Lipid absorption defects in intestine-specific microsomal triglyceride transfer protein and ATP-binding cassette transporter A1-deficient mice

A phenome-wide association and Mendelian randomisation study of alcohol use variants in a diverse cohort comprising over 3 million individuals

BACKGROUND

Alcohol consumption is associated with numerous negative social and health outcomes. These associations may be direct consequences of drinking, or they may reflect common genetic factors that influence both alcohol consumption and other outcomes.

METHODS

We performed exploratory phenome-wide association studies (PheWAS) of three of the best studied protective single nucleotide polymorphisms (SNPs) in genes encoding ethanol metabolising enzymes (ADH1B: rs1229984-T, rs2066702-A; ADH1C: rs698-T) using up to 1109 health outcomes across 28 phenotypic categories (e.g., substance-use, mental health, sleep, immune, cardiovascular, metabolic) from a diverse 23andMe cohort, including European (N ≤ 2,619,939), Latin American (N ≤ 446,646) and African American (N ≤ 146,776) populations to uncover new and perhaps unexpected associations. These SNPs have been consistently implicated by both candidate gene studies and genome-wide association studies of alcohol-related behaviours but have not been investigated in detail for other relevant phenotypes in a hypothesis-free approach in such a large cohort of multiple ancestries. To provide insight into potential causal effects of alcohol consumption on the outcomes significant in the PheWAS, we performed univariable two-sample and one-sample Mendelian randomisation (MR) analyses.

FINDINGS

The minor allele rs1229984-T, which is protective against alcohol behaviours, showed the highest number of PheWAS associations across the three cohorts (N = 232, European; N = 29, Latin American; N = 7, African American). rs1229984-T influenced multiple domains of health. We replicated associations with alcohol-related behaviours, mental and sleep conditions, and cardio-metabolic health. We also found associations with understudied traits related to neurological (migraines, epilepsy), immune (allergies), musculoskeletal (fibromyalgia), and reproductive health (preeclampsia). MR analyses identified evidence of causal effects of alcohol consumption on liability for 35 of these outcomes in the European cohort.

INTERPRETATION

Our work demonstrates that polymorphisms in genes encoding alcohol metabolising enzymes affect multiple domains of health beyond alcohol-related behaviours. Understanding the underlying mechanisms of these effects could have implications for treatments and preventative medicine.

FUNDING

MVJ, NCK, SBB, SSR and AAP were supported by T32IR5226 and 28IR-0070. SSR was also supported by NIDA DP1DA054394. NCK and RBC were also supported by R25MH081482. ASH was supported by funds from NIAAA K01AA030083. JLMO was supported by VA 1IK2CX002095. JLMO and JJMM were also supported by NIDA R21DA050160. JJMM was also supported by the Kavli Postdoctoral Award for Academic Diversity. EGA was supported by K01MH121659 from the NIMH/NIH, the Caroline Wiess Law Fund for Research in Molecular Medicine and the ARCO Foundation Young Teacher-Investigator Fund at Baylor College of Medicine. MSA was supported by the Instituto de Salud Carlos III and co-funded by the European Union Found: Fondo Social Europeo Plus (FSE+) (P19/01224, PI22/00464 and CP22/00128).

Microsomal triglyceride transfer protein is necessary to maintain lipid homeostasis and retinal function

Lipid processing by the retinal pigment epithelium (RPE) is necessary to maintain retinal health and function. Dysregulation of retinal lipid homeostasis due to normal aging or age-related disease triggers lipid accumulation within the RPE, on Bruch's membrane (BrM), and in the subretinal space. In its role as a hub for lipid trafficking into and out of the neural retina, the RPE packages a significant amount of lipid into lipid droplets for storage and into apolipoprotein B (APOB)-containing lipoproteins (Blps) for export. Microsomal triglyceride transfer protein (MTP), encoded by the MTTP gene, is essential for Blp assembly. Herein we test the hypothesis that MTP expression in the RPE is essential to maintain lipid balance and retinal function using the newly generated RPEΔMttp mouse model. Using non-invasive ocular imaging, electroretinography, and histochemical and biochemical analyses we show that genetic depletion of Mttp from the RPE results in intracellular lipid accumulation, increased photoreceptor-associated cholesterol deposits, and photoreceptor cell death, and loss of rod but not cone function. RPE-specific reduction in Mttp had no significant effect on plasma lipids and lipoproteins. While APOB was decreased in the RPE, most ocular retinoids remained unchanged, with the exception of the storage form of retinoid, retinyl ester. Thus suggesting that RPE MTP is critical for Blp synthesis and assembly but is not directly involved in plasma lipoprotein metabolism. These studies demonstrate that RPE-specific MTP expression is necessary to establish and maintain retinal lipid homeostasis and visual function.

Bulky hydrophobic side chains in the β1-sandwich of microsomal triglyceride transfer protein are critical for the transfer of both triglycerides and phospholipids

Hyperlipidemia predisposes individuals to cardiometabolic diseases, the most common cause of global mortality. Microsomal triglyceride transfer protein (MTP) transfers multiple lipids and is essential for the assembly of apolipoprotein B-containing lipoproteins. MTP inhibition lowers plasma lipids but causes lipid retention in the liver and intestine. Previous studies suggested two lipid transfer domains in MTP and that specific inhibition of triglyceride (TG) and not phospholipid (PL) transfer can lower plasma lipids without significant tissue lipid accumulation. However, how MTP transfers different lipids and the domains involved in these activities are unknown. Here, we tested a hypothesis that two different β-sandwich domains in MTP transfer TG and PL. Mutagenesis of charged amino acids in β2-sandwich had no effect on PL transfer activity indicating that they are not critical. In contrast, amino acids with bulky hydrophobic side chains in β1-sandwich were critical for both TG and PL transfer activities. Substitutions of these residues with smaller hydrophobic side chains or positive charges reduced, whereas negatively charged side chains severely attenuated MTP lipid transfer activities. These studies point to a common lipid transfer domain for TG and PL in MTP that is enriched with bulky hydrophobic amino acids. Furthermore, we observed a strong correlation in different MTP mutants with respect to loss of both the lipid transfer activities, again implicating a common binding site for TG and PL in MTP. We propose that targeting of areas other than the identified common lipid transfer domain might reduce plasma lipids without causing cellular lipid retention.

Mitochondrial dysfunction abrogates dietary lipid processing in enterocytes

Digested dietary fats are taken up by enterocytes where they are assembled into pre-chylomicrons in the endoplasmic reticulum followed by transport to the Golgi for maturation and subsequent secretion to the circulation1. The role of mitochondria in dietary lipid processing is unclear. Here we show that mitochondrial dysfunction in enterocytes inhibits chylomicron production and the transport of dietary lipids to peripheral organs. Mice with specific ablation of the mitochondrial aspartyl-tRNA synthetase DARS2 (ref. 2), the respiratory chain subunit SDHA3 or the assembly factor COX10 (ref. 4) in intestinal epithelial cells showed accumulation of large lipid droplets (LDs) in enterocytes of the proximal small intestine and failed to thrive. Feeding a fat-free diet suppressed the build-up of LDs in DARS2-deficient enterocytes, which shows that the accumulating lipids derive mostly from digested fat. Furthermore, metabolic tracing studies revealed an impaired transport of dietary lipids to peripheral organs in mice lacking DARS2 in intestinal epithelial cells. DARS2 deficiency caused a distinct lack of mature chylomicrons concomitant with a progressive dispersal of the Golgi apparatus in proximal enterocytes. This finding suggests that mitochondrial dysfunction results in impaired trafficking of chylomicrons from the endoplasmic reticulum to the Golgi, which in turn leads to storage of dietary lipids in large cytoplasmic LDs. Taken together, these results reveal a role for mitochondria in dietary lipid transport in enterocytes, which might be relevant for understanding the intestinal defects observed in patients with mitochondrial disorders5.

Eugeniin improves cholesterol metabolism in HepG2 cells and Caco-2 cells

Considering the absence of prior studies on the cholesterol metabolism-improving effects of eugeniin, the present investigation aimed to explore the potential impact of eugeniin on cholesterol metabolism. This study sought to elucidate the molecular mechanisms involved in this process using HepG2 and Caco-2 cells treated with 5 µm eugeniin. The intracellular cholesterol levels in HepG2 and Caco-2 cells were significantly decreased in the 24-h eugeniin-treated group. The protein and messenger ribonucleic acid (mRNA) levels of the low-density lipoprotein receptor (LDLR) were increased, while 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase protein and mRNA levels were decreased in HepG2 cells 6 h of the eugeniin-treated group. Additionally, LDLR protein and mRNA levels were increased in HepG2 cells after 24 h of eugeniin treatment. In Caco-2, the protein and mRNA levels of ATP-binding cassette transporter 1 were increased after 24 h eugeniin treatment. This novel finding indicates that eugeniin improves cholesterol metabolism in human cell cultures.

Microsomal triglyceride transfer protein is necessary to maintain lipid homeostasis and retinal function

Lipid processing by the retinal pigment epithelium (RPE) is necessary to maintain retinal health and function. Dysregulation of retinal lipid homeostasis due to normal aging or to age-related disease triggers lipid accumulation within the RPE, on Bruch's membrane (BrM), and in the subretinal space. In its role as a hub for lipid trafficking into and out of the neural retina, the RPE packages a significant amount of lipid into lipid droplets for storage and into apolipoprotein B (apoB)-containing lipoproteins (Blps) for export. Microsomal triglyceride transfer protein (MTP), encoded by the MTTP gene, is essential for Blp assembly. Herein we test the hypothesis that MTP expression in the RPE is essential to maintain lipid balance and retinal function using the newly generated RPEΔMttp mouse model. Using non-invasive ocular imaging, electroretinography, and histochemical and biochemical analyses we show that genetic deletion of Mttp from the RPE results in intracellular lipid accumulation, increased photoreceptor -associated cholesterol deposits and photoreceptor cell death, and loss of rod but not cone function. RPE-specific ablation of Mttp had no significant effect on plasma lipids and lipoproteins. While, apoB was decreased in the RPE, ocular retinoid concentrations remained unchanged. Thus suggesting that RPE MTP is critical for Blp synthesis and assembly but not directly involved in ocular retinoid and plasma lipoprotein metabolism. These studies demonstrate that RPE-specific MTP expression is necessary to establish and maintain retinal lipid homeostasis and visual function.

Pentapeptide IIAEK ameliorates cholesterol metabolism via the suppression of intestinal cholesterol absorption in mice

Dietary protein-derived peptides are effective in improving dyslipidemia and hypercholesterolemia. We previously identified a novel cholesterol-lowering pentapeptide IIAEK from milk beta-lactoglobulin. However, it remains unclear whether IIAEK affects the micellar solubility of cholesterol and the bile acid-binding ability to lower cholesterol. Moreover, there is no direct evidence that IIAEK inhibits intestinal cholesterol absorption and affects hepatic cholesterol and fecal steroid excretion in vivo. Herein, we showed that IIAEK did not affect the micellar solubility of cholesterol and the bile acid-binding ability. However, we found that IIAEK decreased serum and liver cholesterol levels and increased fecal steroid excretion in mice. Interestingly, IIAEK markedly suppressed the intestinal absorption of [3H]-cholesterol in mice. In conclusion, we found that IIAEK ameliorated cholesterol metabolism by suppressing intestinal cholesterol absorption without affecting in vitro micellar solubility of cholesterol and the bile acid-binding ability in mice.

Characterization of feed efficiency-related key signatures molecular in different cattle breeds

Feed efficiency is a major constraint in the beef industry and has a significant negative correlation with residual feed intake (RFI). RFI is widely used as a measure of feed efficiency in beef cattle and is independent of economic traits such as body weight and average daily gain. However, key traits with commonality or specificity among beef cattle breeds at the same level of RFI have not been reported. Accordingly, the present study hypothesized that signatures associated with feed efficiency would have commonality or specificity in the liver of cattle breeds at the same RFI level. By comparing and integrating liver transcriptome data, we investigated the critical signatures closely associated with RFI in beef cattle using weighted co-expression network analysis, consensus module analysis, functional enrichment analysis and protein network interaction analysis. The results showed that the consensus modules in Angus and Charolais cattle were negatively correlated, and four (turquoise, red, tan, yellow) were significantly positively correlated in Angus liver, while (turquoise, red) were significantly negatively correlated in Charolais liver. These consensus modules were found to be primarily involved in biological processes such as substance metabolism, energy metabolism and gene transcription, which may be one of the possible explanations for the difference in feed efficiency between the two beef breeds. This research also identified five key candidate genes, PLA2G12B, LCAT, MTTP, LCAT, ABCA1 and FADS1, which are closely associated with hepatic lipid metabolism. The present study has identified some modules, genes and pathways that may be the major contributors to the variation in feed efficiency among different cattle breeds, providing a new perspective on the molecular mechanisms of feed efficiency in beef cattle and a research basis for investigating molecular markers associated with feed efficiency in beef cattle.

Validation of Knock-Out Caco-2 TC7 Cells as Models of Enterocytes of Patients with Familial Genetic Hypobetalipoproteinemias

Abetalipoproteinemia (FHBL-SD1) and chylomicron retention disease (FHBL-SD3) are rare recessive disorders of lipoprotein metabolism due to mutations in MTTP and SAR1B genes, respectively, which lead to defective chylomicron formation and secretion. This results in lipid and fat-soluble vitamin malabsorption, which induces severe neuro-ophthalmic complications. Currently, treatment combines a low-fat diet with high-dose vitamin A and E supplementation but still fails in normalizing serum vitamin E levels and providing complete ophthalmic protection. To explore these persistent complications, we developed two knock-out cell models of FHBL-SD1 and FHBL-SD3 using the CRISPR/Cas9 technique in Caco-2/TC7 cells. DNA sequencing, RNA quantification and Western blotting confirmed the introduction of mutations with protein knock-out in four clones associated with i) impaired lipid droplet formation and ii) defective triglyceride (-57.0 ± 2.6% to -83.9 ± 1.6%) and cholesterol (-35.3 ± 4.4% to -60.6 ± 3.5%) secretion. A significant decrease in α-tocopherol secretion was also observed in these clones (-41.5 ± 3.7% to -97.2 ± 2.8%), even with the pharmaceutical forms of vitamin E: tocopherol-acetate and tocofersolan (α-tocopheryl polyethylene glycol succinate 1000). MTTP silencing led to a more severe phenotype than SAR1B silencing, which is consistent with clinical observations. Our cellular models thus provide an efficient tool to experiment with therapeutic strategies and will allow progress in understanding the mechanisms involved in lipid metabolism.

ATP-Binding Cassette Protein ABCC10 Deficiency Prevents Diet-Induced Obesity but Not Atherosclerosis in Mice

Excess plasma lipid levels are a risk factor for various cardiometabolic disorders. Studies have shown that improving dyslipidemia lowers the progression of these disorders. In this study, we investigated the role of ATP-binding cassette transporter C10 (ABCC10) in regulating lipid metabolism. Our data indicate that deletion of the Abcc10 gene in male mice results in lower plasma and intestinal triglycerides by around 38% and 36%, respectively. Furthermore, deletion of ABCC10 ameliorates diet-induced obesity in mice and leads to a better response during insulin and glucose tolerance tests. Unexpectedly, ABCC10 deficiency does not affect triglyceride levels or atherosclerosis in ApoE-deficient mice. In addition, our studies demonstrate low oleate uptake by enterocytes (~25-30%) and less absorption (~37%) of triglycerides in the small intestine of ABCC10 knockout mice. Deletion of the Abcc10 gene also alters several lipid metabolism genes in the intestine, suggesting that ABCC10 regulates dietary fat absorption, which may contribute to diet-induced obesity in mice.

Betaine Reduces Lipid Anabolism and Promotes Lipid Transport in Mice Fed a High-Fat Diet by Influencing Intestinal Protein Expression

Betaine is more efficient than choline and methionine methyl donors, as it can increase nitrogen storage, promote fat mobilisation and fatty acid oxidation and change body fat content and distribution. Lipid is absorbed primarily in the small intestine after consumption, which is also the basis of lipid metabolism. This study was conducted to establish a mouse model of obesity in Kunming mice of the same age and similar body weight, and to assess the effect of betaine on the intestinal protein expression profile of mice using a proteomic approach. Analysis showed that betaine supplementation reversed the reduction in expression of proteins related to lipid metabolism and transport in the intestine of mice induced by a high-fat diet (HFD). For example, the addition of betaine resulted in a significant upregulation of microsomal triglyceride transfer protein (Mttp), apolipoprotein A-IV (Apoa4), fatty-acid-binding protein 1 (Fabp1) and fatty-acid-binding protein 2 (Fabp2) expression compared to the HFD group (p < 0.05), which exhibited accelerated lipid absorption and then translocation from the intestine into the body’s circulation, in addition to a significant increase in Acetyl-CoA acyltransferase (Acaa1a) protein expression, hastening lipid metabolism in the intestine (p < 0.05). Simultaneously, a significant reduction in protein expression of alpha-enolase 1 (Eno1) as the key enzyme for gluconeogenesis in mice in the betaine-supplemented group resulted in a reduction in lipid synthesis in the intestine (p < 0.05). These findings provide useful information for understanding the changes in the protein profile of the small intestine in response to betaine supplementation and the potential physiological regulation of diets’ nutrient absorption.

BMP gradient along the intestinal villus axis controls zonated enterocyte and goblet cell states

Intestinal epithelial cells derive from stem cells at the crypt base and travel along the crypt-villus axis to die at the villus tip. The two dominant villus epithelial cell types, absorptive enterocytes and mucous-secreting goblet cells, are mature when they exit crypts. Murine enterocytes switch functional cell states during migration along the villus. Here, we ask whether this zonation is driven by the bone morphogenetic protein (BMP) gradient, which increases toward the villus. Using human intestinal organoids, we show that BMP signaling controls the expression of zonated genes in enterocytes. We find that goblet cells display similar zonation involving antimicrobial genes. Using an inducible Bmpr1a knockout mouse model, we confirm that BMP controls these zonated genes in vivo. Our findings imply that local manipulation of BMP signal strength may be used to reset the enterocyte "rheostat" of carbohydrate versus lipid uptake and to control the antimicrobial response through goblet cells.

Gut microbiota transplantation drives the adoptive transfer of colonic genotype-phenotype characteristics between mice lacking catestatin and their wild type counterparts

The gut microbiota is in continuous interaction with the intestinal mucosa via metabolic, neuro-immunological, and neuroendocrine pathways. Disruption in levels of antimicrobial peptides produced by the enteroendocrine cells, such as catestatin, has been associated with changes in the gut microbiota and imbalance in intestinal homeostasis. However, whether the changes in the gut microbiota have a causational role in intestinal dyshomeostasis has remained elusive. To this end, we performed reciprocal fecal microbial transplantation in wild-type mice and mice with a knockout in the catestatin coding region of the chromogranin-A gene (CST-KO mice). Combined microbiota phylogenetic profiling, RNA sequencing, and transmission electron microscopy were employed. Fecal microbiota transplantation from mice deficient in catestatin (CST-KO) to microbiota-depleted wild-type mice induced transcriptional and physiological features characteristic of a distorted colon in the recipient animals, including impairment in tight junctions, as well as an increased collagen area fraction indicating colonic fibrosis. In contrast, fecal microbiota transplantation from wild-type mice to microbiota-depleted CST-KO mice reduced collagen fibrotic area, restored disrupted tight junction morphology, and altered fatty acid metabolism in recipient CST-KO mice. This study provides a comprehensive overview of the murine metabolic- and immune-related cellular pathways and processes that are co-mediated by the fecal microbiota transplantation and supports a prominent role for the gut microbiota in the colonic distortion associated with the lack of catestatin in mice. Overall, the data show that the gut microbiota may play a causal role in the development of features of intestinal inflammation and metabolic disorders, known to be associated with altered levels of catestatin and may, thus, provide a tractable target in the treatment and prevention of these disorders.

An improved assay to measure the phospholipid transfer activity of microsomal triglyceride transport protein

Microsomal triglyceride transfer protein (MTP) is essential for the assembly and secretion of apolipoprotein B-containing lipoproteins. MTP transfers diverse lipids such as triacylglycerol (TAG) and phospholipids (PLs) between vesicles in vitro. Previously, we described methods to measure these transfer activities using N-7-nitro-2-1,3-benzoxadiazol-4-yl (NBD)-labeled lipids. The NBD-TAG transfer assay is sensitive and can measure MTP activity in cell and tissue homogenates. In contrast, the NBD-PL transfer assay shows high background and is less sensitive; therefore, purified MTP is required to measure its PL transfer activity. Here, we optimized the assay to measure also the PL transfer activity of MTP in cell and tissue homogenates. We found that donor vesicles containing dioleoylphosphoethanolamine and palmitoyloleoylphosphoethanolamine result in a low background signal and are suitable to assay the PL transfer activity of MTP. This assay was capable of measuring protein-dependent and substrate-dependent saturation kinetics. Furthermore, the MTP inhibitor lomitapide blocked this transfer activity. One drawback of the PL transfer assay is that it is less sensitive at physiological temperature than at room temperature, and it requires longer incubation times than the TAG transfer assay. Nevertheless, this significantly improved sensitive assay is simple and easy to perform, involves few steps, can be conducted at room temperature, and is suitable for high-throughput screening to identify inhibitors. This assay can be adapted to measure other PL transfer proteins and to address biological and physiological importance of these activities.

Agomelatine reduces circulating triacylglycerides and hepatic steatosis in fructose-treated rats

Agomelatine (AGO) is an antidepressant drug with agonistic activity at melatonin receptor 1 (MT1) and MT2 and with neutral antagonistic activity at serotonin receptor 5-HT2_C. Although experimental studies show that melatonin reduces hypertriglyceridemia and hepatic steatosis induced by excessive fructose intake, no studies have tested if AGO exerts similar actions. To address this issue we have treated male Wistar rats with fructose (15% in the drinking water) and/or AGO (40 mg/kg/day) for two weeks. AGO reduced body weight gain, feeding efficiency and hepatic lipid levels without affecting caloric intake in fructose-treated rats. AGO has also decreased very low-density lipoprotein (VLDL) production and circulating TAG levels after an oral load with olive oil. Accordingly, treatment with AGO reduced the hepatic expression of fatty acid synthase (Fasn), a limiting step for hepatic de novo lipogenesis (DNLG). The expression of apolipoprotein B (Apob) and microsomal triglyceride transfer protein (Mttp) in the ileum, two crucial proteins for intestinal lipoprotein production, were also downregulated by treatment with AGO. Altogether, the present data show that AGO mimics the metabolic benefits of melatonin when used in fructose-treated rats. This study also suggests that it is relevant to evaluate the potential of AGO to treat metabolic disorders in future clinical trials.

Sar1b mutant mice recapitulate gastrointestinal abnormalities associated with chylomicron retention disease

Chylomicron retention disease (CRD) is an autosomal recessive disorder associated with biallelic Sar1b mutations leading to defects in intracellular chylomicron (CM) trafficking and secretion. To date, a direct cause-effect relationship between CRD and Sar1b mutation has not been established, but genetically modified animal models provide an opportunity to elucidate unrecognized aspects of these mutations. To examine the physiological role and molecular mechanisms of Sar1b function, we generated mice expressing either a targeted deletion or mutation of human Sar1b using the CRISPR-Cas9 system. We found that deletion or mutation of Sar1b in mice resulted in late-gestation lethality of homozygous embryos. Moreover, compared with WT mice, heterozygotes carrying a single disrupted Sar1b allele displayed lower plasma levels of triglycerides, total cholesterol, and HDL-cholesterol, along with reduced CM secretion following gastric lipid gavage. Similarly, decreased expression of apolipoprotein B and microsomal triglyceride transfer protein was observed in correlation with the accumulation of mucosal lipids. Inefficient fat absorption in heterozygotes was confirmed via an increase in fecal lipid excretion. Furthermore, genetically modified Sar1b affected intestinal lipid homeostasis as demonstrated by enhanced fatty acid β-oxidation and diminished lipogenesis through the modulation of transcription factors. This is the first reported mammalian animal model with human Sar1b genetic defects, which reproduces some of the characteristic CRD features and provides a direct cause-effect demonstration.

IIAEK Targets Intestinal Alkaline Phosphatase (IAP) to Improve Cholesterol Metabolism with a Specific Activation of IAP and Downregulation of ABCA1

IIAEK (Ile-Ile-Ala-Glu-Lys, lactostatin) is a novel cholesterol-lowering pentapeptide derived from bovine milk β-lactoglobulin. However, the molecular mechanisms underlying the IIAEK-mediated suppression of intestinal cholesterol absorption are unknown. Therefore, we evaluated the effects of IIAEK on intestinal cholesterol metabolism in a human intestinal model using Caco-2 cells. We found that IIAEK significantly reduced the expression of intestinal cholesterol metabolism-associated genes, particularly that of the ATP-binding cassette transporter A1 (ABCA1). Subsequently, we chemically synthesized a novel molecular probe, IIXEK, which can visualize a complex of target proteins interacting with photoaffinity-labeled IIAEK by fluorescent substances. Through photoaffinity labeling and MS analysis with IIXEK for the rat small intestinal mucosa and intestinal lipid raft fractions of Caco-2 cells, we identified intestinal alkaline phosphatase (IAP) as a specific molecule interacting with IIAEK and discovered the common IIAEK-binding amino acid sequence, GFYLFVEGGR. IIAEK significantly increased IAP mRNA and protein levels while decreasing ABCA1 mRNA and protein levels in Caco-2 cells. In conclusion, we found that IIAEK targets IAP to improve cholesterol metabolism via a novel signaling pathway involving the specific activation of IAP and downregulation of intestinal ABCA1.

Intestine-specific DGAT1 deficiency improves atherosclerosis in apolipoprotein E knockout mice by reducing systemic cholesterol burden

Background and aims

Acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) is the rate-limiting enzyme catalyzing the final step of triglyceride synthesis by esterifying a diglyceride with a fatty acid. We have previously shown that apolipoprotein E-knockout (ApoE^−/−) mice lacking Dgat1 have reduced intestinal cholesterol absorption and potentiated macrophage cholesterol efflux, and consequently, exhibit attenuated atherogenesis. However, he-matopoietic Dgat1 deficiency lacked beneficial effects on atherosclerosis. Due to our recent results on the critical role of intestinal Dgat1 in murine cholesterol homeostasis, we delineated whether intestinal Dgat1 deficiency regulates atherogenesis in mice.

Methods

We generated intestine-specific Dgat1^−/− mice on the ApoE^−/− background (iDgat1^−/−ApoE^−/−) and determined cholesterol homeostasis and atherosclerosis development.

Results

When fed a Western-type diet, iDgat1^−/−ApoE^−/− mice exhibited a substantial decrease in fasting plasma cholesterol content in ApoB-containing lipoproteins. Although lipid absorption was delayed, iDgat1^−/−ApoE^−/− mice had reduced acute and fractional cholesterol absorption coupled with an elevated fecal caloric loss. In line, increased appearance of i.v. administered [3H]cholesterol in duodena and stool of iDgat1^−/−ApoE^−/− animals suggested potentiated cholesterol elimination. Atherosclerotic lesions were markedly smaller with beneficial alterations in plaque composition as evidenced by reduced macrophage infiltration and necrotic core size despite unaltered collagen content, indicating improved plaque stability.

Conclusions

Disruption of Dgat1 activity solely in the small intestine of ApoE^−/− mice strongly decreased plasma cholesterol levels by abrogating the assimilation of dietary cholesterol, partly by reduced absorption and increased excretion. Consequently, the reduced cholesterol burden significantly attenuated atherogenesis and improved the lesion phenotype in iDgat1^−/−ApoE^−/− mice.

Diacylglycerol acyltransferase 1/2 inhibition induces dysregulation of fatty acid metabolism and leads to intestinal barrier failure and diarrhea in mice

The intestinal metabolism and transport of triacylglycerol (TAG) play a critical role in dietary TAG absorption, and defects in the process are associated with congenital diarrhea. The final reaction in TAG synthesis is catalyzed by diacylglycerol acyltransferase (DGAT1 and DGAT2), which uses activated fatty acids (FA) as substrates. Loss-of-function mutations in DGAT1 cause watery diarrhea in humans, but mechanisms underlying the relationship between altered DGAT activity and diarrhea remain largely unclear. Here, the effects of DGAT1 and DGAT2 inhibition, alone or in combination, on dietary TAG absorption and diarrhea in mice were investigated by using a selective DGAT1 inhibitor (PF-04620110) and DGAT2 inhibitor (PF-06424439). Simultaneous administration of a single dosing of these inhibitors drastically decreased intestinal TAG secretion into the blood circulatory system and TAG accumulation in the duodenum at 60 min after lipid gavage. Under 60% high-fat diet (HFD) feeding, their repeated simultaneous administration for 2 days induced severe watery diarrhea and occasionally led to death. The diarrhea was accompanied by enhanced fecal FA excretion, intestinal injury and barrier failure. DGAT1 or DGAT2 inhibition alone did not induce the phenotypic changes observed in DGAT1/2 inhibitor-treated mice. The results demonstrate that DGAT1/2 inhibition alters TAG absorption and results in watery diarrhea in mice. DGAT1/2 inhibition-induced diarrhea may be caused by intestinal barrier dysfunction due to dysregulation of the cytotoxic FA metabolism. These findings suggest that DGAT-mediated intestinal TAG synthesis is a vital step for maintaining intestinal barrier integrity under HFD feeding.

Regulation of intestinal lipid metabolism: current concepts and relevance to disease

Lipids entering the gastrointestinal tract include dietary lipids (triacylglycerols, cholesteryl esters and phospholipids) and endogenous lipids from bile (phospholipids and cholesterol) and from shed intestinal epithelial cells (enterocytes). Here, we comprehensively review the digestion, uptake and intracellular re-synthesis of intestinal lipids as well as their packaging into pre-chylomicrons in the endoplasmic reticulum, their modification in the Golgi apparatus and the exocytosis of the chylomicrons into the lamina propria and subsequently to lymph. We also discuss other fates of intestinal lipids, including intestinal HDL and VLDL secretion, cytosolic lipid droplets and fatty acid oxidation. In addition, we highlight the applicability of these findings to human disease and the development of therapeutics targeting lipid metabolism. Finally, we explore the emerging role of the gut microbiota in modulating intestinal lipid metabolism and outline key questions for future research.

Identification of a novel cholesterol-lowering dipeptide, phenylalanine-proline (FP), and its down-regulation of intestinal ABCA1 in hypercholesterolemic rats and Caco-2 cells

There has been no report about in vivo active cholesterol-lowering dipeptide in any protein origin, despite their potential health benefits. Cattle heart protein hydrolysate ultra-filtrate (HPHU, molecular weight < ca. 1,000 Da peptide mixture) exhibits cholesterol-lowering activity in hypercholesterolemic rats, but the active peptide in HPHU that lowers serum cholesterol levels and its molecular mechanism are unknown. In this study, we separated and purified HPHU to identify a novel cholesterol-lowering dipeptide (phenylalanine-proline, FP) and characterized the mechanism underlying its effects in vivo and in vitro. We identified FP as an active peptide from HPHU by MALDI-TOF mass spectrometry. FP significantly decreased serum total and non-HDL cholesterol and hepatic cholesterol levels in rats. FP significantly increased serum HDL cholesterol, accompanied by a significant decrease in the atherogenic index. FP also significantly increased fecal cholesterol and acidic steroid excretion. Moreover, FP significantly decreased ATP-binding cassette transporter A1 (ABCA1) expression in the rat jejunum and reduced cholesterol absorption in Caco-2 cells. We found a novel cholesterol-lowering dipeptide FP that could improve cholesterol metabolism via the down-regulation of intestinal ABCA1. The cholesterol-lowering action induced by FP was disappeared in PepT1KO mice. FP-induced cholesterol-lowering action is mediated via PepT1 in mice.

A critical role of VMP1 in lipoprotein secretion

Lipoproteins are lipid-protein complexes that are primarily generated and secreted from the intestine, liver, and visceral endoderm and delivered to peripheral tissues. Lipoproteins, which are assembled in the endoplasmic reticulum (ER) membrane, are released into the ER lumen for secretion, but its mechanism remains largely unknown. Here, we show that the release of lipoproteins from the ER membrane requires VMP1, an ER transmembrane protein essential for autophagy and certain types of secretion. Loss of vmp1, but not other autophagy-related genes, in zebrafish causes lipoprotein accumulation in the intestine and liver. Vmp1 deficiency in mice also leads to lipid accumulation in the visceral endoderm and intestine. In VMP1-depleted cells, neutral lipids accumulate within lipid bilayers of the ER membrane, thus affecting lipoprotein secretion. These results suggest that VMP1 is important for the release of lipoproteins from the ER membrane to the ER lumen in addition to its previously known functions.

Small GTPase Arf6 regulates diabetes-induced cholesterol accumulation in podocytes

Podocyte injury is a critical factor for the initiation and progression of diabetic kidney disease (DKD). However, the underlying mechanisms of podocyte injury in DKD have not been completely elucidated. Studies suggested that intracellular cholesterol accumulation was correlated with podocyte injury, but the cause of podocyte cholesterol disorders in DKD are still unknown. ADP-ribosylation factor 6 (Arf6) is a small GTPase with pleiotropic effects and has previously been shown to regulate ATP-binding cassette transporter 1 (ABCA1) recycling, and thus, cholesterol homeostasis. However, Arf6 involvement in cholesterol metabolism in podocytes is scarce. To investigate the role of Arf6 in cholesterol modulation in podocytes, the effect of Arf6 on the regulation of the cholesterol transporter ABCA1 was studied in podocytes in vivo and in vitro. Intracellular cholesterol accumulation was significantly increased in podocytes from streptozotocin-induced diabetic rats and that hyperglycemia downregulated the expression of Arf6. Arf6 knockdown could cause ABCA1 recycling disorders, and thus, further aggravate cholesterol accumulation in podocytes under high-glucose (HG) conditions. Our results demonstrate that HG-induced cholesterol accumulation and cellular injury in podocytes may be related to the recycling disorder of ABCA1 caused by the downexpression of Arf6 in DKD.

The atypical antipsychotic quetiapine induces hyperlipidemia by activating intestinal PXR signaling

Quetiapine, one of the most prescribed atypical antipsychotics, has been associated with hyperlipidemia and an increased risk for cardiovascular disease in patients, but the underlying mechanisms remain unknown. Here, we identified quetiapine as a potent and selective agonist for pregnane X receptor (PXR), a key nuclear receptor that regulates xenobiotic metabolism in the liver and intestine. Recent studies have indicated that PXR also plays an important role in lipid homeostasis. We generated potentially novel tissue-specific PXR-KO mice and demonstrated that quetiapine induced hyperlipidemia by activating intestinal PXR signaling. Quetiapine-mediated PXR activation stimulated the intestinal expression of cholesterol transporter Niemann-Pick C1-Like 1 (NPC1L1) and microsomal triglyceride transfer protein (MTP), leading to increased intestinal lipid absorption. While NPC1L1 is a known PXR target gene, we identified a DR-1-type PXR-response element in the MTP promoter and established MTP as a potentially novel transcriptional target of PXR. Quetiapine's effects on PXR-mediated gene expression and cholesterol uptake were also confirmed in cultured murine enteroids and human intestinal cells. Our findings suggest a potential role of PXR in mediating adverse effects of quetiapine in humans and provide mechanistic insights for certain atypical antipsychotic-associated dyslipidemia.

ATP binding cassette family A protein 1 determines hexosylceramide and sphingomyelin levels in human and mouse plasma

Sphingolipids, including ceramide, SM, and hexosylceramide (HxCer), are carried in the plasma by lipoproteins. They are possible markers of metabolic diseases, but little is known about their control. We previously showed that microsomal triglyceride transfer protein (MTP) is critical to determine plasma ceramide and SM, but not HxCer, levels. In human plasma and mouse models, we examined possible HxCer-modulating pathways, including the role of ABCA1 in determining sphingolipid plasma concentrations. Compared with control samples, plasma from patients with Tangier disease (deficient in ABCA1) had significantly lower HxCer (-69%) and SM (-40%) levels. Similarly, mice deficient in hepatic and intestinal ABCA1 had significantly reduced HxCer (-79%) and SM (-85%) levels. Tissue-specific ablation studies revealed that hepatic ABCA1 determines plasma HxCer and SM levels; that ablation of MTP and ABCA1 in the liver and intestine reduces plasma HxCer, SM, and ceramide levels; and that hepatic and intestinal MTP contribute to plasma ceramide levels, whereas only hepatic MTP modulates plasma SM levels. These results identify the contribution of ABCA1 to plasma SM and HxCer levels and suggest that MTP and ABCA1 are critical determinants of plasma sphingolipid levels.

Fish Oil Supplementation Alleviates the Altered Lipid Homeostasis in Blood, Liver, and Adipose Tissues in High-Fat Diet-Fed Rats

This study investigated the effects of dietary supplementation of fish oil on the signals of lipid metabolism involved in hepatic cholesterol and triglyceride influx and excretion in high-fat diet (HFD)-fed rats. Fish oil (FO) repressed body (HFD, 533 ± 18.2 g; HFD+FO, 488 ± 28.0 g, p < 0.05) and liver weights (HFD, 5.7 ± 0.6 g/100 g of body weight; HFD+FO, 4.8 ± 0.4 g/100 g of body weight, p < 0.05) in HFD-fed rats. Fish oil could also improve HFD-induced imbalance of lipid metabolism in blood, liver, and adipose tissues including the significant decreases in plasma and liver total cholesterol (TC) (plasma-HFD, 113 ± 33.6 mg/dL; HFD+FO, 50.0 ± 5.95 mg/dL, p < 0.05; liver-HFD, 102 ± 13.0 mg/g liver; [corrected] HFD+FO, 86.6 ± 7.81 mg/g liver, [corrected] p < 0.05), blood, liver, and adipose triglyceride (TG) (blood-HFD, 52.5 ± 20.4 mg/dL; HFD+FO, 29.8 ± 4.30 mg/dL, p < 0.05; liver-HFD, 56.2 ± 10.0 mg/g liver; [corrected] HFD+FO, 30.3 ± 5.28 mg/g liver, [corrected] p < 0.05; adipose-HFD, 614 ± 73.2 mg/g liver, [corrected] HFD+FO, 409 ± 334 mg/g of adipose tissue, [corrected] p < 0.05), and low density (HFD, 79.8 ± 40.9 mg/dL; HFD+FO, 16.6 ± 5.47 mg/dL, p < 0.05) and very-low-density (HFD, 49.7 ± 33.3 mg/dL; HFD+FO, 10.4 ± 3.45 mg/dL, p < 0.05) lipoprotein and the significant increases in fecal TC (HFD, 12.2 ± 0.67 mg/g feces; [corrected] HFD+FO, 16.3 ± 2.04 mg/g feces, [corrected] < 0.05) and TG (HFD, 2.09 ± 0.10 mg/g feces; [corrected] HFD+FO, 2.38 ± 0.22 mg/g feces, [corrected] p < 0.05) and lipoprotein lipase activity of adipose tissues (HFD, 16.6 ± 3.64 μM p-nitrophenol; HFD+FO, 24.5 ± 4.19 μM p-nitrophenol, p < 0.05). Moreover, fish oil significantly activated the protein expressions of hepatic lipid metabolism regulators (AMPKα and PPARα) and significantly regulated the lipid-transport-related signaling molecules (ApoE, MTTP, ApoB, Angptl4, ApoCIII, ACOX1, and SREBPF1) in blood or liver of HFD-fed rats. These results suggest that fish oil supplementation improves HFD-induced imbalance of lipid homeostasis in blood, liver, and adipose tissues in rats.

Brown adipose tissue derived ANGPTL4 controls glucose and lipid metabolism and regulates thermogenesis

Objectives

Brown adipose tissue (BAT) controls triglyceride-rich lipoprotein (TRL) catabolism. This process is mediated by the lipoprotein lipase (LPL), an enzyme that catalyzed the hydrolysis of triglyceride (TAG) in glycerol and fatty acids (FA), which are burned to generate heat. LPL activity is regulated by angiopoietin-like 4 (ANGPTL4), a secretory protein produced in adipose tissues (AT), liver, kidney, and muscle. While the role of ANGPTL4 in regulating lipoprotein metabolism is well established, the specific contribution of BAT derived ANGPTL4 in controlling lipid and glucose homeostasis is not well understood.

Methods and results

We generated a novel mouse model lacking ANGPTL4 specifically in brown adipose tissue (BAT-KO). Here, we report that specific deletion of ANGPTL4 in BAT results in enhanced LPL activity, circulating TAG clearance and thermogenesis. Absence of ANGPTL4 in BAT increased FA oxidation and reduced FA synthesis. Importantly, we observed that absence of ANGPTL4 in BAT leads to a remarkable improvement in glucose tolerance in short-term HFD feeding.

Conclusion

Our findings demonstrate an important role of BAT derived ANGPTL4 in regulating lipoprotein metabolism, whole-body lipid and glucose metabolism, and thermogenesis during acute cold exposure.

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Absence of ANGPTL4 in brown adipose tissue reduces circulating triglycerides.

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Loss of ANGPTL4 in brown adipose tissue enhances glucose tolerance and insulin sensitivity.

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Lack of ANGPTL4 in brown adipose tissue improves thermogenesis in response to acute cold exposure.

Cholesterol Transport Revisited: A New Turbo Mechanism to Drive Cholesterol Excretion

A fine-tuned balance between cholesterol uptake and excretion by the body is pivotal to maintain health and to remain free from the deleterious consequences of cholesterol accumulation such as cardiovascular disease. The pathways involved in intracellular and extracellular cholesterol transport are a subject of intense investigation and are being unraveled in increasing detail. In addition, insight into the complex interactions between cholesterol and bile acid metabolism has increased considerably in the last couple of years. This review provides an overview of the mechanisms involved in cholesterol uptake and excretion, with a particular emphasis on the most recent progress in this field. Special attention is given to the transintestinal cholesterol excretion (TICE) pathway, which was recently demonstrated to have a remarkably high transport capacity and to be sensitive to pharmacological modulation.

Does the enterolactone (ENL) affect fatty acid transporters and lipid metabolism in liver?

BACKGROUND

NAFLD as a result of inappropriate diet and obesity, may progress to sever conditions such as: type 2 diabetes mellitus or steatohepatitis, and has recently become a prevalent topic of numerous investigations. Due to its dangerous aftermaths, finding new substances, such as polyphenols and their derivatives, which might reduce liver steatosis is the main target of research into NAFLD treatment. Hence, the aim of the present study was to evaluate the effect(s) of enterolactone (ENL), a metabolite of secoisolariciresinol (SECO), on lipid metabolism together with changes in the expression of fatty acid transporters in fatty liver.

METHODS

The experiments were conducted on HepG2 cells incubated with either ENL and/or palmitic acid during 16 h exposure. The expression of selected fatty acid transport proteins: FATP2, FATP5, CD36, FABPpm, ABCA1, MTP, ACBP and L-FABP, as well as the proteins directly involved in lipogenesis (FAS), oxidation pathway (CPT 1), and lipid metabolism (PPARα, LXR, SREBP1c, pAMPK) was estimated by Western Blot. Intra and extracellular lipid contents were assessed by Gas-Liquid Chromatography. The data was analyzed with two-way analysis of variance (ANOVA), and results were considered to be statistically significant at p ≤ 0.05.

RESULTS

ENL stimulated extracellular efflux of free fatty acids (FFA) and triacylglicerols (TAG) to the medium, while, it had no influence on FATP-family mediated intracellular fatty acid uptake. Moreover, ENL decreased the expression of CPT 1, pAMPK, PPARα, increased SREBP1c and had no effect on LXR, and FAS content.

CONCLUSIONS

The findings of our study demonstrate that ENL had opposite effect on liver steatosis in comparison with other polyphenols what suggests that it may be an inactive metabolite. ENL did not affect significantly the intracellular accumulation of FFA, DAG and TAG, yet it promoted their extracellular efflux. Furthermore, it inhibited ß-oxydation and intracellular lipid metabolism what may contribute to the progression of NAFLD.

Effects of K-877, a novel selective PPARα modulator, on small intestine contribute to the amelioration of hyperlipidemia in low-density lipoprotein receptor knockout mice

Peroxisome proliferator-activated receptor α (PPARα) is a well-known therapeutic target for treating hyperlipidemia. K-877 is a novel selective PPARα modulator (SPPARMα) that enhances PPARα transcriptional activity with high selectivity and potency, resulting in reduced plasma lipid levels. This study aimed to evaluate the effects of K-877 on hyperlipidemia in low-density lipoprotein receptor knockout (Ldlr^-/-) mice, a mouse model of atherosclerosis. We revealed that K-877 administration significantly decreased plasma triglyceride (TG) and total cholesterol (TC) levels and increased plasma high-density lipoprotein cholesterol (HDL-C) levels in Ldlr^-/- mice. K-877 administration to Ldlr^-/- mice efficiently increased the gene expression of PPARα and its target genes related to fatty acid oxidation in the liver and small intestine. The same treatment significantly increased ATP-binding cassette a1 gene expression in the liver and small intestine and reduced Niemann Pick C1-like 1 gene expression in the small intestine, suggesting that K-877 administration induced HDL-C production in the liver and small intestine and reduced cholesterol absorption in the small intestine. In conclusion, K-877 administration had pronounced effects on the liver and small intestine in Ldlr^-/- mice. K-877 is an attractive PPARα-modulating drug for treating hyperlipidemia that works equally well in both the liver and small intestine.

Vascular Disease Is Associated With the Expression of Genes for Intestinal Cholesterol Transport and Metabolism

CONTEXT

Intestinal cholesterol metabolism is important in influencing postprandial lipoprotein concentrations, and might be important in the development of vascular disease.

OBJECTIVE

This study evaluated associations between expression of intestinal cholesterol metabolism genes, postprandial lipid metabolism, and endothelial function/early vascular disease in human subjects.

DESIGN/PATIENTS

One hundred patients undergoing routine oesophago-gastro-duodenoscopy were recruited. mRNA levels of Nieman-Pick C1-like 1 protein (NPC1L1), ABC-G5, ABC-G8, ABC-A1, microsomal tissue transport protein (MTTP), and sterol-regulatory element-binding protein (SREBP)-2 were measured in duodenal biopsies using quantitative reverse transcription polymerase chain reaction. Postprandially, serum lipid and glycemic profiles were measured, endothelial function was assessed using fasting, and postprandial flow-mediated dilatation (FMD) and carotid intima-media thickness (IMT). Subjects were divided into those above and below the median value of relative expression of each gene, and results were compared between the groups.

RESULTS

There were no between-group differences in demographic variables or classical cardiovascular risks. For all genes, the postprandial triglyceride incremental area under the curve was greater (P < 0.05) in the group with greater expression. Postprandial apolipoprotein B48 (ApoB48) levels were greater (P < 0.05) in groups with greater expression of NPC1L1, ABC-G8, and SREBP-2. For all genes, postprandial but not fasting FMD was lower (P < 0.01) in the group with greater expression. Triglyceride and ApoB48 levels correlated significantly with postprandial FMD. Carotid artery IMT was greater (P < 0.05) in groups with greater expression of MTTP, ABC-A1, and SREBP-2.

CONCLUSION

Intestinal cholesterol metabolism gene expression is significantly associated with postprandial increment in triglycerides, intestinal ApoB48, and reduced postprandial FMD. Some genes were also associated with increased IMT. These findings suggest a role of intestinal cholesterol metabolism in development of early vascular disease.

Ezetimibe increases intestinal expression of the LDL receptor gene in dyslipidaemic men with insulin resistance

AIM

To gain further insight into intestinal cholesterol homeostasis in dyslipidaemic men with insulin resistance (IR) by examining the impact of treatment with ezetimibe on the expression of key genes involved in cholesterol synthesis and LDL receptor (R)-mediated uptake of lipoproteins.

METHODS

A total of 25 men with dyslipidaemia and IR were recruited to participate in this double-blind, randomized, crossover, placebo-controlled trial. Participants received 10 mg/day ezetimibe or placebo for periods of 12 weeks each. Intestinal gene expression was measured by quantitative PCR in duodenal biopsy samples collected by gastroduodenoscopy at the end of each treatment.

RESULTS

A total of 20 participants completed the protocol. Treatment with ezetimibe significantly increased intestinal LDLR (+16.2%; P = .01), 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMG-CoAR; +14.0%; P = .04) and acetyl-Coenzyme A acetyltransferase 2 (ACAT-2) mRNA expression (+12.5%; P = .03). Changes in sterol regulatory element-binding transcription factor 2 (SREBP-2) expression were significantly correlated with changes in HMG-CoAR (r = 0.55; P < .05), ACAT-2 (r = 0.69; P < .001) and proprotein convertase substilisin/kexin type 9 (PCSK9) expression (r = 0.45; P < .05).

CONCLUSIONS

These results show that inhibition of intestinal cholesterol absorption by ezetimibe increases expression of the LDLR gene, supporting the concept that increased LDL clearance with ezetimibe treatment occurs not only in the liver but also in the small intestine.

Regulated Inflammation and Lipid Metabolism in Colon mRNA Expressions of Obese Germfree Mice Responding to Enterobacter cloacae B29 Combined with the High Fat Diet

Increased evidences have demonstrated that gut microbiota targeted diet intervention can alleviate obesity and related metabolic disorders. The underlying mechanism of interactions among diet, microbiota, and host still remains unclear. Enterobacter cloacae B29, an endotoxin-producing strain dominated in the gut of a morbidly obese volunteer (weight 174.8 kg, BMI 58.8 kg m^-2) was isolated and transplanted to germfree mice (inoculated 10¹⁰ cells of B29 per day for 1 week). Using deep mRNA sequencing technology, we compared different gene expression profiles in the colon samples of the germfree mice treated with/without B29 and/or high fat diet (HFD) for 16 weeks and identified 279 differential expressed genes in total, including up-regulated genes Apoa4 (fold change, 2.77), Ido1 (2.66), Cyp4a10 (7.01), and down-regulated genes Cyp2e1 (0.11), Cyp26b1 (0.34), Akr1b7 (0.42), Adipoq (0.36), Cyp1a1 (0.11), Apoa1 (0.44), Npc1l1 (0.37), Tff2 (0.13), Apoc1 (0.30), Ctla2a (0.34), Mttp (0.49), Lpl (0.48). Fifty-nine GO biological processes and five KEGG pathways, particularly the peroxisome proliferator-activated receptors signaling pathway, were significantly enriched in response to HFD+B29, which were mainly relevant to inflammation and the metabolism of lipid, lipoprotein, and sterols. These functional changes were consistent with the developed obesity, insulin-resistance, and aggravated inflammatory conditions of the HFD+B29 mice. This work provides insight into the gene expression changes in response to HFD+B29, helping to understand the mechanism of the interactions among HFD, B29 and the germfree mice.

Targeting microsomal triglyceride transfer protein and lipoprotein assembly to treat homozygous familial hypercholesterolemia

Homozygous familial hypercholesterolemia (HoFH) is a polygenic disease arising from defects in the clearance of plasma low-density lipoprotein (LDL), which results in extremely elevated plasma LDL cholesterol (LDL-C) and increased risk of atherosclerosis, coronary heart disease, and premature death. Conventional lipid-lowering therapies, such as statins and ezetimibe, are ineffective at lowering plasma cholesterol to safe levels in these patients. Other therapeutic options, such as LDL apheresis and liver transplantation, are inconvenient, costly, and not readily available. Recently, lomitapide was approved by the Federal Drug Administration as an adjunct therapy for the treatment of HoFH. Lomitapide inhibits microsomal triglyceride transfer protein (MTP), reduces lipoprotein assembly and secretion, and lowers plasma cholesterol levels by over 50%. Here, we explain the steps involved in lipoprotein assembly, summarize the role of MTP in lipoprotein assembly, explore the clinical and molecular basis of HoFH, and review pre-clinical studies and clinical trials with lomitapide and other MTP inhibitors for the treatment of HoFH. In addition, ongoing research and new approaches underway for better treatment modalities are discussed.

Intestinal CREBH overexpression prevents high-cholesterol diet-induced hypercholesterolemia by reducing Npc1l1 expression

Objective

The transcription factor cyclic AMP-responsive element-binding protein H (CREBH, encoded by Creb3l3) is highly expressed in the liver and small intestine. Hepatic CREBH contributes to glucose and triglyceride metabolism by regulating fibroblast growth factor 21 (Fgf21) expression. However, the intestinal CREBH function remains unknown.

Methods

To investigate the influence of intestinal CREBH on cholesterol metabolism, we compared plasma, bile, fecal, and tissue cholesterol levels between wild-type (WT) mice and mice overexpressing active human CREBH mainly in the small intestine (CREBH Tg mice) under different dietary conditions.

Results

Plasma cholesterol, hepatic lipid, and cholesterol crystal formation in the gallbladder were lower in CREBH Tg mice fed a lithogenic diet (LD) than in LD-fed WTs, while fecal cholesterol output was higher in the former. These results suggest that intestinal CREBH overexpression suppresses cholesterol absorption, leading to reduced plasma cholesterol, limited hepatic supply, and greater excretion. The expression of Niemann–Pick C1-like 1 (Npc1l1), a rate-limiting transporter mediating intestinal cholesterol absorption, was reduced in the small intestine of CREBH Tg mice. Adenosine triphosphate-binding cassette transporter A1 (Abca1), Abcg5/8, and scavenger receptor class B, member 1 (Srb1) expression levels were also reduced in CREBH Tg mice. Promoter assays revealed that CREBH directly regulates Npc1l1 expression. Conversely, CREBH null mice exhibited higher intestinal Npc1l1 expression, elevated plasma and hepatic cholesterol, and lower fecal output.

Conclusion

Intestinal CREBH regulates dietary cholesterol flow from the small intestine by controlling the expression of multiple intestinal transporters. We propose that intestinal CREBH could be a therapeutic target for hypercholesterolemia.

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Plasma cholesterol, hepatic lipid, and gallstones were lower in CREBH Tg mice.

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Expression of intestinal Npc1l1 was reduced in CREBH Tg mice.

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CREBH directly down-regulates mouse Npc1l1 promoter activity.

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Intestinal CREBH regulates dietary cholesterol flow from the small intestine.

Exposure to dietary lipid leads to rapid production of cytosolic lipid droplets near the brush border membrane

Background

Intestinal absorption of dietary lipids involves their hydrolysis in the lumen of proximal intestine as well as uptake, intracellular transport and re-assembly of hydrolyzed lipids in enterocytes, leading to the formation and secretion of the lipoproteins chylomicrons and HDL. In this study, we examined the potential involvement of cytosolic lipid droplets (CLD) whose function in the process of lipid absorption is poorly understood.

Methods

Intestinal lipid absorption was studied in mouse after gavage. Three populations of CLD were purified by density ultracentrifugations, as well as the brush border membranes, which were analyzed by western-blots. Immunofluorescent localization of membranes transporters or metabolic enzymes, as well as kinetics of CLD production, were also studied in intestine or Caco-2 cells.

Results

We isolated three populations of CLD (ranging from 15 to 1000 nm) which showed differential expression of the major lipid transporters scavenger receptor BI (SR-BI), cluster of differentiation 36 (CD-36), Niemann Pick C-like 1 (NPC1L1), and the ATP-binding cassette transporters ABCG5/G8 but also caveolin 2 and fatty acid binding proteins. The enzyme monoacylglycerol acyltransferase 2 (MGAT2) was identified in the brush border membrane (BBM) in addition to the endoplasmic reticulum, suggesting local synthesis of triglycerides and CLD at both places.

Conclusions

We show a very fast production of CLD by enterocytes associated with a transfer of apical constituents as lipid transporters. Our findings suggest that following their uptake by enterocytes, lipids can be partially metabolized at the BBM and packaged into CLD for their transportation to the ER.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-016-0107-9) contains supplementary material, which is available to authorized users.

Recent discoveries on absorption of dietary fat: Presence, synthesis, and metabolism of cytoplasmic lipid droplets within enterocytes

Dietary fat provides essential nutrients, contributes to energy balance, and regulates blood lipid concentrations. These functions are important to health, but can also become dysregulated and contribute to diseases such as obesity, diabetes, cardiovascular disease, and cancer. Within enterocytes, the digestive products of dietary fat are re-synthesized into triacylglycerol, which is either secreted on chylomicrons or stored within cytoplasmic lipid droplets (CLDs). CLDs were originally thought to be inert stores of neutral lipids, but are now recognized as dynamic organelles that function in multiple cellular processes in addition to lipid metabolism. This review will highlight recent discoveries related to dietary fat absorption with an emphasis on the presence, synthesis, and metabolism of CLDs within this process.

Invited review: Growth-promoting effects of colostrum in calves based on interaction with intestinal cell surface receptors and receptor-like transporters

The postnatal development and maturation of the gastrointestinal (GI) tract of neonatal calves is crucial for their survival. Major morphological and functional changes in the calf's GI tract initiated by colostrum bioactive substances promote the establishment of intestinal digestion and absorption of food. It is generally accepted that colostrum intake provokes the maturation of organs and systems in young calves, illustrating the significance of the cow-to-calf connection at birth. These postnatal adaptive changes of the GI tissues in neonatal calves are especially induced by the action of bioactive substances such as insulin-like growth factors, hormones, or cholesterol carriers abundantly present in colostrum. These substances interact with specific cell-surface receptors or receptor-like transporters expressed in the GI wall of neonatal calves to elicit their biological effects. Therefore, the abundance and activity of cell surface receptors and receptor-like transporters binding colostral bioactive substances are a key aspect determining the effects of the cow-to-calf connection at birth. The present review compiles the information describing the effects of colostrum feeding on selected serum metabolic and endocrine traits in neonatal calves. In this context, the current paper discusses specifically the consequences of colostrum feeding on the GI expression and activity of cell-receptors and receptor-like transporters binding growth hormone, insulin-like growth factors, insulin, or cholesterol acceptors in neonatal calves.

Hexim1, a Novel Regulator of Leptin Function, Modulates Obesity and Glucose Disposal

Leptin triggers signaling events with significant transcriptional responses that are essential to metabolic processes affecting obesity and glucose disposal. We asked whether hexamethylene bis-acetamide inducible-1 (Hexim1), an inhibitor of RNA II polymerase-dependent transcription elongation, regulates leptin-Janus kinase 2 signaling axis in the hypothalamus. We subjected C57BL6 Hexim1 heterozygous (HT) mice to high-fat diet and when compared with wild type, HT mice were resistant to high-fat diet-induced weight gain and remain insulin sensitive. HT mice exhibited increased leptin-pY(705)Stat3 signaling in the hypothalamus, with normal adipocyte size, increased type I oxidative muscle fiber density, and enhanced glucose transporter 4 expression. We also observed that normal Hexim1 protein level is required to facilitate the expression of CCAAT/enhancer-binding proteins (C/EBPs) required for adipogenesis and inducible suppressor of cytokine signaling 3 (SOCS) expression. Further support on the role of Hexim1 regulating C/EBPs during adipocyte differentiation was shown when HT 3T3L1 fibroblasts failed to undergo adipogenesis. Hexim1 selectively modulates leptin-mediated signal transduction pathways in the hypothalamus, the expression of C/EBPs and peroxisome proliferator-activated receptor-γ (PPAR γ) in skeletal muscle and adipose tissue during the adaptation to metabolic stress. We postulate that Hexim1 might be a novel factor involved in maintaining whole-body energy balance.

Small Intestine but Not Liver Lysophosphatidylcholine Acyltransferase 3 (Lpcat3) Deficiency Has a Dominant Effect on Plasma Lipid Metabolism

Lysophosphatidylcholine acyltransferase 3 (Lpcat3) is involved in phosphatidylcholine remodeling in the small intestine and liver. We investigated lipid metabolism in inducible intestine-specific and liver-specificLpcat3gene knock-out mice. We producedLpcat3-Flox/villin-Cre-ER(T2)mice, which were treated with tamoxifen (at days 1, 3, 5, and 7), to deleteLpcat3specifically in the small intestine. At day 9 after the treatment, we found that Lpcat3 deficiency in enterocytes significantly reduced polyunsaturated phosphatidylcholines in the enterocyte plasma membrane and reduced Niemann-Pick C1-like 1 (NPC1L1), CD36, ATP-binding cassette transporter 1 (ABCA1), and ABCG8 levels on the membrane, thus significantly reducing lipid absorption, cholesterol secretion through apoB-dependent and apoB-independent pathways, and plasma triglyceride, cholesterol, and phospholipid levels, as well as body weight. Moreover, Lpcat3 deficiency does not cause significant lipid accumulation in the small intestine. We also utilized adenovirus-associated virus-Cre to depleteLpcat3in the liver. We found that liver deficiency only reduces plasma triglyceride levels but not other lipid levels. Furthermore, there is no significant lipid accumulation in the liver. Importantly, small intestine Lpcat3 deficiency has a much bigger effect on plasma lipid levels than that of liver deficiency. Thus, inhibition of small intestine Lpcat3 might constitute a novel approach for treating hyperlipidemia.

Dietary fat and carbohydrate modulate the effect of the ATP-binding cassette A1 (ABCA1) R230C variant on metabolic risk parameters in premenopausal women from the Genetics of Atherosclerotic Disease (GEA) Study

Background

Although the R230C-ATP-binding cassette A1 (ABCA1) variant has been consistently associated with HDL-C levels, its association with diabetes and other metabolic parameters is unclear. Estrogen and dietary factors are known to regulate ABCA1 expression in different tissues. Thus, we aimed to explore whether gender, menopausal status and macronutrient proportions of diet modulate the effect of this variant on various metabolic parameters.

Methods

One thousand five hundred ninety-eight controls from the GEA study were included (787 men, 363 premenopausal women and 448 menopausal women), previously assessed for anthropometric and biochemical measurements and visceral to subcutaneous abdominal fat (VAT/SAT) ratio on computed tomography. Taqman assays were performed for genotyping. Diet macronutrient proportions were assessed using a food frequency questionnaire validated for the Mexican population. Multivariate regression models were constructed to assess the interaction between the proportion of dietary macronutrients and the R230C polymorphism on metabolic parameters.

Results

All significant interactions were observed in premenopausal women. Those carrying the risk allele and consuming higher carbohydrate/lower fat diets showed an unfavorable metabolic pattern [lower HDL-C and adiponectin levels, higher VAT/SAT ratio, homeostasis model assessment for insulin resistance (HOMA-IR) and higher gamma-glutamyl transpeptidase (GGT) and alkaline phosphatase (ALP) levels]. Conversely, premenopausal women carrying the risk allele and consuming lower carbohydrate/higher fat diets showed a more favorable metabolic pattern (higher HDL-C and adiponectin levels, and lower VAT/SAT ratio, HOMA-IR, GGT and ALP levels).

Conclusion

This is the first study reporting a gender-specific interaction between ABCA1/R230C variant and dietary carbohydrate and fat percentages affecting VAT/SAT ratio, GGT, ALP, adiponectin levels and HOMA index. Our study confirmed the previously reported gender-specific ABCA1-diet interaction affecting HDL-C levels observed in an independent study. Our results show how gene-environment interactions may help further understand how certain gene variants confer metabolic risk, and may provide information useful to design diet intervention studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-015-0040-3) contains supplementary material, which is available to authorized users.

Microsomal Triglyceride Transfer Protein Transfers and Determines Plasma Concentrations of Ceramide and Sphingomyelin but Not Glycosylceramide

Sphingolipids, a large family of bioactive lipids, are implicated in stress responses, differentiation, proliferation, apoptosis, and other physiological processes. Aberrant plasma levels of sphingolipids contribute to metabolic disease, atherosclerosis, and insulin resistance. They are fairly evenly distributed in high density and apoB-containing lipoproteins (B-lps). Mechanisms involved in the transport of sphingolipids to the plasma are unknown. Here, we investigated the role of microsomal triglyceride transfer protein (MTP), required for B-lp assembly and secretion, in sphingolipid transport to the plasma. Abetalipoproteinemia patients with deleterious mutations in MTP and absence of B-lps had significantly lower plasma ceramide and sphingomyelin but normal hexosylceramide, lactosylceramide, and different sphingosines compared with unaffected controls. Furthermore, similar differential effects on plasma sphingolipids were seen in liver- and intestine-specific MTP knock-out (L,I-Mttp(-/-)) mice, suggesting that MTP specifically plays a role in the regulation of plasma ceramide and sphingomyelin. We hypothesized that MTP deficiency may affect either their synthesis or secretion. MTP deficiency had no effect on ceramide and sphingomyelin synthesis but reduced secretion from primary hepatocytes and hepatoma cells. Therefore, MTP is involved in ceramide and sphingomyelin secretion but not in their synthesis. We also found that MTP transferred these lipids between vesicles in vitro. Therefore, we propose that MTP might regulate plasma ceramide and sphingomyelin levels by transferring these lipids to B-lps in the liver and intestine and facilitating their secretion.

Quercetin represses apolipoprotein B expression by inhibiting the transcriptional activity of C/EBPβ

Quercetin is one of the most abundant polyphenolic flavonoids found in fruits and vegetables and has anti-oxidative and anti-obesity effects. Because the small intestine is a major absorptive organ of dietary nutrients, it is likely that highly concentrated food constituents, including polyphenols, are present in the small intestinal epithelial cells, suggesting that food factors may have a profound effect in this tissue. To identify novel targets of quercetin in the intestinal enterocytes, mRNA profiling using human intestinal epithelial Caco-2 cells was performed. We found that mRNA levels of some apolipoproteins, particularly apolipoprotein B (apoB), are downregulated in the presence of quercetin. On the exposure of Caco-2 cells to quercetin, both mRNA and protein levels of apoB were decreased. Promoter analysis of the human apoB revealed that quercetin response element is localized at the 5′-proximal promoter region, which contains a conserved CCAAT enhancer-binding protein (C/EBP)-response element. We found that quercetin reduces the promoter activity of apoB, driven by the enforced expression of C/EBPβ. Quercetin had no effect on either mRNA or protein levels of C/EBPβ. In contrast, we found that quercetin inhibits the transcriptional activity of C/EBPβ but not its recruitment to the apoB promoter. On the exposure of Caco-2 cells to quercetin 3-O-glucuronide, which is in a cell-impermeable form, no notable change in apoB mRNA was observed, suggesting an intracellular action of quercetin. In vitro interaction experiments using quercetin-conjugated beads revealed that quercetin binds to C/EBPβ. Our results describe a novel regulatory mechanism of transcription of apolipoprotein genes by quercetin in the intestinal enterocytes.

Treatment of low HDL-C subjects with the CETP modulator dalcetrapib increases plasma campesterol only in those without ABCA1 and/or ApoA1 mutations

We investigated the effect of dalcetrapib treatment on phytosterol levels in patients with familial combined hyperlipidemia (FCH) or familial hypoalphalipoproteinemia (FHA) due to mutations in apolipoprotein A1 (ApoA1) or ATP-binding cassette transporter A1 (ABCA1). Patients (n = 40) with FCH or FHA received dalcetrapib 600 mg or placebo in this 4-week, double-blind, crossover study. Lipids, apolipoproteins, cholesteryl ester transfer protein (CETP) activity and mass, and phytosterols were assessed. Dalcetrapib increased high-density lipoprotein cholesterol (HDL-C) and ApoA1 levels to a similar extent in FHA (+22.8, +13.9%) and FCH (+18.4, +12.1%), both p < 0.001 vs. placebo. Changes in CETP activity and mass were comparable for FHA (-31.5, +120.9%) and FCH (-26.6, +111.9%), both p < 0.0001 vs. placebo. Campesterol and lathosterol were unchanged in FHA (+3.8, +3.0%), but only campesterol was markedly increased in FCH (+25.0%, p < 0.0001 vs. placebo). Campesterol increased with dalcetrapib treatment in FCH but not in FHA, despite comparable HDL-C and ApoA1 increases, suggesting that ApoA1 and/or ABCA1 is essential for HDL lipidation by enterocytes in humans.

ATP-binding cassette transporter A1: key player in cardiovascular and metabolic disease at local and systemic level

PURPOSE OF REVIEW

ATP-binding cassette transporter A1 (ABCA1) facilitates cellular cholesterol efflux to lipid-poor apolipoprotein AI (apoAI) and plays a key role in the formation and function of HDL. This review summarizes the advances and new insights in the role of ABCA1 in cardiovascular and metabolic diseases from studies in genetically engineered mice.

RECENT FINDINGS

Recent studies show that low HDL associated with liver-specific deletion of ABCA1 does not affect macrophage reverse cholesterol transport or atherosclerosis susceptibility. In the intestine, ABCA1 contributes to the packaging of dietary cholesterol into HDL. Locally in the arterial wall, ABCA1 influences atherosclerosis by acting not only in bone marrow-derived cells but also in endothelial cells and smooth muscle cells. Furthermore, other than its established role in regulating insulin secretion by β-cells, evidence is provided that adipocyte-specific ABCA1 prevents fat storage and the development of impaired glucose tolerance. Moreover, new insights are provided on the post-transcriptional regulation of ABCA1 expression by microRNAs.

SUMMARY

Recent studies underscore the importance of ABCA1 in the prevention of cardiovascular and metabolic diseases. Furthermore, the discovery of the extensive regulation of ABCA1 expression by microRNAs has unraveled novel therapeutic targets for ABCA1-based strategies for the treatment of these diseases.

Intestinal nuclear receptors in HDL cholesterol metabolism

The intestine plays a pivotal role in cholesterol homeostasis by functioning as an absorptive and secretory organ in the reverse cholesterol transport pathway. Enterocytes control cholesterol absorption, apoAI synthesis, HDL biogenesis, and nonbiliary cholesterol fecal disposal. Thus, intestine-based therapeutic interventions may hold promise in the management of diseases driven by cholesterol overload. Lipid-sensing nuclear receptors (NRs) are highly expressed in the intestinal epithelium and regulate transcriptionally the handling of cholesterol by the enterocytes. Here, we discuss the NR regulation of cholesterol fluxes across the enterocytes with special emphasis on NR exploitation as a bona fide novel HDL-raising strategy.

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.

Intestinal lipid absorption and lipoprotein formation

PURPOSE OF REVIEW

To summarize the evidence for the presence of two lipid absorption pathways and their regulation.

RECENT FINDINGS

Lipid absorption involves hydrolysis of dietary fat in the lumen of the intestine, followed by the uptake of hydrolyzed products by enterocytes. Lipids are resynthesized in the endoplasmic reticulum and are either secreted with chylomicrons and HDLs or stored as cytoplasmic lipid droplets. Lipids in the droplets are hydrolyzed and are secreted at a later time. Secretion of lipids by the chylomicron and HDL pathways are dependent on microsomal triglyceride transfer protein (MTP) and ATP-binding cassette family A protein 1, respectively, and are regulated independently. Gene-ablation studies showed that MTP function and chylomicron assembly is essential for the absorption of triglycerides. Ablation of MTP abolishes triglyceride absorption and results in massive triglyceride accumulation in enterocytes. Although the majority of phospholipid, cholesterol, and vitamin E are absorbed through the chylomicron pathway, a significant amount of these lipids are also absorbed via the HDL pathway. Chylomicron assembly and secretion is increased by the enhanced availability of fatty acids, whereas the HDL pathway is upregulated by liver X receptor agonists.

SUMMARY

Triglycerides are exclusively transported with chylomicrons and this process is critically dependent on MTP. In addition to chylomicrons, absorption of phospholipids, free cholesterol, retinol, and vitamin E also involves HDLs. These two pathways are complementary and are regulated independently. They may be targeted to lower lipid absorption in order to control hyperlipidemia, obesity, metabolic syndrome, steatosis, insulin resistance, atherosclerosis, and other disorders.

Differential impact of hepatic deficiency and total body inhibition of MTP on cholesterol metabolism and RCT in mice

Because apoB-containing lipoproteins are pro-atherogenic and their secretion by liver and intestine largely depends on microsomal triglyceride transfer protein (MTP) activity, MTP inhibition strategies are actively pursued. How decreasing the secretion of apoB-containing lipoproteins affects intracellular rerouting of cholesterol is unclear. Therefore, the aim of the present study was to determine the effects of reducing either systemic or liver-specific MTP activity on cholesterol metabolism and reverse cholesterol transport (RCT) using a pharmacological MTP inhibitor or a genetic model, respectively. Plasma total cholesterol and triglyceride levels were decreased in both MTP inhibitor-treated and liver-specific MTP knockout (L-Mttp(-/-)) mice (each P < 0.001). With both inhibition approaches, hepatic cholesterol as well as triglyceride content was consistently increased (each P < 0.001), while biliary cholesterol and bile acid secretion remained unchanged. A small but significant decrease in fecal bile acid excretion was observed in inhibitor-treated mice (P < 0.05), whereas fecal neutral sterol excretion was substantially increased by 75% (P < 0.001), conceivably due to decreased intestinal absorption. In contrast, in L-Mttp(-/-) mice both fecal neutral sterol and bile acid excretion remained unchanged. However, while total RCT increased in inhibitor-treated mice (P < 0.01), it surprisingly decreased in L-Mttp(-/-) mice (P < 0.05). These data demonstrate that: i) pharmacological MTP inhibition increases RCT, an effect that might provide additional clinical benefit of MTP inhibitors; and ii) decreasing hepatic MTP decreases RCT, pointing toward a potential contribution of hepatocyte-derived VLDLs to RCT.