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.

Inhibition of SREBP by a small molecule, betulin, improves hyperlipidemia and insulin resistance and reduces atherosclerotic plaques

Sterol regulatory element-binding proteins (SREBPs) are major transcription factors activating the expression of genes involved in biosynthesis of cholesterol, fatty acid and triglyceride. In this study, we identified a small molecule, betulin, that specifically inhibited the maturation of SREBP by inducing interaction of SREBP cleavage activating protein (SCAP) and Insig. Inhibition of SREBP by betulin decreased the biosynthesis of cholesterol and fatty acid. In vivo, betulin ameliorated diet-induced obesity, decreased the lipid contents in serum and tissues, and increased insulin sensitivity. Furthermore, betulin reduced the size and improved the stability of atherosclerotic plaques. Our study demonstrates that inhibition SREBP pathway can be employed as a therapeutic strategy to treat metabolic diseases including type II diabetes and atherosclerosis. Betulin, which is abundant in birch bark, could be a leading compound for development of drugs for hyperlipidemia.

The cholesterol absorption inhibitor ezetimibe acts by blocking the sterol-induced internalization of NPC1L1

Niemann-Pick C1-like 1 (NPC1L1) is a polytopic transmembrane protein that plays a critical role in cholesterol absorption. Ezetimibe, a hypocholesterolemic drug, has been reported to bind NPC1L1 and block cholesterol absorption. However, the molecular mechanism of NPC1L1-mediated cholesterol uptake and how ezetimibe inhibits this process are poorly defined. Here we find that cholesterol specifically promotes the internalization of NPC1L1 and that this process requires microfilaments and the clathrin/AP2 complex. Blocking NPC1L1 endocytosis dramatically decreases cholesterol internalization, indicating that NPC1L1 mediates cholesterol uptake via its vesicular endocytosis. Ezetimibe prevents NPC1L1 from incorporating into clathrin-coated vesicles and thus inhibits cholesterol uptake. Together, our data suggest a model wherein cholesterol is internalized into cells with NPC1L1 through clathrin/AP2-mediated endocytosis and ezetimibe inhibits cholesterol absorption by blocking the internalization of NPC1L1.

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.

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'.

Cross-cultural differences in rating hyperactive-disruptive behaviors in children

OBJECTIVE

To determine the extent to which the reported variations across countries in the prevalence of attention-deficit hyperactivity disorder are due to cultural differences among raters, the authors examined the degree to which mental health professionals in four countries differed in their ratings of hyperactive-disruptive behaviors in children.

METHOD

Mental health professionals from China (N = 8), Indonesia (N = 12), Japan (N = 9), and the United States (N = 8) rated the presence and degree of hyperactive-disruptive behaviors in standardized videotape vignettes of four 8-year-old boys participating in individual and group activities.

RESULTS

Chinese and Indonesian clinicians gave significantly higher scores for hyperactive-disruptive behaviors than did their Japanese and American colleagues.

CONCLUSIONS

These findings suggest that perceptions of hyperactivity vary significantly across countries even if uniform rating criteria are applied. Without correction for these perceptual differences, cross-cultural prevalence rates of hyperactivity may not be comparable.

Human acyl-CoA:cholesterol acyltransferase-1 (ACAT-1) gene organization and evidence that the 4.3-kilobase ACAT-1 mRNA is produced from two different chromosomes

Acyl-CoA:cholesterol acyltransferase (ACAT) plays important roles in cellular cholesterol homeostasis. Four human ACAT-1 mRNAs (7.0, 4.3, 3.6, and 2.8 kilobases (kb)) share the same short 5'-untranslated region (exon 1) and coding sequence (exons 2-15). The 4.3-kb mRNA contains an additional 5'-untranslated region (1289 nucleotides in length; exons Xa and Xb) immediately upstream from the exon 1 sequence. One ACAT-1 genomic DNA insert covers exons 1-16 and a promoter (the P1 promoter). A separate insert covers exon Xa (1277 base pairs) and a different promoter (the P7 promoter). Gene mapping shows that exons 1-16 and the P1 promoter sequences are located in chromosome 1, while exon Xa and the P7 promoter sequence are located in chromosome 7. RNase protection assays demonstrate three different protected fragments, corresponding to the 4.3-kb mRNA and the two other mRNAs transcribed from the two promoters. These results are consistent with the interpretation that the 4.3-kb mRNA is produced from two different chromosomes, by a novel RNA recombination mechanism involving trans-splicing of two discontinuous precursor RNAs.

Genome editing with CRISPR/Cas9 in postnatal mice corrects PRKAG2 cardiac syndrome

PRKAG2 cardiac syndrome is an autosomal dominant inherited disease resulted from mutations in the PRKAG2 gene that encodes γ2 regulatory subunit of AMP-activated protein kinase. Affected patients usually develop ventricular tachyarrhythmia and experience progressive heart failure that is refractory to medical treatment and requires cardiac transplantation. In this study, we identify a H530R mutation in PRKAG2 from patients with familial Wolff-Parkinson-White syndrome. By generating H530R PRKAG2 transgenic and knock-in mice, we show that both models recapitulate human symptoms including cardiac hypertrophy and glycogen storage, confirming that the H530R mutation is causally related to PRKAG2 cardiac syndrome. We further combine adeno-associated virus-9 (AAV9) and the CRISPR/Cas9 gene-editing system to disrupt the mutant PRKAG2 allele encoding H530R while leaving the wild-type allele intact. A single systemic injection of AAV9-Cas9/sgRNA at postnatal day 4 or day 42 substantially restores the morphology and function of the heart in H530R PRKAG2 transgenic and knock-in mice. Together, our work suggests that in vivo CRISPR/Cas9 genome editing is an effective tool in the treatment of PRKAG2 cardiac syndrome and other dominant inherited cardiac diseases by selectively disrupting disease-causing mutations.

Cholesterol and fatty acids regulate cysteine ubiquitylation of ACAT2 through competitive oxidation

Ubiquitin linkage to cysteine is an unconventional modification targeting protein for degradation. However, the physiological regulation of cysteine ubiquitylation is still mysterious. Here we found that ACAT2, a cellular enzyme converting cholesterol and fatty acid to cholesteryl esters, was ubiquitylated on Cys277 for degradation when the lipid level was low. gp78-Insigs catalysed Lys48-linked polyubiquitylation on this Cys277. A high concentration of cholesterol and fatty acid, however, induced cellular reactive oxygen species (ROS) that oxidized Cys277, resulting in ACAT2 stabilization and subsequently elevated cholesteryl esters. Furthermore, ACAT2 knockout mice were more susceptible to high-fat diet-associated insulin resistance. By contrast, expression of a constitutively stable form of ACAT2 (C277A) resulted in higher insulin sensitivity. Together, these data indicate that lipid-induced stabilization of ACAT2 ameliorates lipotoxicity from excessive cholesterol and fatty acid. This unconventional cysteine ubiquitylation of ACAT2 constitutes an important mechanism for sensing lipid-overload-induced ROS and fine-tuning lipid homeostasis.

The N-terminal domain of NPC1L1 protein binds cholesterol and plays essential roles in cholesterol uptake

Niemann-Pick C1-like 1 (NPC1L1) is a multitransmembrane protein playing a crucial role in dietary and biliary cholesterol absorption. Cholesterol promotes the formation and endocytosis of NPC1L1-flotillin-cholesterol membrane microdomains, which is an early step in cholesterol uptake. How cholesterol is sensed in this step is unknown. Here, we find that the N-terminal domain (NTD) of NPC1L1 binds cholesterol. Mutation of residue Leu-216 in NPC1L1-NTD eliminates cholesterol binding, decreases the formation of NPC1L1-flotillin-cholesterol membrane microdomains, and prevents NPC1L1-mediated cholesterol uptake in culture cells and mice livers. NPC1L1-NTD specifically binds cholesterol but not plant sterols, which may account for the selective cholesterol absorption in intestine. Furthermore, 25- or 27-hydroxycholesterol competes with cholesterol to bind NPC1L1-NTD and inhibits the cholesterol induced endocytosis of NPC1L1. Together, these results demonstrate that plasma membrane-localized NPC1L1 binds exogenous cholesterol via its NTD, and facilitates the formation of NPC1L1-flotillin-cholesterol membrane microdomains that are then internalized into cells through the clathrin-AP2 pathway. Our study uncovers the mechanism of cholesterol sensing by NPC1L1 and proposes a mechanism for selective cholesterol absorption.

A LIMA1 variant promotes low plasma LDL cholesterol and decreases intestinal cholesterol absorption

A high concentration of low-density lipoprotein cholesterol (LDL-C) is a major risk factor for cardiovascular disease. Although LDL-C levels vary among humans and are heritable, the genetic factors affecting LDL-C are not fully characterized. We identified a rare frameshift variant in the LIMA1 (also known as EPLIN or SREBP3) gene from a Chinese family of Kazakh ethnicity with inherited low LDL-C and reduced cholesterol absorption. In a mouse model, LIMA1 was mainly expressed in the small intestine and localized on the brush border membrane. LIMA1 bridged NPC1L1, an essential protein for cholesterol absorption, to a transportation complex containing myosin Vb and facilitated cholesterol uptake. Similar to the human phenotype, Lima1-deficient mice displayed reduced cholesterol absorption and were resistant to diet-induced hypercholesterolemia. Through our study of both mice and humans, we identify LIMA1 as a key protein regulating intestinal cholesterol absorption.

Ufd1 is a cofactor of gp78 and plays a key role in cholesterol metabolism by regulating the stability of HMG-CoA reductase

The membrane-anchored ubiquitin ligase gp78 promotes degradation of misfolded endoplasmic reticulum (ER) proteins and sterol-regulated degradation of HMG-CoA reductase. It was known previously that Ufd1 plays a critical role in ER-associated degradation (ERAD) together with Npl4 and VCP. The VCP-Ufd1-Npl4 complex recognizes polyubiquitin chains and transfers the ubiquitinated proteins to the proteasome. Here we show that Ufd1 directly interacts with gp78 and functions as a cofactor. Ufd1 enhances the E3 activity of gp78, accelerates the ubiquitination and degradation of reductase, and eventually promotes receptor-mediated uptake of low-density lipoprotein. Furthermore, we demonstrate that the monoubiquitin-binding site in Ufd1 is required for the enhancement of gp78 activity and that the polyubiquitin-binding site in Ufd1 is critical for a postubiquitination step in ERAD. In summary, our study identifies Ufd1 as a cofactor of gp78, reveals an unappreciated function of Ufd1 in the ubiquitination reaction during ERAD, and illustrates that Ufd1 plays a critical role in cholesterol metabolism.

Creation of phosphorylation sites in proteins: construction of a phosphorylatable human interferon alpha

A phosphorylation site was introduced into human interferon alpha A (IFN-alpha A) by site-specific mutation of the coding sequence. Three slightly different phosphorylation sites were created by using the predicted amino acid consensus sequences for phosphorylation by the cAMP-dependent protein kinase. The resultant modified interferons (IFN-alpha A-P) were expressed in Escherichia coli and purified. The purified proteins exhibit antiviral activity on bovine and human cells similar to that of the unmodified IFN-alpha A. The IFN-alpha A-P proteins can be phosphorylated by the catalytic subunit of cAMP-dependent protein kinase with [gamma-32P]ATP to high specific activity (2000-5000 Ci/mmol; 1 Ci = 37 GBq) with retention of biological activity. The 32P-labeled IFN-alpha A-P proteins bind to cells and can be covalently bound to the IFN-alpha/beta receptor with a bifunctional reagent as can human IFN-alpha A. The introduction of phosphorylation sites into proteins provides a procedure to prepare a large variety of radioactive proteins for research and clinical use.

A specific cholesterol metabolic pathway is established in a subset of HCCs for tumor growth

The liver plays a central role in cholesterol homeostasis. It exclusively receives and metabolizes oxysterols, which are important metabolites of cholesterol and are more cytotoxic than free cholesterol, from all extrahepatic tissues. Hepatocellular carcinomas (HCCs) impair certain liver functions and cause pathological alterations in many processes including cholesterol metabolism. However, the link between an altered cholesterol metabolism and HCC development is unclear. Human ACAT2 is abundantly expressed in intestine and fetal liver. Our previous studies have shown that ACAT2 is induced in certain HCC tissues. Here, by investigating tissue samples from HCC patients and HCC cell lines, we report that a specific cholesterol metabolic pathway, involving induction of ACAT2 and esterification of excess oxysterols for secretion to avoid cytotoxicity, is established in a subset of HCCs for tumor growth. Inhibiting ACAT2 leads to the intracellular accumulation of unesterified oxysterols and suppresses the growth of both HCC cell lines and their xenograft tumors. Further mechanistic studies reveal that HCC-linked promoter hypomethylation is essential for the induction of ACAT2 gene expression. We postulate that specifically blocking this HCC-established cholesterol metabolic pathway may have potential therapeutic applications for HCC patients.

Gpnmb secreted from liver promotes lipogenesis in white adipose tissue and aggravates obesity and insulin resistance

Metabolism in mammals is regulated by complex interplay among different organs. Fatty acid synthesis is increased in white adipose tissue (WAT) when it is inhibited in the liver. Here we identify glycoprotein non-metastatic melanoma protein B (Gpnmb) as one liver-WAT cross-talk factor involved in lipogenesis. Inhibition of the hepatic sterol regulatory element-binding protein pathway leads to increased transcription of Gpnmb and promotes processing of the membrane protein to a secreted form. Gpnmb stimulates lipogenesis in WAT and exacerbates diet-induced obesity and insulin resistance. In humans, Gpnmb is tightly associated with body mass index and is a strong risk factor for obesity. Gpnmb inhibition by a neutralizing antibody or liver-specific knockdown improves metabolic parameters, including weight gain reduction and increased insulin sensitivity, probably by promoting the beiging of WAT. These results suggest that Gpnmb is a liver-secreted factor regulating lipogenesis in WAT, and that Gpnmb inhibition may provide a therapeutic strategy in obesity and diabetes.

Inhibition of the sterol regulatory element-binding protein pathway suppresses hepatocellular carcinoma by repressing inflammation in mice

Obesity is a critical risk factor for hepatocellular carcinoma (HCC). However, it remains unknown whether inhibition of de novo lipid biosynthesis can suppress HCC. In this study, we blocked the sterol regulatory element-binding protein (SREBP) pathway, one of the key determinants of lipid homeostasis, by ablating 78-kDa cell-surface glycoprotein or SREBP cleavage-activating protein in hepatocytes, as well as by administering a chemical compound called betulin. We found that either genetically or pharmacologically inhibiting the SREBP pathway dramatically reduced diethylnitrosamine-induced HCC progression by down-regulating tumor-promoting cytokines, including interleukin (IL)-6, tumor necrosis factor alpha, and IL-1β.

CONCLUSION

Inhibition of de novo lipid biosynthesis by suppressing the SREBP pathway prevents HCC. This study identifies a previously underappreciated role of the SREBP pathway in HCC and suggests a novel metabolic strategy to control liver cancer. (Hepatology 2017;65:1936-1947).

Membrane topology of human NPC1L1, a key protein in enterohepatic cholesterol absorption

The Niemann-Pick C1 Like 1 (NPC1L1) is a predicted polytopic membrane protein that is critical for cholesterol absorption. NPC1L1 takes up free cholesterol into cells through vesicular endocytosis. Ezetimibe, a clinically used cholesterol absorption inhibitor, blocks the endocytosis of NPC1L1 thereby inhibiting cholesterol uptake. Human NPC1L1 is a 1,332-amino acid protein with a putative sterol-sensing domain (SSD) that shows sequence homo-logy to HMG-CoA reductase (HMGCR), Niemann-Pick C1 (NPC1), and SREBP cleavage-activating protein (SCAP). Here, we use protease protection and immunofluorescence in selectively permeabilized cells to study the topology of human NPC1L1. Our data indicate that NPC1L1 contains 13 transmembrane helices. The NH2-terminus of NPC1L1 is in the lumen while the COOH-terminus projects to the cytosol. human NPC1L1 contains seven small cytoplasmic loops--four small and three large luminal loops--one of which has been reported to bind ezetimibe. Ezetimibe-glucuronide, the major metabolite of ezetimibe in vivo, can block the internalization of NPC1L1 and cholesterol. The membrane topology of NPC1L1 is similar to that of NPC1, and the putative SSD of NPC1L1 is oriented in the same manner as those of HMGCR, NPC1, and SCAP. The defined topology of NPC1L1 provides necessary information for further dissecting the functions of the different domains of NPC1L1.

Molecular characterization of the NPC1L1 variants identified from cholesterol low absorbers

Niemann-Pick C1-like 1 (NPC1L1) is an essential protein for dietary cholesterol absorption. Nonsynonymous (NS) variants of NPC1L1 in humans have been suggested to associate with cholesterol absorption variations. However, information concerning the characteristics and mechanism of these variants in cholesterol uptake is limited. In this study, we analyzed the cholesterol uptake ability of the 19 reported NS variants of NPC1L1 identified from cholesterol low absorbers. Among these variants, L110F, R306C, A395V, G402S, T413M, R693C, R1214H, and R1268H could partially mediate cellular cholesterol uptake and were categorized as partially dysfunctional variants. The other 11 variants including T61M, N132S, D398G, R417W, G434R, T499M, S620C, I647N, G672R, S881L, and R1108W could barely facilitate cholesterol uptake, and were classified into the severely dysfunctional group. The partially dysfunctional variants showed mild defects in one or multiple aspects of cholesterol-regulated recycling, subcellular localization, glycosylation, and protein stability. The severely dysfunctional ones displayed remarkable defects in all these aspects and were rapidly degraded through the ER-associated degradation (ERAD) pathway. In vivo analyses using adenovirus-mediated expression in mouse liver confirmed that the S881L variant failed to localize to liver canalicular membrane, and the mice showed defects in biliary cholesterol re-absorption, while the G402S variant appeared to be similar to wild-type NPC1L1 in mouse liver. This study suggests that the dysfunction of the 19 variants on cholesterol absorption is due to the impairment of recycling, subcellular localization, glycosylation, or stability of NPC1L1.

Requirement of myosin Vb.Rab11a.Rab11-FIP2 complex in cholesterol-regulated translocation of NPC1L1 to the cell surface

Niemann-Pick C1-like 1 (NPC1L1) plays a critical role in the enterohepatic absorption of free cholesterol. Cellular cholesterol depletion induces the transport of NPC1L1 from the endocytic recycling compartment to the plasma membrane (PM), and cholesterol replenishment causes the internalization of NPC1L1 together with cholesterol via clathrin-mediated endocytosis. Although NPC1L1 has been characterized, the other proteins involved in cholesterol absorption and the endocytic recycling of NPC1L1 are largely unknown. Most of the vesicular trafficking events are dependent on the cytoskeleton and motor proteins. Here, we investigated the roles of the microfilament and microfilament-associated triple complex composed of myosin Vb, Rab11a, and Rab11-FIP2 in the transport of NPC1L1 from the endocytic recycling compartment to the PM. Interfering with the dynamics of the microfilament by pharmacological treatment delayed the transport of NPC1L1 to the cell surface. Meanwhile, inactivation of any component of the myosin Vb.Rab11a.Rab11-FIP2 triple complex inhibited the export of NPC1L1. Expression of the dominant-negative mutants of myosin Vb, Rab11a, or Rab11-FIP2 decreased the cellular cholesterol uptake by blocking the transport of NPC1L1 to the PM. These results suggest that the efficient transport of NPC1L1 to the PM is dependent on the microfilament-associated myosin Vb.Rab11a.Rab11-FIP2 triple complex.

Oral administration of cholera toxin B subunit conjugated to myelin basic protein protects against experimental autoimmune encephalomyelitis by inducing transforming growth factor-beta-secreting cells and suppressing chemokine expression

The efficacy and mechanism of immunosuppression against experimental autoimmune encephalomyelitis (EAE) by oral low-dose administration of myelin basic protein (MBP) conjugated to cholera toxin B subunit (CTB) were investigated in Lewis rats immunized with MBP together with complete Freund's adjuvant 4 days before the start of treatment. Oral treatment with CTB-MBP conjugate gave almost complete protection against disease, an effect that was totally abrogated by including a low dose of cholera holotoxin (CT). The protection by CTB-MBP was associated with a dramatic reduction in the number of leukocytes staining for CD4, CD8, IL-2R or MHC class II in the spinal cord as examined by immunohistochemistry. The mRNA expressions of T(h)1 cytokines IFN-gamma, IL-12 and tumor necrosis factor-alpha, as well as of chemokines monocyte chemotactic protein (MCP)-1 and RANTES in the spinal cord were also reduced by 76-94%, as assessed by in situ hybridization. In contrast, transforming growth factor (TGF)-beta mRNA-expressing cells were strongly increased in the spinal cord from animals treated orally with the CTB-MBP conjugate. In the draining peripheral lymph nodes, the number of MBP-specific TGF-beta mRNA-expressing cells was also increased, whereas there was a decrease in cells expressing T(h)1 or T(h)2 cytokine mRNA. Protection against EAE could be transferred by injection of cells from the mesenteric lymph nodes of animals fed with CTB-MBP into naive animals exposed to encephalitogenic T cells. The results indicate that the protective anti-inflammatory effect by oral treatment with CTB-MBP conjugate is, to a large extent, due to the induction of TGF-beta-secreting suppressive-regulatory T cells and to local down-regulation of MCP-1 and RANTES in the spinal cord.

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.

Human acyl-CoA:cholesterol acyltransferase 2 gene expression in intestinal Caco-2 cells and in hepatocellular carcinoma

Humans express two ACAT (acyl-CoA:cholesterol acyltransferase) genes, ACAT1 and ACAT2. ACAT1 is ubiquitously expressed, whereas ACAT2 is primarily expressed in intestinal mucosa and plays an important role in intestinal cholesterol absorption. To investigate the molecular mechanism(s) responsible for the tissue-specific expression of ACAT2, we identified five cis-elements within the human ACAT2 promoter, four for the intestinal-specific transcription factor CDX2 (caudal type homeobox transcription factor 2), and one for the transcription factor HNF1alpha (hepatocyte nuclear factor 1alpha). Results of luciferase reporter and electrophoretic mobility shift assays show that CDX2 and HNF1alpha exert a synergistic effect, enhancing the ACAT2 promoter activity through binding to these cis-elements. In undifferentiated Caco-2 cells, the ACAT2 expression is increased when exogenous CDX2 and/or HNF1alpha are expressed by co-transfection. In differentiated Caco-2 cells, the ACAT2 expression significantly decreases when the endogenous CDX2 or HNF1alpha expression is suppressed by using RNAi (RNA interference) technology. The expression levels of CDX2, HNF1alpha, and ACAT2 are all greatly increased when the Caco-2 cells differentiate to become intestinal-like cells. These results provide a molecular mechanism for the tissue-specific expression of ACAT2 in intestine. In normal adult human liver, CDX2 expression is not detectable and the ACAT2 expression is very low. In the hepatoma cell line HepG2 the CDX2 expression is elevated, accounting for its elevated ACAT2 expression. A high percentage (seven of fourteen) of liver samples from patients affected with hepatocellular carcinoma exhibited elevated ACAT2 expression. Thus, the elevated ACAT2 expression may serve as a new biomarker for certain form(s) of hepatocellular carcinoma.

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.