Human Acyl-Coenzyme A:Cholesterol Acyltransferase 1 (acat1) Sequences Located in Two Different Chromosomes (7 and 1) Are Required to Produce a Novel ACAT1 Isoenzyme with Additional Sequence at the N Terminus

Trans-spliced long non-coding RNA: an emerging regulator of pluripotency

With dual capacities for unlimited self-renewal and pluripotent differentiation, pluripotent stem cells (PSCs) give rise to many cell types in our body and PSC culture systems provide an unparalleled opportunity to study early human development and disease. Accumulating evidence indicates that the molecular mechanisms underlying pluripotency maintenance in PSCs involve many factors. Among these regulators, recent studies have shown that long non-coding RNAs (lncRNAs) can affect the pluripotency circuitry by cooperating with master pluripotency-associated factors. Additionally, trans-spliced RNAs, which are generated by combining two or more pre-mRNA transcripts to produce a chimeric RNA, have been identified as regulators of various biological processes, including human pluripotency. In this review, we summarize and discuss current knowledge about the roles of lncRNAs, including trans-spliced lncRNAs, in controlling pluripotency.

Targeting Alzheimer's disease with gene and cell therapies

Alzheimer's disease (AD) causes dementia in both young and old people affecting more than 40 million people worldwide. The two neuropathological hallmarks of the disease, amyloid beta (Aβ) plaques and neurofibrillary tangles consisting of protein tau are considered the major contributors to the disease. However, a more complete picture reveals significant neurodegeneration and decreased cell survival, neuroinflammation, changes in protein and energy homeostasis and alterations in lipid and cholesterol metabolism. In addition, gene and cell therapies for severe neurodegenerative disorders have recently improved technically in terms of safety and efficiency and have translated to the clinic showing encouraging results. Here, we review broadly current data within the field for potential targets that could modify AD through gene and cell therapy strategies. We envision that not only Aβ will be targeted in a disease-modifying treatment strategy but rather that a combination of treatments, possibly at different intervention times may prove beneficial in curing this devastating disease. These include decreased tau pathology, neuronal growth factors to support neurons and modulation of neuroinflammation for an appropriate immune response. Furthermore, cell based therapies may represent potential strategies in the future.

Transcriptional-Readthrough RNAs Reflect the Phenomenon of "A Gene Contains Gene(s)" or "Gene(s) within a Gene" in the Human Genome, and Thus Are Not Chimeric RNAs

Tens of thousands of chimeric RNAs, i.e., RNAs with sequences of two genes, have been identified in human cells. Most of them are formed by two neighboring genes on the same chromosome and are considered to be derived via transcriptional readthrough, but a true readthrough event still awaits more evidence and trans-splicing that joins two transcripts together remains as a possible mechanism. We regard those genomic loci that are transcriptionally read through as unannotated genes, because their transcriptional and posttranscriptional regulations are the same as those of already-annotated genes, including fusion genes formed due to genetic alterations. Therefore, readthrough RNAs and fusion-gene-derived RNAs are not chimeras. Only those two-gene RNAs formed at the RNA level, likely via trans-splicing, without corresponding genes as genomic parents, should be regarded as authentic chimeric RNAs. However, since in human cells, procedural and mechanistic details of trans-splicing have never been disclosed, we doubt the existence of trans-splicing. Therefore, there are probably no authentic chimeras in humans, after readthrough and fusion-gene derived RNAs are all put back into the group of ordinary RNAs. Therefore, it should be further determined whether in human cells all two-neighboring-gene RNAs are derived from transcriptional readthrough and whether trans-splicing truly exists.

ACAT1 regulates the dynamics of free cholesterols in plasma membrane which leads to the APP-α-processing alteration

Acyl-CoA:cholesterol acyltransferase 1 (ACAT1) is a key enzyme exclusively using free cholesterols as the substrates in cell and is involved in the cellular cholesterol homeostasis. In this study, we used human neuroblastoma cell line SK-N-SH as a model and first observed that inhibiting ACAT1 can decrease the amyloid precursor protein (APP)-α-processing. Meanwhile, the transfection experiments using the small interfering RNA and expression plasmid of ACAT1 indicated that ACAT1 can dependently affect the APP-α-processing. Furthermore, inhibiting ACAT1 was found to increase the free cholesterols in plasma membrane (PM-FC), and the increased PM-FC caused by inhibiting ACAT1 can lead to the decrease of the APP-α-processing, indicating that ACAT1 regulates the dynamics of PM-FC, which leads to the alteration of the APP-α-processing. More importantly, further results showed that under the ACAT1 inhibition, the alterations of the PM-FC and the subsequent APP-α-processing are not dependent on the cellular total cholesterol level, confirming that ACAT1 regulates the dynamics of PM-FC. Finally, we revealed that even when the Niemann-Pick-Type C-dependent pathway is blocked, the ACAT1 inhibition still obviously results in the PM-FC increase, suggesting that the ACAT1-dependent pathway is responsible for the shuttling of PM-FC to the intracellular pool. Our data provide a novel insight that ACAT1 which enzymatically regulates the dynamics of PM-FC may play important roles in the human neuronal cells.

Acyl-CoA:cholesterol acyltransferases (ACATs/SOATs): Enzymes with multiple sterols as substrates and as activators

Cholesterol is essential to the growth and viability of cells. The metabolites of cholesterol include: steroids, oxysterols, and bile acids, all of which play important physiological functions. Cholesterol and its metabolites have been implicated in the pathogenesis of multiple human diseases, including: atherosclerosis, cancer, neurodegenerative diseases, and diabetes. Thus, understanding how cells maintain the homeostasis of cholesterol and its metabolites is an important area of study. Acyl-coenzyme A:cholesterol acyltransferases (ACATs, also abbreviated as SOATs) converts cholesterol to cholesteryl esters and play key roles in the regulation of cellular cholesterol homeostasis. ACATs are most unusual enzymes because (i) they metabolize diverse substrates including both sterols and certain steroids; (ii) they contain two different binding sites for steroidal molecules. In mammals, there are two ACAT genes that encode two different enzymes, ACAT1 and ACAT2. Both are allosteric enzymes that can be activated by a variety of sterols. In addition to cholesterol, other sterols that possess the 3-beta OH at C-3, including PREG, oxysterols (such as 24(S)-hydroxycholesterol and 27-hydroxycholesterol, etc.), and various plant sterols, could all be ACAT substrates. All sterols that possess the iso-octyl side chain including cholesterol, oxysterols, various plant sterols could all be activators of ACAT. PREG can only be an ACAT substrate because it lacks the iso-octyl side chain required to be an ACAT activator. The unnatural cholesterol analogs epi-cholesterol (with 3-alpha OH in steroid ring B) and ent-cholesterol (the mirror image of cholesterol) contain the iso-octyl side chain but do not have the 3-beta OH at C-3. Thus, they can only serve as activators and cannot serve as substrates. Thus, within the ACAT holoenzyme, there are site(s) that bind sterol as substrate and site(s) that bind sterol as activator; these sites are distinct from each other. These features form the basis to further pursue ACAT structure-function analysis, and can be explored to develop novel allosteric ACAT inhibitors for therapeutic purposes. This article is part of a Special Issue entitled 'Steroid/Sterol signaling'.

A chimeric transcript containing Psy1 and a potential mRNA is associated with yellow flesh color in tomato accession PI 114490

Carotenoid content is the primary determinant of fruit color that affects nutritional value and appearance in tomato. Phytoene synthase (PSY) is the key regulatory enzyme in the carotenoid biosynthesis pathway. Absent function of PSY1 in tomato fruit results in yellow flesh phenotype. We, here, report that two different transcripts, a wild-type (Psy1) and a chimeric mRNA (Psy1/Unknown), exist in a yellow-fruited tomato accession PI 114490. Psy1/Unknown is generated by joining exons from two different genes, Psy1 and an unknown gene, transcribed using both complementary DNA strands. The Psy1 shows low expression in the fruit of PI 114490, while the expression of Psy1/Unknown in the fruit of PI 114490 shows the same pattern as Psy1 in red fruit. The PSY1/Unknown has a lower function than PSY1 in a bacterial expression system. Coincidence of one single-nucleotide polymorphism (SNP) in the fourth intron and one simple sequence repeat (SSR) with 19 AT repeats in the downstream sequence of Psy1 gene with Psy1/Unknown in a set of yellow-fruited tomato lines indicates that Psy1/Unknown might be caused by the SNP and/or SSR. One possible explanation of these observations is trans-splicing. Severely reduced Psy1 transcript caused by Psy1/Unknown results in low accumulation of carotenoid and yellow flesh in PI 114490.

MiR-9 reduces human acyl-coenzyme A:cholesterol acyltransferase-1 to decrease THP-1 macrophage-derived foam cell formation

MicroRNAs (miRNAs) post-transcriptionally regulate gene expression by targeting mRNAs and control a wide range of biological functions. Recent studies have indicated that miRNAs can regulate lipid and cholesterol metabolism in mammals. Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is a key enzyme in cellular cholesterol metabolism. The accumulated cholesteryl esters are mainly synthesized by ACAT1 during the formation of foam cell, a hallmark of early atherosclerotic lesions. Here, we revealed that miR-9 could target the 3'-untranslated region of human ACAT1 mRNA, specifically reduce human ACAT1 or reporter firefly luciferase (Fluc) proteins but not their mRNAs in different human cell lines, and functionally decrease the formation of foam cells from THP-1-derived macrophages. Our findings suggest that miR-9 might be an important regulator in cellular cholesterol homeostasis and decrease the formation of foam cells in vivo by reducing ACAT1 proteins.

Exo-endo trans splicing: a new way to link

In addition to canonical cis splicing, which joins exons from a single pre-mRNA, various forms of trans splicing have been described, whereby two separate precursor transcripts are linked with each other. A new study by Hu et al. in Cell Research characterizes a novel and unusual splicing variation, called exo-endo trans splicing.

Production of ACAT1 56-kDa isoform in human cells via trans-splicing involving the ampicillin resistance gene

Trans-splicing, a process involving the cleavage and joining of two separate transcripts, can expand the transcriptome and proteome in eukaryotes. Chimeric RNAs generated by trans-splicing are increasingly described in literatures. The widespread presence of antibiotic resistance genes in natural environments and human intestines is becoming an important challenge for public health. Certain antibiotic resistance genes, such as ampicillin resistance gene (Amp^r), are frequently used in recombinant plasmids. Until now, trans-splicing involving recombinant plasmid-derived exogenous transcripts and endogenous cellular RNAs has not been reported. Acyl-CoA:cholesterol acyltransferase 1 (ACAT1) is a key enzyme involved in cellular cholesterol homeostasis. The 4.3-kb human ACAT1 chimeric mRNA can produce 50-kDa and 56-kDa isoforms with different enzymatic activities. Here, we show that human ACAT1 56-kDa isoform is produced from an mRNA species generated through the trans-splicing of an exogenous transcript encoded by the antisense strand of Amp^r (asAmp) present in common Amp^r-plasmids and the 4.3-kb endogenous ACAT1 chimeric mRNA, which is presumably processed through a prior event of interchromosomal trans-splicing. Strikingly, DNA fragments containing the asAmp with an upstream recombined cryptic promoter and the corresponding exogenous asAmp transcripts have been detected in human cells. Our findings shed lights on the mechanism of human ACAT1 56-kDa isoform production, reveal an exogenous-endogenous trans-splicing system, in which recombinant plasmid-derived exogenous transcripts are linked with endogenous cellular RNAs in human cells, and suggest that exogenous DNA might affect human gene expression at both DNA and RNA levels.

Novel female-specific trans-spliced and alternative splice forms of dsx in the silkworm Bombyx mori

The Bombyx mori doublesex gene (Bmdsx) plays an important role in somatic sexual development. Its pre-mRNA splices in a sex-specific manner to generate two female-specific and one male-specific splice forms. The present study investigated six novel dsx variants generated by trans-splicing between female dsx transcripts and two additional novel genes, dsr1 and dsr2. Expression analysis indicated that Bmdsx-dsr1 represented splicing noise, whereas dsr2, which trans-spliced with dsx to generate five variants, regulated the expression of the female-specific B. mori dsx transcript Bmdsx(F)s. We unexpectedly found a novel exon 2n insertion during Bmdsx transcription, which did not influence the validity of the novel protein, BmDSX(F3). Ectopic expression of BmDSX(F3) repressed the pheromone-binding protein gene and the testis-specific gene A2 in males, and activated of the storage protein 1 gene. Our findings suggest that trans-splicing is a novel regulatory function of Bmdsx, which participates in female sexual development by regulating the expression of three BmDSX(F) proteins.

Identification and characterization of a novel non-coding RNA involved in sperm maturation

A long and ever-expanding roster of small (∼20–30 nucleotides) RNAs has emerged during the last decade, and most can be subsumed under the three main headings of microRNAs(miRNAs), Piwi-interacting RNAs(piRNAs), and short interferingRNAs(siRNAs). Among the three categories, miRNAs is the most quickly expanded group. The most recent number of identified miRNAs is 16,772 (Sanger miRbase, April 2011). However, there are insufficient publications on their primary forms, and no tissue-specific small RNAs precursors have been reported in the epididymis. Here, we report the identification in rats of an epididymis-specific, chimeric, noncoding RNA that is spliced from two different chromosomes (chromosomes 5 and 19), which we named HongrES2. HongrES2 is a 1.6 kb mRNA-like precursor that gives rise to a new microRNA-like small RNA (mil-HongrES2) in rat epididymis. The generation of mil-HongrES2 is stimulated during epididymitis. An epididymis-specific carboxylesterase named CES7 had 100% cDNA sequence homology at the 3′end with HongrES2 and its protein product could be downregulated by HongrES2 via mil-HongrES2. This was confirmed in vivo by initiating mil-HongrES2 over-expression in rats and observing an effect on sperm capacitation.

Escaping the cut by restriction enzymes through single-strand self-annealing of host-edited 12-bp and longer synthetic palindromes

Palindromati, the massive host-edited synthetic palindromic contamination found in GenBank, is illustrated and exemplified. Millions of contaminated sequences with portions or tandems of such portions derived from the ZAP adaptor or related linkers are shown (1) by the 12-bp sequence reported elsewhere, exon Xb, 5' CCCGAATTCGGG 3', (2) by a 22-bp related sequence 5' CTCGTGCCGAATTCGGCACGAG 3', and (3) by a longer 44-bp related sequence: 5' CTCGTGCCGAATTCGGCACGAGCTCGTGCCGAATTCGGCACGAG 3'. Possible reasons for why those long contaminating sequences continue in the databases are presented here: (1) the recognition site for the plus strand (+) is single-strand self-annealed; (2) the recognition site for the minus strand (-) is not only single-strand self-annealed but also located far away from the single-strand self-annealed plus strand, rendering impossible the formation of the active EcoRI enzyme dimer to cut on 5' G/AATTC 3', its target sequence. As a possible solution, it is suggested to rely on at least two or three independent results, such as sequences obtained by independent laboratories with the use, preferably, of independent sequencing methodologies. This information may help to develop tools for bioinformatics capable to detect/remove these contaminants and to infer why some damaged sequences which cause genetic diseases escape detection by the molecular quality control mechanism of cells and organisms, being undesirably transferred unchecked through the generations.

Acyl-coenzyme A:cholesterol acyltransferases

The enzymes acyl-coenzyme A (CoA):cholesterol acyltransferases (ACATs) are membrane-bound proteins that utilize long-chain fatty acyl-CoA and cholesterol as substrates to form cholesteryl esters. In mammals, two isoenzymes, ACAT1 and ACAT2, encoded by two different genes, exist. ACATs play important roles in cellular cholesterol homeostasis in various tissues. This chapter summarizes the current knowledge on ACAT-related research in two areas: 1) ACAT genes and proteins and 2) ACAT enzymes as drug targets for atherosclerosis and for Alzheimer's disease.

RNA secondary structures located in the interchromosomal region of human ACAT1 chimeric mRNA are required to produce the 56-kDa isoform

We have previously reported that the human ACAT1 gene produces a chimeric mRNA through the interchromosomal processing of two discontinuous RNAs transcribed from chromosomes 1 and 7. The chimeric mRNA uses AUG(1397-1399) and GGC(1274-1276) as translation initiation codons to produce normal 50-kDa ACAT1 and a novel enzymatically active 56-kDa isoform, respectively, with the latter being authentically present in human cells, including human monocyte-derived macrophages. In this work, we report that RNA secondary structures located in the vicinity of the GGC(1274-1276) codon are required for production of the 56-kDa isoform. The effects of the three predicted stem-loops (nt 1255-1268, 1286-1342 and 1355-1384) were tested individually by transfecting expression plasmids into cells that contained the wild-type, deleted or mutant stem-loop sequences linked to a partial ACAT1 AUG open reading frame (ORF) or to the ORFs of other genes. The expression patterns were monitored by western blot analyses. We found that the upstream stem-loop(1255-1268) from chromosome 7 and downstream stem-loop(1286-1342) from chromosome 1 were needed for production of the 56-kDa isoform, whereas the last stem-loop(1355-1384) from Chromosome 1 was dispensable. The results of experiments using both monocistronic and bicistronic vectors with a stable hairpin showed that translation initiation from the GGC(1274-1276) codon was mediated by an internal ribosome entry site (IRES). Further experiments revealed that translation initiation from the GGC(1274-1276) codon requires the upstream AU-constituted RNA secondary structure and the downstream GC-rich structure. This mechanistic work provides further support for the biological significance of the chimeric nature of the human ACAT1 transcript.

TNF-alpha stimulates the ACAT1 expression in differentiating monocytes to promote the CE-laden cell formation

High levels of the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) are present in atherosclerotic lesions. TNF-alpha regulates expression of multiple genes involved in various stages of atherosclerosis, and it exhibits proatherosclerotic and antiatherosclerotic properties. ACAT catalyzes the formation of cholesteryl esters (CE) in monocytes/macrophages, and it promotes the foam cell formation at the early stage of atherosclerosis. We hypothesize that TNF-alpha may be involved in regulating the ACAT gene expression in monocytes/macrophages. In this article, we show that in cultured, differentiating human monocytes, TNF-alpha enhances the expression of the ACAT1 but not ACAT2 gene, increases the cholesteryl ester accumulation, and promotes the lipid-laden cell formation. Several other proinflammatory cytokines tested do not affect the ACAT1 gene expression. The stimulation effect is consistent with a receptor-dependent process, and is blocked by using nuclear factor-kappa B (NF-kappa B) inhibitors. A functional and unique NF-kappa B element located within the human ACAT1 gene proximal promoter is required to mediate the action of TNF-alpha. Our data demonstrate that TNF-alpha, through the NF-kappa B pathway, specifically enhances the expression of human ACAT1 gene to promote the CE-laden cell formation from the differentiating monocytes, and our data support the hypothesis that TNF-alpha is proatherosclerotic during early phase of lesion development.

The optional long 5'-untranslated region of human ACAT1 mRNAs impairs the production of ACAT1 protein by promoting its mRNA decay

We have previously reported that human ACAT1 mRNAs produce the 50 kDa protein using the AUG(11397-1399) initiation codon, and also a minor 56 kDa isoform using the upstream in-frame GGC(1274-1276) initiation codon. The GGC(1274-1276) codon is located at the optional long 5'-untranslated region (5'-UTR, nt 1-1396) of the mRNAs. The DNA sequences corresponding to this 5'-UTR are located in two different chromosomes, 7 and 1. In the current work, we report that the optional long 5'-UTR significantly impairs the production of human ACAT1 protein initiated from the AUG(1397-1399)codon, mainly by promoting its mRNA decay. The western blot analyses indicated that the optional long 5'-UTR potently impaired the production of different proteins initiated from the AUG(1397-1399) codon, meaning that this impairing effect was not influenced by the 3'-UTR or the coding sequence of ACAT1 mRNA. The results of reverse transcription-quantitative polymerase chain reaction demonstrated that this 5'- UTR dramatically reduced the contents of its linked mRNAs. Analyses of the protein to mRNA ratios showed that this 5'-UTR mainly decreased its mRNA stability rather than altering its translational efficiency. We next performed the plasmid transfection experiments and used actinomycin D to inhibit transcription. The results showed that this 5'-UTR promoted its mRNA decay. Additional transfection and nucleofection experiments using RNAs prepared in vitro illustrated that, in both the cytoplasm and the nucleus of cells, the optional long 5'-UTR-linked mRNAs decayed faster than those without the link. Overall, our study brings new insight to the regulation of the human ACAT1 gene expression at the post-transcription level.

Mammalian transcription in support of hybrid mRNA and protein synthesis in testis and lung

Post-transcriptional mechanisms including differential splicing expand the protein repertoire beyond that provided by the one gene-one protein model. Trans-splicing has been observed in mammalian systems but is low level (sometimes referred to as noise), and a contribution to hybrid protein expression is unclear. In the study of rat sperm tail proteins a cDNA, called 1038, was isolated representing a hybrid mRNA derived in part from the ornithine decarboxylase antizyme 3 (Oaz3) gene located on rat chromosome 2 fused to sequences encoded by a novel gene on chromosome 4. Cytoplasmic Oaz3 mRNA is completely testis specific. However, in several tissues Oaz3 is transcribed and contributes to hybrid 1038 mRNA synthesis, without concurrent Oaz3 mRNA synthesis. 1038 mRNA directs synthesis of a hybrid 14-kDa protein, part chromosome 2- and part chromosome 4-derived as shown in vitro and in transfected cells. Antisera that recognize a chromosome 4-encoded C-terminal peptide confirm the hybrid character of endogenous 14-kDa protein and its presence in sperm tail structures and 1038-positive tissue. Our data suggest that the testis-specific OAZ3 gene may be an example of a mammalian gene that in several tissues is transcribed to contribute to a hybrid mRNA and protein. This finding expands the repertoire of known mechanisms available to cells to generate proteome diversity.

CTCF mediates interchromosomal colocalization between Igf2/H19 and Wsb1/Nf1

Gene transcription may be regulated by remote enhancer or insulator regions through chromosome looping. Using a modification of chromosome conformation capture (3C) and fluorescence in situ hybridization, we found that one allele of the insulin-like growth factor 2 (Igf2)/H19 imprinting control region (ICR) on chromosome 7 colocalized with one allele of Wsb1/Nf1 on chromosome 11. Omission of CCCTC-binding factor (CTCF) or deletion of the maternal ICR abrogated this association and altered Wsb1/Nf1 gene expression. These findings demonstrate that CTCF mediates an interchromosomal association, perhaps by directing distant DNA segments to a common transcription factory, and the data provide a model for long-range allele-specific associations between gene regions on different chromosomes that suggest a framework for DNA recombination and RNA trans-splicing.

Alternative trans-splicing: a novel mode of pre-mRNA processing

Alternative splicing is an important process contributing to proteome diversity without involving an increase in the number of genes. In some cases, alternative splicing is carried out under 'trans-mode', called alternative trans-splicing, in which exons located on separate pre-mRNA molecules are selectively joined to produce mature mRNAs encoding proteins with distinct structures and functions. However, it is not known how widespread or how frequently trans-splicing occurs in vivo. Recently, trans-allelic trans-splicing has been unambiguously demonstrated in Drosophila using a SNP (single nucleotide polymorphism) as a marker. In this review, we provide an overview of alternative trans-splicing in Drosophila and mammals, and discuss its mechanisms.

Human ACAT1 gene expression and its involvement in the development of atherosclerosis

Atherosclerosis is caused by a series of pathologic changes at the cellular level, with formation of macrophage-derived foam cells occurring at an early stage. Most of the cholesteryl esters in macrophage foam cells are produced by the enzyme acyl-coenzyme A:cholesterol acyltransferase (ACAT). Two ACAT genes, Acat1 and Acat2, exist in mammals. In the monocyte–macrophages, ACAT1 is the major isoenzyme and is a drug target for atherosclerosis treatment. Various proatherogenic stimuli, including interferon-γ and dexamethasone, cause upregulation of human Acat1 expression in macrophages. Thus, it should be possible to find antagonist(s) to downregulate human Acat1 expression. A greater understanding of human Acat1 expression may provide scientists with opportunities for novel therapeutic approaches to combat atherosclerosis.

Two human ACAT2 mRNA variants produced by alternative splicing and coding for novel isoenzymes

Acyl coenzyme A:cholesterol acyltransferase 2 (ACAT2) plays an important role in cholesterol absorption. Human ACAT2 is highly expressed in small intestine and fetal liver, but its expression is greatly diminished in adult liver. The full-length human ACAT2 mRNA encodes a protein, designated ACAT2a, with 522 amino acids. We have previously reported the organization of the human ACAT2 gene and the differentiation-dependent promoter activity in intestinal Caco-2 cells. In the current work, two human ACAT2 mRNA variants produced by alternative splicing are cloned and predicted to encode two novel ACAT2 isoforms, named ACAT2b and ACAT2c, with 502 and 379 amino acids, respectively. These mRNA variants differ from ACAT2a mRNA by lack of the exon 4 (ACAT2b mRNA) and exons 4-5 plus 8-9-10 (ACAT2c mRNA). Significantly, comparable amounts of the alternatively spliced ACAT2 mRNA variants were detected by RT-PCR, and Western blot analysis confirmed the presence of their corresponding proteins in human liver and intestinecells. Furthermore, phosphorylation and enzymatic activity analyses demonstrated that the novel isoenzymes ACAT2b and ACAT2c lacked the phosphorylatable site SLLD, and their enzymatic activities reduced to 25%-35% of that of ACAT2a. These evidences indicate that alternative splicing produces two human ACAT2 mRNA variants that encode the novel ACAT2 isoenzymes. Our findings might help to understand the regulation of the ACAT2 gene expression under certain physiological and pathological conditions.

Potential role of acyl-coenzyme A:cholesterol transferase (ACAT) Inhibitors as hypolipidemic and antiatherosclerosis drugs

Acyl-coenzyme A:cholesterol transferase (ACAT) is an integral membrane protein localized in the endoplasmic reticulum. ACAT catalyzes the formation of cholesteryl esters from cholesterol and fatty acyl coenzyme A. The cholesteryl esters are stored as cytoplasmic lipid droplets inside the cell. This process is very important to the organism as high cholesterol levels have been associated with cardiovascular disease. In mammals, two ACAT genes have been identified, ACAT1 and ACAT2. ACAT1 is ubiquitous and is responsible for cholesteryl ester formation in brain, adrenal glands, macrophages, and kidneys. ACAT2 is expressed in the liver and intestine. The inhibition of ACAT activity has been associated with decreased plasma cholesterol levels by suppressing cholesterol absorption and by diminishing the assembly and secretion of apolipoprotein B-containing lipoproteins such as very low density lipoprotein (VLDL). ACAT inhibition also prevents the conversion of macrophages into foam cells in the arterial walls, a critical event in the development of atherosclerosis. This review paper will focus on the role of ACAT in cholesterol metabolism, in particular as a target to develop novel therapeutic agents to control hypercholesterolemia, atherosclerosis, and Alzheimer's disease.

The active site His-460 of human acyl-coenzyme A:cholesterol acyltransferase 1 resides in a hitherto undisclosed transmembrane domain

Human acyl-coenzyme A:cholesterol acyltransferase 1 (hACAT1) esterifies cholesterol at the endoplasmic reticulum (ER). We had previously reported that hACAT1 contains seven transmembrane domains (TMD) (Lin, S., Cheng, D., Liu, M. S., Chen, J., and Chang, T. Y. (1999) J. Biol. Chem. 274, 23276-23285) and nine cysteines. The Cys near the N-terminal is located at the cytoplasm; the two cysteines near the C-terminal form a disulfide bond and are located in the ER lumen. The other six free cysteines are located in buried region(s) of the enzyme (Guo, Z.-Y., Chang, C. C. Y., Lu, X., Chen, J., Li, B.-L., and Chang, T.-Y. (2005) Biochemistry 44, 6537-6548). In the current study, we show that the conserved His-460 is a key active site residue for hACAT1. We next performed Cys-scanning mutagenesis within the region of amino acids 354-493, expressed these mutants in Chinese hamster ovary cells lacking ACAT1, and prepared microsomes from transfected cells. The microsomes are either left intact or permeabilized with detergent. The accessibility of the engineered cysteines of microsomal hACAT1 to various maleimide derivatives, including mPEG(5000)-maleimide (large, hydrophilic, and membrane-impermeant), N-ethylmaleimide, 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid (small, hydrophilic, and ER membrane-permeant), and N-phenylmaleimide (small, hydrophobic, and ER membrane-permeant), were monitored by Western blot analysis. The results led us to construct a revised, nine-TMD model, with the active site His-460 located within a hitherto undisclosed transmembrane domain, between Arg-443 and Tyr-462.

Pre-mRNA trans-splicing: from kinetoplastids to mammals, an easy language for life diversity

Since the discovery that genes are split into intron and exons, the studies of the mechanisms involved in splicing pointed to presence of consensus signals in an attempt to generalize the process for all living cells. However, as discussed in the present review, splicing is a theme full of variations. The trans-splicing of pre-mRNAs, the joining of exons from distinct transcripts, is one of these variations with broad distribution in the phylogenetic tree. The biological meaning of this phenomenon is discussed encompassing reactions resembling a possible noise to mechanisms of gene expression regulation. All of them however, can contribute to the generation of life diversity.

Modulation of endosomal cholesteryl ester metabolism by membrane cholesterol

Cells acquire cholesterol in part by endocytosis of cholesteryl ester containing lipoproteins. In endosomes and lysosomes cholesteryl ester is hydrolyzed by acidic cholesteryl ester hydrolase producing cholesterol and fatty acids. Under certain pathological conditions, however, such as in atherosclerosis, excessive levels of cholesteryl ester accumulate in lysosomes for reasons that are poorly understood. Here, we have studied endosomal and lysosomal cholesteryl ester metabolism in cultured mouse macrophages and with cell-free extracts. We show that net hydrolysis of cholesteryl ester is coupled to the transfer of cholesterol to membranes. When membrane cholesterol levels are low, absorption of cholesterol effectively drives cholesteryl ester hydrolysis. When cholesterol levels in acceptor membranes approach saturation or when cholesterol export is blocked, cholesterol is re-esterified in endosomes. These results reveal a new facet of cellular cholesterol homeostasis and provide a potential explanation for cholesteryl ester accumulation in lysosomes of atherosclerotic cells.