B-100 of LDL followed by covalent disulfide linkage of two free cysteine residues on both proteins.

Insights into apolipoprotein B biology from transgenic and gene-targeted mice

Over the past five years, several laboratories have used transgenic and gene-targeted mice to study apolipoprotein (apo) B biology. Genetically modified mice have proven useful for investigating the genetic and environmental factors affecting atherogenesis, for defining apoB structure/function relationships, for understanding the regulation of the apoB gene expression in the intestine, for defining the "physiologic rationale" for the existence of the two different forms of apoB (apoB48 and apoB100) in mammalian metabolism and for providing mechanistic insights into the human apoB deficiency syndrome, familial hypobetalipoproteinemia. This review will provide several examples of how genetically modified mice have contributed to our understanding of apoB biology, including our new discovery that human heart myocytes secrete nascent apoB-containing lipoproteins.

Sequences within the amino terminus of ApoB100 mediate its noncovalent association with apo(a)

Although sequences within the C terminus of apolipoprotein B (apoB) have been implicated in the formation of covalent lipoprotein(a) [Lp(a)] particles, sequences in apoB that mediate initial noncovalent interaction with apo(a) remain to be characterized. To address this question, we have used an affinity chromatography method in which 2 recombinant forms of apo(a) [r-apo(a); either a 17-kringle form (17K) or a derivative containing apo(a) kringle IV types 5-8] have been immobilized onto Sepharose beads. Conditioned media from rat hepatoma (McA-RH7777) cell lines stably expressing various carboxyl-terminally truncated forms of human apoB (ranging from full-length apoB to apoB15) were applied to the r-apo(a) affinity columns; the columns were subsequently washed and eluted with epsilon-aminocaproic acid (epsilon-ACA). Specific binding was quantified by Western blot analysis of column fractions. Of the apoB truncations examined, apoB94, apoB42, apoB37, and apoB29 exhibited complete specific binding to 17K r-apo(a). Only approximately 50% binding was observed for apoB18, whereas essentially no detectable binding was observed with apoB15. In all cases, similar results were obtained when the r-apo(a) kringle IV types 5-8-Sepharose column was used. Additionally, substitution of proline for epsilon-ACA as the eluent resulted in similar column profiles with either r-apo(a) affinity column. We also demonstrated that apoB48 present in chylomicrons bound completely to the 17K column in an epsilon-ACA-dependent manner. Taken together, these results represent the first demonstration that N-terminal sequences in apoB between amino acid residues 680 (apoB15) and 781 (apoB18) are essential for noncovalent association with apo(a) and that these sequences interact with domain(s) present within apo(a) kringle IV types 5-8.

A molecular approach to understanding human sterol metabolism using yeast genetics

The availability of the sequenced genome of Saccharomyces cerevisiae (yeast) has culminated in the use of this model eukaryote to study human diseases at a basic level. This article describes the advantages of studying lipid metabolism in this genetically facile organism, including examples of conserved functions and genetic approaches to identifying new components of cholesterol homeostasis.

Expression of large genomic clones in transgenic mice: new insights into apolipoprotein B structure, function and regulation

Extensive manipulation of the apolipoprotein B gene in yeast and bacterial artificial chromosome clones and subsequent expression of these clones in transgenic mice have provided fresh insights into several aspects of apolipoprotein B biology, including the identification of sequences important for lipoprotein (a) assembly, the demonstration that intestinal expression of apolipoprotein B is controlled by DNA sequences > 50 kb from the gene, and the extraordinary finding that apolipoprotein B is expressed in the heart.