Absence of microsomal triglyceride transfer protein in individuals with abetalipoproteinemia

Transport functions of intestinal lymphatic vessels

Lymphatic vessels are crucial for fluid absorption and the transport of peripheral immune cells to lymph nodes. However, in the small intestine, the lymphatic fluid is rich in diet-derived lipids incorporated into chylomicrons and gut-specific immune cells. Thus, intestinal lymphatic vessels have evolved to handle these unique cargoes and are critical for systemic dietary lipid delivery and metabolism. This Review covers mechanisms of lipid absorption from epithelial cells to the lymphatics as well as unique features of the gut microenvironment that affect these functions. Moreover, we discuss details of the intestinal lymphatics in gut immune cell trafficking and insights into the role of inter-organ communication. Lastly, we highlight the particularities of fat absorption that can be harnessed for efficient lipid-soluble drug distribution for novel therapies, including the ability of chylomicron-associated drugs to bypass first-pass liver metabolism for systemic delivery. In all, this Review will help to promote an understanding of intestinal lymphatic-systemic interactions to guide future research directions.

The biogenesis and transport of triglyceride-rich lipoproteins

Triglyceride-rich lipoproteins (TRLs) play essential roles in human health and disease by transporting bulk lipids into the circulation. This review summarizes the fundamental mechanisms and diverse factors governing lipoprotein production, secretion, and regulation. Emphasizing the broader implications for human health, we outline the intricate landscape of lipoprotein research and highlight the potential coordination between the biogenesis and transport of TRLs in physiology, particularly the unexpected coupling of metabolic enzymes and transport machineries. Challenges and opportunities in lipoprotein biology with respect to inherited diseases and viral infections are also discussed. Further characterization of the biogenesis and transport of TRLs will advance both basic research in lipid biology and translational medicine for metabolic diseases.

High carrier frequency for abetalipoproteinemia and evidence of a founder variant in a French-Canadian population

Abetalipoproteinemia (ABL) is a rare recessive genetic disease caused by bi-allelic pathogenic variants in the microsomal triglyceride transfer protein (MTTP) gene. This disease is characterized by a deficiency in the secretion of apolipoprotein B-containing lipoproteins. Patients with ABL present with neurological, hematological, and gastrointestinal symptoms due to fat malabsorption and a deficiency in liposoluble vitamins. In this report, we present a total of four ABL cases, including three new cases, all originating from the same French-Canadian founder population in Saguenay-Lac-Saint-Jean, Québec, Canada. These individuals are homozygous for the same pathogenic variant in the MTTP gene (c.419dup, p.Asn140Lysfs*2). We found that this variant is more common than anticipated in this population, with an estimated carrier frequency of 1:203. Early diagnosis is essential to initiate treatment known to prevent complications associated with ABL. Population carrier screening or newborn screening for ABL should be considered in this French-Canadian founder population.

An overview of the cholesterol metabolism and its proinflammatory role in the development of MASLD

Cholesterol is an essential lipid molecule in mammalian cells. It is not only involved in the formation of cell membranes but also serves as a raw material for the synthesis of bile acids, vitamin D, and steroid hormones. Additionally, it acts as a covalent modifier of proteins and plays a crucial role in numerous life processes. Generally, the metabolic processes of cholesterol absorption, synthesis, conversion, and efflux are strictly regulated. Excessive accumulation of cholesterol in the body is a risk factor for metabolic diseases such as cardiovascular disease, type 2 diabetes, and metabolic dysfunction-associated steatotic liver disease (MASLD). In this review, we first provide an overview of the discovery of cholesterol and the fundamental process of cholesterol metabolism. We then summarize the relationship between dietary cholesterol intake and the risk of developing MASLD, and also the animal models of MASLD specifically established with a cholesterol-containing diet. In the end, the role of cholesterol-induced inflammation in the initiation and development of MASLD is discussed.

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.

ANGPTL3 deficiency impairs lipoprotein production and produces adaptive changes in hepatic lipid metabolism

Angiopoietin-like protein 3 (ANGPTL3) is a hepatically secreted protein and therapeutic target for reducing plasma triglyceride-rich lipoproteins and low-density lipoprotein (LDL) cholesterol. Although ANGPTL3 modulates the metabolism of circulating lipoproteins, its role in triglyceride-rich lipoprotein assembly and secretion remains unknown. CRISPR-associated protein 9 (CRISPR/Cas9) was used to target ANGPTL3 in HepG2 cells (ANGPTL3-/-) whereupon we observed ∼50% reduction of apolipoprotein B100 (ApoB100) secretion, accompanied by an increase in ApoB100 early presecretory degradation via a predominantly lysosomal mechanism. Despite defective particle secretion in ANGPTL3-/- cells, targeted lipidomic analysis did not reveal neutral lipid accumulation in ANGPTL3-/- cells; rather ANGPTL3-/- cells demonstrated decreased secretion of newly synthesized triglycerides and increased fatty acid oxidation. Furthermore, RNA sequencing demonstrated significantly altered expression of key lipid metabolism genes, including targets of peroxisome proliferator-activated receptor α, consistent with decreased lipid anabolism and increased lipid catabolism. In contrast, CRISPR/Cas9 LDL receptor (LDLR) deletion in ANGPTL3-/- cells did not result in a secretion defect at baseline, but proteasomal inhibition strongly induced compensatory late presecretory degradation of ApoB100 and impaired its secretion. Additionally, these ANGPTL3-/-;LDLR-/- cells rescued the deficient LDL clearance of LDLR-/- cells. In summary, ANGPTL3 deficiency in the presence of functional LDLR leads to the production of fewer lipoprotein particles due to early presecretory defects in particle assembly that are associated with adaptive changes in intrahepatic lipid metabolism. In contrast, when LDLR is absent, ANGPTL3 deficiency is associated with late presecretory regulation of ApoB100 degradation without impaired secretion. Our findings therefore suggest an unanticipated intrahepatic role for ANGPTL3, whose function varies with LDLR status.

Conditional hepatocyte ablation of PDIA1 uncovers indispensable roles in both APOB and MTTP folding to support VLDL secretion

OBJECTIVES

The assembly and secretion of hepatic very low-density lipoprotein (VLDL) plays pivotal roles in hepatic and plasma lipid homeostasis. Protein disulfide isomerase A1 (PDIA1/P4HB) is a molecular chaperone whose functions are essential for protein folding in the endoplasmic reticulum. Here we investigated the physiological requirement in vivo for PDIA1 in maintaining VLDL assembly and secretion.

METHODS

Pdia1/P4hb was conditionally deleted in adult mouse hepatocytes and the phenotypes characterized. Mechanistic analyses in primary hepatocytes determined how PDIA1 ablation alters MTTP synthesis and degradation as well as altering synthesis and secretion of Apolipoprotein B (APOB), along with complementary expression of intact PDIA1 vs a catalytically inactivated PDIA1 mutant.

RESULTS

Hepatocyte-specific deletion of Pdia1/P4hb inhibited hepatic MTTP expression and dramatically reduced VLDL production, leading to severe hepatic steatosis and hypolipidemia. Pdia1-deletion did not affect mRNA expression or protein stability of MTTP but rather prevented Mttp mRNA translation. We demonstrate an essential role for PDIA1 in MTTP synthesis and function and show that PDIA1 interacts with APOB in an MTTP-independent manner via its molecular chaperone function to support APOB folding and secretion.

CONCLUSIONS

PDIA1 plays indispensable roles in APOB folding, MTTP synthesis and activity to support VLDL assembly. Thus, like APOB and MTTP, PDIA1 is an obligatory component of hepatic VLDL production.

Carotenoids in familial hypobetalipoproteinemia disorders: Malabsorption in Caco2 cell models and severe deficiency in patients

BACKGROUND

Familial hypobetalipoproteinemias (FHBL) are rare genetic diseases characterized by lipid malabsorption. We focused on abetalipoproteinemia (FHBL-SD1) and chylomicron retention disease (FHBL-SD3), caused by mutations in microsomal triglyceride transfer protein (MTTP) and SAR1B genes, respectively. Treatments include a low-fat diet and high-dose fat-soluble vitamin supplementations. However, patients are not supplemented in carotenoids, a group of lipid-soluble pigments essential for eye health.

OBJECTIVE

Our aim was to evaluate carotenoid absorption and status in the context of hypobetalipoproteinemia.

METHODS

We first used knock-out Caco-2/TC7 cell models of FHBL-SD1 and FHBL-SD3 to evaluate carotenoid absorption. We then characterized FHBL-SD1 and FHBL-SD3 patient status in the main dietary carotenoids and compared it to that of control subjects.

RESULTS

In vitro results showed a significant decrease in basolateral secretion of α- and β-carotene, lutein, and zeaxanthin (-88.8 ± 2.2 % to -95.3 ± 5.8 %, -79.2 ± 4.4 % to -96.1 ± 2.6 %, -91.0 ± 4.5 % to -96.7 ± 0.3 % and -65.4 ± 3.6 % to -96.6 ± 1.9 %, respectively). Carotenoids plasma levels in patients confirmed significant deficiencies, with decreases ranging from -89 % for zeaxanthin to -98 % for α-carotene, compared to control subjects.

CONCLUSION

Given the continuous loss in visual function despite fat-soluble vitamin treatment in some patients, carotenoid supplementation may be of clinical utility. Future studies should assess the correlation between carotenoid status and visual function in aging patients and investigate whether carotenoid supplementation could prevent their visual impairment.

Serum opacity factor normalizes erythrocyte morphology in Scarb1-/- mice in an HDL-free cholesterol-dependent way

Compared with WT mice, HDL receptor-deficient (Scarb1-/-) mice have higher plasma levels of free cholesterol (FC)-rich HDL and exhibit multiple pathologies associated with a high mol% FC in ovaries, platelets, and erythrocytes, which are reversed by lowering HDL. Bacterial serum opacity factor (SOF) catalyzes the opacification of plasma by targeting and quantitatively converting HDL to neo HDL (HDL remnant), a cholesterol ester-rich microemulsion, and lipid-free APOA1. SOF delivery with an adeno-associated virus (AAVSOF) constitutively lowers plasma HDL-FC and reverses female infertility in Scarb1-/- mice in an HDL-dependent way. We tested whether AAVSOF delivery to Scarb1-/- mice will normalize erythrocyte morphology in an HDL-FC-dependent way. We determined erythrocyte morphology and FC content (mol%) in three groups-WT, untreated Scarb1-/- (control), and Scarb1-/- mice receiving AAVSOF-and correlated these with their respective HDL-mol% FC. Plasma-, HDL-, and tissue-lipid compositions were also determined. Plasma- and HDL-mol% FC positively correlated across all groups. Among Scarb1-/- mice, AAVSOF treatment normalized reticulocyte number, erythrocyte morphology, and erythrocyte-mol% FC. Erythrocyte-mol% FC positively correlated with HDL-mol% FC and with both the number of reticulocytes and abnormal erythrocytes. AAVSOF treatment also reduced FC of extravascular tissues to a lesser extent. HDL-FC spontaneously transfers from plasma HDL to cell membranes. AAVSOF treatment lowers erythrocyte-FC and normalizes erythrocyte morphology and lipid composition by reducing HDL-mol% FC.

Generation of a familial hypercholesterolemia model in non-human primate

Familial hypercholesterolemia (FH) is an inherited autosomal dominant disorder that is associated with a high plasma level of low-density lipoprotein (LDL) cholesterol, leading to an increased risk of cardiovascular diseases. To develop basic and translational research on FH, we here generated an FH model in a non-human primate (cynomolgus monkeys) by deleting the LDL receptor (LDLR) gene using the genome editing technique. Six LDLR knockout (KO) monkeys were produced, all of which were confirmed to have mutations in the LDLR gene by sequence analysis. The levels of plasma cholesterol and triglyceride were quite high in the monkeys, and were similar to those in FH patients with homozygous mutations in the LDLR gene. In addition, periocular xanthoma was observed only 1 year after birth. Lipoprotein profile analysis showed that the plasma very low-density lipoprotein and LDL were elevated, while the plasma high density lipoprotein was decreased in LDLR KO monkeys. The LDLR KO monkeys were also strongly resistant to medications for hypercholesterolemia. Taken together, we successfully generated a non-human primate model of hypercholesterolemia in which the phenotype is similar to that of homozygous FH patients.

Identification and characterisation of a rare MTTP variant underlying hereditary non-alcoholic fatty liver disease

Background & Aims

Non-alcoholic fatty liver disease (NAFLD) is a complex trait with an estimated prevalence of 25% globally. We aimed to identify the genetic variant underlying a four-generation family with progressive NAFLD leading to cirrhosis, decompensation, and development of hepatocellular carcinoma in the absence of common risk factors such as obesity and type 2 diabetes.

Methods

Exome sequencing and genome comparisons were used to identify the likely causal variant. We extensively characterised the clinical phenotype and post-prandial metabolic responses of family members with the identified novel variant in comparison with healthy non-carriers and wild-type patients with NAFLD. Variant-expressing hepatocyte-like cells (HLCs) were derived from human-induced pluripotent stem cells generated from homozygous donor skin fibroblasts and restored to wild-type using CRISPR-Cas9. The phenotype was assessed using imaging, targeted RNA analysis, and molecular expression arrays.

Results

We identified a rare causal variant c.1691T>C p.I564T (rs745447480) in MTTP, encoding microsomal triglyceride transfer protein (MTP), associated with progressive NAFLD, unrelated to metabolic syndrome and without characteristic features of abetalipoproteinaemia. HLCs derived from a homozygote donor had significantly lower MTP activity and lower lipoprotein ApoB secretion than wild-type cells, while having similar levels of MTP mRNA and protein. Cytoplasmic triglyceride accumulation in HLCs triggered endoplasmic reticulum stress, secretion of pro-inflammatory mediators, and production of reactive oxygen species.

Conclusions

We have identified and characterised a rare causal variant in MTTP, and homozygosity for MTTP p.I564T is associated with progressive NAFLD without any other manifestations of abetalipoproteinaemia. Our findings provide insights into mechanisms driving progressive NAFLD.

Impact and Implications

A rare genetic variant in the gene MTTP has been identified as responsible for the development of severe non-alcoholic fatty liver disease in a four-generation family with no typical disease risk factors. A cell line culture created harbouring this variant gene was characterised to understand how this genetic variation leads to a defect in liver cells, which results in accumulation of fat and processes that promote disease. This is now a useful model for studying the disease pathways and to discover new ways to treat common types of fatty liver disease.

Association Between Vitamin Deficiencies and Ophthalmological Conditions

Vitamin deficiencies can have adverse effects on health, including on the visual system. The ocular manifestations of a vitamin deficiency are related to the underlying biochemical function of the particular nutrient. While vitamin deficiencies are not common in developed counties, they are still prevalent in parts of the developing world and in specific, vulnerable populations. Vitamin deficiencies can cause or contribute to many ophthalmological conditions and eye diseases may even be the first presenting finding of a vitamin deficiency. As such, it is important for ophthalmologists to be aware of the ocular manifestations of vitamin deficiencies, especially given that the complications can be severe and effectively treated if identified early. This review summarizes the literature on the main vitamins known to have characteristic ocular manifestations: vitamins A, B1, B2, B9, B12, C, D, E and K. The function, epidemiology, manifestations, workup, and management of each vitamin is discussed in detail.

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.

Microsomal triglyceride transfer protein regulates intracellular lipolysis in adipocytes independent of its lipid transfer activity

BACKGROUND

The triglyceride (TG) transfer activity of microsomal triglyceride transfer protein (MTP) is essential for lipoprotein assembly in the liver and intestine; however, its function in adipose tissue, which does not assemble lipoproteins, is unknown. Here we have elucidated the function of MTP in adipocytes.

APPROACH AND RESULTS

We demonstrated that MTP is present on lipid droplets in human adipocytes. Adipose-specific MTP deficient (A-Mttp^-/-) male and female mice fed an obesogenic diet gained less weight than Mttp^f/f mice, had less fat mass, smaller adipocytes and were insulin sensitive. A-Mttp^-/- mice showed higher energy expenditure than Mttp^f/f mice. During a cold challenge, A-Mttp^-/- mice maintained higher body temperature by mobilizing more fatty acids. Biochemical studies indicated that MTP deficiency de-repressed adipose triglyceride lipase (ATGL) activity and increased TG lipolysis. Both wild type MTP and mutant MTP deficient in TG transfer activity interacted with and inhibited ATGL activity. Thus, the TG transfer activity of MTP is not required for ATGL inhibition. C-terminally truncated ATGL that retains its lipase activity interacted less efficiently than full-length ATGL.

CONCLUSION

Our findings demonstrate that adipose-specific MTP deficiency increases ATGL-mediated TG lipolysis and enhances energy expenditure, thereby resisting diet-induced obesity. We speculate that the regulatory function of MTP involving protein-protein interactions might have evolved before the acquisition of TG transfer activity in vertebrates. Adipose-specific inhibition of MTP-ATGL interactions may ameliorate obesity while avoiding the adverse effects associated with inhibition of the lipid transfer activity of MTP.

NAFLD: Mechanisms, Treatments, and Biomarkers

Nonalcoholic fatty liver disease (NAFLD), recently renamed metabolic-associated fatty liver disease (MAFLD), is one of the most common causes of liver diseases worldwide. NAFLD is growing in parallel with the obesity epidemic. No pharmacological treatment is available to treat NAFLD, specifically. The reason might be that NAFLD is a multi-factorial disease with an incomplete understanding of the mechanisms involved, an absence of accurate and inexpensive imaging tools, and lack of adequate non-invasive biomarkers. NAFLD consists of the accumulation of excess lipids in the liver, causing lipotoxicity that might progress to metabolic-associated steatohepatitis (NASH), liver fibrosis, and hepatocellular carcinoma. The mechanisms for the pathogenesis of NAFLD, current interventions in the management of the disease, and the role of sirtuins as potential targets for treatment are discussed here. In addition, the current diagnostic tools, and the role of non-coding RNAs as emerging diagnostic biomarkers are summarized. The availability of non-invasive biomarkers, and accurate and inexpensive non-invasive diagnosis tools are crucial in the detection of the early signs in the progression of NAFLD. This will expedite clinical trials and the validation of the emerging therapeutic treatments.

Histopathology of intestinal villi in neonatal and paediatric age: main features with clinical correlation - Part I

The neonatal and paediatric spectrum of small bowel disorders encompass a wide variety of conditions, ranging from food allergies to life-threatening surgical emergencies or life-long medical conditions and, as such, it comes with a whole set of diagnostic challenges for the non-paediatric pathologist. Histologic examination is a cornerstone of diagnosis in a large number of diseases and may still provide important diagnostic clues in the appropriate clinical context. In this review, divided in two sections, we aim to provide a comprehensive histopathological summary of paediatric small bowel alteration and their differential diagnoses with a reference to the main clinical aspects required for appropriate interpretation. Specifically, in this first part, we describe congenital and metabolic disorders, intestinal lymphangiectasia, immunodeficiencies, GVHD, and necrotising enterocolitis.

Rare primary dyslipidaemias associated with low LDL and HDL cholesterol values in Portugal

Background: Dyslipidaemia represents a group of disorders of lipid metabolism, characterized by either an increase or decrease in lipid particles, usually associated with triglycerides, LDL cholesterol (LDL-C) and/or HDL cholesterol (HDL-C). Most hyperlipidaemias and HDL deficiencies confer an increased cardiovascular risk, while hypolipidaemia, such as abeta or hypobetalipoproteinemia, may present different manifestations ranging from poor weight progression to neurological manifestations. The aim of this study is to present 7 cases with rare dyslipidaemias associated with low LDL or low HDL cholesterol values, referred to our laboratory for the genetic identification of the cause of the dyslipidaemia. Methods: Lipid profile was determined for each individual in an automated equipment Integra Cobas (Roche). Molecular analysis was performed by NGS with a target panel of 57 genes involved in lipid metabolism (Sure select QXT, Agilent) and samples were run in a NextSEQ Sequencer (Illumina). Only genes associated to rare forms of low HDL-c or LDL-c were analysed for this work, namely: ABCA1, APOA1, LCAT, SCARB1, APOB, PCSK9, MTTP, SAR1B, and ANGPTL3. All rare variants (MAF<5%) found in these genes were confirmed by Sanger sequencing. Results and discussion: This study includes 7 index cases (IC), with the following clinical diagnoses: Fish Eye Disease (1), Hypoalphalipoproteinemia (1) and Abetalipoproteinemia (ABL) / Familial Hypobetalipoproteinemia (FHBL) (5). We have identified one IC with a compound heterozygosity in LCAT causing Fish Eye Disease and one IC with a variant in ABCA1 in homozygosity causing Tangier disease. We found variants causing homozygous FHBL in 2 IC, one of whom has an undescribed pathogenic variant in homozygosity in APOB (c.12087+1G>A) and the other is a possible compound heterozygous for APOB variants c.2604+1G>A and c.4651C>T/p.(Gln1551*). In two patients only a variant in heterozygosity (c.3365delG/p.(Gly1122Vfs*62) and c.11095A>T/p.(Arg3699*)). In the remaining patient, no variants were identified. NGS proved to be a fundamental key for genetic testing of rare lipid disorders, allowing us to find the genetic cause of disease in 6/7 patients with low HDL-c and LDL-c. Patients with these rare conditions should be identified as early as possible in order to minimize or prevent clinical manifestations. The unsolved case is still under investigation.

Hepatic TM6SF2 Is Required for Lipidation of VLDL in a Pre-Golgi Compartment in Mice and Rats

BACKGROUND & AIMS

Substitution of lysine for glutamic acid at residu 167 in Transmembrane 6 superfamily member 2 (TM6SF2) is associated with fatty liver disease and reduced plasma lipid levels. Tm6sf2^-/- mice replicate the human phenotype but were not suitable for detailed mechanistic studies. As an alternative model, we generated Tm6sf2^-/- rats to determine the subcellular location and function of TM6SF2.

METHODS

Two lines of Tm6sf2^-/- rats were established using gene editing. Lipids from tissues and from newly secreted very low density lipoproteins (VLDLs) were quantified using enzymatic assays and mass spectrometry. Neutral lipids were visualized in tissue sections using Oil Red O staining. The rate of dietary triglyceride (TG) absorption and hepatic VLDL-TG secretion were compared in Tm6sf2^-/- mice and in their wild-type littermates. The intracellular location of TM6SF2 was determined by cell fractionation. Finally, TM6SF2 was immunoprecipitated from liver and enterocytes to identify interacting proteins.

RESULTS

Tm6sf2^-/- rats had a 6-fold higher mean hepatic TG content (56.1 ± 28.9 9 vs 9.8 ± 3.9 mg/g; P < .0001) and lower plasma cholesterol levels (99.0 ± 10.5 vs 110.6 ± 14.0 mg/dL; P = .0294) than their wild-type littermates. Rates of appearance of dietary and hepatic TG into blood were reduced significantly in Tm6sf2^-/- rats (P < .001 and P < .01, respectively). Lipid content of newly secreted VLDLs isolated from perfused livers was reduced by 53% (TG) and 62% (cholesterol) (P = .005 and P = .01, respectively) in Tm6sf2^-/- mice. TM6SF2 was present predominantly in the smooth endoplasmic reticulum and endoplasmic reticulum-Golgi intermediate compartments, but not in Golgi. Both apolipoprotein B-48 and acyl-CoA synthetase long chain family member 5 physically interacted with TM6SF2.

CONCLUSIONS

TM6SF2 acts in the smooth endoplasmic reticulum to promote bulk lipidation of apolipoprotein B-containing lipoproteins, thus preventing fatty liver disease.

Vitamin E: How much is enough, too much and why!

α-Tocopherol (α-T) is a required dietary nutrient for humans and thus is a vitamin. This narrative review focuses on vitamin E structures, functions, biological determinants and its deficiency symptoms in humans. The mechanisms for the preferential α-T tissue enrichment in the human body include the α-T transfer protein (TTPA) and the preferential metabolism of non-α-T forms. Potential new α-T biomarkers, pharmacokinetic data, and whether there are better approaches to evaluate and set the α-T dietary requirement are discussed. Finally, the possible role of α-T supplements in delay of chronic diseases and the evaluation of vitamin E safety are considered.

Absorption, transportation, and distribution of vitamin E homologs

Vitamin E has eight different naturally occurring forms: four tocopherols and four tocotrienols. Because α-tocopherol has three asymmetric carbons, both natural α-tocopherol (RRR-α-tocopherol) and synthetic α-tocopherol (all-rac-α-tocopherol) are utilized in both pharmaceutical products and food additives. Therefore, determining the distribution of vitamin E in the body is very important. With regard to absorption, and transportation of vitamin E, it is suggested that the pathways mediated by three proteins (CD36, SR-BI, and NPC1L1) as well as passive diffusion affect absorption of vitamin E. Vitamin E homologs are mainly transported by very low-density lipoprotein (VLDL) with the α-tocopherol being recognized by the α-tocopherol transfer protein in liver. However, it is also suggested that chylomicrons (CMs) and high-density lipoprotein (HDL) are involved in transportation of vitamin E homologs from the small intestine to each section of peripheral tissue. In particular, it is speculated that vitamin E homologs transportation by CMs and HDL from enterocytes to peripheral tissues such as adipose tissue greatly affects the distribution of vitamin E homologs, excluding α-tocopherol. However, how lipoprotein lipase affects the incorporation of vitamin E homologs containing lipoprotein into peripheral tissues is unclear. Whether there is biodiscrimination when vitamin E homologs are incorporated into peripheral tissues from lipoprotein is an interesting question. It is likely that future research will reveal how individual vitamin E homologs are incorporated into peripheral tissue, especially the brain, adipose tissue, and skin.

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.

DGAT2 stability is increased in response to DGAT1 inhibition in gene edited HepG2 cells

In eukaryotic organisms, two unrelated acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes, DGAT1 and DGAT2, catalyze the final step of the triacylglycerol biosynthetic pathway. Both enzymes are highly expressed in lipogenic tissues, such as adipose tissue, small intestine and the liver. DGAT2 has a prominent role in hepatocyte lipid metabolism synthesizing triacylglycerols that are utilized for very low-density lipoprotein assembly. However, due to the lack of useful antibodies to detect endogenous DGAT2 protein, it has been difficult to determine how this enzyme functions at the cellular level. We have unsuccessfully tested many commercial antibodies as well as our own "in-house" antibodies. There is currently no evidence that DGAT2 undergoes processing such that antigenic epitopes to these antibodies are removed. As an alternative, many studies have utilized epitope tagged versions of DGAT2 overexpressed in cells. These approaches can provide valuable information about a protein, but can be subject to artifacts, such as mislocalization, misregulation, protein aggregation and abnormal protein-protein interactions. In this study, we used gene editing with CRISPR/Cas9 to add three consecutive FLAG epitopes to the C-terminus of endogenous DGAT2 in HepG2 cells. HepG2 cells, derived from a human hepatocellular carcinoma, have been routinely used as a cell model to study human hepatocyte lipid and lipoprotein metabolism. Using this system allowed us to successfully detect DGAT2 expressed from its endogenous locus in HepG2 cells by immunoblotting with anti-FLAG antibodies.

Corticosterone pattern-dependent glucocorticoid receptor binding and transcriptional regulation within the liver

Ultradian glucocorticoid rhythms are highly conserved across mammalian species, however, their functional significance is not yet fully understood. Here we demonstrate that pulsatile corticosterone replacement in adrenalectomised rats induces a dynamic pattern of glucocorticoid receptor (GR) binding at ~3,000 genomic sites in liver at the pulse peak, subsequently not found during the pulse nadir. In contrast, constant corticosterone replacement induced prolonged binding at the majority of these sites. Additionally, each pattern further induced markedly different transcriptional responses. During pulsatile treatment, intragenic occupancy by active RNA polymerase II exhibited pulsatile dynamics with transient changes in enrichment, either decreased or increased depending on the gene, which mostly returned to baseline during the inter-pulse interval. In contrast, constant corticosterone exposure induced prolonged effects on RNA polymerase II occupancy at the majority of gene targets, thus acting as a sustained regulatory signal for both transactivation and repression of glucocorticoid target genes. The nett effect of these differences were consequently seen in the liver transcriptome as RNA-seq analysis indicated that despite the same overall amount of corticosterone infused, twice the number of transcripts were regulated by constant corticosterone infusion, when compared to pulsatile. Target genes that were found to be differentially regulated in a pattern-dependent manner were enriched in functional pathways including carbohydrate, cholesterol, glucose and fat metabolism as well as inflammation, suggesting a functional role for dysregulated glucocorticoid rhythms in the development of metabolic dysfunction.

Adrenal glucocorticoid hormones are released in a characteristic ultradian rhythm that becomes dysregulated during chronic stress, disease, or synthetic corticosteroid treatment. Metabolic dysfunction is a comorbidity associated with all these conditions, but the role that altered glucocorticoid dynamics play is unknown. As the liver is a major site of glucocorticoid action on metabolic homeostasis regulated by the glucocorticoid receptor, we have assessed how different patterns of hormone replacement in adrenalectomised rats differentially regulate gene pathways involved in type II diabetes, cirrhosis, and fatty liver development, via altering the pattern of glucocorticoid receptor binding to regulatory sites. We believe our findings have important implications for therapies that can reproduce the endogenous glucocorticoid rhythm and thus minimize adverse metabolic side-effects in patients.

Dietary egg white protein hydrolysate improves orotic acid-induced fatty liver in rats by promoting hepatic phospholipid synthesis and microsomal triglyceride transfer protein expression

We investigated the effects of egg white protein hydrolysates (EWH) on orotic acid (OA)-induced nonalcoholic fatty liver (NAFL) in rats. Effects of the egg white protein (EWP) and EWH were also compared. Four groups of male Sprague-Dawley rats were separately fed AIN-76-based diets, supplemented with 20% casein for control, or with 1% OA, together with either 20% casein (OA), 20% EWP, or 20% EWH, respectively, for 3 d (developing stage) and 14 d (developed stage). In both feeding periods, animals from the OA group showed higher accumulation hepatic triacylglycerol (TAG) compared with those from the control group. In the 14-d experiment, dietary EWP and EWH significantly reduced the hepatic TAG levels. Intake of EWP reduced liver fat in OA-fed rats by 61%, while EWH reduced it by 92%. In addition, EWH restored the OA-induced high serum-TAG level to that seen in the control group. The 3 d experiment showed that consumption of EWH improved the expression of hepatic MTP, that was reduced by OA, without changing Mttp gene expression. It also increased the hepatic synthesis of PC and PE by enhancing the transcription of Pcyt1 and Pemt genes. Inclusion of EWP and EWH in the diet improves the OA-induced NAFL. EWH reduces the liver TAG better than EWP, and works more rapidly. Dietary EWH ameliorates OA-induced NAFL by promoting the secretion of hepatic TAG.

Ellagic Acid Suppresses ApoB Secretion and Enhances ApoA-1 Secretion from Human Hepatoma Cells, HepG2

The effect of ellagic acid (EA), a naturally occurring polyphenolic compound, on the secretion of apolipoproteins from human hepatocytes, HepG2, was investigated. The levels of apoB and apoA-1 secreted in the cell culture medium were determined by sandwich ELISA. EA did not affect cell viability at the tested concentrations (up to 50 µM). EA suppressed the secretion of apoB and enhanced that of apoA-1 from HepG2 cells. However, cellular apoB levels were increased, suggesting that EA inhibited the trafficking of apoB during the process of secretion. In contrast, the increase in the cellular levels of apoA-1 was consistent with its secreted levels. These results indicate that EA inhibits the secretion of apoB from hepatocytes and increases the secretion of apoA-1. Both of these effects are beneficial for lipoprotein metabolism in the prevention of lifestyle-related diseases. The detailed mechanism underlying these effects of EA on lipoprotein metabolism should be elucidated in the future, but this naturally occurring polyphenolic compound might be antihyperlipidemic. Based on these results, EA is suggested as a candidate food-derived compound for the prevention of hyperlipidemia.

Current Diagnosis and Management of Abetalipoproteinemia

Abetalipoproteinemia (ABL) is a rare autosomal recessive disorder caused by biallelic pathogenic mutations in the MTTP gene. Deficiency of microsomal triglyceride transfer protein (MTTP) abrogates the assembly of apolipoprotein (apo) B-containing lipoprotein in the intestine and liver, resulting in malabsorption of fat and fat-soluble vitamins and severe hypolipidemia. Patients with ABL typically manifest steatorrhea, vomiting, and failure to thrive in infancy. The deficiency of fat-soluble vitamins progressively develops into a variety of symptoms later in life, including hematological (acanthocytosis, anemia, bleeding tendency, etc.), neuromuscular (spinocerebellar ataxia, peripheral neuropathy, myopathy, etc.), and ophthalmological symptoms (e.g., retinitis pigmentosa). If left untreated, the disease can be debilitating and even lethal by the third decade of life due to the development of severe complications, such as blindness, neuromyopathy, and respiratory failure. High dose vitamin supplementation is the mainstay for treatment and may prevent, delay, or alleviate the complications and improve the prognosis, enabling some patients to live to the eighth decade of life. However, it cannot fully prevent or restore impaired function. Novel therapeutic modalities that improve quality of life and prognosis are awaited. The aim of this review is to 1) summarize the pathogenesis, clinical signs and symptoms, diagnosis, and management of ABL, and 2) propose diagnostic criteria that define eligibility to receive financial support from the Japanese government for patients with ABL as a rare and intractable disease. In addition, our diagnostic criteria and the entry criterion of low-density lipoprotein cholesterol (LDL-C) ＜15 mg/dL and apoB ＜15 mg/dL can be useful in universal or opportunistic screening for the disease. Registry research on ABL is currently ongoing to better understand the disease burden and unmet needs of this life-threatening disease with few therapeutic options.

Cellular and sub-cellular mechanisms of lipid transport from gut to lymph

Although the general structure of the barrier between the gut and the blood is well known, many details are still missing. Here, we analyse the literature and our own data related to lipid transcytosis through adult mammalian enterocytes, and their absorption into lymph at the tissue level of the intestine. After starvation, the Golgi complex (GC) of enterocytes is in a resting state. The addition of lipids in the form of chyme leads to the initial appearance of pre-chylomicrons (ChMs) in the tubules of the smooth endoplasmic reticulum, which are attached at the basolateral plasma membrane, immediately below the 'belt' of the adhesive junctions. Then pre-ChMs move into the cisternae of the rough endoplasmic reticulum and then into the expansion of the perforated Golgi cisternae. Next, they pass through the GC, and are concentrated in the distensions of the perforated cisternae on the trans-side of the GC. The arrival of pre-ChMs at the GC leads to the transition of the GC to a state of active transport, with formation of intercisternal connections, attachment of cis-most and trans-most perforated cisternae to the medial Golgi cisternae, and disappearance of COPI vesicles. Post-Golgi carriers then deliver ChMs to the basolateral plasma membrane, fuse with it, and secret ChMs into the intercellular space between enterocytes at the level of their interdigitating contacts. Finally, ChMs are squeezed out into the interstitium through pores in the basal membrane, most likely due to the function of the actin-myosin 'cuff' around the interdigitating contacts. These pores appear to be formed by protrusions of the dendritic cells and the enterocytes per se. ChMs are absorbed from the interstitium into the lymphatic capillaries through the special oblique contacts between endothelial cells, which function as valves through the contraction-relaxation of bundles of smooth muscle cells in the interstitium. Lipid overloading of enterocytes results in accumulation of cytoplasmic lipid droplets, an increase in diameter of ChMs, inhibition of intra-Golgi transport, and fusion of ChMs in the interstitium. Here, we summarise and analyse recent findings, and discuss their functional implications.

General Perspectives for the Treatment of Atherosclerosis

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Atherosclerosis, a cardiovascular disease, is at the top of the list among the diseases leading to death. Although the biochemical and pathophysiological cascades involved within the development of atherosclerosis have been identified clearly, its nature is quite complex to be treated with a single agent targeting a pathway. Therefore, many natural and synthetic compounds have been suggested for the treatment of the disease. The majority of the drugs employed target one of the single components of the pathological outcomes, resulting in many times less effective and longterm treatments. In most cases, treatment options prevent further worsening of the symptoms rather than a radical treatment. Consequently, the current review has been prepared to focus on the validated and non-validated targets of atherosclerosis as well as the alternative treatment options such as hydroxymethyl glutaryl coenzyme A (HMG-CoA) reductase inhibitors, acyl-CoA cholesterol acyl transferase (ACAT) inhibitors, lipoprotein lipase stimulants, bile acid sequestrants, and some antioxidants. Related to the topic, both synthetic compounds designed employing medicinal chemistry skills and natural molecules becoming more popular in drug development are scrutinized in this mini review.

New and Emerging Therapies for Reduction of LDL-Cholesterol and Apolipoprotein B: JACC Focus Seminar 1/4

Adding to the foundation of statins, ezetimibe and proprotein convertase subtilisin-kexin type 9 inhibitors (PCSK9i), novel, emerging low-density lipoprotein cholesterol (LDL-C)-lowering therapies are under development for the prevention of cardiovascular disease. Inclisiran, a small interfering RNA molecule that inhibits PCSK9, only needs to be dosed twice a year and has the potential to help overcome current barriers to persistence and adherence to lipid-lowering therapies. Bempedoic acid, which lowers LDL-C upstream from statins, provides a novel alternative for patients with statin intolerance. Angiopoetin-like 3 protein (ANGPTL3) inhibitors have been shown to provide potent LDL-C lowering in patients with homozygous familial hypercholesterolemia without major adverse effects as seen with lomitapide and mipomersen, and may reduce the need for apheresis. Finally, CETP inhibitors may yet be effective with the development of obicetrapib. These novel agents provide the clinician the tools to effectively lower LDL-C across the entire range of LDL-C-induced elevation of cardiovascular risk, from primary prevention and secondary prevention to null-null homozygous familial hypercholesterolemia patients.

Effect of liver total sphingomyelin synthase deficiency on plasma lipid metabolism

Sphingomyelin (SM) is one major phospholipids on lipoproteins. It is enriched on apolipoprotein B-containing particles, including very low-density lipoprotein (VLDL) and its catabolites, low-density lipoprotein (LDL). SM is synthesized by sphingomyelin synthase 1 and 2 (SMS1 and SMS2) which utilizes ceramide and phosphatidylcholine, as two substrates, to produce SM and diacylglyceride. SMS1 and SMS2 activities are co-expressed in all tested tissues, including the liver where VLDL is produced. Thus, neither Sms1 gene knockout (KO) nor Sms2 KO approach is sufficient to evaluate the effect of SMS on VLDL metabolism. We prepared liver-specific Sms1 KO/global Sms2 KO mice to evaluate the effect of hepatocyte SM biosynthesis in lipoprotein metabolism. We found that hepatocyte total SMS depletion significantly reduces cellular sphingomyelin levels. Also, we found that the deficiency induces cellular glycosphingolipid levels which is specifically related with SMS1 but not SMS2 deficiency. To our surprise, hepatocyte total SMS deficiency has marginal effect on hepatocyte ceramide, diacylglyceride, and phosphatidylcholine levels. Importantly, total SMS deficiency decreases plasma triglyceride but not apoB levels and reduces larger VLDL concentration. The reduction of triglyceride levels also was observed when the animals were on a high fat diet. Our results show that hepatocyte total SMS blocking can reduce VLDL-triglyceride production and plasma triglyceride levels. This phenomenon could be related with a reduction of atherogenicity.

From Congenital Disorders of Fat Malabsorption to Understanding Intra-Enterocyte Mechanisms Behind Chylomicron Assembly and Secretion

During the last two decades, a large body of information on the events responsible for intestinal fat digestion and absorption has been accumulated. In particular, many groups have extensively focused on the absorptive phase in order to highlight the critical "players" and the main mechanisms orchestrating the assembly and secretion of chylomicrons (CM) as essential vehicles of alimentary lipids. The major aim of this article is to review understanding derived from basic science and clinical conditions associated with impaired packaging and export of CM. We have particularly insisted on inborn metabolic pathways in humans as well as on genetically modified animal models (recapitulating pathological features). The ultimate goal of this approach is that "experiments of nature" and in vivo model strategy collectively allow gaining novel mechanistic insight and filling the gap between the underlying genetic defect and the apparent clinical phenotype. Thus, uncovering the cause of disease contributes not only to understanding normal physiologic pathway, but also to capturing disorder onset, progression, treatment and prognosis.

PRAP1 is a novel lipid-binding protein that promotes lipid absorption by facilitating MTTP-mediated lipid transport

Microsomal triglyceride transfer protein (MTTP) is an endoplasmic reticulum resident protein that is essential for the assembly and secretion of triglyceride (TG)-rich, apoB-containing lipoproteins. Although the function and structure of mammalian MTTP have been extensively studied, how exactly MTTP transfers lipids to lipid acceptors and whether there are other biomolecules involved in MTTP-mediated lipid transport remain elusive. Here we identify a role in this process for the poorly characterized protein PRAP1. We report that PRAP1 and MTTP are partially colocalized in the endoplasmic reticulum. We observe that PRAP1 directly binds to TG and facilitates MTTP-mediated lipid transfer. A single amino acid mutation at position 85 (E85V) impairs PRAP1's ability to form a ternary complex with TG and MTTP, as well as impairs its ability to facilitate MTTP-mediated apoB-containing lipoprotein assembly and secretion, suggesting that the ternary complex formation is required for PRAP1 to facilitate MTTP-mediated lipid transport. PRAP1 is detectable in chylomicron/VLDL-rich plasma fractions, suggesting that MTTP recognizes PRAP1-bound TG as a cargo and transfers TG along with PRAP1 to lipid acceptors. Both PRAP1-deficient and E85V knock-in mutant mice fed a chow diet manifested an increase in the length of their small intestines, likely to compensate for challenges in absorbing lipid. Interestingly, both genetically modified mice gained significantly less body weight and fat mass when on high-fat diets compared with littermate controls and were prevented from hepatosteatosis. Together, this study provides evidence that PRAP1 plays an important role in MTTP-mediated lipid transport and lipid absorption.

Membrane-bound sn-1,2-diacylglycerols explain the dissociation of hepatic insulin resistance from hepatic steatosis in MTTP knockout mice

Microsomal triglyceride transfer protein (MTTP) deficiency results in a syndrome of hypolipidemia and accelerated NAFLD. Animal models of decreased hepatic MTTP activity have revealed an unexplained dissociation between hepatic steatosis and hepatic insulin resistance. Here, we performed comprehensive metabolic phenotyping of liver-specific MTTP knockout (L-Mttp-/-) mice and age-weight matched wild-type control mice. Young (10-12-week-old) L-Mttp-/- mice exhibited hepatic steatosis and increased DAG content; however, the increase in hepatic DAG content was partitioned to the lipid droplet and was not increased in the plasma membrane. Young L-Mttp-/- mice also manifested normal hepatic insulin sensitivity, as assessed by hyperinsulinemic-euglycemic clamps, no PKCε activation, and normal hepatic insulin signaling from the insulin receptor through AKT Ser/Thr kinase. In contrast, aged (10-month-old) L-Mttp-/- mice exhibited glucose intolerance and hepatic insulin resistance along with an increase in hepatic plasma membrane sn-1,2-DAG content and PKCε activation. Treatment with a functionally liver-targeted mitochondrial uncoupler protected the aged L-Mttp-/- mice against the development of hepatic steatosis, increased plasma membrane sn-1,2-DAG content, PKCε activation, and hepatic insulin resistance. Furthermore, increased hepatic insulin sensitivity in the aged controlled-release mitochondrial protonophore-treated L-Mttp-/- mice was not associated with any reductions in hepatic ceramide content. Taken together, these data demonstrate that differences in the intracellular compartmentation of sn-1,2-DAGs in the lipid droplet versus plasma membrane explains the dissociation of NAFLD/lipid-induced hepatic insulin resistance in young L-Mttp-/- mice as well as the development of lipid-induced hepatic insulin resistance in aged L-Mttp-/- mice.

Alisol B 23-acetate activates ABCG5/G8 in the jejunum via the LXRα/ACAT2 pathway to relieve atherosclerosis in ovariectomized ApoE-/- mice

Phytosterols have been shown to improve blood lipid levels and treat atherosclerosis. This research investigated the effects of phytosterol Alisol B 23-acetate (AB23A) on jejunum lipid metabolism and atherosclerosis. The results show that intragastric administration of AB23A can significantly reduce atherosclerotic plaque area and lipid accumulation in the jejunum of ovariectomized ApoE-/- mice fed a high-fat diet and can also improve the lipid mass spectra of the plasma and jejunum. In vitro studies have shown that AB23A can increase cholesterol outflow in Caco-2 cells exposed to high fat concentrations and increase the expression of ATP-binding cassette transfer proteins G5/G8 (ABCG5/G8), the liver X receptor α (LXRα). Furthermore, inhibition of LXRα can significantly eliminate the active effect of AB23A on decreasing intracellular lipid accumulation. We also confirmed that AB23A has a negative effect on Acyl-CoA cholesterol acyltransferase 2 (ACAT2) in Caco-2 cells cultured in the high concentrations of fat, and we found that AB23A further reduces ACAT2 expression in cells treated with the ACAT2 inhibitor pyripyropene or transfected with ACAT2 siRNA. In conclusion, we confirmed that AB23A can reduce the absorption of dietary lipids in the jejunum by affecting the LXRα-ACAT2-ABCG5/G8 pathway and ultimately exert an anti-atherosclerotic effect.

CTRP12 inhibits triglyceride synthesis and export in hepatocytes by suppressing HNF-4α and DGAT2 expression

C1q/TNF-related protein 12 (CTRP12) is an antidiabetic adipokine whose circulating levels are reduced in obesity and diabetes. Although partial and complete loss-of-function mouse models suggest a role for CTRP12 in modulating lipid metabolism and adiposity, its effect on cellular lipid metabolism remains poorly defined. Here, we demonstrate a direct action of CTRP12 in regulating lipid synthesis and secretion. In hepatoma cells and primary mouse hepatocytes, CTRP12 treatment inhibits triglyceride synthesis by suppressing glycerophosphate acyltransferase (GPAT) and diacylglycerol acyltransferase (DGAT) expression. CTRP12 treatment also downregulates the expression of hepatocyte nuclear factor-4α (HNF-4α) and its target gene microsomal triglyceride transfer protein (MTTP), leading to reduced very-low-density lipoprotein (VLDL)-triglyceride export from hepatocytes. Consistent with the in vitro findings, overexpressing CTRP12 lowers fasting and postprandial serum triglyceride levels in mice. These results underscore the important function of CTRP12 in lipid metabolism in hepatocytes.

A point mutation decouples the lipid transfer activities of microsomal triglyceride transfer protein

Apolipoprotein B-containing lipoproteins (B-lps) are essential for the transport of hydrophobic dietary and endogenous lipids through the circulation in vertebrates. Zebrafish embryos produce large numbers of B-lps in the yolk syncytial layer (YSL) to move lipids from yolk to growing tissues. Disruptions in B-lp production perturb yolk morphology, readily allowing for visual identification of mutants with altered B-lp metabolism. Here we report the discovery of a missense mutation in microsomal triglyceride transfer protein (Mtp), a protein that is essential for B-lp production. This mutation of a conserved glycine residue to valine (zebrafish G863V, human G865V) reduces B-lp production and results in yolk opacity due to aberrant accumulation of cytoplasmic lipid droplets in the YSL. However, this phenotype is milder than that of the previously reported L475P stalactite (stl) mutation. MTP transfers lipids, including triglycerides and phospholipids, to apolipoprotein B in the ER for B-lp assembly. In vitro lipid transfer assays reveal that while both MTP mutations eliminate triglyceride transfer activity, the G863V mutant protein unexpectedly retains ~80% of phospholipid transfer activity. This residual phospholipid transfer activity of the G863V mttp mutant protein is sufficient to support the secretion of small B-lps, which prevents intestinal fat malabsorption and growth defects observed in the mttpstl/stl mutant zebrafish. Modeling based on the recent crystal structure of the heterodimeric human MTP complex suggests the G865V mutation may block triglyceride entry into the lipid-binding cavity. Together, these data argue that selective inhibition of MTP triglyceride transfer activity may be a feasible therapeutic approach to treat dyslipidemia and provide structural insight for drug design. These data also highlight the power of yolk transport studies to identify proteins critical for B-lp biology.

Lipoproteins and lipids in cardiovascular disease: from mechanistic insights to therapeutic targeting

With cardiovascular disease being the leading cause of morbidity and mortality worldwide, effective and cost-efficient therapies to reduce cardiovascular risk are highly needed. Lipids and lipoprotein particles crucially contribute to atherosclerosis as underlying pathology of cardiovascular disease and influence inflammatory processes as well as function of leukocytes, vascular and cardiac cells, thereby impacting on vessels and heart. Statins form the first-line therapy with the aim to block cholesterol synthesis, but additional lipid-lowering drugs are sometimes needed to achieve low-density lipoprotein (LDL) cholesterol target values. Furthermore, beyond LDL cholesterol, also other lipid mediators contribute to cardiovascular risk. This review comprehensively discusses low- and high-density lipoprotein cholesterol, lipoprotein (a), triglycerides as well as fatty acids and derivatives in the context of cardiovascular disease, providing mechanistic insights into their role in pathological processes impacting on cardiovascular disease. Also, an overview of applied as well as emerging therapeutic strategies to reduce lipid-induced cardiovascular burden is provided.

The association between SNPs rs1800591 and rs3816873 of the MTTP gene and nonalcoholic fatty liver disease: A meta-analysis

BACKGROUND/AIMS

: The role of two polymorphisms rs1800591 and rs3816873 of the microsomal triglyceride transfer protein (MTTP) gene in the development of nonalcoholic fatty liver disease (NAFLD) remains controversial. A meta-analysis was conducted to determine the correlation between these MTTP polymorphisms and NAFLD.

MATERIALS AND METHODS

: A systematic search was carried out using PubMed, Embase, and Cochrane Library to retrieve English studies that reported the relationship between MTTP polymorphisms (rs1800591 and rs3816873) and NAFLD published before February 18, 2020. Odds ratio (OR) and 95% confidence interval (CI) were used to appraise the risk of MTTP polymorphism in NAFLD.

RESULTS

: A total of 10 case-control studies, including 1388 cases and 1690 healthy subjects, were included. No significant correlation between the rs1800591 (G vs. T: OR = 1.08, 95% CI = 0.68-1.70, P = 0.76) and rs3816873 (CT + CC vs. TT: OR = 1.23, 95% CI = 0.76-2.01, P = 0.398) polymorphisms of MTTP and NAFLD was found in any of the models. However, when NASH patients confirmed by liver biopsy were extracted alone for rs1800591 polymorphism analysis, it was found that the G allele significantly increased the risk of NASH under the heterozygote model (GT vs. TT: OR = 3.16, 95% CI = 1.13-8.83, P = 0.028) and dominant model (GT + GG vs. TT: OR = 3.03, 95% CI = 1.13-8.09, P = 0.027).

CONCLUSION

The present meta-analysis revealed that the rs1800591 and rs3816873 polymorphisms of the MTTP gene are uncommon in NAFLD. However, the G allele of rs1800591 was more likely to be correlated to NASH susceptibility.

Influences of glycyrrhetinic acid (GA) dietary supplementation on growth, feed utilization, and expression of lipid metabolism genes in channel catfish (Ictalurus punctatus) fed a high-fat diet

An 8-week feeding trial was performed to test the effects of glycyrrhetinic acid (GA) on growth and some gene expression of hepatic lipid metabolism in channel catfish (initial body weight, 3.5 ± 0.02 g) fed high-fat diets. Fish were fed the control diet, high-fat diet (HFD), and HFD supplemented with 0.4, 0.8, and 1.2 mg/kg GA in 15 tanks at a stocking density of 21 fish/tank. Fish fed HFD were significantly lower in body weight gain and specific growth rate but higher in feed intake and feed conversion ratio in comparison to the control. Supplement of GA at 1.2 mg/kg remarkably improved these parameters as compared to the control diet. High levels of cholesterol (TC), triglycerides (TG), low-density lipoprotein (LDL), and low levels of high-density lipoprotein (HDL) in plasma were observed in fish fed HFD; the opposite was observed for fish fed HFD supplemented with GA. The transcription of fatty acid synthase (FAS), sterol regulatory element-binding protein-1 (SREBP1), liver X receptor alpha (LXRα), and hormone-sensitive lipase (HSL) was upregulated, while that of carnitine palmitoyltransferase I (CPT1), peroxisome proliferator-activated receptors alpha (PPARα), acyl-CoA oxidase (ACO), and microsomal triglyceride transfer protein (MTTP) mRNA expression were downregulated in fish fed HFD. The opposite was observed in fish fed HFD supplemented with GA as well as the control group. In conclusion, supplementing the HFD with GA at 1.2 mg/kg could improve the growth performance and lipid metabolism of channel catfish consuming HFD.

Histological changes, lipid metabolism, and oxidative and endoplasmic reticulum stress in the liver of laying hens exposed to cadmium concentrations

The objective of this study was to determine the effects of cadmium (Cd) on histological changes, lipid metabolism, and oxidative and endoplasmic reticulum (ER) stress in the liver of layers. A total of 480 hens at 38 wk of age were randomly assigned in 5 groups that were fed a basal diet or basal diet supplemented with CdCl₂ 2.5H₂O at 7.5, 15, 30, and 60 mg Cd/kg feed for 9 wk. The results showed that accumulation of Cd was the greatest in the kidney, followed by the liver, pancreas, and lung. Diet contaminated with 30 mg Cd/kg induced antioxidant defenses accompanied by the increase of the activities of antioxidant enzymes in the liver, while dietary supplementation with 60 mg Cd/kg decreased the antioxidant levels significantly (P < 0.05). Immunofluorescence assay showed Cd induced reactive oxygen species production and endoplasmic reticulum stress in hepatocytes. Exposure to 60 mg Cd/kg significantly upregulated the expression of cytochrome C, caspase 3, caspase 9, caspase 7, Grp78, and Chop (P < 0.05). Histopathology and quantitative real-time PCR results presented periportal fibrosis, bile duct hyperplasia, and periportal inflammatory cell infiltration in the liver accompanied by upregulating the expression of tumor necrosis factor-α, IL-6 and IL-10 in the 30- or 60-mg Cd/kg groups. Oil Red O staining and RT-qPCR results showed dietary supplementation with 7.5, 15, and 30 mg Cd/kg promoted the synthesis of lipid droplets and upregulated the expression of fatty acid synthase, while dietary supplementation with 60 mg Cd/kg attenuated the synthesis of lipid droplets and downregulated the expression of acyl-CoA oxidase 1, carnitine palmitoyltransferase-1, and perixisome proliferation-activated receptor α (P < 0.05). Besides, the expression of vitellogenin (VTG) II and microsomal triglyceride transfer protein were upregulated in the 7.5-mg Cd/kg group, and the expressions of apolipoprotein B, vitellogenin II, and apolipoprotein very-low-density lipoprotein-II were downregulated in the 30- and/or 60-mg Cd/kg groups (P < 0.05). Conclusively, although low-dose Cd exposure promoted the synthesis of lipids and lipoproteins in the liver, the increase of Cd exposure could trigger liver injury through inducing oxidative and endoplasmic reticulum stress and negatively affect lipid metabolism and yolk formation in laying hens.

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.

miR-130b is a potent stimulator of hepatic very-low-density lipoprotein assembly and secretion via marked induction of microsomal triglyceride transfer protein

miR-130b is a microRNA whose expression is particularly elevated within adipose tissue and in the circulation in diabetic states. Hepatic miR-130b expression has been linked to hepatocellular carcinoma and changes in lipid metabolism. Here, we investigated the role of miR-130b in hepatic lipid homeostasis and lipoprotein export. We observed that overexpression of miR-130b-3p or -5p in HepG2 cells markedly enhanced the secretion of very-low-density lipoprotein (VLDL) particles, enhanced the secretion of [³H]glycerol metabolically labeled triglyceride (TG), and significantly increased the number or the average size of lipid droplets (LDs), respectively. Overexpression of miR-130b also altered the expression of key genes involved in lipid metabolism and in particular markedly increased both mRNA and protein expression levels of microsomal triglyceride transfer protein (MTP). Conversely, the miR-130b inhibitor decreased mRNA levels of MTP and fatty acid synthase (FAS) in HepG2 cells. However, dual-luciferase reporter assays indicated that MTP is not a direct target of miR-130b-3p. miR-130b overexpression did not alter de novo synthesized TG or the stability and secretion of apolipoprotein B 100. Interestingly, knockdown of phosphatase and tensin homolog (PTEN) blocked the upregulation of MTP mRNA induced by miR-130b. Finally, miR-130b-induced stimulation of VLDL secretion was also observed in a second hepatocyte cell culture model, immortalized human hepatocytes, confirming the effects observed in HepG2 cells. Overall, these data suggest a potential role for miR-130b in promoting hepatic VLDL assembly and secretion mediated by marked stimulation of MTP expression and TG mobilization. Thus miR-130b overexpression corrects the defect in VLDL production in HepG2 cells.

Endoplasmic reticulum quality control in lipoprotein metabolism

Lipids play a critical role in energy metabolism, and a suite of proteins is required to deliver lipids to tissues. Several of these proteins require an intricate endoplasmic reticulum (ER) quality control (QC) system and unique secondary chaperones for folding. Key examples include apolipoprotein B (apoB), which is the primary scaffold for many lipoproteins, dimeric lipases, which hydrolyze triglycerides from circulating lipoproteins, and the low-density lipoprotein receptor (LDLR), which clears cholesterol-rich lipoproteins from the circulation. ApoB requires specialized proteins for lipidation, dimeric lipases lipoprotein lipase (LPL) and hepatic lipase (HL) require a transmembrane maturation factor for secretion, and the LDLR requires several specialized, domain-specific chaperones. Deleterious mutations in these proteins or their chaperones may result in dyslipidemias, which are detrimental to human health. Here, we review the ER quality control systems that ensure secretion of apoB, LPL, HL, and LDLR with a focus on the specialized chaperones required by each protein.

Lomitapide and Mipomersen-Inhibiting Microsomal Triglyceride Transfer Protein (MTP) and apoB100 Synthesis

PURPOSE OF REVIEW

The goal of this review is to evaluate the role of inhibiting the synthesis of lipoproteins when there is no or little residual LDL-receptor function as in patients with homozygous familial hypercholesterolaemia. Lomitapide is administered orally once a day while mipomersen is given by subcutaneous injection once a week. Lomitapide inhibits microsomal triglyceride transfer protein while mipomersen is an antisense oligonucleotide directed against apoB100.

RECENT FINDINGS

The pivotal registration trials for lomitapide and mipomersen were published in 2013 and 2010, respectively. More recently published data from extension trials and cohort studies provides additional information on long-term safety and efficacy. The mean LDL cholesterol reduction was 50% with lomitapide in its single-arm open-label registration trial. Mipomersen reduced LDL cholesterol by approximately 25% in its double-blind, placebo-controlled registration study. Both lomitapide and mipomersen therapy are associated with variable increases in hepatic fat content. The long-term safety of increased hepatic fat content in patients receiving these therapies is uncertain and requires further study. Both drugs may cause elevated transaminase in some patients, but no cases of severe liver injury have been reported. Lomitapide may also cause gastrointestinal discomfort and diarrhoea, especially if patients consume high-fat meals and patients are advised to follow a low-fat diet supplemented with essential fatty acids and fat-soluble vitamins. Mipomersen may cause injection-site and influenza-like reactions. The effect of lomitapide and mipomersen on cardiovascular outcomes has not been studied, but circumstantial evidence suggests that the LDL cholesterol lowering achieved with these two agents may reduce cardiovascular event rates.