Characterization of two human genes encoding acyl coenzyme A:cholesterol acyltransferase-related enzymes

Mutations in the midway gene disrupt a Drosophila acyl coenzyme A: diacylglycerol acyltransferase

During Drosophila oogenesis, defective or unwanted egg chambers are eliminated during mid-oogenesis by programmed cell death. In addition, final cytoplasm transport from nurse cells to the oocyte depends upon apoptosis of the nurse cells. To study the regulation of germline apoptosis, we analyzed the midway mutant, in which egg chambers undergo premature nurse cell death and degeneration. The midway gene encodes a protein similar to mammalian acyl coenzyme A: diacylglycerol acyltransferase (DGAT), which converts diacylglycerol (DAG) into triacylglycerol (TAG). midway mutant egg chambers contain severely reduced levels of neutral lipids in the germline. Expression of midway in insect cells results in high levels of DGAT activity in vitro. These results show that midway encodes a functional DGAT and that changes in acylglycerol lipid metabolism disrupt normal egg chamber development in Drosophila.

Role of the N-terminal hydrophilic domain of acyl-coenzyme A:cholesterol acyltransferase 1 on the enzyme's quaternary structure and catalytic efficiency

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an enzyme involved in cellular cholesterol homeostasis and atherosclerosis. ACAT1 is an allosteric enzyme responding to its substrate cholesterol in a sigmoidal manner. It is a homotetrameric protein that spans the membrane multiple times, with its N-terminal 131 hydrophilic amino acids residing at the cytoplasmic side of the endoplasmic reticulum. This region contains two closely linked putative alpha-helices. Our current studies show that this region contains a dimer-forming motif. Adding this motif to the bacterial glutathione S-transferase (GST) converted the homodimeric GST to a tetrameric fusion protein. Conversely, deleting this motif from the full-length ACAT1 converted the enzyme from a homotetramer to a homodimer. The dimeric ACAT1 remains enzymatically active. Its biochemical characteristics, including the sigmoidal response to cholesterol, the IC(50) value toward a specific ACAT inhibitor, and sensitivity toward heat inactivation, are essentially unaltered. On the other hand, the dimeric ACAT1 exhibits a 5-10-fold increase in the V(max) of the overall reaction and a 2.2-fold increase in the K(m) for oleoyl-coenzyme. Thus, deleting the dimer-forming motif near the N-terminus changes ACAT1 from its tetrameric form to a dimeric form and increases its catalytic efficiency.

The DGA1 gene determines a second triglyceride synthetic pathway in yeast

Diacylglycerol esterification provides an excellent target for the pharmacological reduction of triglyceride accumulation in several human disease states. We have used Saccharomyces cerevisiae as a model system to study this critical component of triglyceride synthesis. Recent studies of an oleaginous fungus, Mortierella ramanniana, identified a new family of enzymes with in vitro acyl-CoA:diacylglycerol acyltransferase activity. We show here that DGA1, the sole member of this gene family in yeast, has a physiological role in triglyceride synthesis. Metabolic labeling of DGA1 deletion strains with triglyceride precursors detected significant reductions in triglyceride synthesis. Triglyceride synthesis was virtually abolished in four different growth conditions when DGA1 was deleted in concert with LRO1, an enzyme that esterifies diacylglycerol from a phospholipid acyl donor. The relative contributions of the two enzymes depended on growth conditions. The residual synthesis was lost when ARE2, encoding an acyl-CoA:sterol acyltransferase, was deleted. In vitro microsomal assays verified that DGA1 and ARE2 mediate acyl-CoA:diacylglycerol acyltransferase reactions. Three enzymes can thus account for diacylglycerol esterification in yeast. Yeast strains deficient in both diacylglycerol and sterol esterification showed only a slight growth defect indicating that neutral lipid synthesis is dispensable under common laboratory conditions.

Cholesterol absorption inhibitors for the treatment of hypercholesterolaemia

The benefits of lipid lowering therapy on coronary heart disease have been clearly established in many clinical trials on primary and secondary prevention. Despite the availability of potent lipid lowering drugs, many patients do not reach the current treatment goals. This paper reviews new therapeutic approaches in lipid lowering drugs focusing on compounds which lower cholesterol absorption. The role of plant sterols and stanols, new acyl-CoA:cholesterol O-acyl transferase (ACAT) inhibitors, microsomal triglyceride transfer protein (MTP) inhibitors, and ezetimibe are summarised. Although the lipid lowering effect of plant sterols and plant stanols is only moderate, their use as functional foods is beneficial for patients with mild hypercholesterolaemia and is able to enhance the lipid lowering effect of HMG-CoA reductase inhibitors (statins). The role of ACAT inhibitors that might also inhibit cholesterol absorption remains unclear. Avasimibe, the first oral bioavailable ACAT inhibitor, has entered phase III trials. However, the presently available data in humans do not indicate a clear clinical benefit. The role of MTP inhibitors, which exhibit remarkable effects on all plasma lipids, also remains unclear, as safety concerns must first be addressed. Ezetimibe, the first available 2-azetidinone, succeeded in phase III trials showing remarkable effects in inhibition of cholesterol absorption as well as cholesterol lowering. The synergistic effect of co-administration of ezetimibe with statins seemingly offers a new approach in reaching the therapeutic goals.

The influence of chylomicron remnants on cholesteryl ester metabolism in cultured rat hepatocytes: comparison of the effects of particles enriched in n-3 or n-6 polyunsaturated fatty acids

The effect of chylomicron remnants derived from fish oil (rich in n-3 polyunsaturated fatty acids) or corn oil (rich in n-6 polyunsaturated fatty acids) on the formation and hydrolysis of cholesteryl esters in cultured rat hepatocytes was investigated. Hepatocytes were incubated with or without fish or corn oil chylomicron remnants (0.25-0.75 mM triacylglycerol), and the activity of acyl-CoA:cholesterol acyltranferase (ACAT) and cholesteryl ester hydrolases in the cytosol (cCEH) and endoplasmic reticulum (erCEH), and the expression of mRNA for ACAT1, ACAT2 and cCEH, and of enzyme protein for erCEH was determined. Addition of either type of remnants to hepatocyte cultures resulted in a decreased activity of erCEH, cCEH (after 6 and 19 h incubation), and of ACAT (after 6 h only). Hepatocyte levels of mRNA encoding ACAT1 and ACAT2 were not affected by either type of chylomicron remnants after 6 h of incubation, while ACAT2 mRNA levels were down-regulated by fish oil remnants as compared with corn oil remnants, and also with control cells in the long term (19 h). In contrast, cCEH mRNA levels were down-regulated by chylomicron remnants derived from corn oil but not fish oil. The expression of erCEH protein was induced in response to the inhibitory effect of both types of remnants on the activity of the enzyme, with corn oil remnants having a significantly greater effect. These findings demonstrate that dietary polyunsaturated fatty acids when delivered to hepatocytes in chylomicron remnants regulate the activity of the enzymes governing the intracellular cholesteryl ester balance, and suggest that dietary n-3 and n-6 polyunsaturated fatty acids or a metabolite thereof have differential effects on the expression of their genes at the mRNA and post-transcriptional levels.

Human acyl-CoA:diacylglycerol acyltransferase is a tetrameric protein

Acyl-CoA:diacylglycerol acyltransferase (DGAT) is an integral membrane enzyme that catalyses the last step of triacylglycerol synthesis from diacylglycerol and acyl-CoA. Here we provide experimental evidence that DGAT is a homotetramer. Although the predicted molecular mass of human DGAT protein is 55 kDa, CHAPS-solubilized recombinant human DGAT was eluted in fractions over 150 kDa on gel-filtration chromatography. Cross-linking of recombinant DGAT in membranes with disuccinimidyl suberate yielded bands corresponding to the dimer (108 kDa) and the tetramer (214 kDa) in SDS/PAGE. Finally, when two differently epitope-tagged forms of DGAT were co-transfected into mammalian cells, they could be co-immunoprecipitated. From a human adipose tissue cDNA library we cloned a cDNA encoding a novel splice variant of DGAT (designated DGATsv) that contained a 77 nt insert of unspliced intron with an in-frame stop codon. This resulted in a truncated form of DGAT that terminated at Arg-387, deleting 101 residues from the C-terminus containing the putative active site. DGATsv was enzymically inactive when transfected in HEK-293E cells but was still able to form dimer and tetramer on cross-linking, indicating that the ability to form tetramers resides in the N-terminal region. When co-expressed in HEK-293E cells, DGATsv did not inhibit the activity of full-length DGAT, suggesting that the subunits of DGAT catalyse triacylglycerol synthesis independently.

Cloning of DGAT2, a second mammalian diacylglycerol acyltransferase, and related family members

Studies involving the cloning and disruption of the gene for acyl-CoA:diacylglycerol acyltransferase (DGAT) have shown that alternative mechanisms exist for triglyceride synthesis. In this study, we cloned and characterized a second mammalian DGAT, DGAT2, which was identified by its homology to a DGAT in the fungus Mortierella rammaniana. DGAT2 is a member of a gene family that has no homology with DGAT1 and includes several mouse and human homologues that are candidates for additional DGAT genes. The expression of DGAT2 in insect cells stimulated triglyceride synthesis 6-fold in assays with cellular membranes, and DGAT2 activity was dependent on the presence of fatty acyl-CoA and diacylglycerol, indicating that this protein is a DGAT. Activity was not observed for acyl acceptors other than diacylglycerol. DGAT2 activity was inhibited by a high concentration (100 mm) of MgCl(2) in an in vitro assay, a characteristic that distinguishes DGAT2 from DGAT1. DGAT2 is expressed in many tissues with high expression levels in the liver and white adipose tissue, suggesting that it may play a significant role in mammalian triglyceride metabolism.

Acyl-coenzyme A: cholesterol acyltransferase modulates the generation of the amyloid beta-peptide

The pathogenic event common to all forms of Alzheimer's disease is the abnormal accumulation of the amyloid beta-peptide (Abeta). Here we provide strong evidence that intracellular cholesterol compartmentation modulates the generation of Abeta. Using genetic, biochemical and metabolic approaches, we found that cholesteryl-ester levels are directly correlated with Abeta production. Acyl-coenzyme A:cholesterol acyltransferase (ACAT), the enzyme that catalyses the formation of cholesteryl esters, modulates the generation of Abeta through the tight control of the equilibrium between free cholesterol and cholesteryl esters. We also show that pharmacological inhibitors of ACAT, developed for the treatment of atherosclerosis, are potent modulators of Abeta generation, indicating their potential for use in the treatment of Alzheimer's disease.

Transcriptional regulation of the two sterol esterification genes in the yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae transcribes two genes, ARE1 and ARE2, that contribute disproportionately to the esterification of sterols. Are2p is the major enzyme isoform in a wild-type cell growing aerobically. This likely results from a combination of differential transcription initiation and transcript stability. By using ARE1 and ARE2 promoter fusions to lacZ reporters, we demonstrated that transcriptional initiation from the ARE1 promoter is significantly reduced compared to that from the ARE2 promoter. Furthermore, the half-life of the ARE2 mRNA is approximately 12 times as long as that of the ARE1 transcript. We present evidence that the primary role of the minor sterol esterification isoform encoded by ARE1 is to esterify sterol intermediates, whereas the role of the ARE2 enzyme is to esterify ergosterol, the end product of the pathway. Accordingly, the ARE1 promoter is upregulated in strains that accumulate ergosterol precursors. Furthermore, ARE1 and ARE2 are oppositely regulated by heme. Under heme-deficient growth conditions, ARE1 was upregulated fivefold while ARE2 was down-regulated. ARE2 requires the HAP1 transcription factor for optimal expression, and both ARE genes are derepressed in a rox1 (repressor of oxygen) mutant genetic background. We further report that the ARE genes are not subject to end product inhibition; neither ARE1 nor ARE2 transcription is altered in an are mutant background, nor does overexpression of either ARE gene alter the response of the ARE-lacZ reporter constructs. Our observations are consistent with an important physiological role for Are1p during anaerobic growth when heme is limiting and sterol precursors may accumulate. Conversely, Are2p is optimally required during aerobiosis when ergosterol is plentiful.

A role for smooth endoplasmic reticulum membrane cholesterol ester in determining the intracellular location and regulation of sterol-regulatory-element-binding protein-2

Cellular cholesterol homoeostasis is regulated through proteolysis of the membrane-bound precursor sterol-regulatory-element-binding protein (SREBP) that releases the mature transcription factor form, which regulates gene expression. Our aim was to identify the nature and intracellular site of the putative sterol-regulatory pool which regulates SREBP proteolysis in hamster liver. Cholesterol metabolism was modulated by feeding hamsters control chow, or a cholesterol-enriched diet, or by treatment with simvastatin or with the oral acyl-CoA:cholesterol acyltransferase inhibitor C1-1011 plus cholesterol. The effects of the different treatments on SREBP activation were confirmed by determination of the mRNAs for the low-density lipoprotein receptor and hydroxymethylglutaryl-CoA (HMG-CoA) reductase and by measurement of HMG-CoA reductase activity. The endoplasmic reticulum was isolated from livers and separated into subfractions by centrifugation in self-generating iodixanol gradients. Immunodetectable SREBP-2 accumulated in the smooth endoplasmic reticulum of cholesterol-fed animals. Cholesterol ester levels of the smooth endoplasmic reticulum membrane (but not the cholesterol levels) increased after cholesterol feeding and fell after treatment with simvastatin or C1-1011. The results suggest that an increased cellular cholesterol load causes accumulation of SREBP-2 in the smooth endoplasmic reticulum and, therefore, that membrane cholesterol ester may be one signal allowing exit of the SREBP-2/SREBP-cleavage-regulating protein complex to the Golgi.

Synergistic transcriptional activation of human Acyl-coenzyme A: cholesterol acyltransterase-1 gene by interferon-gamma and all-trans-retinoic acid THP-1 cells

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an intracellular enzyme involved in cellular cholesterol homeostasis and in atherosclerotic foam cell formation. Human ACAT-1 gene contains two promoters (P1 and P7), each located in a different chromosome (1 and 7) (Li, B. L., Li, X. L., Duan, Z. J., Lee, O., Lin, S., Ma, Z. M., Chang, C. C., Yang, X. Y., Park, J. P., Mohandas, T. K., Noll, W., Chan, L., and Chang, T. Y. (1999) J. Biol Chem. 274, 11060-11071). Interferon-gamma (IFN-gamma), a cytokine that exerts many pro-atherosclerotic effects in vivo, causes up-regulation of ACAT-1 mRNA in human blood monocyte-derived macrophages and macrophage-like cells but not in other cell types. To examine the molecular nature of this observation, we identified within the ACAT-1 P1 promoter a 159-base pair core region. This region contains 4 Sp1 elements and an IFN-gamma activated sequence (GAS) that overlaps with the second Sp1 element. In the monocytic cell line THP-1 cell, the combination of IFN-gamma and all-trans-retinoic acid (a known differentiation agent) enhances the ACAT-1 P1 promoter but not the P7 promoter. Additional experiments showed that all-trans-retinoic acid causes large induction of the transcription factor STAT1, while IFN-gamma causes activation of STAT1 such that it binds to the GAS/Sp1 site in the ACAT-1 P1 promoter. Our work provides a molecular mechanism to account for the effect of IFN-gamma in causing transcriptional activation of ACAT-1 in macrophage-like cells.

Roles of acyl-coenzyme A:cholesterol acyltransferase-1 and -2

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an intracellular enzyme that produces cholesteryl esters in various tissues. In mammals, two ACAT genes (ACAT1 and ACAT2) have been identified. Together, these two enzymes are involved in storing cholesteryl esters as lipid droplets, in macrophage foam-cell formation, in absorbing dietary cholesterol, and in supplying cholesteryl esters as part of the core lipid for lipoprotein synthesis and assembly. The key difference in tissue distribution of ACAT1 and ACAT2 between humans, mice and monkeys is that, in adult human liver (including hepatocytes and bile duct cells), the major enzyme is ACAT1, rather than ACAT2. There is compelling evidence implicating a role for ACAT1 in macrophage foam-cell formation, and for ACAT2 in intestinal cholesterol absorption. However, further studies at the biochemical and cell biological levels are needed in order to clarify the functional roles of ACAT1 and ACAT2 in the VLDL or chylomicron synthesis/assembly process.

Seed-specific over-expression of an Arabidopsis cDNA encoding a diacylglycerol acyltransferase enhances seed oil content and seed weight

We recently reported the cloning and characterization of an Arabidopsis (ecotype Columbia) diacylglycerol acyltransferase cDNA (Zou et al., 1999) and found that in Arabidopsis mutant line AS11, an ethyl methanesulfonate-induced mutation at a locus on chromosome II designated as Tag1 consists of a 147-bp insertion in the DNA, which results in a repeat of the 81-bp exon 2 in the Tag1 cDNA. This insertion mutation is correlated with an altered seed fatty acid composition, reduced diacylglycerol acyltransferase (DGAT; EC 2.3.1.20) activity, reduced seed triacylglycerol content, and delayed seed development in the AS11 mutant. The effect of the insertion mutation on microsomal acyl-coenzyme A-dependent DGAT is examined with respect to DGAT activity and its substrate specificity in the AS11 mutant relative to wild type. We demonstrate that transformation of mutant AS11 with a single copy of the wild-type Tag1 DGAT cDNA can complement the fatty acid and reduced oil phenotype of mutant AS11. More importantly, we show for the first time that seed-specific over-expression of the DGAT cDNA in wild-type Arabidopsis enhances oil deposition and average seed weight, which are correlated with DGAT transcript levels. The DGAT activity in developing seed of transgenic lines was enhanced by 10% to 70%. Thus, the current study confirms the important role of DGAT in regulating the quantity of seed triacylglycerols and the sink size in developing seeds.

Structure of the human acyl-CoA:cholesterol acyltransferase-2 (ACAT-2) gene and its relation to dyslipidemia

Acyl-CoA:cholesterol acyltransferase (ACAT) catalyzes cholesterol esterification in mammalian cells. Two isoforms of ACAT have been reported to date (ACAT-1 and ACAT-2). ACAT-1 is ubiquitously expressed in tissues except the intestine. In contrast, ACAT-2 is expressed mainly in the intestine in humans. To investigate the relationship between ACAT-2 and dyslipidemia, we determined the structure of the human ACAT-2 gene and then studied the relationship between mutations of the ACAT-2 gene and dyslipidemia. To isolate human ACAT-2 genomic DNA, we designed primers based on the human ACAT-2 cDNA sequence: forward primer 5'-ACACCTCGATCTTGGTCCTGCCATA-3' and reverse primer 5'-GGAATGCAGACAGGGAGTCCT-3'. Using these primers, a human P1-derived artificial chromosome (PAC) library was screened by PCR-based procedures. Isolated PAC clones were completely digested with BamHI and subcloned into plasmid vector. Subclones that contained exons were screened by dot-blot hybridization using partial ACAT-2 cDNA fragments. The coding region of the ACAT-2 gene was encoded in 15 exons from 51 to 265 base pairs on a 21 kilobase span of genomic DNA. The exonic sequences coincided completely with that of ACAT-2 cDNA, and each exon-intron junction conserved splicing consensus sequences. Next, 187 (91 dyslipidemic and 96 normolipidemic) subjects were screened by PCR single-strand conformational polymorphism analysis of the ACAT-2 gene. Three mutations were identified by DNA sequencing: two missense mutations (E14G in exon 1 and T254I in exon 7) and a point mutation in intron 7 (-35G-->A). Mutations in exon 1 and intron 7 were not associated with plasma concentrations of lipids and apolipoproteins (apo). However, plasma apoC-III levels in T254I heterozygotes were significantly higher than those in subjects without mutation. Plasma triglyceride (TG) levels in T254I heterozygotes were similar to those in subjects without mutation. Although further studies are needed, our data suggest that ACAT-2 may contribute to apoC-III gene expression and the assembly of apoC-III and TG, possibly in the intestine.

Differential modulation of ACAT1 and ACAT2 transcription and activity by long chain free fatty acids in cultured cells

Fatty acyl CoA and cholesterol are the substrates for cholesteryl ester synthesis by acyl coenzyme A:cholesterol acyltransferase (ACAT). Two ACAT genes have been identified; ACAT1 is expressed ubiquitously while ACAT2 is primarily expressed in intestine and liver. We tested effects of different free fatty acids (FFAs) on ACAT1 and ACAT2 expression and activity in HepG2 human hepatocytes and THP1 human macrophages. Incubation of oleic acid, arachidonic acid, or eicosapentaenoic acid, but not 25-hydroxycholesterol, induced ACAT1 mRNA levels 1.5--2-fold in HepG2, with no affect on ACAT2 mRNA. FFA had no affect on ACAT1 mRNA in THP1 cells. To determine if FFAs affect ACAT1 or ACAT2 posttranscriptionally, cells were labeled with [(3)H]cholesterol in the presence of the different FFAs for 1--5 h. Both HepG2 and THP1 cells showed the greatest cholesteryl ester production with oleic acid. This was also confirmed by the observation that more [(3)H]oleic acid incorporated into CE compared to [(3)H]eicosapentaenoic acid, even though there was no difference in the total uptake of these FFAs. In ACAT-deficient SRD4, CHO cells stably transfected with human ACAT1 or ACAT2, ACAT1 expressing cells showed a strong preference for oleic acid while ACAT2 expressing cells utilized unsaturated FFAs. Acyl CoA substrate specificity was further tested in microsomes isolated from these cells as well as HepG2 and THP1. THP1 and ACAT1 cells utilized oleoyl CoA preferentially. In contrast, HepG2 and ACAT2 microsomes utilized linolenoyl CoA as well. We conclude that FFAs increase ACAT1 mRNA levels in a cell specific manner, and furthermore that the ACAT reactions exhibit differential FFA utilization.

Very-low-density lipoprotein assembly and secretion

The assembly of apolipoprotein B (apoB) into VLDL is broadly divided into two steps. The first involves transfer of lipid by the microsomal triglyceride transfer protein (MTP) to apoB during translation. The second involves fusion of apoB-containing precursor particles with triglyceride droplets to form mature VLDL. ApoB and MTP are homologs of the egg yolk storage protein, lipovitellin. Homodimerization surfaces in lipovitellin are reutilized in apoB and MTP to achieve apoB-MTP interactions necessary for first step assembly. Structural modeling predicts a small lipovitellin-like lipid binding cavity in MTP and a transient lipovitellin-like cavity in apoB important for nucleation of lipid sequestration. The formation of triglyceride droplets in the endoplasmic reticulum requires MTP however, their fusion with apoB may be MTP-independent. Second step assembly is modulated by phospholipase D and A2. Phospholipases may prime membrane transport steps required for second step fusion and/or channel phospholipids into a pathway for VLDL triglyceride production. The enzymology of VLDL triglyceride synthesis is still poorly understood; however, it appears that ACAT2 is the sole source of cholesterol esters for VLDL and chylomicron assembly. VLDL production is controlled primarily at the level of presecretory degradation. Recently, it was discovered that the LDL receptor modulates VLDL production through its interactions with nascent VLDL in the secretory pathway.

Acyl coenzyme A: cholesterol acyltransferase types 1 and 2: structure and function in atherosclerosis

Two enzymes are responsible for cholesterol ester formation in tissues, acyl coenzyme A:cholesterol acyltransferase types 1 and 2 (ACAT1 and ACAT2). The available evidence suggests different cell locations, membrane orientations, and metabolic functions for each enzyme. ACAT1 and ACAT2 gene disruption experiments in mice have shown complementary results, with ACAT1 being responsible for cholesterol homeostasis in the brain, skin, adrenal, and macrophages. ACAT1 -/- mice have less atherosclerosis than their ACAT1 +/+ counterparts, presumably because of the decreased ACAT activity in the macrophages. By contrast, ACAT2 -/- mice have limited cholesterol absorption in the intestine, and decreased cholesterol ester content in the liver and plasma lipoproteins. Almost no cholesterol esterification was found when liver and intestinal microsomes from ACAT2 -/- mice were assayed. Studies in non-human primates have shown the presence of ACAT1 primarily in the Kupffer cells of the liver, in non-mucosal cell types in the intestine, and in kidney and adrenal cortical cells, whereas ACAT2 is present only in hepatocytes and in intestinal mucosal cells. The membrane topology for ACAT1 and ACAT2 is also apparently different, with ACAT1 having a serine essential for activity on the cytoplasmic side of the endoplasmic reticulum membrane, whereas the analogous serine is present on the lumenal side of the endoplasmic reticulum for ACAT2. Taken together, the data suggest that cholesterol ester formation by ACAT1 supports separate functions compared with cholesterol esterification by ACAT2. The latter enzyme appears to be responsible for cholesterol ester formation and secretion in lipoproteins, whereas ACAT1 appears to function to maintain appropriate cholesterol availability in cell membranes.

Organization of human ACAT-2 gene and its cell-type-specific promoter activity

Acyl-CoA:cholesterol acyltransferase (ACAT) plays important roles in cellular cholesterol homeostasis. Two ACAT genes exist in mammals. We report here the genomic organization of human ACAT-2 gene and analysis of its promoter activity in various cell lines. The human ACAT-2 gene spans over 18 kb and contains 15 exons. Three transcription start sites and one poly(A) site are identified by the 5'/3'-RACE. In addition, the human ACAT-2 gene is linked to the insulin-like growth factor binding protein 6 (IGFBP-6) gene in a head-to-tail manner with a small intergenic region of about 1.2 kb. The 5'-flanking region of human ACAT-2 gene contains many potential cis-acting elements for multiple transcriptional regulatory factors but lacks TATA and CCAAT boxes. Using promoter-luciferase reporter assays, we demonstrate the transcriptional activity of ACAT-2 gene promoter is high in Caco-2 cells, especially after these cells become postconfluent and behave as intestinal enterocytes.

APOLIPOPROTEIN B: mRNA editing, lipoprotein assembly, and presecretory degradation

Apolipoprotein (apo)B circulates in two distinct forms, apoB100 and apoB48. Human liver secretes apoB100, the product of a large mRNA encoding 4536 residues. The small intestine of all mammals secretes apoB48, which arises following C-to-U deamination of a single cytidine base in the nuclear apoB transcript, introducing a translational stop codon. This process, referred to as apoB RNA editing, operates through a multicomponent enzyme complex that contains a single catalytic subunit, apobec-1, in addition to other protein factors that have yet to be cloned. ApoB RNA editing also exhibits stringent cis-acting requirements that include both structural and sequence-specific elements-specifically efficiency elements that flank the minimal cassette, an AU-rich RNA context, and an 11-nucleotide mooring sequence-located in proximity to a suitably positioned (usually upstream) cytidine. C-to-U RNA editing may become unconstrained under circumstances where apobec-1 is overexpressed, in which case multiple cytidines in apoB RNA, as well as in other transcripts, undergo C-to-U editing. ApoB RNA editing is eliminated following targeting of apobec-1, establishing that there is no genetic redundancy in this function. Under physiological circumstances, apoB RNA editing exhibits developmental, hormonal, and nutritional regulation, in some cases related to transcriptional regulation of apobec-1 mRNA. ApoB and the microsomal triglyceride transfer protein (MTP) are essential for the assembly and secretion of apoB-containing lipoproteins. MTP functions by transferring lipid to apoB during its translation and by transporting triglycerides into the endoplasmic reticulum to form apoB-free lipid droplets. These droplets fuse with nascent apoB-containing particles to form mature, very low-density lipoproteins or chylomicrons. In cultured hepatic cells, lipid availability dictates the rate of apoB production. Unlipidated or underlipidated forms of apoB are subjected to presecretory degradation, a process mediated by retrograde transport from the lumen of the endoplasmic reticulum to the cytosol, coupled with multiubquitination and proteasomal degradation. Although control of lipid secretion in vivo is primarily achieved at the level of lipoprotein particle size, regulation of apoB production by presecretory degradation may be relevant in some dyslipidemic states.

Physiological and nutritional regulation of enzymes of triacylglycerol synthesis

Although triacylglycerol stores play the critical role in an organism's ability to withstand fuel deprivation and are strongly associated with such disorders as diabetes, obesity, and atherosclerotic heart disease, information concerning the enzymes of triacylglycerol synthesis, their regulation by hormones, nutrients, and physiological conditions, their mechanisms of action, and the roles of specific isoforms has been limited by a lack of cloned cDNAs and purified proteins. Fortunately, molecular tools for several key enzymes in the synthetic pathway are becoming available. This review summarizes recent studies of these enzymes, their regulation under varying physiological conditions, their purported roles in synthesis of triacylglycerol and related glycerolipids, the possible functions of different isoenzymes, and the evidence for specialized cellular pools of triacylglycerol and glycerolipid intermediates.

Increased atherosclerosis in LDL receptor-null mice lacking ACAT1 in macrophages

During atherogenesis, circulating macrophages migrate into the subendothelial space, internalize cholesterol-rich lipoproteins, and become foam cells by progressively accumulating cholesterol esters. The inhibition of macrophage acyl coenzyme A:cholesterol acyltransferase (ACAT), which catalyzes the formation of cholesterol esters, has been proposed as a strategy to reduce foam cell formation and to treat atherosclerosis. We show here, however, that hypercholesterolemic LDL receptor-deficient (LDLR(-/-)) mice reconstituted with ACAT1-deficient macrophages unexpectedly develop larger atherosclerotic lesions than control LDLR(-/-) mice. The ACAT1-deficient lesions have reduced macrophage immunostaining and more free cholesterol than control lesions. Our findings suggest that selective inhibition of ACAT1 in lesion macrophages in the setting of hyperlipidemia can lead to the accumulation of free cholesterol in the artery wall, and that this promotes, rather than inhibits, lesion development.

Conservation of eukaryotic sterol homeostasis: new insights from studies in budding yeast

The model eukaryote Saccharomyces cerevisiae (budding yeast) has provided significant insight into sterol homeostasis. The study of sterol metabolism in a genetically amenable model organism such as yeast is likely to have an even greater impact and relevance to human disease with the advent of the complete human genome sequence. In addition to definition of the sterol biosynthetic pathway, almost to completion, the remarkable conservation of other components of sterol homeostasis are described in this review.

Resistance to diet-induced hypercholesterolemia and gallstone formation in ACAT2-deficient mice

The importance of cholesterol ester synthesis by acyl CoA:cholesterol acyltransferase (ACAT) enzymes in intestinal and hepatic cholesterol metabolism has been unclear. We now demonstrate that ACAT2 is the major ACAT in mouse small intestine and liver, and suggest that ACAT2 deficiency has profound effects on cholesterol metabolism in mice fed a cholesterol-rich diet, including complete resistance to diet-induced hypercholesterolemia and cholesterol gallstone formation. The underlying mechanism involves the lack of cholesterol ester synthesis in the intestine and a resultant reduced capacity to absorb cholesterol. Our results indicate that ACAT2 has an important role in the response to dietary cholesterol, and suggest that ACAT2 inhibition may be a useful strategy for treating hypercholesterolemia or cholesterol gallstones.

ACAT1 and ACAT2 membrane topology segregates a serine residue essential for activity to opposite sides of the endoplasmic reticulum membrane

A second form of the enzyme acyl-CoA:cholesterol acyltransferase, ACAT2, has been identified. To explore the hypothesis that the two ACAT enzymes have separate functions, the membrane topologies of ACAT1 and ACAT2 were examined. A glycosylation reporter and FLAG epitope tag sequence was appended to a series of ACAT cDNAs truncated after each predicted transmembrane domain. Fusion constructs were assembled into microsomal membranes, in vitro, and topologies were determined based on glycosylation site use and accessibility to exogenous protease. The accessibility of the C-terminal FLAG epitope in constructs was determined by immunofluorescence microscopy of permeabilized transfected cells. Both ACAT1 and ACAT2 span the membrane five times with their N termini in the cytosol and C termini in the ER lumen. The fourth transmembrane domain is located in a different region for each protein, placing the putative active site ACAT1 serine (Ser(269)) in the cytosol and the analogous residue in ACAT2 (Ser(249)) in the ER lumen. Mutation of these serines inactivated the ACAT enzymes. The outcome is consistent with the hypothesis that cholesterol ester formation by ACAT2 may be coupled to lipoprotein particle assembly and secretion, whereas ACAT1 may function primarily to maintain the balance of free and esterified cholesterol intracellularly.

Glibenclamide acts as an inhibitor of acyl-CoA:cholesterol acyltransferase enzyme

Sulfonylureas are used in the treatment of non-insulin-dependent diabetes mellitus. Little is known, however, about their effects on cholesterol metabolism. We tested in the present study the effects of glibenclamide (GB) on cholesterol esterification (CE) in macrophage-derived cells. GB inhibited intracellular accumulation of CE induced by acetylated LDL or oxidized LDL in J774 cells, but no such effect on total cholesterol, suggesting that the target of GB was acyl-CoA:cholesterol acyltransferase (ACAT). In the cell-free reconstitution ACAT assay, GB inhibited the ACAT activity with an IC(50) value of 20 microM. Furthermore, GB effectively inhibited the ACAT activity of PMA-stimulated THP-1 cells to the undifferentiated level of THP-1. In the whole-cell ACAT assay using CHO cells overexpressed with ACAT-1 or ACAT-2, GB inhibited the activity of both isozymes with similar potency. Our in vitro data suggest that sulfonylurea could be a potential seed for a new generation of ACAT inhibitors.

Immunological quantitation and localization of ACAT-1 and ACAT-2 in human liver and small intestine

By using specific anti-ACAT-1 antibodies in immunodepletion studies, we previously found that ACAT-1, a 50-kDa protein, plays a major catalytic role in the adult human liver, adrenal glands, macrophages, and kidneys but not in the intestine. Acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity in the intestine may be largely derived from a different ACAT protein. To test this hypothesis, we produced specific polyclonal anti-ACAT-2 antibodies that quantitatively immunodepleted human ACAT-2, a 46-kDa protein expressed in Chinese hamster ovary cells. In hepatocyte-like HepG2 cells, ACAT-1 comprises 85-90% of the total ACAT activity, with the remainder attributed to ACAT-2. In adult intestines, most of the ACAT activity can be immunodepleted by anti-ACAT-2. ACAT-1 and ACAT-2 do not form hetero-oligomeric complexes. In differentiating intestinal enterocyte-like Caco-2 cells, ACAT-2 protein content increases by 5-10-fold in 6 days, whereas ACAT-1 protein content remains relatively constant. In the small intestine, ACAT-2 is concentrated at the apices of the villi, whereas ACAT-1 is uniformly distributed along the villus-crypt axis. In the human liver, ACAT-1 is present in both fetal and adult hepatocytes. In contrast, ACAT-2 is evident in fetal but not adult hepatocytes. Our results collectively suggest that in humans, ACAT-2 performs significant catalytic roles in the fetal liver and in intestinal enterocytes.

Absence of ACAT-1 attenuates atherosclerosis but causes dry eye and cutaneous xanthomatosis in mice with congenital hyperlipidemia

Acyl-CoA:cholesterol acyltransferase (ACAT) catalyzes esterification of cellular cholesterol. To investigate the role of ACAT-1 in atherosclerosis, we have generated ACAT-1 null (ACAT-1-/-) mice. ACAT activities were present in the liver and intestine but were completely absent in adrenal, testes, ovaries, and peritoneal macrophages in our ACAT-1-/- mice. The ACAT-1-/- mice had decreased openings of the eyes because of atrophy of the meibomian glands, a modified form of sebaceous glands normally expressing high ACAT activities. This phenotype is similar to dry eye syndrome in humans. To determine the role of ACAT-1 in atherogenesis, we crossed the ACAT-1-/- mice with mice lacking apolipoprotein (apo) E or the low density lipoprotein receptor (LDLR), hyperlipidemic models susceptible to atherosclerosis. High fat feeding resulted in extensive cutaneous xanthomatosis with loss of hair in both ACAT-1-/-:apo E-/- and ACAT-1-/-:LDLR-/- mice. Free cholesterol content was significantly increased in their skin. Aortic fatty streak lesion size as well as cholesteryl ester content were moderately reduced in both double mutant mice compared with their respective controls. These results indicate that the local inhibition of ACAT activity in tissue macrophages is protective against cholesteryl ester accumulation but causes cutaneous xanthomatosis in mice that lack apo E or LDLR.

Triglyceride synthesis: insights from the cloning of diacylglycerol acyltransferase

Although the biochemistry of triglyceride synthesis has been studied for decades, an understanding of the molecular processes involved has been lacking. The recent cloning of a gene encoding acyl coenzyme A : diacylglycerol acyltransferase, an enzyme that catalyses the final step in triglyceride synthesis, has opened this area to molecular investigation and has begun to provide new insights into triglyceride metabolism.

A lecithin cholesterol acyltransferase-like gene mediates diacylglycerol esterification in yeast

The terminal step in triglyceride biosynthesis is the esterification of diacylglycerol. To study this reaction in the model eukaryote, Saccharomyces cerevisiae, we investigated five candidate genes with sequence conservation to mammalian acyltransferases. Four of these genes are similar to the recently identified acyl-CoA diacylglycerol acyltransferase and, when deleted, resulted in little or no decrease in triglyceride synthesis as measured by incorporation of radiolabeled oleate or glycerol. By contrast, deletion of LRO1, a homolog of human lecithin cholesterol acyltransferase, resulted in a dramatic reduction in triglyceride synthesis, whereas overexpression of LRO1 yielded a significant increase in triglyceride production. In vitro microsomal assays determined that Lro1 mediated the esterification of diacylglycerol using phosphatidylcholine as the acyl donor. The residual triglyceride biosynthesis that persists in the LRO1 deletion strain is mainly acyl-CoA-dependent and mediated by a gene that is structurally distinct from the previously identified mammalian diacylglycerol acyltransferase. These mechanisms may also exist in mammalian cells.

Massive xanthomatosis and altered composition of atherosclerotic lesions in hyperlipidemic mice lacking acyl CoA:cholesterol acyltransferase 1

Inhibitors of acyl CoA:cholesterol acyltransferase (ACAT) have attracted considerable interest as a potential treatment for atherosclerosis. Currently available inhibitors probably act nonselectively against the two known ACATs. One of these enzymes, ACAT1, is highly expressed in macrophages in atherosclerotic lesions, where it contributes to foam-cell formation. In this study, we examined the effects of selective ACAT1 deficiency in two mouse models of atherosclerosis. In the setting of severe hypercholesterolemia caused by deficiency in apoE or the LDL receptor (LDLR), total ACAT1 deficiency led to marked alterations in cholesterol homeostasis and extensive deposition of unesterified cholesterol in the skin and brain. Bone marrow transplantation experiments demonstrated that ACAT1 deficiency in macrophages was sufficient to cause dermal xanthomas in hyperlipidemic LDLR-deficient mice. ACAT1 deficiency did not prevent the development of atherosclerotic lesions in either apoE-deficient or LDLR-deficient mice, despite causing relatively lower serum cholesterol levels. However, the lesions in ACAT1-deficient mice were atypical in composition, with reduced amounts of neutral lipids and a paucity of macrophages in advanced lesions. Although the latter findings may be associated with increased lesion stability, the marked alterations in cholesterol homeostasis indicate that selectively inhibiting ACAT1 in the setting of severe hyperlipidemia may have detrimental consequences.

Contribution of Are1p and Are2p to steryl ester synthesis in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, two acyl-CoA:sterol acyltransferases (ASATs) that catalyze the synthesis of steryl esters have been identified, namely Are2p (Sat1p) and Are1p (Sat2p). Deletion of either ARE1 or ARE2 has no effect on cell viability, and are1are2 double mutants grow in a similar manner to wild-type despite the complete lack of cellular ASAT activity and steryl ester formation [Yang, H., Bard, M., Bruner, D. A., Gleeson, A., Deckelbaum, R. J., Aljinovic, G., Pohl, T. M., Rothstein, R. & Sturley, S. L. (1996) Science 272, 1353-1356; Yu, C., Kennedy, J., Chang, C. C. Y. & Rothblatt, J. A. (1996) J. Biol. Chem. 271, 24157-24163]. Here we show that both Are2p and Are1p reside in the endoplasmic reticulum as demonstrated by measuring ASAT activity in subcellular fractions of are1 and are2 deletion strains. This localization was confirmed by fluorescence microscopy using hybrid proteins of Are2p and Are1p fused to green fluorescent protein (GFP). Lipid analysis of are1 and are2 deletion strains revealed that Are2p and Are1p utilize sterol substrates in vivo with different efficiency; Are2p has a significant preference for ergosterol as a substrate, whereas Are1p esterifies sterol precursors, mainly lanosterol, as well as ergosterol. The specificity towards fatty acids is similar for both isoenzymes. The lack of steryl esters in are1are2 mutant cells is largely compensated by an increased level of free sterols. Nevertheless, terbinafine, an inhibitor of ergosterol biosynthesis, inhibits growth of are1are2 cells more efficiently than growth of wild-type. In a growth competition experiment are1are2 cells grow more slowly than wild-type after several rounds of cultivation, suggesting that Are1p and Are2p or steryl esters, the product formed by these two enzymes, are more important in the natural environment than under laboratory conditions.

Expression in yeast and tobacco of plant cDNAs encoding acyl CoA:diacylglycerol acyltransferase

During the course of a search for cDNAs encoding plant sterol acyltransferases, an expressed sequence tag clone presenting substantial identity with yeast and animal acyl CoA:cholesterol acyltransferases was used to screen cDNA libraries from Arabidopsis and tobacco. This resulted in the isolation of two full-length cDNAs encoding proteins of 520 and 532 amino acids, respectively. Attempts to complement the yeast double-mutant are1 are2 defective in acyl CoA:cholesterol acyltransferase were unsuccessful, showing that neither gene encodes acyl CoA:cholesterol acyltransferase. Their deduced amino acid sequences were then shown to have 40 and 38% identity, respectively, with a murine acyl CoA:diacylglycerol acyltransferase and their expression in are1 are2 or wild-type yeast resulted in a strong increase in the incorporation of oleyl CoA into triacylglycerols. Incorporation was 2-3 times higher in microsomes from yeast transformed with these plant cDNAs than in yeast transformed with the void vector, clearly showing that these cDNAs encode acyl CoA:diacylglycerol acyltransferases. Moreover, during the preparation of microsomes from the Arabidopsis DGAT-transformed yeast, a floating layer was observed on top of the 100 000 g supernatant. This fraction was enriched in triacylglycerols and exhibited strong acyl CoA:diacylglycerol acyltransferase activity, whereas almost no activity was detected in the corresponding clear fraction from the control yeast. Thanks to the use of this active fraction and dihexanoylglycerol as a substrate, the de novo synthesis of 1,2-dihexanoyl 3-oleyl glycerol by AtDGAT could be demonstrated. Transformation of tobacco with AtDGAT was also performed. Analysis of 19 primary transformants allowed detection, in several individuals, of a marked increase (up to seven times) of triacylglycerol content which correlated with the AtDGAT mRNA expression. Furthermore, light-microscopy observations of leaf epidermis cells, stained with a lipid-specific dye, showed the presence of lipid droplets in the cells of triacylglycerol-overproducer plants, thus illustrating the potential application of acyl CoA:diacylglycerol acyltransferase-transformed plants.

Localization of human acyl-coenzyme A: cholesterol acyltransferase-1 (ACAT-1) in macrophages and in various tissues

To investigate the distribution of acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT-1) in various human tissues, we examined tissues of autopsy cases immunohistochemically. ACAT-1 was demonstrated in macrophages, antigen-presenting cells, steroid hormone-producing cells, neurons, cardiomyocytes, smooth muscle cells, mesothelial cells, epithelial cells of the urinary tracts, thyroid follicles, renal tubules, pituitary, prostatic, and bronchial glands, alveolar and intestinal epithelial cells, pancreatic acinar cells, and hepatocytes. These findings showed that ACAT-1 is present in a variety of human tissues examined. The immunoreactivities are particularly prominent in the macrophages, steroid hormone-producing cells, followed by hepatocytes, and intestinal epithelia. In cultured human macrophages, immunoelectron microscopy revealed that ACAT-1 was located mainly in the tubular rough endoplasmic reticulum; immunoblot analysis showed that the ACAT-1 protein content did not change with or without cholesterol loading; however, on cholesterol loading, about 30 to 40% of the total immunoreactivity appeared in small-sized vesicles. These vesicles were also enriched in 78-kd glucose-regulated protein (GRP 78), a specific marker for the endoplasmic reticulum. Immunofluorescent microscopy demonstrated extensive colocalization of ACAT-1 and GRP 78 signals in both the tubular and vesicular endoplasmic reticulum before and after cholesterol loading. These results raise the possibility that foam cell formation may activate an endoplasmic reticulum vesiculation process, producing vesicles enriched in the ACAT-1 protein.

Postprandial hyperlipidemia in streptozotocin-induced diabetic rats is due to abnormal increase in intestinal acyl coenzyme A:cholesterol acyltransferase activity

Postprandial hyperlipidemia (PH) is recognized as a significant risk factor for cardiovascular disease. The present study, involving rats with streptozotocin (STZ)-induced diabetes, was performed to establish a PH model and to examine the relation between small intestinal acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity and serum lipid levels in the postprandial state. The small intestinal ACAT activities in normal rats during the experimental period were 4 to 5 pmol/mg protein per minute. In contrast, in the diabetic rats, the ACAT activities were 2 to 3 times higher than activities seen in normal rats from 7 to 21 days after the STZ injection in the absence of a high fat diet and hyperplasia in the gut. In an oral fat-loading test that used diabetic rats that had been injected with STZ (60 mg/kg) intravenously 14 days previously, the postloading changes in the serum concentrations of total cholesterol (TC) and triglyceride (TG) were significantly greater in the diabetic rats than in normal rats. Single oral administration of (1s,2s)-2-[3-(2,2-dimethylpropyl)-3-nonylureido]cyclohexane- 1-yl 3-[(4R)-N-(2,2,5,5-tetramethyl-1, 3-dioxane-4-carbonyl)amino]propionate (F-1394, 3 to 30 mg/kg), a potent ACAT inhibitor, suppressed the post-fat-loading elevation of serum TC levels in the diabetic rats in a dose-dependent manner without affecting serum glucose levels. Furthermore, the small intestinal ACAT activity, serum TG levels, and lymphatic absorption of TC and TG in the rats that were administered F-1394 (30 mg/kg) were reduced by approximately 90%, 70%, 30%, and 15%, respectively. This is the first evidence that elevated ACAT activity in the gut, unlike hyperplasia and hyperphagia, induces PH in rats. Our results strongly suggest that F-1394 may be a potential treatment for PH in humans.

Human acyl-CoA:cholesterol acyltransferase-1 is a homotetrameric enzyme in intact cells and in vitro

Acyl-CoA:cholesterol acyltransferase (ACAT) is a key enzyme in cellular cholesterol homeostasis and in atherosclerosis. ACAT-1 may function as an allosteric enzyme. We took a multifaceted approach to investigate the subunit composition of ACAT-1. When ACAT-1 with two different tags were co-expressed in the same Chinese hamster ovary cells, antibody specific to one tag caused co-immunoprecipitation of both types of ACAT-1 proteins. Radioimmunoprecipitations of cells expressing the untagged ACAT-1 or the 6-histidine-tagged ACAT-1 yielded a single radiolabeled band of predicted size on SDS-polyacrylamide gel electrophoresis. These results show that ACAT-1 exists as homo-oligomers in intact Chinese hamster ovary cells. We solubilized HisACAT-1 with the detergent deoxycholate or CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonic acid), performed gel filtration chromatography and sucrose density gradient centrifugations in H(2)O and D(2)O, and determined the Stokes radii and sedimentation coefficients of the HisACAT1-detergent complexes. The estimated molecular mass of HisACAT-1 is 263 kDa, which is 4 times that of the HisACAT-1 monomer (69 kDa). Finally, cross-linking experiments in intact cells and in vitro show that the increase in cross-linker concentrations causes an increase in size of the HisACAT-1-positive signals, forming material(s) 4 times the size of the monomer, supporting the conclusion that ACAT-1 is a homotetrameric enzyme.

Evidence for triacylglycerol synthesis in the lumen of microsomes via a lipolysis-esterification pathway involving carnitine acyltransferases

In this study a pathway for the synthesis of triacylglycerol (TAG) within the lumen of the endoplasmic reticulum has been identified, using microsomes that had been preconditioned by depleting their endogenous substrates and then fusing them with biotinylated phosphatidylserine liposomes containing CoASH and Mg(2+). Incubating these fused microsomes with tri[(3)H] oleoylglycerol and [(14)C]oleoyl-CoA yielded microsome-associated triacylglycerol, which resisted extensive washing and had a [(3)H]:[(14)C] ratio close to 2:1. The data suggest that the precursor tri[(3)H]oleoylglycerol was hydrolyzed by microsomal lipase to membrane-bound di[(3)H]oleoylglycerol and subsequently re-esterified with luminal [(14)C]oleoyl-CoA. The accumulation of TAG within the microsomes, even when overt diacylglycerol acyltransferase (DGAT I) was inactive, is consistent with the existence of a latent diacylglycerol acyltransferase (DGAT II) within the microsomal lumen. Moreover, because luminal synthesis of TAG was carnitine-dependent and markedly reduced by glybenclamide, a potent carnitine acyltransferase inhibitor, microsomal carnitine acyltransferase appears to be essential for trafficking the [(14)C]oleoyl-CoA into the microsomal lumen for subsequent incorporation into newly synthesized TAG. This study thus provides the first direct demonstration of an enzymatic process leading to the synthesis of luminal triacylglycerol, which is a major component of very low density lipoproteins.

The Arabidopsis thaliana TAG1 mutant has a mutation in a diacylglycerol acyltransferase gene

In Arabidopsis thaliana (ecotype Columbia) mutant line AS11, an EMS-induced mutation at a locus on chromosome II results in a reduced diacylglycerol acyltransferase (DGAT; EC 2.3.1.20) activity, reduced seed triacylglycerol, an altered seed fatty acid composition, and delayed seed development. A mutation has been identified in AS11 in a gene, which we designated as TAG1, that encodes a protein with an amino acid sequence which is similar to a recently reported mammalian DGAT, and, to a lesser extent, to acyl CoA:cholesterol acyltransferases. Molecular analysis revealed that the mutant allele in AS11 has a 147 bp insertion located at the central region of intron 2. At the RNA level, an 81 bp insertion composed entirely of an exon 2 repeat was found in the transcript. While the seed triacylglycerol content is reduced by the lesion in AS11, there is no apparent effect on sterol ester content in the mutant seed. The TAG1 cDNA was over-expressed in yeast, and its activity as a microsomal DGAT confirmed. Therefore, the TAG1 locus encodes a diacylglycerol acyltransferase, and the insertion mutation in the TAG1 gene in mutant AS11 results in its altered lipid phenotype.

Human acyl-CoA:cholesterol acyltransferase-1 in the endoplasmic reticulum contains seven transmembrane domains

Acyl-CoA:cholesterol acyltransferase (ACAT) plays important roles in cellular cholesterol homeostasis and is involved in atherosclerosis. ACAT-1 protein is located mainly in the ER. The hydropathy plot suggests that ACAT-1 protein contains multiple transmembrane segments. We inserted either the hemagglutinin tag or the HisT7 tag at various hydrophilic regions within the human ACAT-1 protein and used immunofluorescence microscopy to determine the topography of the tagged proteins expressed in mutant Chinese hamster ovary cells lacking endogenous ACAT. All of the tagged proteins are located mainly in the ER and retain full or partial enzyme activities. None of the tagged proteins produces detectable intracellular degradation intermediates. Treating cells with digitonin at 5 micrograms/ml permeabilizes the plasma membranes while leaving the ER membranes sealed; in contrast, treating cells with 0.25% Triton X-100 or with cold methanol permeabilizes both the plasma membranes and the ER membranes. After appropriate permeabilization, double immunostaining using antibodies against the N-terminal region and against the inserted tag were used to visualize various regions of the tagged protein. The results show that human ACAT-1 in the ER contains seven transmembrane domains.