The ever-expanding role of degradation in the regulation of apolipoprotein B metabolism

The intracellular chylomicron highway: novel insights into chylomicron biosynthesis, trafficking, and secretion

PURPOSE OF REVIEW

Chylomicron biosynthesis plays a vital role in supplying essential lipids and lipid soluble vitamins to peripheral tissues for various functions. Despite this, the intracellular synthesis, trafficking, and secretion of chylomicrons remains only partly understood. The purpose of this review is to summarize the role of established proteins in this process and bring attention to recently identified proteins to provide an up-to-date model of chylomicron biosynthesis.

RECENT FINDINGS

Recently, several proteins have been shown to play a role in the initial formation and lipidation of chylomicrons at the endoplasmic reticulum (ER), which include: TM6SF2, PLA2G12B, PRAP1, and SURF4. In addition, mitochondria have been implicated in chylomicron metabolism, but mechanistic insight is missing. The trafficking of chylomicrons from the ER to the Golgi, and the subsequent trafficking from the Golgi to the basolateral side of enterocytes, however, remains a mystery.

SUMMARY

Progress in the chylomicron biosynthesis field is largely associated with findings in VLDL biosynthesis. In addition, increased insight in events after prechylomicrons leave the ER is needed. Given the important role of chylomicron biosynthesis in whole-body lipid metabolism, further research into the molecular mechanisms is warranted.

Alterations in phosphatidylethanolamine metabolism impacts hepatocellular lipid storage, energy homeostasis, and proliferation

Phosphatidylethanolamine (PE) is the second most abundant glycerophospholipid in eukaryotic membranes and is involved in several cellular processes. An important pathway for de novo PE synthesis is the Kennedy Pathway. The rate limiting enzyme in the pathway, CTP:phosphoethanolamine cytidyltransferase, catalyzes the synthesis of CDP-ethanolamine from phosphoethanolamine (pEtn) and CTP. Ethanolamine phosphate phospholyase (ETNPPL) has the potential to breakdown pEtn by catabolizing it to form acetaldehyde, ammonium, and inorganic phosphate. Research on this enzyme is limited and it is unclear how its activity affects PE synthesis; therefore, an investigation into ETNPPL's biological effects is required. ETNPPL was expressed in human hepatoma cell line (Huh7) by stable transfection, allowing for long-term expression in cells without ETNPPL. We show that ETNPPL reduces cellular pEtn synthesized from ethanolamine, which decreased synthesis of PE and an increased PC:PE ratio, which has been shown to be associated with metabolic dysfunction-associated steatotic liver disease (MASLD) and impaired mitochondrial function. Experiments conducted show increased neutral lipid storage accompanied by decreased ATP production and oxygen consumption; however, no differences in triglyceride secretion were seen, although ApoB100 secretion was reduced. Huh7 cells expressing ETNPPL proliferate at a slower rate than control and have increased mRNA expression of p53 and tumor suppressor genes (CDKN1A, BBC3, BAX, BRCA1), implicating ETNPPL in cell proliferation, cancer development and/or tumor progression. Overall, ETNPPL rewires hepatic lipid metabolism, altering several processes including increasing lipid storage and decreasing proliferation. The impacts observed in this study may create a link between hepatic ETNPPL expression and MASLD/HCC pathophysiology.

Paraoxonase-like APMAP maintains endoplasmic-reticulum-associated lipid and lipoprotein homeostasis

Oxidative stress perturbs lipid homeostasis and contributes to metabolic diseases. Though ignored when compared with mitochondrial oxidation, the endoplasmic reticulum (ER) generates reactive oxygen species requiring antioxidant quality control. Using multi-organismal profiling featuring Drosophila, zebrafish, and mammalian hepatocytes, here we characterize the paraoxonase-like C20orf3/adipocyte plasma-membrane-associated protein (APMAP) as an ER-localized antioxidant that suppresses ER lipid oxidation to safeguard ER function. APMAP-depleted cells exhibit defective ER morphology, ER stress, and lipid peroxidation dependent on ER-oxidoreductase 1α (ERO1A), as well as sensitivity to ferroptosis and defects in ApoB-lipoprotein homeostasis. Similarly, organismal APMAP depletion in Drosophila and zebrafish perturbs ApoB-lipoprotein homeostasis. Strikingly, APMAP loss is rescued with chemical antioxidant N-acetyl-cysteine (NAC). Lipidomics identifies that APMAP loss elevates phospholipid peroxidation and boosts ceramides-signatures of lipid stress. Collectively, we propose that APMAP is an ER-localized antioxidant that promotes lipid and lipoprotein homeostasis in the ER network.

Clinical significance of small dense low-density lipoprotein cholesterol measurement in type 2 diabetes

Low-density lipoprotein cholesterol (LDL-C) is known to be a causal substance of atherosclerosis, but its usefulness as a predictive biomarker for atherosclerotic cardiovascular disease (ASCVD) is limited. In patients with type 2 diabetes (T2D), LDL-C concentrations do not markedly increase, while triglycerides (TG) concentrations are usually elevated. Although TG is associated with ASCVD risk, they do not play a direct role in the formation of atheromatous plaques. TG changes the risk of ASCVD in a way that is dependent on LDL-C, and TG is the primary factor in reducing LDL particle size. Small dense (sd)LDL, a potent atherogenic LDL subfraction, best explains the "Atherogenic Duo" of TG and LDL-C. Although hypertriglyceridemia is associated with small-sized LDL, patients with severe hypertriglyceridemia and low LDL-C rarely develop ASCVD. This suggests that quantifying sdLDL is more clinically relevant than measuring LDL size. We developed a full-automated direct sdLDL-C assay, and it was proven that sdLDL-C is a better predictor of ASCVD than LDL-C. The sdLDL-C level is specifically elevated in patients with metabolic syndrome and T2D who have insulin resistance. Due to its clear link to metabolic dysfunction, sdLDL-C could be named "metabolic LDL-C." Insulin resistance/hyperinsulinemia promotes TG production in the liver, causing steatosis and overproduction of VLDL1, a precursor of sdLDL. sdLDL-C is closely associated with steatotic liver disease and chronic kidney disease, which are common complications in T2D. This review focuses on T2D and discusses the clinical significance of sdLDL-C including its composition, pathophysiology, measurements, association with ASCVD, and treatments.

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.

Walking the VLDL tightrope in cardiometabolic diseases

Very-low-density lipoprotein (VLDL), a triglyceride-rich lipoprotein secreted by hepatocytes, is pivotal for supplying peripheral tissues with fatty acids for energy production. As if walking on a tightrope, perturbations in the balance of VLDL metabolism contribute to cardiometabolic dysfunction, promoting pathologies such as cardiovascular disease (CVD) or metabolic dysfunction-associated steatotic liver disease (MASLD). Despite the advent of lipid-lowering therapies, including statins and proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, risks for cardiovascular events persist. With limitations to currently available CVD therapeutics and no US Food and Drug Administration (FDA)-approved treatment for MASLD, this review summarizes the current understanding of VLDL metabolism that sheds light on novel therapeutic avenues to pursue for cardiometabolic disorders.

A whole-animal phenotypic drug screen identifies suppressors of atherogenic lipoproteins

Lipoproteins are essential for lipid transport in all bilaterians. A single Apolipoprotein B (ApoB) molecule is the inseparable structural scaffold of each ApoB-containing lipoprotein (B-lps), which are responsible for transporting lipids to peripheral tissues. The cellular mechanisms that regulate ApoB and B-lp production, secretion, transport, and degradation remain to be fully defined. In humans, elevated levels of vascular B-lps play a causative role in cardiovascular disease. Previously, we have detailed that human B-lp biology is remarkably conserved in the zebrafish using an in vivo chemiluminescent reporter of ApoB (LipoGlo) that does not disrupt ApoB function. Thus, the LipoGlo model is an ideal system for identifying novel mechanisms of ApoB modulation and, due to the ability of zebrafish to generate many progeny, is particularly amenable to large-scale phenotypic drug screening. Here, we report a screen of roughly 3000 compounds that identified 49 unique ApoB-lowering hits. Nineteen hits passed orthogonal screening criteria. A licorice root component, enoxolone, significantly lowered B-lps only in animals that express a functional allele of the nuclear hormone receptor Hepatocyte Nuclear Factor 4α (HNF4α). Consistent with this result, inhibitors of HNF4α also reduce B-lp levels. These data demonstrate that mechanism(s) of action can be rapidly determined from a whole animal zebrafish phenotypic screen. Given the well documented role of HNF4α in human B-lp biology, these data validate the LipoGlo screening platform for identifying small molecule modulators of B-lps that play a critical role in a leading cause of worldwide mortality.

A suite of genome-engineered hepatic cells provides novel insights into the spatiotemporal metabolism of apolipoprotein B and apolipoprotein B-containing lipoprotein secretion

AIMS

Apolipoprotein B (APOB)-containing very LDL (VLDL) production, secretion, and clearance by hepatocytes is a central determinant of hepatic and circulating lipid levels. Impairment of any of the aforementioned processes is associated with the development of multiple diseases. Despite the discovery of genes and processes that govern hepatic VLDL metabolism, our understanding of the different mechanistic steps involved is far from complete. An impediment to these studies is the lack of tractable hepatocyte-based systems to interrogate and follow APOB in cells, which the current study addresses.

METHODS AND RESULTS

To facilitate the cellular study of VLDL metabolism, we generated human hepatic HepG2 and Huh-7 cell lines in which CRISPR/Cas9-based genome engineering was used to introduce the fluorescent protein mNeonGreen into the APOB gene locus. This results in the production of APOB100-mNeon that localizes predominantly to the endoplasmic reticulum (ER) and Golgi by immunofluorescence and electron microscopy imaging. The production and secretion of APOB100-mNeon can be quantitatively followed in medium over time and results in the production of lipoproteins that are taken up via the LDL receptor pathway. Importantly, the production and secretion of APOB-mNeon is sensitive to established pharmacological and physiological treatments and to genetic modifiers known to influence VLDL production in humans. As a showcase, we used HepG2-APOBmNeon cells to interrogate ER-associated degradation of APOB. The use of a dedicated sgRNA library targeting all established membrane-associated ER-resident E3 ubiquitin ligases led to the identification of SYNV1 as the E3 responsible for the degradation of poorly lipidated APOB in HepG2 cells.

CONCLUSIONS

In summary, the engineered cells reported here allow the study of hepatic VLDL assembly and secretion and facilitate spatiotemporal interrogation induced by pharmacologic and genetic perturbations.

Basic and translational evidence supporting the role of TM6SF2 in VLDL metabolism

PURPOSE OF REVIEW

Transmembrane 6 superfamily member 2 ( TM6SF2 ) gene was identified through exome-wide studies in 2014. A genetic variant from glutamic acid to lysine substitution at amino acid position 167 (NM_001001524.3:c.499G> A) (p.Gln167Lys/p.E167K, rs58542926) was discovered (p.E167K) to be highly associated with increased hepatic fat content and reduced levels of plasma triglycerides and LDL cholesterol. In this review, we focus on the discovery of TM6SF2 and its role in VLDL secretion pathways. Human data suggest TM6SF2 is linked to hepatic steatosis and cardiovascular disease (CVD), hence understanding its metabolic pathways is of high scientific interest.

RECENT FINDINGS

Since its discovery, completed research studies in cell, rodent and human models have defined the role of TM6SF2 and its links to human disease. TM6SF2 resides in the endoplasmic reticulum (ER) and the ER-Golgi interface and helps with the lipidation of nascent VLDL, the main carrier of triglycerides from the liver to the periphery. Consistent results from cells and rodents indicated that the secretion of triglycerides is reduced in carriers of the p.E167K variant or when hepatic TM6SF2 is deleted. However, data for secretion of APOB, the main protein of VLDL particles responsible for triglycerides transport, are inconsistent.

SUMMARY

The identification of genetic variants that are highly associated with human disease presentation should be followed by the validation and investigation into the pathways that regulate disease mechanisms. In this review, we highlight the role of TM6SF2 and its role in processing of liver triglycerides.

Nrf2 Signaling Pathway as a Key to Treatment for Diabetic Dyslipidemia and Atherosclerosis

Diabetes mellitus (DM) is a chronic endocrine disorder that affects more than 20 million people in the United States. DM-related complications affect multiple organ systems and are a significant cause of morbidity and mortality among people with DM. Of the numerous acute and chronic complications, atherosclerosis due to diabetic dyslipidemia is a condition that can lead to many life-threatening diseases, such as stroke, coronary artery disease, and myocardial infarction. The nuclear erythroid 2-related factor 2 (Nrf2) signaling pathway is an emerging antioxidative pathway and a promising target for the treatment of DM and its complications. This review aims to explore the Nrf2 pathway's role in combating diabetic dyslipidemia. We will explore risk factors for diabetic dyslipidemia at a cellular level and aim to elucidate how the Nrf2 pathway becomes a potential therapeutic target for DM-related atherosclerosis.

FoxO6-mediated ApoC3 upregulation promotes hepatic steatosis and hyperlipidemia in aged rats fed a high-fat diet

FoxO6, an identified factor, induces hyperlipidemia and hepatic steatosis during aging by activating hepatic lipoprotein secretion and lipogenesis leading to increased ApoC3 concentrations in the bloodstream. However, the intricate mechanisms underlying hepatic steatosis induced by elevated FoxO6 under hyperglycemic conditions remain intricate and require further elucidation. In order to delineate the regulatory pathway involving ApoC3 controlled by FoxO6 and its resultant functional impacts, we employed a spectrum of models including liver cell cultures, aged rats subjected to HFD, transgenic mice overexpressing FoxO6 (FoxO6-Tg), and FoxO6 knockout mice (FoxO6-KO). Our findings indicate that FoxO6 triggered ApoC3-driven lipid accumulation in the livers of aged rats on an HFD and in FoxO6-Tg, consequently leading to hepatic steatosis and hyperglycemia. Conversely, the absence of FoxO6 attenuated the expression of genes involved in lipogenesis, resulting in diminished hepatic lipid accumulation and mitigated hyperlipidemia in murine models. Additionally, the upregulation of FoxO6 due to elevated glucose levels led to increased ApoC3 expression, consequently instigating cellular triglyceride mediated lipid accumulation. The transcriptional activation of FoxO6 induced by both the HFD and high glucose levels resulted in hepatic steatosis by upregulating ApoC3 and genes associated with gluconeogenesis in aged rats and liver cell cultures. Our conclusions indicate that the upregulation of ApoC3 by FoxO6 promotes the development of hyperlipidemia, hyperglycemia, and hepatic steatosis in vivo, and in vitro. Taken together, our findings underscore the significance of FoxO6 in driving hyperlipidemia and hepatic steatosis specifically under hyperglycemic states by enhancing the expression of ApoC3 in aged rats.

Unveiling the crucial neuronal role of the proteasomal ATPase subunit gene PSMC5 in neurodevelopmental proteasomopathies

Neurodevelopmental proteasomopathies represent a distinctive category of neurodevelopmental disorders (NDD) characterized by genetic variations within the 26S proteasome, a protein complex governing eukaryotic cellular protein homeostasis. In our comprehensive study, we identified 23 unique variants in PSMC5 , which encodes the AAA-ATPase proteasome subunit PSMC5/Rpt6, causing syndromic NDD in 38 unrelated individuals. Overexpression of PSMC5 variants altered human hippocampal neuron morphology, while PSMC5 knockdown led to impaired reversal learning in flies and loss of excitatory synapses in rat hippocampal neurons. PSMC5 loss-of-function resulted in abnormal protein aggregation, profoundly impacting innate immune signaling, mitophagy rates, and lipid metabolism in affected individuals. Importantly, targeting key components of the integrated stress response, such as PKR and GCN2 kinases, ameliorated immune dysregulations in cells from affected individuals. These findings significantly advance our understanding of the molecular mechanisms underlying neurodevelopmental proteasomopathies, provide links to research in neurodegenerative diseases, and open up potential therapeutic avenues.

VLDL Biogenesis and Secretion: It Takes a Village

The production and secretion of VLDLs (very-low-density lipoproteins) by hepatocytes has a direct impact on liver fat content, as well as the concentrations of cholesterol and triglycerides in the circulation and thus affects both liver and cardiovascular health, respectively. Importantly, insulin resistance, excess caloric intake, and lack of physical activity are associated with overproduction of VLDL, hepatic steatosis, and increased plasma levels of atherogenic lipoproteins. Cholesterol and triglycerides in remnant particles generated by VLDL lipolysis are risk factors for atherosclerotic cardiovascular disease and have garnered increasing attention over the last few decades. Presently, however, increased risk of atherosclerosis is not the only concern when considering today's cardiometabolic patients, as they often also experience hepatic steatosis, a prevalent disorder that can progress to steatohepatitis and cirrhosis. This duality of metabolic risk highlights the importance of understanding the molecular regulation of the biogenesis of VLDL, the lipoprotein that transports triglycerides and cholesterol out of the liver. Fortunately, there has been a resurgence of interest in the intracellular assembly, trafficking, degradation, and secretion of VLDL by hepatocytes, which has led to many exciting new molecular insights that are the topic of this review. Increasing our understanding of the biology of this pathway will aid to the identification of novel therapeutic targets to improve both the cardiovascular and the hepatic health of cardiometabolic patients. This review focuses, for the first time, on this duality.

Intestinal SURF4 is essential for apolipoprotein transport and lipoprotein secretion

OBJECTIVE

Lipoprotein assembly and secretion in the small intestine are critical for dietary fat absorption. Surfeit locus protein 4 (SURF4) serves as a cargo receptor, facilitating the cellular transport of multiple proteins and mediating hepatic lipid secretion in vivo. However, its involvement in intestinal lipid secretion is not fully understood. In this study, we investigated the role of SURF4 in intestinal lipid absorption.

METHODS

We generated intestine-specific Surf4 knockout mice and characterized the phenotypes. Additionally, we investigated the underlying mechanisms of SURF4 in intestinal lipid secretion using proteomics and cellular models.

RESULTS

We unveiled that SURF4 is indispensable for apolipoprotein transport and lipoprotein secretion. Intestine-specific Surf4 knockout mice exhibited ectopic lipid deposition in the small intestine and hypolipidemia. Deletion of SURF4 impeded the transport of apolipoprotein A1 (ApoA1), proline-rich acidic protein 1 (PRAP1), and apolipoprotein B48 (ApoB48) and hindered the assembly and secretion of chylomicrons and high-density lipoproteins.

CONCLUSIONS

SURF4 emerges as a pivotal regulator of intestinal lipid absorption via mediating the secretion of ApoA1, PRAP1 and ApoB48.

PCSK7: A novel regulator of apolipoprotein B and a potential target against non-alcoholic fatty liver disease

BACKGROUND

Epidemiological evidence links the proprotein convertase subtilisin/kexin 7 (PCSK7) to triglyceride (TG) metabolism. We associated the known PCSK7 gain-of-function non-coding SNP rs236918 with higher levels of plasma apolipoprotein B (apoB) and the loss-of-function coding variant p.Pro777Leu (SNP rs201598301) with lower apoB and TG. Herein, we aimed to unravel the in vivo role of liver PCSK7.

METHODS

We biochemically defined the functional role of PCSK7 in lipid metabolism using hepatic cell lines and Pcsk7-/- mice. Our findings were validated following subcutaneous administration of hepatocyte-targeted N-acetylgalactosamine (GalNAc)-antisense oligonucleotides (ASOs) against Pcsk7.

RESULTS

Independent of its proteolytic activity, membrane-bound PCSK7 binds apoB100 in the endoplasmic reticulum and enhances its secretion. Mechanistically, the loss of PCSK7/Pcsk7 leads to apoB100 degradation, triggering an unfolded protein response, autophagy, and β-oxidation, eventually reducing lipid accumulation in hepatocytes. Non-alcoholic fatty liver disease (NAFLD) was induced by a 12-week high fat/fructose/cholesterol diet in wild type (WT) and Pcsk7-/- mice that were then allowed to recover on a 4-week control diet. Pcsk7-/- mice recovered more effectively than WT mice from all NAFLD-related liver phenotypes. Finally, subcutaneous administration of GalNAc-ASOs targeting hepatic Pcsk7 to WT mice validated the above results.

CONCLUSIONS

Our data reveal hepatic PCSK7 as one of the major regulators of apoB, and its absence reduces apoB secretion from hepatocytes favoring its ubiquitination and degradation by the proteasome. This results in a cascade of events, eventually reducing hepatic lipid accumulation, thus supporting the notion of silencing PCSK7 mRNA in hepatocytes for targeting NAFLD.

Sex-Specific Differences in Lipoprotein Production and Clearance

Therapeutic approaches to reduce atherogenic lipid and lipoprotein levels remain the most effective and assessable strategies to prevent and treat cardiovascular disease. The discovery of novel research targets linked to pathways associated with cardiovascular disease development has enhanced our ability to decrease disease burden; however, residual cardiovascular disease risks remain. Advancements in genetics and personalized medicine are essential to understand some of the factors driving residual risk. Biological sex is among the most relevant factors affecting plasma lipid and lipoprotein profiles, playing a pivotal role in the development of cardiovascular disease. This minireview summarizes the most recent preclinical and clinical studies covering the effect of sex on plasma lipid and lipoprotein levels. We highlight the recent advances in the mechanisms regulating hepatic lipoprotein production and clearance as potential drivers of disease presentation. We focus on using sex as a biological variable in studying circulating lipid and lipoprotein levels.

TM6SF2 Determines Both the Degree of Lipidation and the Number of VLDL Particles Secreted by the Liver

In 2014, exome-wide studies identified a glutamine176lysine (p.E167K) substitution in a protein of unknown function named transmembrane 6 superfamily member 2 (TM6SF2). The p.E167K variant was associated with increased hepatic fat content and reduced levels of plasma TG and LDL cholesterol. Over the next several years, additional studies defined the role of TM6SF2, which resides in the ER and the ER-Golgi interface, in the lipidation of nascent VLDL to generate mature, more TG-rich VLDL. Consistent results from cells and rodents indicated that the secretion of TG was reduced in the p.E167K variant or when hepatic TM6SF2 was deleted. However, data for secretion of APOB was inconsistent, either reduced or increased secretion was observed. A recent study of people homozygous for the variant demonstrated reduced in vivo secretion of large, TG-rich VLDL1 into plasma; both TG and APOB secretion were reduced. Here we present new results demonstrating increased secretion of VLDL APOB with no change in TG secretion in p.E167K homozygous individuals from the Lancaster Amish community compared to their wild-type siblings. Our in vivo kinetic tracer results are supported by in vitro experiments in HepG2 and McA cells with knock-down or Crispr-deletions of TM6SF2, respectively. We offer a model to potentially explain all of the prior data and our new results.

In vivo PAR-CLIP (viP-CLIP) of liver TIAL1 unveils targets regulating cholesterol synthesis and secretion

System-wide cross-linking and immunoprecipitation (CLIP) approaches have unveiled regulatory mechanisms of RNA-binding proteins (RBPs) mainly in cultured cells due to limitations in the cross-linking efficiency of tissues. Here, we describe viP-CLIP (in vivo PAR-CLIP), a method capable of identifying RBP targets in mammalian tissues, thereby facilitating the functional analysis of RBP-regulatory networks in vivo. We applied viP-CLIP to mouse livers and identified Insig2 and ApoB as prominent TIAL1 target transcripts, indicating an important role of TIAL1 in cholesterol synthesis and secretion. The functional relevance of these targets was confirmed by showing that TIAL1 influences their translation in hepatocytes. Mutant Tial1 mice exhibit altered cholesterol synthesis, APOB secretion and plasma cholesterol levels. Our results demonstrate that viP-CLIP can identify physiologically relevant RBP targets by finding a factor implicated in the negative feedback regulation of cholesterol biosynthesis.

Lipid droplet biogenesis and functions in health and disease

Ubiquitous yet unique, lipid droplets are intracellular organelles that are increasingly being recognized for their versatility beyond energy storage. Advances uncovering the intricacies of their biogenesis and the diversity of their physiological and pathological roles have yielded new insights into lipid droplet biology. Despite these insights, the mechanisms governing the biogenesis and functions of lipid droplets remain incompletely understood. Moreover, the causal relationship between the biogenesis and function of lipid droplets and human diseases is poorly resolved. Here, we provide an update on the current understanding of the biogenesis and functions of lipid droplets in health and disease, highlighting a key role for lipid droplet biogenesis in alleviating cellular stresses. We also discuss therapeutic strategies of targeting lipid droplet biogenesis, growth or degradation that could be applied in the future to common diseases, such as cancer, hepatic steatosis and viral infection.

Glycerolipid Synthesis and Lipid Droplet Formation in the Endoplasmic Reticulum

More than 60 years ago, Eugene Kennedy and coworkers elucidated the endoplasmic reticulum (ER)-based pathways of glycerolipid synthesis, including the synthesis of phospholipids and triacylglycerols (TGs). The reactions of the Kennedy pathway were identified by studying the conversion of lipid intermediates and the isolation of biochemical enzymatic activities, but the molecular basis for most of these reactions was unknown. With recent progress in the cell biology, biochemistry, and structural biology in this area, we have a much more mechanistic understanding of this pathway and its reactions. In this review, we provide an overview of molecular aspects of glycerolipid synthesis, focusing on recent insights into the synthesis of TGs. Further, we go beyond the Kennedy pathway to describe the mechanisms for storage of TG in cytosolic lipid droplets and discuss how overwhelming these pathways leads to ER stress and cellular toxicity, as seen in diseases linked to lipid overload and obesity.

A human iPSC-derived hepatocyte screen identifies compounds that inhibit production of Apolipoprotein B

Familial hypercholesterolemia (FH) patients suffer from excessively high levels of Low Density Lipoprotein Cholesterol (LDL-C), which can cause severe cardiovascular disease. Statins, bile acid sequestrants, PCSK9 inhibitors, and cholesterol absorption inhibitors are all inefficient at treating FH patients with homozygous LDLR gene mutations (hoFH). Drugs approved for hoFH treatment control lipoprotein production by regulating steady-state Apolipoprotein B (apoB) levels. Unfortunately, these drugs have side effects including accumulation of liver triglycerides, hepatic steatosis, and elevated liver enzyme levels. To identify safer compounds, we used an iPSC-derived hepatocyte platform to screen a structurally representative set of 10,000 small molecules from a proprietary library of 130,000 compounds. The screen revealed molecules that could reduce the secretion of apoB from cultured hepatocytes and from humanized livers in mice. These small molecules are highly effective, do not cause abnormal lipid accumulation, and share a chemical structure that is distinct from any known cholesterol lowering drug.

Physiological Bases for the Superiority of Apolipoprotein B Over Low-Density Lipoprotein Cholesterol and Non-High-Density Lipoprotein Cholesterol as a Marker of Cardiovascular Risk

In 2019, the European Society of Cardiology/European Atherosclerosis Society stated that apolipoprotein B (apoB) was a more accurate marker of cardiovascular risk than low-density lipoprotein cholesterol (LDL-C) and non-high-density lipoprotein cholesterol. Since then, the evidence has continued to mount in favor of apoB. This review explicates the physiological mechanisms responsible for the superiority of apoB as a marker of the cardiovascular risk attributable to the atherogenic apoB lipoprotein particles chylomicron remnants, very low-density lipoprotein, and low-density lipoprotein particles. First, the nature and relative numbers of these different apoB particles will be outlined. This will make clear why low-density lipoprotein particles are almost always the major determinants of cardiovascular risk and why the concentrations of triglycerides and LDL-C may obscure this relation. Next, the mechanisms that govern the number of very low-density lipoprotein and low-density lipoprotein particles will be outlined because, except for dysbetalipoproteinemia, the total number of apoB particles determines cardiovascular risk, Then, the mechanisms that govern the cholesterol mass within very low-density lipoprotein and low-density lipoprotein particles will be reviewed because these are responsible for the discordance between the mass of cholesterol within apoB particles, measured either as LDL-C or non-high-density lipoprotein cholesterol, and the number of apoB particles measured as apoB, which creates the superior predictive power of apoB over LDL-C and non-high-density lipoprotein cholesterol. Finally, the major apoB dyslipoproteinemias will be briefly outlined. Our objective is to provide a physiological framework for health care givers to understand why apoB is a more accurate marker of cardiovascular risk than LDL-C or non-high-density lipoprotein cholesterol.

Intestinal lipid absorption and transport in type 2 diabetes

Postprandial hyperlipidaemia is an important feature of diabetic dyslipidaemia and plays an important role in the development of cardiovascular disease in individuals with type 2 diabetes. Postprandial hyperlipidaemia in type 2 diabetes is secondary to increased chylomicron production by the enterocytes and delayed catabolism of chylomicrons and chylomicron remnants. Insulin and some intestinal hormones (e.g. glucagon-like peptide-1 [GLP-1]) influence intestinal lipid metabolism. In individuals with type 2 diabetes, insulin resistance and possibly reduced GLP-1 secretion are involved in the pathophysiology of postprandial hyperlipidaemia. Several factors are involved in the overproduction of chylomicrons: (1) increased expression of microsomal triglyceride transfer protein, which is a key enzyme in chylomicron synthesis; (2) higher stability and availability of apolipoprotein B-48; and (3) increased de novo lipogenesis. Individuals with type 2 diabetes present with disorders of cholesterol metabolism in the enterocytes with reduced absorption and increased synthesis. The increased production of chylomicrons in type 2 diabetes is also associated with a reduction in their catabolism, mostly because of a reduction in activity of lipoprotein lipase. Modification of the microbiota, which is observed in type 2 diabetes, may also generate disorders of intestinal lipid metabolism, but human data remain limited. Some glucose-lowering treatments significantly influence intestinal lipid absorption and transport. Postprandial hyperlipidaemia is reduced by metformin, pioglitazone, alpha-glucosidase inhibitors, dipeptidyl peptidase 4 inhibitors and GLP-1 agonists. The most pronounced effect is observed with GLP-1 agonists, which reduce chylomicron production significantly in individuals with type 2 diabetes and have a direct effect on the intestine by reducing the expression of genes involved in intestinal lipoprotein metabolism. The effect of sodium-glucose cotransporter 2 inhibitors on intestinal lipid metabolism needs to be clarified.

Lack of VMP1 impairs hepatic lipoprotein secretion and promotes non-alcoholic steatohepatitis

BACKGROUND & AIMS

Vacuole membrane protein 1 (VMP1) is an endoplasmic reticulum (ER) transmembrane protein that regulates the formation of autophagosomes and lipid droplets. Recent evidence suggests that VMP1 plays a critical role in lipoprotein secretion in zebra fish and cultured cells. However, the pathophysiological roles and mechanisms by which VMP1 regulates lipoprotein secretion and lipid accumulation in non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are unknown.

METHODS

Liver-specific and hepatocyte-specific Vmp1 knockout mice as well as Vmp1 knock-in mice were generated by crossing Vmp1flox or Vmp1KI mice with albumin-Cre mice or by injecting AAV8-TBG-cre, respectively. Lipid and energy metabolism in these mice were characterized by metabolomic and transcriptome analyses. Mice with hepatic overexpression of VMP1 who were fed a NASH diet were also characterized.

RESULTS

Hepatocyte-specific deletion of Vmp1 severely impaired VLDL secretion resulting in massive hepatic steatosis, hepatocyte death, inflammation and fibrosis, which are hallmarks of NASH. Mechanistically, loss of Vmp1 led to decreased hepatic levels of phosphatidylcholine and phosphatidylethanolamine as well as to changes in phospholipid composition. Deletion of Vmp1 in mouse liver also led to the accumulation of neutral lipids in the ER bilayer and impaired mitochondrial beta-oxidation. Overexpression of VMP1 ameliorated steatosis in diet-induced NASH by improving VLDL secretion. Importantly, we also showed that decreased liver VMP1 is associated with NAFLD/NASH in humans.

CONCLUSIONS

Our results provide novel insights on the role of VMP1 in regulating hepatic phospholipid synthesis and lipoprotein secretion in the pathogenesis of NAFLD/NASH.

LAY SUMMARY

Non-alcoholic fatty liver disease and its more severe form, non-alcoholic steatohepatitis, are associated with a build-up of fat in the liver (steatosis). However, the exact mechanisms that underly steatosis in patients are not completely understood. Herein, the authors identified that the lack of a protein called VMP1 impairs the secretion and metabolism of fats in the liver and could therefore contribute to the development and progression of non-alcoholic fatty liver disease.

Metabolism of triglyceride-rich lipoproteins in health and dyslipidaemia

Accumulating evidence points to the causal role of triglyceride-rich lipoproteins and their cholesterol-enriched remnants in atherogenesis. Genetic studies in particular have not only revealed a relationship between plasma triglyceride levels and the risk of atherosclerotic cardiovascular disease, but have also identified key proteins responsible for the regulation of triglyceride transport. Kinetic studies in humans using stable isotope tracers have been especially useful in delineating the function of these proteins and revealing the hitherto unappreciated complexity of triglyceride-rich lipoprotein metabolism. Given that triglyceride is an essential energy source for mammals, triglyceride transport is regulated by numerous mechanisms that balance availability with the energy demands of the body. Ongoing investigations are focused on determining the consequences of dysregulation as a result of either dietary imprudence or genetic variation that increases the risk of atherosclerosis and pancreatitis. The identification of molecular control mechanisms involved in triglyceride metabolism has laid the groundwork for a 'precision-medicine' approach to therapy. Novel pharmacological agents under development have specific molecular targets within a regulatory framework, and their deployment heralds a new era in lipid-lowering-mediated prevention of disease. In this Review, we outline what is known about the dysregulation of triglyceride transport in human hypertriglyceridaemia.

Sortilin restricts secretion of apolipoprotein B-100 by hepatocytes under stressed but not basal conditions

Genetic variants at the SORT1 locus in humans, which cause increased SORT1 expression in the liver, are significantly associated with reduced plasma levels of LDL cholesterol and apolipoprotein B (apoB). However, the role of hepatic sortilin remains controversial, as genetic deletion of sortilin in mice has resulted in variable and conflicting effects on apoB secretion. Here, we found that Sort1-KO mice on a chow diet and several Sort1-deficient hepatocyte lines displayed no difference in apoB secretion. When these models were challenged with high-fat diet or ER stress, the loss of Sort1 expression resulted in a significant increase in apoB-100 secretion. Sort1-overexpression studies yielded reciprocal results. Importantly, carriers of SORT1 variant with diabetes had larger decreases in plasma apoB, TG, and VLDL and LDL particle number as compared with people without diabetes with the same variants. We conclude that, under basal nonstressed conditions, loss of sortilin has little effect on hepatocyte apoB secretion, whereas, in the setting of lipid loading or ER stress, sortilin deficiency leads to increased apoB secretion. These results are consistent with the directionality of effect in human genetics studies and suggest that, under stress conditions, hepatic sortilin directs apoB toward lysosomal degradation rather than secretion, potentially serving as a quality control step in the apoB secretion pathway in hepatocytes.

Hello from the other side: Membrane contact of lipid droplets with other organelles and subsequent functional implications

Lipid droplets (LDs) are ubiquitous organelles that play crucial roles in response to physiological and environmental cues. The identification of several neutral lipid synthesizing and regulatory protein complexes have propelled significant advance on the mechanisms of LD biogenesis in the endoplasmic reticulum (ER). Increasing evidence suggests that distinct proteins and regulatory factors, which localize to membrane contact sites (MCS), are involved not only in interorganellar lipid exchange and transport, but also function in other important cellular processes, including autophagy, mitochondrial dynamics and inheritance, ion signaling and inter-regulation of these MCS. More and more tethers and molecular determinants are associated to MCS and to a diversity of cellular and pathophysiological processes, demonstrating the dynamics and importance of these junctions in health and disease. The conjugation of lipids with proteins in supramolecular complexes is known to be paramount for many biological processes, namely membrane biosynthesis, cell homeostasis, regulation of organelle division and biogenesis, and cell growth. Ultimately, this physical organization allows the contact sites to function as crucial metabolic hubs that control the occurrence of chemical reactions. This leads to biochemical and metabolite compartmentalization for the purposes of energetic efficiency and cellular homeostasis. In this review, we will focus on the structural and functional aspects of LD-organelle interactions and how they ensure signaling exchange and metabolites transfer between organelles.

Naringin improves lipid metabolism in a tissue-engineered liver model of NAFLD and the underlying mechanisms

AIMS

Nonalcoholic fatty liver disease (NAFLD) is a lipid metabolism disorder. Naringin (a main active ingredient in Ganshuang granules) is a flavanone that has been demonstrated to exert hepatoprotective and antifibrotic effects. The present study aimed to use a novel tissue-engineered fatty liver model to assess the effects and mechanisms of naringin on NAFLD.

MAIN METHODS

Intracellular triglyceride (TG) was examined by oil red O staining and commercial kits. The proteins associated with lipid metabolism were measured by western blotting and/or qPCR. Very low-density lipoprotein (VLDL) was measured by ELISA. A CCK8 assay was used to assess the cytotoxicity of naringin. Molecular docking was used to predict the interactions and binding patterns between naringin and target proteins.

KEY FINDINGS

Naringin significantly reduced intracellular TG accumulation by 52.7% in tissue-engineered fatty (TEF) livers, and also the level of pyruvate dehydrogenase kinase 4. Naringin downregulated CD36 and proliferator activated-receptor γ expression, reducing the uptake of FFAs; naringin also downregulated de novo liposynthetases by reducing acetyl CoA carboxylase, fatty acid synthetase etc. in TEF livers. Moreover, naringin increased the expression of proliferator activated-receptor α (PPAR-α) and carnitine palmitoyltransferase 1 to improve the oxidation of fatty acids. The levels of VLDL secreted from TEF livers were reduced by 24.7% after naringin treatment. Molecular docking analyses determined the bioactivity of naringin through its specific binding to CD36 and PPAR-α.

SIGNIFICANCE

Naringin improved lipid metabolism disorders in TEF livers by reducing fatty acid uptake and de novo lipogenesis and increasing fatty acid oxidation. CD36 and PPAR-α might be specific targets of naringin.

Lipid Droplets as Regulators of Metabolism and Immunity

Abstract

A hallmark of sterile and nonsterile inflammation is the increased accumulation of cytoplasmic lipid droplets (LDs) in non-adipose cells. LDs are ubiquitous organelles specialized in neutral lipid storage and hydrolysis. Originating in the ER, LDs are comprised of a core of neutral lipids (cholesterol esters, triglycerides) surrounded by a phospholipid monolayer and several LD-associated proteins. The perilipin (PLIN1-5) family are the most abundant structural proteins present on the surface of LDs. While PLIN1 is primarily expressed in adipocytes, PLIN2 and PLIN3 are ubiquitously expressed. LDs also acquire a host of enzymes and proteins that regulate LD metabolism. Amongst these are neutral lipases and selective lipophagy factors that promote hydrolysis of LD-associated neutral lipid. In addition, LDs physically associate with other organelles such as mitochondria through inter-organelle membrane contact sites that facilitate lipid transport. Beyond serving as a source of energy storage, LDs participate in inflammatory and infectious diseases, regulating both innate and adaptive host immune responses. Here, we review recent studies on the role of LDs in the regulation of immunometabolism.

A Unique Junctional Interface at Contact Sites Between the Endoplasmic Reticulum and Lipid Droplets

Lipid droplets (LDs) constitute compartments dedicated to the storage of metabolic energy in the form of neutral lipids. LDs originate from the endoplasmic reticulum (ER) with which they maintain close contact throughout their life cycle. These ER-LD junctions facilitate the exchange of both proteins and lipids between these two compartments. In recent years, proteins that are important for the proper formation of LDs and localize to ER-LD junctions have been identified. This junction is unique as it is generally believed to invoke a transition from the ER bilayer membrane to a lipid monolayer that delineates LDs. Proper formation of this junction requires the ordered assembly of proteins and lipids at specialized ER subdomains. Without such a well-ordered assembly of LD biogenesis factors, neutral lipids are synthesized throughout the ER membrane, resulting in the formation of aberrant LDs. Such ectopically formed LDs impact ER and lipid homeostasis, resulting in different types of lipid storage diseases. In response to starvation, the ER-LD junction recruits factors that tether the vacuole to these junctions to facilitate LD degradation. In addition, LDs maintain close contacts with peroxisomes and mitochondria for metabolic channeling of the released fatty acids toward beta-oxidation. In this review, we discuss the function of different components that ensure proper functioning of LD contact sites, their role in lipogenesis and lipolysis, and their relation to lipid storage diseases.

The Nuclear Envelope in Lipid Metabolism and Pathogenesis of NAFLD

Simple Summary

The liver is a major organ regulating lipid metabolism and a proper liver function is essential to health. Nonalcoholic fatty liver disease (NAFLD) is a condition with abnormal fat accumulation in the liver without heavy alcohol use. NAFLD is becoming one of the most common liver diseases with the increase in obesity in many parts of the world. There is no approved cure for the disease and a better understanding of disease mechanism is needed for effective prevention and treatment. The nuclear envelope, a membranous structure that surrounds the cell nucleus, is connected to the endoplasmic reticulum where the vast majority of cellular lipids are synthesized. Growing evidence indicates that components in the nuclear envelope are involved in cellular lipid metabolism. We review published studies with various cell and animal models, indicating the essential roles of nuclear envelope proteins in lipid metabolism. We also discuss how defects in these proteins affect cellular lipid metabolism and possibly contribute to the pathogenesis of NAFLD.

Abstract

Nonalcoholic fatty liver disease (NAFLD) is a burgeoning public health problem worldwide. Despite its tremendous significance for public health, we lack a comprehensive understanding of the pathogenic mechanisms of NAFLD and its more advanced stage, nonalcoholic steatohepatitis (NASH). Identification of novel pathways or cellular mechanisms that regulate liver lipid metabolism has profound implications for the understanding of the pathology of NAFLD and NASH. The nuclear envelope is topologically connected to the ER, where protein synthesis and lipid synthesis occurs. Emerging evidence points toward that the nuclear lamins and nuclear membrane-associated proteins are involved in lipid metabolism and homeostasis. We review published reports that link these nuclear envelope proteins to lipid metabolism. In particular, we focus on the recent work demonstrating the essential roles for the nuclear envelope-localized torsinA/lamina-associated polypeptide (LAP1) complex in hepatic steatosis, lipid secretion, and NASH development. We also discuss plausible pathogenic mechanisms by which the loss of either protein in hepatocytes leads to hepatic dyslipidemia and NASH development.

ApoB48 as an Efficient Regulator of Intestinal Lipid Transport

Fatty meals induce intestinal secretion of chylomicrons (CMs) containing apolipoprotein (Apo) B48. These CMs travel via the lymphatic system before entering the circulation. ApoB48 is produced after post-transcriptional RNA modification by Apobec-1 editing enzyme, exclusively in the small intestine of humans and most other mammals. In contrast, in the liver where Apobec-1 editing enzyme is not expressed (except in rats and mice), the unedited transcript encodes a larger protein, ApoB100, which is used in the formation of very low-density lipoproteins (VLDL) to transport liver-synthesized fat to peripheral tissues. Apobec-1 knockout (KO) mice lack the ability to perform ApoB RNA editing, and thus, express ApoB100 in the intestine. These mice, maintained on either a chow diet or high fat diet, have body weight gain and food intake comparable to their wildtype (WT) counterparts on the respective diet; however, they secrete larger triglyceride (TG)-rich lipoprotein particles and at a slower rate than the WT mice. Using a lymph fistula model, we demonstrated that Apobec-1 KO mice also produced fewer CMs and exhibited reduced lymphatic transport of TG in response to duodenal infusion of TG at a moderate dose; in contrast, the Apobec-1 KO and WT mice had similar lymphatic transport of TG when they received a high dose of TG. Thus, the smaller, energy-saving ApoB48 appears to play a superior role in comparison with ApoB100 in the control of intestinal lipid transport in response to dietary lipid intake, at least at low to moderate lipid levels.

An unbiased approach de-livers unexpected insight into torsin biology

Mutations affecting the integrity of the essential torsin ATPase/cofactor system have been identified in a steadily increasing number of congenital disorders. Since most of these mutations affect brain function, much of the research has focused on deciphering disease etiology in the brain. However, torsin is expressed in a wide variety of nonneural tissues and is strictly conserved across species, including the lowest metazoans, suggesting that it plays roles extending beyond neurons. In this issue of the JCI, Shin et al. explored torsin function in the mammalian liver. The group reports major defects in hepatic lipid metabolism when the torsin system is compromised in mice. Remarkably, conditional deletion of either torsinA or its cofactor, lamina-associated polypeptide 1 (LAP1), resulted in fatty liver disease and steatohepatitis, likely from a secretion defect of VLDLs. This study considerably expands our understanding of torsin biology, while providing defined opportunities for future investigations of torsin function and dysfunction in human pathologies.

ARHGAP21 deficiency impairs hepatic lipid metabolism and improves insulin signaling in lean and obese mice

ARHGAP21 is a Rho-GAP that controls GTPases activity in several tissues, but its role on liver lipid metabolism is unknown. Thus, to achieve the Rho-GAP role in the liver, control and ARHGAP21-haplodeficient mice were fed chow (Ctl and Het) or high-fat diet (Ctl-HFD and Het-HFD) for 12 weeks, and pyruvate and insulin tolerance tests, insulin signaling, liver glycogen and triglycerides content, gene and protein expression, and very-low-density lipoprotein secretion were measured. Het mice displayed reduced body weight and plasma triglycerides levels, and increased liver insulin signaling. Reduced gluconeogenesis and increased glycogen content were observed in Het-HFD mice. Gene and protein expression of microsomal triglyceride transfer protein were reduced in both Het mice, while the lipogenic genes SREBP-1c and ACC were increased. ARHGAP21 knockdown resulted in hepatic steatosis through increased hepatic lipogenesis activity coupled with decreases in CPT1a expression and very-low-density lipoprotein export. In conclusion, liver of ARHGAP21-haplodeficient mice are more insulin sensitive, associated with higher lipid synthesis and lower lipid export.

Apolipoproteins and cancer

The role of apolipoproteins in cardiovascular disease has been well investigated, but their participation in cancer has only been explored in a few published studies which showed a close link with certain kinds of cancer. In this review, we focused on the function of different kinds of apolipoproteins in cancers, autophagy, oxidative stress, and drug resistance. The potential application of apolipoproteins as biomarkers for cancer diagnosis and prognosis was highlighted, together with an investigation of their potential as drug targets for cancer treatment. Many important roles of apolipoproteins and their mechanisms in cancers were reviewed in detail and future perspectives of apolipoprotein research were discussed.

Nuclear envelope-localized torsinA-LAP1 complex regulates hepatic VLDL secretion and steatosis

Deciphering novel pathways regulating liver lipid content has profound implications for understanding the pathophysiology of nonalcoholic fatty liver disease and nonalcoholic steatohepatitis. Recent evidence suggests that the nuclear envelope is a site of regulation of lipid metabolism but there is limited appreciation of the responsible mechanisms and molecular components within this organelle. We showed that conditional hepatocyte deletion of the inner nuclear membrane protein lamina-associated polypeptide 1 (LAP1) caused defective VLDL secretion and steatosis, including intranuclear lipid accumulation. LAP1 binds to and activates torsinA, an AAA+ ATPase that resides in the perinuclear space and continuous main ER. Deletion of torsinA from mouse hepatocytes caused even greater reductions in VLDL secretion and profound steatosis. Both of these mutant mouse lines developed hepatic steatosis and subsequent steatohepatitis on a regular chow diet in the absence of whole-body insulin resistance or obesity. Our results establish an essential role for the nuclear envelope-localized torsinA-LAP1 complex in hepatic VLDL secretion and suggest that the torsinA pathway participates in the pathophysiology of nonalcoholic fatty liver disease.

Role of sortilin in lipid metabolism

PURPOSE OF REVIEW

Sortilin, encoded SORT1 gene at chromosome 1p13.3, is a multiligand receptor that traffics protein from the Golgi to the endosomes, secretory vesicles, and the cell surface. Genome-wide association studies (GWAS) revealed an association between sortilin and reduced plasma LDL-cholesterol (LDL-C) as well as reduced coronary artery disease (CAD). This review explores the various lipid metabolism pathways that are affected by alterations in sortilin expression.

RECENT FINDINGS

The effects of increased hepatic sortilin on plasma LDL-C levels are mediated by increased clearance of LDL-C and decreased very LDL (VLDL) secretion because of increased autophagy-mediated lysosomal degradation of apolipoproteinB100. Sort1 knockout models have shown opposite VLDL secretion phenotypes as well as whole body lipid metabolism in response to diet challenges, leading to confusion about the true role of sortilin in the liver and other tissues.

SUMMARY

The regulation of VLDL secretion by hepatic sortilin is complex and remains incompletely understood. Further investigation to determine the specific conditions under which both hepatic sortilin and total body sortilin cause changes in lipid metabolism pathways is needed.

Different acute effects of fructose and glucose administration on hepatic fat content

BACKGROUND

Diets rich in fat and added sugars (especially fructose) play an important role in the pathogenesis of nonalcoholic liver disease (NAFLD), but there is only limited information on the acute effects of these nutrients on hepatic fat content (HFC).

OBJECTIVES

We therefore explored how the administration of high-fat load, glucose, fructose, and combinations thereof affects HFC measured in vivo using proton magnetic resonance spectroscopy (1H-MRS) in healthy subjects.

METHODS

Ten healthy nonsteatotic male volunteers (age 38.5 ± 9.6 y, body mass index [BMI, kg/m2] 26.9 ± 2.7) underwent, in random order, 6 experiments, each lasting 8 h, that included: 1) fasting; 2) a high-fat load (150 g of fat [dairy cream] at time 0); 3) glucose (3 doses of 50 g at 0, 2, and 4 h); 4) a high-fat load with glucose; 5) fructose (3 doses of 50 g at 0, 2, and 4 h); and 6) a high-fat load with fructose. HFC was measured using 1H-MRS prior to test meal administration (before time 0) and at 3 and 6 h. Plasma concentrations of triglycerides, nonesterified fatty acids, glucose, and insulin were monitored throughout each experiment.

RESULTS

HFC increased to 119 ± 19% (P < 0.05) and 117 ± 17% (P < 0.01) of baseline when subjects consumed a high-fat load alone or a high-fat load with fructose, respectively, but was not affected when glucose was coadministered with a high-fat load. HFC was not affected when subjects had fasted or had consumed repeated doses of fructose. When subjects were administered 3 doses of glucose, HFC dropped to 85 ± 13% (P < 0.05) of baseline.

CONCLUSIONS

Our results demonstrate that fructose and glucose have a different immediate impact on HFC in humans in vivo. Clinical trial registry: The study was registered at clinicaltrials.gov and obtained clinicaltrials.gov identifier: NCT03680248.

Hsp104 facilitates the endoplasmic-reticulum-associated degradation of disease-associated and aggregation-prone substrates

Misfolded proteins in the endoplasmic reticulum (ER) are selected for ER-associated degradation (ERAD). More than 60 disease-associated proteins are substrates for the ERAD pathway due to the presence of missense or nonsense mutations. In yeast, the Hsp104 molecular chaperone disaggregates detergent-insoluble ERAD substrates, but the spectrum of disease-associated ERAD substrates that may be aggregation prone is unknown. To determine if Hsp104 recognizes aggregation-prone ERAD substrates associated with human diseases, we developed yeast expression systems for a hydrophobic lipid-binding protein, apolipoprotein B (ApoB), along with a chimeric protein harboring a nucleotide-binding domain from the cystic fibrosis transmembrane conductance regulator (CFTR) into which disease-causing mutations were introduced. We discovered that Hsp104 facilitates the degradation of ER-associated ApoB as well as a truncated CFTR chimera in which a premature stop codon corresponds to a disease-causing mutation. Chimeras containing a wild-type version of the CFTR domain or a different mutation were stable and thus Hsp104 independent. We also discovered that the detergent solubility of the unstable chimera was lower than the stable chimeras, and Hsp104 helped retrotranslocate the unstable chimera from the ER, consistent with disaggregase activity. To determine why the truncated chimera was unstable, we next performed molecular dynamics simulations and noted significant unraveling of the CFTR nucleotide-binding domain. Because human cells lack Hsp104, these data indicate that an alternate disaggregase or mechanism facilitates the removal of aggregation-prone, disease-causing ERAD substrates in their native environments.

Heparin-binding EGF-like growth factor (HB-EGF) antisense oligonucleotide protected against hyperlipidemia-associated atherosclerosis

BACKGROUND AND AIMS

Heparin-binding EGF-like growth factor (HB-EGF) is a representative EGF family member that interacts with EGFR under diverse stress environment. Previously, we reported that the HB-EGF-targeting using antisense oligonucleotide (ASO) effectively suppressed an aortic aneurysm in the vessel wall and circulatory lipid levels. In this study, we further examined the effects of the HB-EGF ASO administration on the development of hyperlipidemia-associated atherosclerosis using an atherogenic mouse model.

METHODS AND RESULTS

The male and female LDLR deficient mice under Western diet containing 21% fat and 0.2% cholesterol content were cotreated with control and HB-EGF ASOs for 12 weeks. We observed that the HB-EGF ASO administration effectively downregulated circulatory VLDL- and LDL-associated lipid levels in circulation; concordantly, the HB-EGF targeting effectively suppressed the development of atherosclerosis in the aorta. An EGFR blocker BIBX1382 administration suppressed the hepatic TG secretion rate, suggesting a positive role of the HB-EGF signaling for the hepatic VLDL production. We newly observed that there was a significant improvement of the insulin sensitivity by the HB-EGF ASO administration in a mouse model under the Western diet as demonstrated by the improvement of the glucose and insulin tolerances.

CONCLUSION

The HB-EGF ASO administration effectively downregulated circulatory lipid levels by suppressing hepatic VLDL production rate, which leads to effective protection against atherosclerosis in the vascular wall.

Nuclear lipid droplets derive from a lipoprotein precursor and regulate phosphatidylcholine synthesis

The origin and physiological significance of lipid droplets (LDs) in the nucleus is not clear. Here we show that nuclear LDs in hepatocytes are derived from apolipoprotein B (ApoB)-free lumenal LDs, a precursor to very low-density lipoproprotein (VLDL) generated in the ER lumen by microsomal triglyceride transfer protein. ApoB-free lumenal LDs accumulate under ER stress, grow within the lumen of the type I nucleoplasmic reticulum, and turn into nucleoplasmic LDs by disintegration of the surrounding inner nuclear membrane. Oleic acid with or without tunicamycin significantly increases the formation of nucleoplasmic LDs, to which CTP:phosphocholine cytidylyltransferase α (CCTα) is recruited, resulting in activation of phosphatidylcholine (PC) synthesis. Perilipin-3 competes with CCTα in binding to nucleoplasmic LDs, and thus, knockdown and overexpression of perilipin-3 increases and decreases PC synthesis, respectively. The results indicate that nucleoplasmic LDs in hepatocytes constitute a feedback mechanism to regulate PC synthesis in accordance with ER stress.

The origin and physiological significance of lipid droplets (LDs) in the nucleus is not clear. Here authors show that nucleoplasmic LDs in hepatocytes are derived from apolipoprotein B (ApoB)-free lumenal LDs and constitute a feedback mechanism to regulate PC synthesis in accordance with ER stress.

Elevated lipolysis in adipose tissue by doxorubicin via PPARα activation associated with hepatic steatosis and insulin resistance

Adipose dysfunction is tightly associated with hepatic insulin resistance and steatosis condition. Doxorubicin would disturb the lipid metabolism both in adipose and liver. Here we projected that doxorubicin would impede lipogenesis and elevated lipolysis in adipose tissue would elevate the circulatory lipid profile and leads to insulin resistance. Further exacerbated lipid profile in circulation would impair the lipid metabolism in hepatic tissue which leads to fatty liver condition and consequently related disease during doxorubicin treatment. Doxorubicin impairs the lipogenesis through PPARγ and augments lipolysis and fatty acid oxidation through ATGL and PPARα in adipose tissue. Increased fatty acid level by adipose tissue in circulation would translocate into the liver and dysregulates AHR, PXR, PPARγ, ATGL and Apo B,which further develop insulin resistance and hepatic steatosis condition. The findings add to the mechanistic role of association between adipose tissue dysfunction and hepatic dysfunction.

Effects of mipomersen, an apolipoprotein B100 antisense, on lipoprotein (a) metabolism in healthy subjects

Elevated lipoprotein (a) [Lp(a)] levels increase the risk for CVD. Novel treatments that decrease LDL cholesterol (LDL-C) have also shown promise for reducing Lp(a) levels. Mipomersen, an antisense oligonucleotide that inhibits apoB synthesis, is approved for the treatment of homozygous familial hypercholesterolemia. It decreases plasma levels of LDL-C by 25% to 39% and lowers levels of Lp(a) by 21% to 39%. We examined the mechanisms for Lp(a) lowering during mipomersen treatment. We enrolled 14 healthy volunteers who received weekly placebo injections for 3 weeks followed by weekly injections of mipomersen for 7 weeks. Stable isotope kinetic studies were performed using deuterated leucine at the end of the placebo and mipomersen treatment periods. The fractional catabolic rate (FCR) of Lp(a) was determined from the enrichment of a leucine-containing peptide specific to apo(a) by LC/MS. The production rate (PR) of Lp(a) was calculated from the product of Lp(a) FCR and Lp(a) concentration (converted to pool size). In a diverse population, mipomersen reduced plasma Lp(a) levels by 21%. In the overall study group, mipomersen treatment resulted in a 27% increase in the FCR of Lp(a) with no significant change in PR. However, there was heterogeneity in the response to mipomersen therapy, and changes in both FCRs and PRs affected the degree of change in Lp(a) concentrations. Mipomersen treatment decreases Lp(a) plasma levels mainly by increasing the FCR of Lp(a), although changes in Lp(a) PR were significant predictors of reductions in Lp(a) levels in some subjects.

Can modulators of apolipoproteinB biogenesis serve as an alternate target for cholesterol-lowering drugs?

Understanding the molecular defects underlying cardiovascular disease is necessary for the development of therapeutics. The most common method to lower circulating lipids, which reduces the incidence of cardiovascular disease, is statins, but other drugs are now entering the clinic, some of which have been approved. Nevertheless, patients cannot tolerate some of these therapeutics, the drugs are costly, and/or the treatments are approved for only rare forms of disease. Efforts to find alternative treatments have focused on other factors, such as apolipoproteinB (apoB), which transports cholesterol in the blood stream. The levels of apoB are regulated by endoplasmic reticulum (ER) associated degradation as well as by a post ER degradation pathway in model systems, and we suggest that these events provide novel therapeutic targets. We discuss first how cardiovascular disease arises and how cholesterol is regulated, and then summarize the mechanisms of action of existing treatments for cardiovascular disease. We then review the apoB biosynthetic pathway, focusing on steps that might be amenable to therapeutic interventions.

A VCP inhibitor substrate trapping approach (VISTA) enables proteomic profiling of endogenous ERAD substrates

A new substrate trapping strategy that couples VCP inhibition and quantitative ubiquitin proteomics identifies endogenous ERAD substrates, expanding the available toolbox of strategies for global analysis of the ERAD substrate landscape.

Endoplasmic reticulum (ER)–associated degradation (ERAD) mediates the proteasomal clearance of proteins from the early secretory pathway. In this process, ubiquitinated substrates are extracted from membrane-embedded dislocation complexes by the AAA ATPase VCP and targeted to the cytosolic 26S proteasome. In addition to its well-established role in the degradation of misfolded proteins, ERAD also regulates the abundance of key proteins such as enzymes involved in cholesterol synthesis. However, due to the lack of generalizable methods, our understanding of the scope of proteins targeted by ERAD remains limited. To overcome this obstacle, we developed a VCP inhibitor substrate trapping approach (VISTA) to identify endogenous ERAD substrates. VISTA exploits the small-molecule VCP inhibitor CB5083 to trap ERAD substrates in a membrane-associated, ubiquitinated form. This strategy, coupled with quantitative ubiquitin proteomics, identified previously validated (e.g., ApoB100, Insig2, and DHCR7) and novel (e.g., SCD1 and RNF5) ERAD substrates in cultured human hepatocellular carcinoma cells. Moreover, our results indicate that RNF5 autoubiquitination on multiple lysine residues targets it for ubiquitin and VCP-­dependent clearance. Thus, VISTA provides a generalizable discovery method that expands the available toolbox of strategies to elucidate the ERAD substrate landscape.

Pathophysiology of Nonalcoholic Fatty Liver Disease/Nonalcoholic Steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) encompasses a spectrum of liver disorders ranging from hepatic steatosis to nonalcoholic steatohepatitis (NASH) and ultimately may lead to cirrhosis. Hepatic steatosis or fatty liver is defined as increased accumulation of lipids in hepatocytes and results from increased production or reduced clearance of hepatic triglycerides or fatty acids. Fatty liver can progress to NASH in a significant proportion of subjects. NASH is a necroinflammatory liver disease governed by multiple pathways that are not completely elucidated. This review describes the main mechanisms that have been reported to contribute to the pathophysiology of NAFLD and NASH.