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

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.

Gene structure and expression of the mouse APOBEC-1 complementation factor: multiple transcriptional initiation sites and a spliced variant with a premature stop translation codon

Editing of apolipoprotein (apo) B mRNA is mediated by an enzyme-complex that consists of the catalytic cytidine deaminase APOBEC-1 and the mRNA binding protein APOBEC-1 complementation factor or APOBEC-1 stimulating protein (ACF/ASP). Here we describe the detailed characterization of the structure, expression and splicing pattern of the mouse ACF/ASP gene. ACF/ASP mRNA is mainly expressed in mouse liver, small intestine and kidney. The deduced protein sequences of ACF/ASP from mouse and man share an identity of 93%. The mouse ACF/ASP gene consists of 12 exons and gives rise predominantly to full-length transcripts. To a minor extent (<10%) ACF/ASP mRNA with unspliced exon 8 is generated in liver, kidney and small intestine that encodes a truncated protein with a predicted molecular weight of 43 kDa. The promoter of the mouse ACF/ASP gene lacks a canonical TATA-box, but contains a cluster of Sp1 binding sites and uses multiple transcriptional initiation sites. Transfection studies demonstrated a preference of this promoter for cell lines derived from the gastrointestinal tract and proved the location of the promoter core region. The high sequence identity between man and mouse-much higher as observed for APOBEC-1-indicates a strong evolutionary constraint on the structure-function relationship of ACF/ASP, most probably due to a central role in editing and processing of apo B mRNA.

Regulation of alpha1G T-type calcium channel gene (CACNA1G) expression during neuronal differentiation

Down-regulation of T-type Ca channel current and mRNA occurs following differentiation of Y79 retinoblastoma cells. To understand how the decrease in expression is linked to cell differentiation, we examined transcriptional regulation of the Cav3.1 Ca channel gene, CACNA1G. We identified two putative promoters (A and B) in 1.3 kb of cloned genomic DNA. Reverse transcriptase-polymerase chain reaction and 5' rapid amplification of cDNA ends-polymerase chain reaction analyses demonstrated that two transcripts with different 5' untranslated regions are generated by different transcription start sites, with promoter A favoured in undifferentiated cells and promoter B favoured in differentiated cells. Functional analyses of the promoter sequence revealed that both promoters are active. Enhancer and repressor sequences were identified upstream of promoter A and B, respectively. These results suggest that the down-regulation of alpha1G mRNA in differentiated Y79 cells is mediated primarily by decreased activity of promoter A, which could occur in conjunction with repression of the activity of promoter B. The decrease in T-type Ca channel expression in Y79 cells may be an essential signal affecting phenotypic maturation and expression of other ion channel subtypes in the differentiated cells.

Reduced expression and increased CpG dinucleotide methylation of the rat APOBEC-1 promoter in transgenic rabbits

Editing of apolipoprotein (apo) B mRNA in liver limits the plasma LDL levels in horses, dogs, rats or mice. Species such as man or rabbit do not edit the hepatic apo B mRNA and are therefore susceptible to atherosclerosis and coronary artery disease due to elevated plasma LDL levels. The catalytic subunit APOBEC-1 is the only missing component of the apo B mRNA editing enzyme complex in the human or rabbit liver. Here we describe the generation of transgenic rabbits in which APOBEC-1 expression is mediated by the proximal promoter of the rat APOBEC-1 gene. These transgenic rabbits are healthy and fertile, and rat APOBEC-1 mRNA is expressed in liver, intestine, kidney, lung, brain and muscle. The transgenic APOBEC-1 expression is low and not sufficient to induce editing in rabbit liver. In rat, the proximal APOBEC-1 promoter demonstrates a progressive loss of CpG dinucleotide methylation towards the core promoter region that is entirely unmethylated. In the transgenic rabbits, this distinct pattern of CpG methylation is lost, and throughout the entire rat APOBEC-1 promoter, >90% of the CpGs are methylated. Thus, the weak proximal rat APOBEC-1 promoter appears to be down-regulated in the rabbit and may be species-specific.

C-->U editing of neurofibromatosis 1 mRNA occurs in tumors that express both the type II transcript and apobec-1, the catalytic subunit of the apolipoprotein B mRNA-editing enzyme

C-->U RNA editing of neurofibromatosis 1 (NF1) mRNA changes an arginine (CGA) to a UGA translational stop codon, predicted to result in translational termination of the edited mRNA. Previous studies demonstrated varying degrees of C-->U RNA editing in peripheral nerve-sheath tumor samples (PNSTs) from patients with NF1, but the basis for this heterogeneity was unexplained. In addition, the role, if any, of apobec-1, the catalytic deaminase that mediates C-->U editing of mammalian apolipoprotein B (apoB) RNA, was unresolved. We have examined these questions in PNSTs from patients with NF1 and demonstrate that a subset (8/34) manifest C-->U editing of RNA. Two distinguishing characteristics were found in the PNSTs that demonstrated editing of NF1 RNA. First, these tumors express apobec-1 mRNA, the first demonstration, in humans, of its expression beyond the luminal gastrointestinal tract. Second, PNSTs with C-->U editing of RNA manifest increased proportions of an alternatively spliced exon, 23A, downstream of the edited base. C-->U editing of RNA in these PNSTs was observed preferentially in transcripts containing exon 23A. These findings were complemented by in vitro studies using synthetic RNA templates incubated in the presence of recombinant apobec-1, which again confirmed preferential editing of transcripts containing exon 23A. Finally, adenovirus-mediated transfection of HepG2 cells revealed induction of editing of apoB RNA, along with preferential editing of NF1 transcripts containing exon 23A. Taken together, the data support the hypothesis that C-->U RNA editing of the NF1 transcript occurs both in a subset of PNSTs and in an alternatively spliced form containing a downstream exon, presumably an optimal configuration for enzymatic deamination by apobec-1.

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.

RNA editing: cytidine to uridine conversion in apolipoprotein B mRNA

RNA editing is a post-transcriptional process that changes the informational capacity within the RNA. These processes include alterations made by nucleotide deletion, insertion and base conversion. A to I and C to U conversion occurs in mammals and these editing events are catalysed by RNA binding deaminases. C to U editing of apoB mRNA was the first mammalian editing event to be identified. The minimal protein complex necessary for apoB mRNA editing has been determined and consists of APOBEC-1 and ACF. Overexpression of APOBEC-1 in transgenic animals caused liver dysplasia and APOBEC-1 has been identified in neurofibromatosis type 1 tumours, suggesting that RNA editing may be another mechanism for tumourigenesis. Several APOBEC-1-like proteins have been identified, including a family of APOBEC-1-related proteins with unknown function on chromosome 22. This review summarises the different types of RNA editing and discusses the current status of C to U apoB mRNA editing. This knowledge is very important in understanding the structure and function of these related proteins and their role in biology.

Purification and molecular cloning of a novel essential component of the apolipoprotein B mRNA editing enzyme-complex

Editing of apolipoprotein B (apoB) mRNA requires the catalytic component APOBEC-1 together with "auxiliary" proteins that have not been conclusively characterized so far. Here we report the purification of these additional components of the apoB mRNA editing enzyme-complex from rat liver and the cDNA cloning of the novel APOBEC-1-stimulating protein (ASP). Two proteins copurified into the final active fraction and were characterized by peptide sequencing and mass spectrometry: KSRP, a 75-kDa protein originally described as a splicing regulating factor, and ASP, a hitherto unknown 65-kDa protein. Separation of these two proteins resulted in a reduction of APOBEC-1-stimulating activity. ASP represents a novel type of RNA-binding protein and contains three single-stranded RNA-binding domains in the amino-terminal half and a putative double-stranded RNA-binding domain at the carboxyl terminus. Purified recombinant glutathione S-transferase (GST)-ASP, but not recombinant GST-KSRP, stimulated recombinant GST-APOBEC-1 to edit apoB RNA in vitro. These data demonstrate that ASP is the second essential component of the apoB mRNA editing enzyme-complex. In rat liver, ASP is apparently associated with KSRP, which may confer stability to the editing enzyme-complex with its substrate apoB RNA serving as an additional auxiliary component.

Absence of APOBEC-1 mediated mRNA editing in human carcinomas

The transgene expression of the catalytic subunit APOBEC-1 of the apo B mRNA editing enzyme-complex can cause hepatocellular carcinoma in mice and rabbits. It has been proposed that aberrant editing of mRNA may represent a novel oncogenic principle. This investigation aimed to define whether such aberrant hyperediting mediated by APOBEC-1 occurs in human carcinomas. Editing and hyperediting of apo B, NAT1 or NF1 mRNA was not identified in any of 28 resected tumor specimens, including hepatocellular, bile duct, gastric, colorectal, pancreatic adeno- and neuroendocrine, lung adeno-, medullary thyroid and breast carcinoma, soft tissue sarcoma and neuroblastoma. In most types of carcinoma, significant levels for full-length APOBEC-1 mRNA could not be detected. Low level expression of APOBEC-1 was found in colorectal and gastric carcinoma where most of the APOBEC-1 mRNA is inactivated by alternate splicing. The 'auxiliary' components of the apo B mRNA editing enzyme-complex are missing in many tumors including colorectal and gastric carcinoma, but are highly expressed in hepatocellular, lung adeno- and breast carcinoma all of which lack APOBEC-1. Taken together, either APOBEC-1 or the 'auxiliary' components of the apo B mRNA editing enzyme-complex or both are missing in human carcinomas resulting in the absence of mRNA editing. Currently, there is no evidence that aberrant editing mediated by APOBEC-1 contributes to the tumorigenesis of natural human carcinomas.

C-->U editing of apolipoprotein B mRNA in marsupials: identification and characterisation of APOBEC-1 from the American opossum Monodelphus domestica

The C->U editing of RNA is widely found in plant and animal species. In mammals it is a discrete process confined to the editing of apolipoprotein B (apoB) mRNA in eutherians and the editing of the mitochondrial tRNA for glycine in marsupials. Here we have identified and characterised apoB mRNA editing in the American opossum Monodelphus domestica. The apoB mRNA editing site is highly conserved in the opossum and undergoes complete editing in the small intestine, but not in the liver or other tissues. Opossum APOBEC-1 cDNA was cloned, sequenced and expressed. The encoded protein is similar to APOBEC-1 of eutherians. Motifs previously identified as involved in zinc binding, RNA binding and catalysis, nuclear localisation and a C-terminal leucine-rich domain are all conserved. Opossum APOBEC-1 contains a seven amino acid C-terminal extension also found in humans and rabbits, but not present in rodents. The opossum APOBEC-1 gene has the same intron/exon organisation in the coding sequence as the eutherian gene. Northern blot and RT-PCR analyses and an editing assay indicate that no APOBEC-1 was expressed in the liver. Thus the far upstream promoter responsible for hepatic expression in rodents does not operate in the opossum. An APOBEC-1-like enzyme such as might be involved in C->U RNA editing of tRNA in marsupial mitochondria was not demonstrated. The activity of opossum APOBEC-1 in the presence of both chicken and rodent auxiliary editing proteins was comparable to that of other mammals. These studies extend the origins of APOBEC-1 back 170 000 000 years to marsupials and help bridge the gap in the origins of this RNA editing process between birds and eutherian mammals.

Synoretin--A new protein belonging to the synuclein family

Aoffa-Synuclein, a presynaptic nerve terminal protein, may be an important component of Lewy bodies in Parkinson's disease, dementia with Lewy bodies, and other neurodegenerative diseases. Additionally, recent genetic studies based on linkage analysis and cosegregation of A53T and A30P missense mutations demonstrated that the alpha-synuclein gene may be responsible for the development of at least some cases of familial Parkinson's disease. Despite intense interest in the members of the synuclein family, their function(s) and exact role in the diseases remained unknown. Here we describe a new member of the synuclein family, which we term synoretin, and show that it is expressed in different retinal cells, as well as in the brain, and it may affect the regulation of signal transduction through activation of the Elk1 pathway.

Distinct promoters induce APOBEC-1 expression in rat liver and intestine

The expression of apolipoprotein (apo) B can be modulated by mRNA editing, a unique posttranscriptional base change in the apo B mRNA. Apo B-48, the translation product of edited apo B mRNA, is not a precursor of the atherogenic low density lipoproteins and lipoprotein(a). In humans and various other mammals, the apo B mRNA is edited in the intestine but not in the liver, which exclusively secretes apo B-100-containing lipoproteins as precursors for low density lipoprotein formation. In species such as the rat, mouse, dog, and horse, apo B mRNA is also edited in the liver, resulting in low plasma levels of low density lipoprotein. Editing of the apo B mRNA is mediated by the apo B mRNA-editing enzyme complex, of which the catalytic subunit APOBEC-1 is not expressed in the liver of species without hepatic editing. To understand the molecular basis for liver-specific expression of APOBEC-1 and the editing of hepatic apo B mRNA, the expression pattern and genomic organization of the rat APOBEC-1 gene have been characterized. The rat APOBEC-1 gene contains 6 exons and 2 promoters with distinct activities. The expression of APOBEC-1 in the rat liver is the result of a promoter located upstream, with tissue-specific exon use and alternate splicing within the 5'-untranslated region of APOBEC-1 mRNA encoded by exon 2. In addition to the liver, this promoter also induces APOBEC-1 expression in the spleen, lung, kidney, heart, and skeletal muscle. The promoter located downstream belongs to a new class of TATA-less promoters and is responsible for the abundant expression of APOBEC-1 in the intestine. Mapping of the transcriptional start sites and deletion analysis of the promoter regions by using luciferase as the reporter gene have defined the regulatory elements of both promoters. The downstream, intestine-specific promoter contains a negative regulatory element between -1100 and -500, which appears to restrict its activity to the intestine. The upstream, liver-specific promoter of the rat APOBEC-1 gene induces APOBEC-1 expression and editing of apo B mRNA in human hepatoma HuH-7 and Hep G2 cells. Understanding the molecular basis for the liver-specific expression of APOBEC-1 in the rat promises new strategies to induce APOBEC-1 expression in the human liver for the reduction of atherogenic lipoprotein levels by hepatic apo B mRNA editing.

Low expression of the apolipoprotein B mRNA-editing transgene in mice reduces LDL levels but does not cause liver dysplasia or tumors

Hepatic expression of apolipoprotein (apo) B mRNA-editing enzyme catalytic polypeptide 1 (APOBEC-1) has been proposed as a gene therapy approach for lowering plasma low density lipoprotein (LDL) levels. However, high-level expression of APOBEC-1 in transgenic mouse and rabbit livers causes liver dysplasia and hepatocellular carcinoma. To determine the physiological and pathological effects of low-level hepatic expression of APOBEC-1, we used a 52-kb rat APOBEC-1 genomic clone (RE4) to generate transgenic mice expressing low levels of APOBEC-1 (2 to 5 times those in nontransgenic mice). Liver function, liver histology, editing of apoB mRNA at the normal editing site (C6666), and abnormal editing at multiple sites (hyperediting) in these mice were compared with those in transgenic mice expressing intermediate (I-20) or high (I-28) levels of APOBEC-1 in the liver. Hyperediting of mRNA coding for the novel APOBEC-1 target 1 (NAT1) was also examined. In the high-expressing I-28 line, 50% of the mice had palpable tumors at 15 weeks of age, whereas in the intermediate-expressing I-20 line, 50% of the mice had evidence of liver tumors after 1 year. In contrast, low-expressing RE4 mice had normal liver function and histology and did not develop liver tumors when examined at 3 to 17 months of age. Moreover, hyperediting of apoB and NAT1 mRNA in the liver was robust in the I-20 mice but barely detectable in the RE4 mice. The low-level expression resulted in sufficient APOBEC-1 to edit essentially all apoB mRNA at the normal editing site, virtually eliminating apoB-100 and LDL in the plasma of RE4 mice. When RE4 mice were crossed with human apoB transgenic mice, which possess high plasma LDL concentrations, plasma LDL levels in the offspring were reduced to very low levels. These results indicates that long-term hepatic expression of APOBEC-1 at low levels sufficient to eliminate LDL does not cause apparent liver damage or liver tumors in transgenic mice. RE4 APOBEC-1 transgenic mice should prove valuable for studying the roles of apoB-containing lipoproteins in lipid metabolism and atherosclerosis.

Genomic DNA sequence, promoter expression, and chromosomal mapping of rat muscle carnitine palmitoyltransferase I

Carnitine palmitoyltransferase I (CPT-I) is a key enzyme involved in the regulation of fatty acid oxidation. CPT-IA and CPT-IB are isoforms of carnitine palmitoyltransferase I, of which CPT-IA is expressed in liver, kidney, fibroblasts, and heart and CPT-IB is expressed in skeletal muscle, heart, brown and white adipocytes, and testes. Although the genomic DNA sequence of human CPT-IB is available, the transcription start site and upstream regulatory sequences are not known. For rat CPT-IB, only the cDNA sequence has been published. We have cloned the entire rat CPT-IB gene from a Lambda fix II rat kidney genomic library. The genomic structure contains 19 exons, with the transcription start site for CPT-IB located in a short first exon, which is a 13-bp extension to the previously published cDNA 5' sequence. The coding sequence is identical with the rat muscle cDNA. The rat CPT-IB gene contains 18 introns and 19 exons, the latter 18 exons showing 85% homology to the human CPT-IB cDNA. CPT-IB maps to rat chromosome 7 at band q34. A putative promoter region was identified to within 391 bp of the transcription start site. The muscle specificity of the 5' flanking region was verified by comparison of luciferase expression to that of beta-galactosidase in cardiac myocytes and in HepG2 cells.