Inhibition of acyl-coenzyme A:cholesterol acyltransferase 2 (ACAT2) prevents dietary cholesterol-associated steatosis by enhancing hepatic triglyceride mobilization

Lipidomics reveals serum lipid metabolism disorders in CTD-induced liver injury

BACKGROUND

Cantharidin (CTD), the main toxic component of Mylabris, has been extensively used for tumor treatment in recent years. CTD-induced liver toxicity has attracted significant interest in clinic.

METHODS

In this study, biochemical parameters and liver pathological changes were analyzed after CTD was administered to mice by gavage. Subsequently, a lipidomic approach was used to investigate serum lipid metabolism disorders, and the mechanism underlying CTD-induced liver injury in mice was explored.

RESULTS

The results showed that the levels of TC and LDL-C were significantly increased after CTD intervention. Besides, pathological results showed inflammatory cell infiltration and hepatocyte necrosis in the liver. Furthermore, lipidomics found that a total of 18 lipid metabolites were increased and 40 were decreased, including LPC(20:4), LPC(20:3), PC(22:6e/2:0), PE(14:0e/21:2), PC(18:2e/22:6), glycerophospholipids, CE(16:0), CE(18:0) Cholesterol esters and TAG(12:0/12:0/22:3), TAG(16:1/16:2/20:4), TAG(18:1/18:1/20:0), TAG(16:2/18:2/18:2), TAG(18:0/18:0/20:0), TAG(13:1/19:0/19:0) glycerolipids. Metabolic pathway analysis found that glycerophospholipid, glycerol ester and glycosylphosphatidylinositol (GPI)-anchored biosynthetic metabolic pathways were dysregulated and the increase in PE caused by glycophoric metabololism and GPI may be the source of lipid metabolism disorders caused by CTD. Overall, the present study provided new insights into the mechanism of CTD-induced liver injury and increased drug safety during clinical application.

[Response to Primary Biliary Cholangitis Treatment: Influencing Factors and the Role in Prognosis Prediction]

Objective

To examine the influencing factors and prognostic features of poor response to ursodeoxycholic acid (UDCA) treatment in primary biliary cholangitis (PBC) patients with dyslipidemia.

Methods

A retrospective study was conducted, covering 512 patients who had a confirmed diagnosis of PBC, and who received treatment at West China Hospital, Sichuan University between January 2009 and March 2022. According to their actual response to UDCA treatment, patients were divided into two groups, UDCA full-response group ( n=305) and UDCA non-responding group ( n=207). The data from the two groups were compared to predict the adverse factors influencing patient response and the area under the curve ( AUC) of the receiver operating characteristic (ROC) curve, identify the cut-off value of total cholesterol (TC), and analyze the differences in baseline laboratory test findings and the rate of responses to treatment. According to the TC cut-off value, patients were divided into a group with TC≥5.415 mmol/L and another group with TC<5.415 mmol/L. In addition, differences in the prognosis of the two groups were assessed by comparing the UK-PBC and GLOBE scores.

Results

The baseline data, including alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bilirubin (TB), alkaline phosphatase (ALP), gamma-glutamyl transpeptidase (GGT), triglycerides (TG), TC, high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C), were significantly increased in the UDCA non-responding group compared to those in the full-response group (all P<0.005), while the albumin level of the UDCA non-responding group was decreased compared to that of the full-response group ( P=0.012). Findings of multi-factor logistic regression analysis suggested that TC (odds ratio [ OR]=1.501, 95% confidence interval [ CI]: 1.275-1.767, P<0.01) and ALP ( OR=1.005, 95% CI: 1.003-1.006, P<0.01) were independent risk factors influencing patient response. The ROC curve analysis suggested worse prognosis for patients with TC≥5.415 mmol/L ( AUC: 0.727, 95% CI: 0.680-0.775, 63.8% sensitivity, 76.4% specificity). In addition, the UK-PBC risk score at 1 year of treatment was higher in the high-TC group (TC≥5.415 mmol/L) than that in the low-TC group (TC<5.415 mmol/L) ( P<0.05).

Conclusions

Hypercholesterolemia is an independent risk factor for poor response to UDCA in PBC patients. When the baseline TC is equal to or higher than 5.415 mmol/L, PBC patients have a relatively poor response to UDCA and poor prognosis.

Soat2 ties cholesterol metabolism to β-oxidation and glucose tolerance in male mice

BACKGROUND

Sterol O-acyltransferase 2 (Soat2) encodes acyl-coenzyme A:cholesterol acyltransferase 2 (ACAT2), which synthesizes cholesteryl esters in hepatocytes and enterocytes fated either to storage or to secretion into nascent triglyceride-rich lipoproteins.

OBJECTIVES

We aimed to unravel the molecular mechanisms leading to reduced hepatic steatosis when Soat2 is depleted in mice.

METHODS

Soat2-/- and wild-type mice were fed a high-fat, a high-carbohydrate, or a chow diet, and parameters of lipid and glucose metabolism were assessed.

RESULTS

Glucose, insulin, homeostatic model assessment for insulin resistance (HOMA-IR), oral glucose tolerance (OGTT), and insulin tolerance tests significantly improved in Soat2-/- mice, irrespective of the dietary regimes (2-way ANOVA). The significant positive correlations between area under the curve (AUC) OGTT (r = 0.66, p < 0.05), serum fasting insulin (r = 0.86, p < 0.05), HOMA-IR (r = 0.86, p < 0.05), Adipo-IR (0.87, p < 0.05), hepatic triglycerides (TGs) (r = 0.89, p < 0.05), very-low-density lipoprotein (VLDL)-TG (r = 0.87, p < 0.05) and the hepatic cholesteryl esters in wild-type mice disappeared in Soat2-/- mice. Genetic depletion of Soat2 also increased whole-body oxidation by 30% (p < 0.05) compared to wild-type mice.

CONCLUSION

Our data demonstrate that ACAT2-generated cholesteryl esters negatively affect the metabolic control by retaining TG in the liver and that genetic inhibition of Soat2 improves liver steatosis via partitioning of lipids into secretory (VLDL-TG) and oxidative (fatty acids) pathways.

Hepatic cholesterol transport and its role in non-alcoholic fatty liver disease and atherosclerosis

Non-alcoholic fatty liver disease (NAFLD) is a quickly emerging global health problem representing the most common chronic liver disease in the world. Atherosclerotic cardiovascular disease represents the leading cause of mortality in NAFLD patients. Cholesterol metabolism has a crucial role in the pathogenesis of both NAFLD and atherosclerosis. The liver is the major organ for cholesterol metabolism. Abnormal hepatic cholesterol metabolism not only leads to NAFLD but also drives the development of atherosclerotic dyslipidemia. The cholesterol level in hepatocytes reflects the dynamic balance between endogenous synthesis, uptake, esterification, and export, a process in which cholesterol is converted to neutral cholesteryl esters either for storage in cytosolic lipid droplets or for secretion as a major constituent of plasma lipoproteins, including very-low-density lipoproteins, chylomicrons, high-density lipoproteins, and low-density lipoproteins. In this review, we describe decades of research aimed at identifying key molecules and cellular players involved in each main aspect of hepatic cholesterol metabolism. Furthermore, we summarize the recent advances regarding the biological processes of hepatic cholesterol transport and its role in NAFLD and atherosclerosis.

Efficacy of nobiletin in improving hypercholesterolemia and nonalcoholic fatty liver disease in high-cholesterol diet-fed mice

BACKGROUND/OBJECTIVES

Nobiletin (NOB), a citrus flavonoid, is reported to have beneficial effects on cardiovascular and metabolic health. However, there is limited research investigating the effect of long-term supplementation with low-dose NOB on high-cholesterol diet (HCD)-induced hypercholesterolemia and non-obese nonalcoholic fatty liver disease (NAFLD). Therefore, we investigated the influence of NOB on hypercholesterolemia and NAFLD in HCD-fed mice.

SUBJECTS/METHODS

C57BL/6J mice were fed a normal diet (ND) or HCD (35 kcal% fat, 1.25% cholesterol, 0.5% cholic acid) with or without NOB (0.02%) for 20 weeks.

RESULTS

HCD feeding markedly reduced the final body weight compared to ND feeding, with no apparent energy intake differences. NOB supplementation suppressed HCD-induced weight loss without altering energy intake. Moreover, NOB significantly decreased the total cholesterol (TC) levels and the low-density lipoprotein (LDL)/very-LDL-cholesterol to TC ratio, and increased the high-density lipoprotein-cholesterol/TC ratio in plasma, compared to those for HCD feeding alone. The plasma levels of inflammatory and atherosclerosis markers (C-reactive protein, oxidized LDL, interleukin [IL]-1β, IL-6, and plasminogen activator inhibitor-1) were significantly lower, whereas those of anti-atherogenic adiponectin and paraoxonase were higher in the NOB-supplemented group than in the HCD control group. Furthermore, NOB significantly decreased liver weight, hepatic cholesterol and triglyceride contents, and lipid droplet accumulation by inhibiting messenger RNA expression of hepatic genes and activity levels of cholesterol synthesis-, esterification-, and fatty acid synthesis-associated enzymes, concomitantly enhancing fatty acid oxidation-related gene expression and enzyme activities. Dietary NOB supplementation may protect against hypercholesterolemia and NAFLD via regulation of hepatic lipid metabolism in HCD-fed mice; these effects are associated with the amelioration of inflammation and reductions in the levels of atherosclerosis-associated cardiovascular markers.

CONCLUSIONS

The present study suggests that NOB may serve as a potential therapeutic agent for the treatment of HCD-induced hypercholesterolemia and NAFLD.

Hepatic Synaptotagmin 1 is involved in the remodelling of liver plasma- membrane lipid composition and gene expression in male Apoe-deficient mice consuming a Western diet

BACKGROUND AND AIMS

The molecular mechanisms by which the liver develops steatotic disease still remain unclear. Previous studies using nutritional and genetic models of hepatic steatosis in mice showed that liver synaptotagmin 1 (Syt1) expression was associated with lipid droplet area. Hepatic Syt1 overexpression was used as a tool to explore its effect on hepatic and plasma lipids.

METHODS AND RESULTS

To find out a cause-effect, hepatic mouse Syt1 mRNA was cloned into a vector driving hepatocyte-specific expression and administered by hydrodynamic injection to male Apoe-deficient mice fed on a Western diet, the latter as a model of rapid spontaneous steatosis development. Hepatic microsomal, large vesicle, lysosomal and plasma membrane fractions were enriched in SYT1 protein following gene overexpression. In these conditions, very low density lipoprotein esterified cholesterol increased. Likewise, the transgene caused an alteration in lipid droplet surface and a positive correlation between Syt1 expression and hepatic total cholesterol content. A lipidomic approach evidenced a decrease in lysophosphatidylcholine, phosphatidylcholine and triglycerides in isolated plasma membrane fraction. Expressions of genes involved in biosynthesis of bile acids, fatty acid metabolism, lipoprotein dynamics and vesicular transport were modified by the increased SYT1 expression.

CONCLUSIONS

These results indicate that this protein is involved in hepatic management of lipids and in the regulation of genes involved in lipid metabolism.

New Terpendole Congeners, Inhibitors of Sterol O-Acyltransferase, Produced by Volutella citrinella BF-0440

New terpendoles N-P (1–3) were isolated along with 8 structurally related known compounds including terpendoles and voluhemins from a culture broth of the fungus Volutella citrinella BF-0440. The structures of 1–3 were elucidated using various spectroscopic experiments including 1D- and 2D-NMR. All compounds 1–3 contained a common indole–diterpene backbone. Compounds 2 and 3 had 7 and 6 consecutive ring systems with an indole ring, respectively, whereas 1 had a unique indolinone plus 4 consecutive ring system. Compounds 2 and 3 inhibited both sterol O-acyltransferase 1 and 2 isozymes, but 1 lost the inhibitory activity. Structure–activity relationships of fungal indole–diterpene compounds are discussed.

Generation of new hepatocyte-like in vitro models better resembling human lipid metabolism

In contrast to human hepatocytes in vivo, which solely express acyl-coenzyme A:cholesterol acyltransferase (ACAT) 2, both ACAT1 and ACAT2 (encoded by SOAT1 and SOAT2) are expressed in primary human hepatocytes and in human hepatoma cell lines. Here, we aimed to create hepatocyte-like cells expressing the ACAT2, but not the ACAT1, protein to generate a model that - at least in this regard - resembles the human condition in vivo and to assess the effects on lipid metabolism. Using the Clustered Regularly Interspaced Short Palindromic Repeats technology, we knocked out SOAT1 in HepG2 and Huh7.5 cells. The wild type and SOAT2-only-cells were cultured with fetal bovine or human serum and the effects on lipoprotein and lipid metabolism were studied. In SOAT2-only-HepG2 cells, increased levels of cholesterol, triglycerides, apolipoprotein B and lipoprotein(a) in the cell media were detected; this was likely dependent of the increased expression of key genes involved in lipid metabolism (e.g. MTP, APOB, HMGCR, LDLR, ACACA, and DGAT2). Opposite effects were observed in SOAT2-only-Huh7.5 cells. Our study shows that the expression of SOAT1 in hepatocyte-like cells contributes to the distorted phenotype observed in HepG2 and Huh7.5 cells. As not only parameters of lipoprotein and lipid metabolism but also some markers of differentiation/maturation increase in the SOAT2-only-HepG2 cells cultured with HS, this cellular model represent an improved model for studies of lipid metabolism.

Differential gene expression in bovine endometrial epithelial cells after challenge with LPS; specific implications for genes involved in embryo maternal interactions

Lipopolysaccharide (LPS) expressed on the surface of Gram-negative bacteria activates pro-inflammatory pathways, dys-regulates the function of endometrial cells and is a key player in the mechanisms involved in endometritis. This study aimed to investigate the effects of LPS on bovine endometrial epithelial cells (bEEC) from whole transcriptome with a special focus on genes involved in embryo-maternal interactions. Following in vitro culture, bEEC from three cows were exposed to 0, 2, and 8 μg/mL LPS for 24h. RNA samples extracted at 0 and 24 hours were analyzed by RNA sequencing (RNA-seq). At 24h, 2035 differentially expressed genes (DEGs) were identified between controls and samples treated with 2 μg/mL LPS. Gene ontology analysis showed that over-expressed DEGs were associated to immune response, response to stress and external stimuli, catalytic activity, and cell cycle. Genes associated with cell membrane and cell adhesion pathways were under-expressed. LPS induced changes in expression of specific genes related to embryo-maternal interactions including under-expression of eight members of the cadherin superfamily, over-expression of six members of the mucin family, and differential expression of a large set of genes binding the above molecules and of more than 20 transcripts coding for cytokines and their receptors. Type I interferon-τ dependent genes were also over-expressed. From a sub-set of 19 genes, (biological replicates of bEEC from cows taken at time 6 (n = 3), 24 (n = 6) and 48 hours (n = 3), and 2 technical replicates per sample) differential gene expression was confirmed by RT²-qPCR (r² between fold changes at 24 hours by RT²-qPCR and RNA-seq = 0.97). These results indicate that LPS affects the function of bEEC in many ways by differential transcription, glycolytic metabolism and oxidative stress. Many transcriptomic signatures related to implantation and embryo maternal interactions were strongly affected by LPS. These results pave the way for further studies to investigate the duration of these changes and their possible impact on endometrial function and fertility.

Lipid Droplet Formation and Lipophagy in Fatty Liver Disease

Lipid droplets (LDs) are key sites of neutral lipid storage that can be found in all cells. Metabolic imbalances between the synthesis and degradation of LDs can result in the accumulation of significant amounts of lipid deposition, a characteristic feature of hepatocytes in patients with fatty liver disease, a leading indication for liver transplant in the United States. In this review, the authors highlight new literature related to the synthesis and autophagic catabolism of LDs, discussing key proteins and machinery involved in these processes. They also discuss recent findings that have revealed novel genetic risk factors associated with LD biology that contribute to lipid retention in the diseased liver.

Genetic depletion of Soat2 diminishes hepatic steatosis via genes regulating de novo lipogenesis and by GLUT2 protein in female mice

Depletion of the cholesterol esterifying enzyme acyl-Coenzyme A: cholesterol acyltransferase 2 (ACAT2, encoded by Soat2) protects mice from atherosclerosis, diet-induced hypercholesterolemia, and hepatic steatosis when fed high-cholesterol diet. The glucose transporter 2 (GLUT2) represents the main gate of glucose uptake by the liver. Lipid synthesis from glucose (de novo lipogenesis; DNL) plays a pivotal role in the development of hepatic steatosis. Inhibition of DNL is a successful approach to reverse hepatic steatosis, as shown by different studies in mice and humans. Here we aimed to investigate whether depletion of Soat2 per se can reduce hepatic steatosis, also in the presence of very low levels of cholesterol in the diet, and the underlying mechanisms. Female Soat2-/- and wild type mice were either fed high-fat or high-carbohydrate diet and both contained <0.05% (w/w) cholesterol. Analysis in serum, liver, muscles and adipose tissues were performed. We found Soat2-/- mice fed high-fat, low-cholesterol diet to have less hepatic steatosis, decreased expression of genes involved in DNL and lower hepatic GLUT2. Similar findings were found in Soat2-/- mice fed high-carbohydrate, low-cholesterol diet. CONCLUSION: Depletion of Soat2 reduces hepatic steatosis independently of the presence of high levels of cholesterol in the diet. Our study provides a link between hepatic cholesterol esterification, DNL, and GLUT2.

Inhibition of Adenosine Monophosphate-Activated Protein Kinase-3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase Signaling Leads to Hypercholesterolemia and Promotes Hepatic Steatosis and Insulin Resistance

Adenosine monophosphate-activated protein kinase (AMPK) regulates multiple signaling pathways involved in glucose and lipid metabolism in response to changes in hormonal and nutrient status. Cell culture studies have shown that AMPK phosphorylation and inhibition of the rate-limiting enzyme in the mevalonate pathway 3-hydroxy-3-methylglutaryl (HMG) coenzyme A (CoA) reductase (HMGCR) at serine-871 (Ser871; human HMGCR Ser872) suppresses cholesterol synthesis. In order to evaluate the role of AMPK-HMGCR signaling in vivo, we generated mice with a Ser871-alanine (Ala) knock-in mutation (HMGCR KI). Cholesterol synthesis was significantly suppressed in wild-type (WT) but not in HMGCR KI hepatocytes in response to AMPK activators. Liver cholesterol synthesis and cholesterol levels were significantly up-regulated in HMGCR KI mice. When fed a high-carbohydrate diet, HMGCR KI mice had enhanced triglyceride synthesis and liver steatosis, resulting in impaired glucose homeostasis. Conclusion: AMPK-HMGCR signaling alone is sufficient to regulate both cholesterol and triglyceride synthesis under conditions of a high-carbohydrate diet. Our findings highlight the tight coupling between the mevalonate and fatty acid synthesis pathways as well as revealing a role of AMPK in suppressing the deleterious effects of a high-carbohydrate diet.

Celludinones, new inhibitors of sterol O-acyltransferase, produced by Talaromyces cellulolyticus BF-0307

New indanones, designated celludinones A ((±)-1) and B (2), were isolated from the culture broth of the fungal strain Talaromyces cellulolyticus BF-0307. The structures of celludinones were elucidated by spectroscopic data, including 1D and 2D NMR. Celludinone A was found to be a mixture of racemic isomers ((±)-1), which were isolated by a chiral column. Compounds (+)-1 and (-)-1 inhibited the sterol O-acyltransferase (SOAT) 1 and 2 isozymes in a cell-based assay using SOAT1- and SOAT2-expressing Chinese hamster ovary (CHO) cells, while 2 selectively inhibited the SOAT2 isozyme.

Adipose tissues of MPC1± mice display altered lipid metabolism-related enzyme expression levels

Mitochondrial pyruvate carrier 1 (MPC1) is a component of the MPC1/MPC2 heterodimer that facilitates the transport of pyruvate into mitochondria. Pyruvate plays a central role in carbohydrate, fatty, and amino acid catabolism. The present study examined epididymal white adipose tissue (eWAT) and intrascapular brown adipose tissue (iBAT) from MPC1^± mice following 24 weeks of feeding, which indicated low energy accumulation as evidenced by low body and eWAT weight and adipocyte volume. To characterize molecular changes in energy metabolism, we analyzed the transcriptomes of the adipose tissues using RNA-Sequencing (RNA-Seq). The results showed that the fatty acid oxidation pathway was activated and several genes involved in this pathway were upregulated. Furthermore, qPCR and western blotting indicated that numerous genes and proteins that participate in lipolysis were also upregulated. Based on these findings, we propose that the energy deficiency caused by reduced MPC1 activity can be alleviated by activating the lipolytic pathway.

Niemann-Pick C1-deficient mice lacking sterol O-acyltransferase 2 have less hepatic cholesterol entrapment and improved liver function

Cholesteryl esters are generated at multiple sites in the body by sterol O-acyltransferase (SOAT) 1 or SOAT2 in various cell types and lecithin cholesterol acyltransferase in plasma. Esterified cholesterol and triacylglycerol contained in lipoproteins cleared from the circulation via receptor-mediated or bulk-phase endocytosis are hydrolyzed by lysosomal acid lipase within the late endosomal/lysosomal (E/L) compartment. Then, through the successive actions of Niemann-Pick C (NPC) 2 and NPC 1, unesterified cholesterol (UC) is exported from the E/L compartment to the cytosol. Mutations in either NPC1 or NPC2 lead to continuing entrapment of UC in all organs, resulting in multisystem disease, which includes hepatic dysfunction and in some cases liver failure. These studies investigated primarily whether elimination of SOAT2 in NPC1-deficient mice impacted hepatic UC sequestration, inflammation, and transaminase activities. Measurements were made in 7-wk-old mice fed a low-cholesterol chow diet or one enriched with cholesterol starting 2 wk before study. In the chow-fed mice, NPC1:SOAT2 double knockouts, compared with their littermates lacking only NPC1, had 20% less liver mass, 28% lower hepatic UC concentrations, and plasma alanine aminotransferase and aspartate aminotransferase activities that were decreased by 48% and 36%, respectively. mRNA expression levels for several markers of inflammation were all significantly lower in the NPC1 mutants lacking SOAT2. The existence of a new class of potent and selective SOAT2 inhibitors provides an opportunity for exploring if suppression of this enzyme could potentially become an adjunctive therapy for liver disease in NPC1 deficiency. NEW & NOTEWORTHY In Niemann-Pick type C1 (NPC1) disease, the entrapment of unesterified cholesterol (UC) in the endosomal/lysosomal compartment of all cells causes multiorgan disease, including neurodegeneration, pulmonary dysfunction, and liver failure. Some of this sequestered UC entered cells initially in the esterified form. When sterol O-acyltransferase 2, a cholesterol esterifying enzyme present in enterocytes and hepatocytes, is eliminated in NPC1-deficient mice, there is a reduction in their hepatomegaly, hepatic UC content, and cellular injury.

Dose-Specific Effects of Di-Isononyl Phthalate on the Endocannabinoid System and on Liver of Female Zebrafish

Phthalates, used as plasticizers, have become a ubiquitous contaminant and have been reported for their potential to induce toxicity in living organisms. Among them, di-isononyl phthalate (DiNP) has been recently used to replace di(2-ethylhexyl) phthalate (DEHP). Nowadays, there is evidence that DiNP is an endocrine-disrupting chemical; however, little is known about its effects on the endocannabinoid system (ECS) and lipid metabolism. Hence, the aim of our study was to investigate the effects of DiNP on the ECS in zebrafish liver and brain and on hepatic lipid storage. To do so, adult female zebrafish were exposed to three concentrations (0.42 µg/L, 4.2 µg/L, and 42 µg/L) of DiNP via water for 3 weeks. Afterwards, we investigated transcript levels for genes involved in the ECS of the brain and liver as well as liver histology and image analysis, Fourier-transform infrared spectroscopy imaging, and measurement of endocannabinoid levels. Our results demonstrate that DiNP upregulates orexigenic signals and causes hepatosteatosis together with deregulation of the peripheral ECS and lipid metabolism. A decrease in the levels of ECS components at the central level was observed after exposure to the highest DiNP concentration tested. These findings suggest that replacement of DEHP with DiNP should be considered with caution because of observed adverse DiNP effects on aquatic organisms.

Shunts, channels and lipoprotein endosomal traffic: a new model of cholesterol homeostasis in the hepatocyte

The liver directs cholesterol metabolism in the organism. All the major fluxes of cholesterol within the body involve the liver: dietary cholesterol is directed to the liver; cholesterol from peripheral cells goes to the liver; the liver is a major site of cholesterol synthesis for the organism; cholesterol is secreted from the liver within the bile, within apoB lipoproteins and translocated to nascent HDL. The conventional model of cholesterol homeostasis posits that cholesterol from any source enters a common, rapidly exchangeable pool within the cell, which is in equilibrium with a regulatory pool. Increased influx of cholesterol leads rapidly to decreased synthesis of cholesterol. This model was developed based on in vitro studies in the fibroblast and validated only for LDL particles. The challenges the liver must meet in vivo to achieve cholesterol homeostasis are far more complex. Our model posits that the cholesterol derived from three different lipoproteins endosomes has three different fates: LDL-derived cholesterol is largely recycled within VLDL with most of the cholesterol shunted through the hepatocyte without entering the exchangeable pool of cholesterol; high density lipoprotein-derived CE is transcytosed into bile; and chylomicron remnant-derived cholesterol primarily enters the regulatory pool within the hepatocyte. These endosomal channels represent distinct physiological pathways and hepatic homeostasis represents the net result of the outcomes of these distinct channels. Our model takes into account the distinct physiological challenges the hepatocyte must meet, underlie the pathophysiology of many of the apoB dyslipoproteinemias and account for the sustained effectiveness of therapeutic agents such as statins.

Apigenin Ameliorates Dyslipidemia, Hepatic Steatosis and Insulin Resistance by Modulating Metabolic and Transcriptional Profiles in the Liver of High-Fat Diet-Induced Obese Mice

Several in vitro and in vivo studies have reported the anti-inflammatory, anti-diabetic and anti-obesity effects of the flavonoid apigenin. However, the long-term supplementary effects of low-dose apigenin on obesity are unclear. Therefore, we investigated the protective effects of apigenin against obesity and related metabolic disturbances by exploring the metabolic and transcriptional responses in high-fat diet (HFD)-induced obese mice. C57BL/6J mice were fed an HFD or apigenin (0.005%, w/w)-supplemented HFD for 16 weeks. In HFD-fed mice, apigenin lowered plasma levels of free fatty acid, total cholesterol, apolipoprotein B and hepatic dysfunction markers and ameliorated hepatic steatosis and hepatomegaly, without altering food intake and adiposity. These effects were partly attributed to upregulated expression of genes regulating fatty acid oxidation, tricarboxylic acid cycle, oxidative phosphorylation, electron transport chain and cholesterol homeostasis, downregulated expression of lipolytic and lipogenic genes and decreased activities of enzymes responsible for triglyceride and cholesterol ester synthesis in the liver. Moreover, apigenin lowered plasma levels of pro-inflammatory mediators and fasting blood glucose. The anti-hyperglycemic effect of apigenin appeared to be related to decreased insulin resistance, hyperinsulinemia and hepatic gluconeogenic enzymes activities. Thus, apigenin can ameliorate HFD-induced comorbidities via metabolic and transcriptional modulations in the liver.

Influence of the Alcohol Present in a Phytotherapic Tincture on Male Rat Lipid Profiles and Renal Function

This study evaluated the influence of the alcohol present in a formulation of the antiophidic phytotherapic tincture, Específico-Pessôa, on rat blood biochemical and hematological parameters, and on organ histology. Three groups of rats were treated orally for 10, 15, or 30 days; one group received the tincture, the other received alcohol alone, and the third was a control group. The results of this study indicated that cholesterol levels were significantly increased after 10 days in the alcohol and tincture groups, although these decreased after 30 days in the tincture group. Triglyceride levels were significantly reduced after 15 days in the tincture group and after 30 days in the alcohol and tincture groups. A higher creatinine level was observed in the alcohol and tincture groups after 15 and 30 days. The uric acid levels in these groups were reduced at 10 and 30 days, although this metabolite was elevated at 15 days in the alcohol group. Hydropic multifocal degeneration with lymphohistiocytic infiltration and some polymorphonuclear cells was observed in the livers of rats treated with either the tincture or alcohol. These data demonstrate the importance of considering the potential actions of the alcohol present in pharmaceutical formulations.

Sex-Specific Differences in Hepatic Fat Oxidation and Synthesis May Explain the Higher Propensity for NAFLD in Men

CONTEXT AND OBJECTIVE

In most populations a greater proportion of men have hepatic steatosis than women. Sex-specific differences in hepatic dietary fatty acid (FA) metabolism have not been well characterized. We compared fasting and postprandial hepatic FA synthesis (de novo lipogenesis [DNL]) and oxidation in men and women.

PARTICIPANTS AND METHODS

Fasting and postprandial hepatic FA metabolism was studied in 22 healthy men (n = 11) and women with similar age, body mass index, and liver fat content using metabolic substrates labeled with stable-isotope tracers ((2)H2O and [U(13)C]palmitate). Dietary FA oxidation was assessed by appearance of (13)C into plasma 3-hydroxybutyrate and breath CO2 as markers of liver and whole-body FA oxidation, respectively.

RESULTS

Despite similar liver fat content, fasting and postprandial plasma triacylglycerol (TG) concentrations were significantly (P < .05) higher in men compared with women. The appearance of (13)C from dietary FA into plasma 3-hydroxybutyrate and breath CO2 was greater (P < .05) in women compared with men. Although the contribution of DNL into very low-density lipoprotein (VLDL)-TG was similar (∼ 10%) in the fasting state, there was a divergence in pattern over the course of the study, with men maintaining a higher contribution of DNL to VLDL-TG than women (P = .006 time x sex interaction).

CONCLUSIONS

The combination of lower dietary FA oxidation and a prolonged increase in DNL observed in men may represent partitioning of FA into esterification and storage pathways within the liver, leading to greater VLDL-TG production, and predispose to the sex difference in hepatic steatosis.

PRD125, a potent and selective inhibitor of sterol O-acyltransferase 2 markedly reduces hepatic cholesteryl ester accumulation and improves liver function in lysosomal acid lipase-deficient mice

In most organs, the bulk of cholesterol is unesterified, although nearly all possess a varying capability of esterifying cholesterol through the action of either sterol O-acyltransferase (SOAT) 1 or, in the case of hepatocytes and enterocytes, SOAT2. Esterified cholesterol (EC) carried in plasma lipoproteins is hydrolyzed by lysosomal acid lipase (LAL) when they are cleared from the circulation. Loss-of-function mutations in LIPA, the gene that encodes LAL, result in Wolman disease or cholesteryl ester storage disease (CESD). Hepatomegaly and a massive increase in tissue EC levels are hallmark features of both disorders. While these conditions can be corrected with enzyme replacement therapy, the question arose as to whether pharmacological inhibition of SOAT2 might reduce tissue EC accretion in CESD. When weaned at 21 days, Lal(-/-) mice, of either gender, had a whole liver cholesterol content that was 12- to 13-fold more than that of matching Lal(+/+) littermates (23 versus 1.8 mg, respectively). In Lal(-/-) males given the selective SOAT2 inhibitor PRD125 1,11-O-o-methylbenzylidene-7-O-p-cyanobenzoyl-1,7,11-trideacetylpyripyropene A in their diet (∼10 mg/day per kg body weight) from 21 to 53 days, whole liver cholesterol content was 48.6 versus 153.7 mg in untreated 53-day-old Lal(-/-) mice. This difference reflected a 59% reduction in hepatic EC concentration (mg/g), combined with a 28% fall in liver mass. The treated mice also showed a 63% reduction in plasma alanine aminotransferase activity, in parallel with decisive falls in hepatic mRNA expression levels for multiple proteins that reflect macrophage presence and inflammation. These data implicate SOAT2 as a potential target in CESD management.

Hepatic overexpression of methionine sulfoxide reductase A reduces atherosclerosis in apolipoprotein E-deficient mice

Methionine sulfoxide reductase A (MsrA), a specific enzyme that converts methionine-S-sulfoxide to methionine, plays an important role in the regulation of protein function and the maintenance of redox homeostasis. In this study, we examined the impact of hepatic MsrA overexpression on lipid metabolism and atherosclerosis in apoE-deficient (apoE^−/−) mice. In vitro study showed that in HepG2 cells, lentivirus-mediated human MsrA (hMsrA) overexpression upregulated the expression levels of several key lipoprotein-metabolism-related genes such as liver X receptor α, scavenger receptor class B type I, and ABCA1. ApoE^−/− mice were intravenously injected with lentivirus to achieve high-level hMsrA expression predominantly in the liver. We found that hepatic hMsrA expression significantly reduced plasma VLDL/LDL levels, improved plasma superoxide dismutase, and paraoxonase-1 activities, and decreased plasma serum amyloid A level in apoE^−/− mice fed a Western diet, by significantly altering the expression of several genes in the liver involving cholesterol selective uptake, conversion and excretion into bile, TG biosynthesis, and inflammation. Moreover, overexpression of hMsrA resulted in reduced hepatic steatosis and aortic atherosclerosis. These results suggest that hepatic MsrA may be an effective therapeutic target for ameliorating dyslipidemia and reducing atherosclerosis-related cardiovascular diseases.

Antisense oligonucleotides, microRNAs, and antibodies

The specificity of Watson-Crick base pairing and the development of several chemical modifications to oligonucleotides have enabled the development of novel drug classes for the treatment of different human diseases. This review focuses on promising results of recent preclinical or clinical studies on targeting HDL metabolism and function by antisense oligonucleotides and miRNA-based therapies. Although many hurdles regarding basic mechanism of action, delivery, specificity, and toxicity need to be overcome, promising results from recent clinical trials and recent approval of these types of therapy to treat dyslipidemia suggest that the treatment of HDL dysfunction will benefit from these unique clinical opportunities. Moreover, an overview of monoclonal antibodies (mAbs) developed for the treatment of dyslipidemia and cardiovascular disease and currently being tested in clinical studies is provided. Initial studies have shown that these compounds are generally safe and well tolerated, but ongoing large clinical studies will assess their long-term safety and efficacy.

Acyltransferase inhibitors: a patent review (2010-present)

INTRODUCTION

Acyltransferase (AT) catalyzes the transfer of an acyl moiety from acyl-coenzyme A (acyl-CoA) to an acceptor. ATs play important roles in the maintenance of homeostasis in the human body and have been linked to various diseases; therefore, several ATs have been proposed as potential targets for the treatment or prevention of such diseases. The AT family includes acyl-CoA:cholesterol AT (ACAT), diacylglycerol AT (DGAT), and monoacylglycerol AT (MGAT) for the metabolism of lipids. Furthermore, recent molecular biological studies revealed the existence of their isozymes with distinct functions in the body.

AREAS COVERED

This review summarized patent filings published between 2010 and the present date that claimed isozyme-selective inhibitors of ACAT, DGAT and MGAT, which are involved in neutral lipid metabolism.

EXPERT OPINION

Isozymes of ACAT, DGAT and MGAT play distinct functions in neutral lipid metabolism in the human body and have been considered as potential therapeutic targets. Accordingly, isozyme-selective inhibitors that could be used in the treatment or prevention of lipid metabolism disorders were searched for. Of these, pyripyropene A derivatives, ACAT2-selective inhibitors, may be potential therapeutics for the treatment of atherosclerosis, homozygous familial hypercholesterolemia and nonalcoholic fatty liver disease.

Impaired VLDL assembly: a novel mechanism contributing to hepatic lipid accumulation following ovariectomy and high-fat/high-cholesterol diets?

The aim of the present study was to identify molecular mechanisms involved in liver fat and cholesterol accumulation in ovariectomised (Ovx) rats fed with high-cholesterol diets. VLDL assembly and bile acid metabolism were specifically targeted. After being either Ovx or sham-operated, the rats were fed a standard diet or a high-fat diet containing 0, 0·25 or 0·5 % cholesterol for 6 weeks. Although Ovx rats exposed to dietary cholesterol intake accumulated the greatest amount of hepatic fat and cholesterol, plasma cholesterol levels were lower (P< 0·05) in these animals than in the corresponding control rats. Accompanying this observation, ovariectomy and dietary cholesterol intake resulted in a down-regulation (P< 0·05) of the expression of genes associated with VLDL assembly, including microsomal TAG transfer protein, diacylglycerol acyltransferase 2, acyl-CoA:cholesterol acyltransferase 2 and apoB-100 as well as genes associated with bile acid metabolism including farnesoid X receptor and bile salt export pump (P< 0·01). These results indicate that high-fat/high-cholesterol diets and ovariectomy concomitantly disrupt hepatic lipid output through defects in VLDL assembly and, most probably, secretion. The results also point to a defect in hepatic bile acid secretion. The present study offers novel insights into intrahepatic lipid metabolism, which may be relevant to metabolic complications found in postmenopausal women.

Deletion of sterol O-acyltransferase 2 (SOAT2) function in mice deficient in lysosomal acid lipase (LAL) dramatically reduces esterified cholesterol sequestration in the small intestine and liver

Sterol O-acyltransferase 2 (SOAT2), also known as ACAT2, is the major cholesterol esterifying enzyme in the liver and small intestine (SI). Esterified cholesterol (EC) carried in certain classes of plasma lipoproteins is hydrolyzed by lysosomal acid lipase (LAL) when they are cleared from the circulation. Loss-of-function mutations in LIPA, the gene that encodes LAL, result in Wolman disease (WD) or cholesteryl ester storage disease (CESD). Hepatomegaly and a massive increase in tissue EC levels are hallmark features of both disorders. While these conditions can be corrected with enzyme replacement therapy, the question arose as to what effect the loss of SOAT2 function might have on tissue EC sequestration in LAL-deficient mice. When weaned at 21 days, Lal(-)(/)(-):Soat2(+)(/)(+) mice had a whole liver cholesterol content (mg/organ) of 24.7 mg vs 1.9mg in Lal(+/+):Soat2(+/+) littermates, with almost all the excess sterol being esterified. Over the next 31 days, liver cholesterol content in the Lal(-)(/)(-):Soat2(+)(/)(+) mice increased to 145 ± 2 mg but to only 29 ± 2 mg in their Lal(-)(/)(-):Soat2(-)(/)(-) littermates. The level of EC accumulation in the SI of the Lal(-)(/)(-):Soat2(-)(/)(-) mice was also much less than in their Lal(-)(/)(-):Soat2(+)(/)(+) littermates. In addition, there was a >70% reduction in plasma transaminase activities in the Lal(-)(/)(-):Soat2(-)(/)(-) mice. These studies illustrate how the severity of disease in a mouse model for CESD can be substantially ameliorated by elimination of SOAT2 function.

Cholesterol esters (CE) derived from hepatic sterol O-acyltransferase 2 (SOAT2) are associated with more atherosclerosis than CE from intestinal SOAT2

RATIONALE

Cholesterol esters (CE), especially cholesterol oleate, generated by hepatic and intestinal sterol O-acyltransferase 2 (SOAT2) play a critical role in cholesterol homeostasis. However, it is unknown whether the contribution of intestine-derived CE from SOAT2 would have similar effects in promoting atherosclerosis progression as for liver-derived CE.

OBJECTIVE

To test whether, in low-density lipoprotein receptor null (LDLr(-/-)) mice, the conditional knockout of intestinal SOAT2 (SOAT2(SI-/SI-)) or hepatic SOAT2 (SOAT2(L-/L-)) would equally limit atherosclerosis development compared with the global deletion of SOAT2 (SOAT2(-/-)).

METHODS AND RESULTS

SOAT2 conditional knockout mice were bred with LDLr(-/-) mice creating LDLr(-/-) mice with each of the specific SOAT2 gene deletions. All mice then were fed an atherogenic diet for 16 weeks. SOAT2(SI-/SI-)LDLr(-/-) and SOAT2(-/-)LDLr(-/-) mice had significantly lower levels of intestinal cholesterol absorption, more fecal sterol excretion, and lower biliary cholesterol levels. Analysis of plasma LDL showed that all mice with SOAT2 gene deletions had LDL CE with reduced percentages of cholesterol palmitate and cholesterol oleate. Each of the LDLr(-/-) mice with SOAT2 gene deletions had lower accumulations of total cholesterol and CE in the liver compared with control mice. Finally, aortic atherosclerosis development was significantly lower in all mice with global or tissue-restricted SOAT2 gene deletions. Nevertheless, SOAT2(-/-)LDLr(-/-) and SOAT2(L-/L-)LDLr(-/-) mice had less aortic CE accumulation and smaller aortic lesions than SOAT2(SI-/SI-)LDLr(-/-) mice.

CONCLUSIONS

SOAT2-derived CE from both the intestine and liver significantly contribute to the development of atherosclerosis, although the CE from the hepatic enzyme appeared to promote more atherosclerosis development.

Hepatitis C virus infection mediates cholesteryl ester synthesis to facilitate infectious particle production

Cholesterol is a critical component of the hepatitis C virus (HCV) life cycle, as demonstrated by its accumulation within infected hepatocytes and lipoviral particles. To cope with excess cholesterol, hepatic enzymes ACAT1 and ACAT2 produce cholesteryl esters (CEs), which are destined for storage in lipid droplets or for secretion as apolipoproteins. Here we demonstrate in vitro that cholesterol accumulation following HCV infection induces upregulation of the ACAT genes and increases CE synthesis. Analysis of human liver biopsy tissue showed increased ACAT2 mRNA expression in liver infected with HCV genotype 3, compared with genotype 1. Inhibiting cholesterol esterification using the potent ACAT inhibitor TMP-153 significantly reduced production of infectious virus, but did not inhibit virus RNA replication. Density gradient analysis showed that TMP-153 treatment caused a significant increase in lipoviral particle density, suggesting reduced lipidation. These data suggest that cholesterol accumulation following HCV infection stimulates the production of CE, a major component of lipoviral particles. Inhibition of CE synthesis reduces HCV particle density and infectivity, suggesting that CEs are required for optimal infection of hepatocytes.

Synthesis, biological evaluation, and molecular docking studies of xanthone sulfonamides as ACAT inhibitors

Three series of xanthone sulfonamides were synthesized, and their inhibitory activities against acyl-CoA: cholesterol acyltransferase (ACAT) were evaluated. Results showed that most of the title compounds exhibited strong inhibitory activity against ACAT, of which compounds 1c, 1e, 1f, 2d, 2e, and 3d were proved to be more active than the positive control Sandoz 58-035. Computational docking experiments indicated that the interaction between inhibitors and ACAT contained the H-bond interaction, the hydrophobic interaction, and the narrow hydrophobic cleft.

Role of TG-interacting factor (Tgif) in lipid metabolism

TG interacting factors (Tgifs) 1 and 2 are members of the TALE (three-amino-acid loop extension) superfamily of homeodomain proteins. These two proteins bind to the same DNA sequence and share a conserved C-terminal repression domain. Mutations in TGIF1 have been linked to holoprosencephaly, which is a human genetic disease that affects craniofacial development. As these proteins can interact with the ligand binding domain of retinoid X receptor α, a common heterodimeric partner of several nuclear receptors [e.g., liver X receptors (LXRs) and peroxisome proliferator-activated receptors (PPARs)], Tgif1 and Tgif2 might repress other transcriptional pathways activated by lipids. In line with this, Tgif1 interacts with LXRα and Tgif1 null mice have increased expression of the two Lxrα target genes apolipoproteins (Apo) c2 and a4. Also, we have recently identified Tgif1 to function as a transcriptional repressor of the cholesterol esterifying enzyme acyl-coenzyme A:cholesterol acyltransferase 2 (gene name SOAT2). As no studies yet have shown involvement of Tgif2 in the lipid metabolism, this review will focus on the role of Tgif1 in lipid and cholesterol metabolism. This article is part of a Special Issue entitled: Linking transcription to physiology in lipodomics.

An approach to identify SNPs in the gene encoding acetyl-CoA acetyltransferase-2 (ACAT-2) and their proposed role in metabolic processes in pig

The novel liver protein acetyl-CoA acetyltransferase-2 (ACAT2) is involved in the beta-oxidation and lipid metabolism. Its comprehensive relative expression, in silico non-synonymous single nucleotide polymorphism (nsSNP) analysis, as well as its annotation in terms of metabolic process with another protein from the same family, namely, acetyl-CoA acyltransferase-2 (ACAA2) was performed in Sus scrofa. This investigation was conducted to understand the most important nsSNPs of ACAT2 in terms of their effects on metabolic activities and protein conformation. The two most deleterious mutations at residues 122 (I to V) and 281 (R to H) were found in ACAT2. Validation of expression of genes in the laboratory also supported the idea of differential expression of ACAT2 and ACAA2 conceived through the in silico analysis. Analysis of the relative expression of ACAT2 and ACAA2 in the liver tissue of Jeju native pig showed that the former expressed significantly higher (P<0.05). Overall, the computational prediction supported by wet laboratory analysis suggests that ACAT2 might contribute more to metabolic processes than ACAA2 in swine. Further associations of SNPs in ACAT2 with production traits might guide efforts to improve growth performance in Jeju native pigs.

Intestine-specific MTP and global ACAT2 deficiency lowers acute cholesterol absorption with chylomicrons and HDLs

Intestinal cholesterol absorption involves the chylomicron and HDL pathways and is dependent on microsomal triglyceride transfer protein (MTP) and ABCA1, respectively. Chylomicrons transport free and esterified cholesterol, whereas HDLs transport free cholesterol. ACAT2 esterifies cholesterol for secretion with chylomicrons. We hypothesized that free cholesterol accumulated during ACAT2 deficiency may be secreted with HDLs when chylomicron assembly is blocked. To test this, we studied cholesterol absorption in mice deficient in intestinal MTP, global ACAT2, and both intestinal MTP and global ACAT2. Intestinal MTP ablation significantly increased intestinal triglyceride and cholesterol levels and reduced their transport with chylomicrons. In contrast, global ACAT2 deficiency had no effect on triglyceride absorption but significantly reduced cholesterol absorption with chylomicrons and increased cellular free cholesterol. Their combined deficiency reduced cholesterol secretion with both chylomicrons and HDLs. Thus, contrary to our hypothesis, free cholesterol accumulated in the absence of MTP and ACAT2 is unavailable for secretion with HDLs. Global ACAT2 deficiency causes mild hypertriglyceridemia and reduces hepatosteatosis in mice fed high cholesterol diets by increasing hepatic lipoprotein production by unknown mechanisms. We show that this phenotype is preserved in the absence of intestinal MTP in global ACAT2-deficient mice fed a Western diet. Further, we observed increases in hepatic MTP activity in these mice. Thus, ACAT2 deficiency might increase MTP expression to avoid hepatosteatosis in cholesterol-fed animals. Therefore, ACAT2 inhibition might avert hepatosteatosis associated with high cholesterol diets by increasing hepatic MTP expression and lipoprotein production.

Acute sterol o-acyltransferase 2 (SOAT2) knockdown rapidly mobilizes hepatic cholesterol for fecal excretion

The primary risk factor for atherosclerotic cardiovascular disease is LDL cholesterol, which can be reduced by increasing cholesterol excretion from the body. Fecal cholesterol excretion can be driven by a hepatobiliary as well as a non-biliary pathway known as transintestinal cholesterol efflux (TICE). We previously showed that chronic knockdown of the hepatic cholesterol esterifying enzyme sterol O-acyltransferase 2 (SOAT2) increased fecal cholesterol loss via TICE. To elucidate the initial events that stimulate TICE, C57Bl/6 mice were fed a high cholesterol diet to induce hepatic cholesterol accumulation and were then treated for 1 or 2 weeks with an antisense oligonucleotide targeting SOAT2. Within 2 weeks of hepatic SOAT2 knockdown (SOAT2HKD), the concentration of cholesteryl ester in the liver was reduced by 70% without a reciprocal increase in hepatic free cholesterol. The rapid mobilization of hepatic cholesterol stores resulted in a ∼ 2-fold increase in fecal neutral sterol loss but no change in biliary cholesterol concentration. Acute SOAT2HKD increased plasma cholesterol carried primarily in lipoproteins enriched in apoB and apoE. Collectively, our data suggest that acutely reducing SOAT2 causes hepatic cholesterol to be swiftly mobilized and packaged onto nascent lipoproteins that feed cholesterol into the TICE pathway for fecal excretion.

Hepatic ACAT2 knock down increases ABCA1 and modifies HDL metabolism in mice

OBJECTIVES

ACAT2 is the exclusive cholesterol-esterifying enzyme in hepatocytes and enterocytes. Hepatic ABCA1 transfers unesterified cholesterol (UC) to apoAI, thus generating HDL. By changing the hepatic UC pool available for ABCA1, ACAT2 may affect HDL metabolism. The aim of this study was to reveal whether hepatic ACAT2 influences HDL metabolism.

DESIGN

WT and LXRα/β double knockout (DOKO) mice were fed a western-type diet for 8 weeks. Animals were i.p. injected with an antisense oligonucleotide targeted to hepatic ACAT2 (ASO6), or with an ASO control. Injections started 4 weeks after, or concomitantly with, the beginning of the diet.

RESULTS

ASO6 reduced liver cholesteryl esters, while not inducing UC accumulation. ASO6 increased hepatic ABCA1 protein independently of the diet conditions. ASO6 affected HDL lipids (increased UC) only in DOKO, while it increased apoE-containing HDL in both genotypes. In WT mice ASO6 led to the appearance of large HDL enriched in apoAI and apoE.

CONCLUSIONS

The use of ASO6 revealed a new pathway by which the liver may contribute to HDL metabolism in mice. ACAT2 seems to be a hepatic player affecting the cholesterol fluxes fated to VLDL or to HDL, the latter via up-regulation of ABCA1.

TG-interacting factor 1 acts as a transcriptional repressor of sterol O-acyltransferase 2

Acat2 [gene name: sterol O-acyltransferase 2 (SOAT2)] esterifies cholesterol in enterocytes and hepatocytes. This study aims to identify repressor elements in the human SOAT2 promoter and evaluate their in vivo relevance. We identified TG-interacting factor 1 (Tgif1) to function as an important repressor of SOAT2. Tgif1 could also block the induction of the SOAT2 promoter activity by hepatocyte nuclear factor 1α and 4α. Women have ∼ 30% higher hepatic TGIF1 mRNA compared with men. Depletion of Tgif1 in mice increased the hepatic Soat2 expression and resulted in higher hepatic lipid accumulation and plasma cholesterol levels. Tgif1 is a new player in human cholesterol metabolism.

The regulation of alfalfa saponin extract on key genes involved in hepatic cholesterol metabolism in hyperlipidemic rats

To investigate the cholesterol-lowering effects of alfalfa saponin extract (ASE) and its regulation mechanism on some key genes involved in cholesterol metabolism, 40 healthy 7 weeks old male Sprague Dawley (SD) rats were randomly divided into four groups with 10 rats in each group: control group, hyperlipidemic group, ASE treatment group, ASE prevention group. The body weight gain, relative liver weight and serum lipid 1evels of rats were determined. Total cholesterol (TC) and total bile acids (TBA) levels in liver and feces were also measured. Furthermore, the activity and mRNA expressions of Hmgcr, Acat2, Cyp7a1 and Ldlr were investigated. The results showed the following: (1) The abnormal serum lipid levels in hyperlipidemic rats were ameliorated by ASE administration (both ASE prevention group and treatment group) (P<0.05). (2) Both ASE administration to hyperlipidemic rats significantly reduced liver TC and increased liver TBA level (P<0.05). TC and TBA levels in feces of hyperlipidemic rats were remarkably elevated by both ASE administration (P<0.05). (3) mRNA expressions of Hmgcr and Acat2 in the liver of hyperlipidemic rats were remarkably down-regulated (P<0.05), as well as mRNA expressions of Cyp7a1 and Ldlr were dramatically up-regulated by both ASE administration (P<0.05). The activities of these enzymes also paralleled the observed changes in mRNA levels. (4) There was no significant difference between ASE treatment and ASE prevention group for most parameters evaluated. Our present study indicated that ASE had cholesterol-lowering effects. The possible mechanism could be attributed to (1) the down-regulation of Hmgcr and Acat2, as well as up-regulation of Cyp7a1 and Ldlr in the liver of hyperlipidemic rats, which was involved in cholesterol biosynthesis, uptake, and efflux pathway; (2) the increase in excretion of cholesterol. The findings in our study suggested ASE had great potential usefulness as a natural agent for treating hyperlipidemia.

Reduction of VLDL secretion decreases cholesterol excretion in niemann-pick C1-like 1 hepatic transgenic mice

An effective way to reduce LDL cholesterol, the primary risk factor of atherosclerotic cardiovascular disease, is to increase cholesterol excretion from the body. Our group and others have recently found that cholesterol excretion can be facilitated by both hepatobiliary and transintestinal pathways. However, the lipoprotein that moves cholesterol through the plasma to the small intestine for transintestinal cholesterol efflux (TICE) is unknown. To test the hypothesis that hepatic very low-density lipoproteins (VLDL) support TICE, antisense oligonucleotides (ASO) were used to knockdown hepatic expression of microsomal triglyceride transfer protein (MTP), which is necessary for VLDL assembly. While maintained on a high cholesterol diet, Niemann-Pick C1-like 1 hepatic transgenic (L1Tg) mice, which predominantly excrete cholesterol via TICE, and wild type (WT) littermates were treated with control ASO or MTP ASO. In both WT and L1Tg mice, MTP ASO decreased VLDL triglyceride (TG) and cholesterol secretion. Regardless of treatment, L1Tg mice had reduced biliary cholesterol compared to WT mice. However, only L1Tg mice treated with MTP ASO had reduced fecal cholesterol excretion. Based upon these findings, we conclude that VLDL or a byproduct such as LDL can move cholesterol from the liver to the small intestine for TICE.

The endoplasmic reticulum coat protein II transport machinery coordinates cellular lipid secretion and cholesterol biosynthesis

Triglycerides and cholesterol are essential for life in most organisms. Triglycerides serve as the principal energy storage depot and, where vascular systems exist, as a means of energy transport. Cholesterol is essential for the functional integrity of all cellular membrane systems. The endoplasmic reticulum is the site of secretory lipoprotein production and de novo cholesterol synthesis, yet little is known about how these activities are coordinated with each other or with the activity of the COPII machinery, which transports endoplasmic reticulum cargo to the Golgi. The Sar1B component of this machinery is mutated in chylomicron retention disorder, indicating that this Sar1 isoform secures delivery of dietary lipids into the circulation. However, it is not known why some patients with chylomicron retention disorder develop hepatic steatosis, despite impaired intestinal fat malabsorption, and why very severe hypocholesterolemia develops in this condition. Here, we show that Sar1B also promotes hepatic apolipoprotein (apo) B lipoprotein secretion and that this promoting activity is coordinated with the processes regulating apoB expression and the transfer of triglycerides/cholesterol moieties onto this large lipid transport protein. We also show that although Sar1A antagonizes the lipoprotein secretion-promoting activity of Sar1B, both isoforms modulate the expression of genes encoding cholesterol biosynthetic enzymes and the synthesis of cholesterol de novo. These results not only establish that Sar1B promotes the secretion of hepatic lipids but also adds regulation of cholesterol synthesis to Sar1B's repertoire of transport functions.

Thyroid hormone negatively regulates CDX2 and SOAT2 mRNA expression via induction of miRNA-181d in hepatic cells

Thyroid hormones (THs) regulate transcription of many metabolic genes in the liver through its nuclear receptors (TRs). Although the molecular mechanisms for positive regulation of hepatic genes by TH are well understood, much less is known about TH-mediated negative regulation. Recently, several nuclear hormone receptors were shown to downregulate gene expression via miRNAs. To further examine the potential role of miRNAs in TH-mediated negative regulation, we used a miRNA microarray to identify miRNAs that were directly regulated by TH in a human hepatic cell line. In our screen, we discovered that miRNA-181d is a novel hepatic miRNA that was regulated by TH in hepatic cell culture and in vivo. Furthermore, we identified and characterized two novel TH-regulated target genes that were downstream of miR-181d signaling: caudal type homeobox 2 (CDX2) and sterol O-acyltransferase 2 (SOAT2 or ACAT2). CDX2, a known positive regulator of hepatocyte differentiation, was regulated by miR-181d and directly activated SOAT2 gene expression. Since SOAT2 is an enzyme that generates cholesteryl esters that are packaged into lipoproteins, our results suggest miR-181d plays a significant role in the negative regulation of key metabolic genes by TH in the liver.

Prediction of bioactivity of ACAT2 inhibitors by multilinear regression analysis and support vector machine

Two quantitative structure-activity relationships (QSAR) models for predicting 95 compounds inhibiting Acyl-coenzyme A: cholesterol acyltransferase2 (ACAT2) were developed. The whole data set was randomly split into a training set including 72 compounds and a test set including 23 compounds. The molecules were represented by 11 descriptors calculated by software ADRIANA.Code. Then the inhibitory activity of ACAT2 inhibitors was predicted using multilinear regression (MLR) analysis and support vector machine (SVM) method, respectively. The correlation coefficients of the models for the test sets were 0.90 for MLR model, and 0.91 for SVM model. Y-randomization was employed to ensure the robustness of the SVM model. The atom charge and electronegativity related descriptors were important for the interaction between the inhibitors and ACAT2.

Making, baking, and breaking: the synthesis, storage, and hydrolysis of neutral lipids

The esterification of amphiphilic alcohols with fatty acids is a ubiquitous strategy implemented by eukaryotes and some prokaryotes to conserve energy and membrane progenitors and simultaneously detoxify fatty acids and other lipids. This key reaction is performed by at least four evolutionarily unrelated multigene families. The synthesis of this "neutral lipid" leads to the formation of a lipid droplet, which despite the clear selective advantage it confers is also a harbinger of cellular and organismal malaise. Neutral lipid deposition as a cytoplasmic lipid droplet may be thermodynamically favored but nevertheless is elaborately regulated. Optimal utilization of these resources by lipolysis is similarly multigenic in determination and regulation. We present here a perspective on these processes that originates from studies in model organisms, and we include our thoughts on interventions that target reductions in neutral lipids as therapeutics for human diseases such as obesity and diabetes.

Label-free quantitative proteomic analysis of right ventricular remodeling in infant Tetralogy of Fallot patients

Tetralogy of Fallot (TOF) results in chronic progressive right ventricular (RV) pressure overload and shunt hypoxemia. We investigated the global changes in the proteome of RV among infant patients with and without TOF to gain an insight into early RV remodeling. One hundred and thirty-six differentially expressed proteins were identified using label-free LC-ESI-MS/MS analysis. Western blot results revealed that the expression of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 2 (PFKFB2) increased significantly in TOF patients; and levels of lysocardiolipin acyltransferase 1 (LCLAT1), lumican (LUM), and versican (VCAN) decreased significantly. QRT-PCR analysis showed that levels of PFKFB2 mRNA were markedly increased, but those of LCLAT1 and LUM were significantly decreased. VCAN mRNA showed no significant change in response to pathophysiology of TOF. The results of immunohistochemical staining were similar to those of Western blot analysis. Results of the proteomic analysis indicated that the level of glycolysis-related proteins had increased and levels of lipid-metabolism-related proteins had decreased. ECM proteins were found to be more down-regulated in TOF in the present study than in previous reports. Taken together, our findings may provide clues to both the metabolic inflexibility and ECM remodeling during the early RV remodeling, which occur in response to chronic hypoxia and long-term pressure overload in TOF patients.

Atherosclerosis: lessons from LXR and the intestine

Modulation of the cholesterol-sensing liver X receptors (LXRs) and their downstream targets has emerged as promising therapeutic avenues in atherosclerosis. The intestine is important for its unique capabilities to act as a gatekeeper for cholesterol absorption and to participate in the process of cholesterol elimination in the feces and reverse cholesterol transport (RCT). Pharmacological and genetic intestine-specific LXR activation have been shown to protect against atherosclerosis. In this review we discuss the LXR-targeted molecular players in the enterocytes as well as the intestine-driven pathways contributing to cholesterol homeostasis with therapeutic potential as targets in the prevention and treatment of atherosclerosis..

Carvacrol Protects against Hepatic Steatosis in Mice Fed a High-Fat Diet by Enhancing SIRT1-AMPK Signaling

We investigated the protective effect of carvacrol against high-fat-diet-induced hepatic steatosis in mice and the potential underlying molecular mechanisms. Mice were fed a normal diet, high-fat diet, or carvacrol-supplemented high-fat diet for 10 weeks. Compared to mice fed the high-fat diet, those fed the carvacrol-supplemented diet showed significantly lower hepatic lipid levels and reduced plasma activities of alanine aminotransferase and aspartate aminotransferase and plasma concentrations of monocyte chemoattractant protein 1 and tumor necrosis factor α. Carvacrol decreased the expression of LXRα, SREBP1c, FAS, leptin, and CD36 genes and phosphorylation of S6 kinase 1 protein involved in lipogenesis, whereas it increased the expression of SIRT1 and CPT1 genes and phosphorylation of liver kinase B1, AMP-activated protein kinase, and acetyl-CoA carboxylase proteins involved in fatty acid oxidation in the liver of mice fed the high-fat diet. These results suggest that carvacrol prevents HFD-induced hepatic steatosis by activating SIRT1-AMPK signaling.

Using self-organizing map (SOM) and support vector machine (SVM) for classification of selectivity of ACAT inhibitors

Using a self-organizing map (SOM) and support vector machine, two classification models were built to predict whether a compound is a selective inhibitor toward the two Acyl-coenzyme A: cholesterol acyltransferase (ACAT) isozymes, ACAT-1 and ACAT-2. A dataset of 97 ACAT inhibitors was collected. For each molecule, the global descriptors, 2D and 3D property autocorrelation descriptors and autocorrelation of surface properties were calculated from the program ADRIANA.Code. The prediction accuracies of the models (based on the training/ test set splitting by SOM method) for the test sets are 88.9 % for SOM1, 92.6 % for SVM1 model. In addition, the extended connectivity fingerprints (ECFP_4) for all the molecules were calculated and the structure-activity relationship of selective ACAT inhibitors was summarized, which may help find important structural features of inhibitors relating to the selectivity of ACAT isozymes.

Cholesterol metabolism and the pathogenesis of non-alcoholic steatohepatitis

Emerging experimental and human evidence has linked altered hepatic cholesterol homeostasis and free cholesterol (FC) accumulation to the pathogenesis of non-alcoholic steatohepatits (NASH). This review focuses on cellular mechanisms of cholesterol toxicity involved in liver injury and on alterations in cholesterol homeostasis promoting hepatic cholesterol overload in NASH. FC accumulation injures hepatocytes directly, by disrupting mitochondrial and endoplasmic reticulum (ER) membrane integrity, triggering mitochondrial oxidative injury and ER stress, and by promoting generation of toxic oxysterols, and indirectly, by inducing adipose tissue dysfunction. Accumulation of oxidized LDL particles may also activate Kupffer and hepatic stellate cells, promoting liver inflammation and fibrogenesis. Hepatic cholesterol accumulation is driven by a deeply deranged cellular cholesterol homeostasis, characterized by elevated cholesterol synthesis and uptake from circulating lipoproteins and by a reduced cholesterol excretion. Extensive dysregulation of cellular cholesterol homeostasis by nuclear transcription factors sterol regulatory binding protein (SREBP)-2, liver X-receptor (LXR)-α and farnesoid X receptor (FXR) plays a key role in hepatic cholesterol accumulation in NASH. The therapeutic implications and opportunities for normalizing cellular cholesterol homeostasis in these patients are also discussed.

Tissue-specific knockouts of ACAT2 reveal that intestinal depletion is sufficient to prevent diet-induced cholesterol accumulation in the liver and blood

Acyl-CoA:cholesterol acyltransferase 2 (ACAT2) generates cholesterol esters (CE) for packaging into newly synthesized lipoproteins and thus is a major determinant of blood cholesterol levels. ACAT2 is expressed exclusively in the small intestine and liver, but the relative contributions of ACAT2 expression in these tissues to systemic cholesterol metabolism is unknown. We investigated whether CE derived from the intestine or liver would differentially affect hepatic and plasma cholesterol homeostasis. We generated liver-specific (ACAT2(L-/L-)) and intestine-specific (ACAT2(SI-/SI-)) ACAT2 knockout mice and studied dietary cholesterol-induced hepatic lipid accumulation and hypercholesterolemia. ACAT2(SI-/SI-) mice, in contrast to ACAT2(L-/L-) mice, had blunted cholesterol absorption. However, specific deletion of ACAT2 in the intestine generated essentially a phenocopy of the conditional knockout of ACAT2 in the liver, with reduced levels of plasma very low-density lipoprotein and hepatic CE, yet hepatic-free cholesterol does not build up after high cholesterol intake. ACAT2(L-/L-) and ACAT2(SI-/SI-) mice were equally protected from diet-induced hepatic CE accumulation and hypercholesterolemia. These results suggest that inhibition of intestinal or hepatic ACAT2 improves atherogenic hyperlipidemia and limits hepatic CE accumulation in mice and that depletion of intestinal ACAT2 is sufficient for most of the beneficial effects on cholesterol metabolism. Inhibitors of ACAT2 targeting either tissue likely would be beneficial for atheroprotection.