Apolipoprotein B Secretory Regulation by Degradation

Curcumin inhibits the progression of hyperlipidemia via OGT mediated O-GlcNAcylation modulation of APOC3

The etiology of hyperlipidemia is complex, and our understanding of its underlying mechanisms is limited. Effective therapeutic strategies for hyperlipidemia remain elusive. This study aimed to confirm the effect of curcumin on hyperlipidemia treatment and elucidate the precise mechanism. A high-fat diet-induced hyperlipidemia model using C57BL/6J mice and HaCaT cells was established. Co-immunoprecipitation and immunofluorescence were performed to detect protein interactions, and immunoprecipitation coupled with Western blotting was used to assess protein succinylation. 40 μM of curcumin administration promoted cell viability, increased the levels of glutathione peroxidase, glutathione, catalase, and superoxide dismutase, while reducing reactive oxygen species activity and the levels of triglycerides and malondialdehyde. Additionally, curcumin attenuated the development of hyperlipidemia in vivo. Mechanistically, 100 mg/kg of curcumin promoted O-GlcNAcylation and increased the expression of O-linked N-acetylglucosamine transferase in HaCaT cells. Furthermore, apolipoprotein C3 was identified as a substrate of O-linked N-acetylglucosamine transferase, and O-GlcNAcylation of apolipoprotein C3 enhanced its stability. Rescue experiments further verified that curcumin exerts its effects by regulating apolipoprotein C3 expression. In conclusion, these findings provide novel insights into the treatment of hyperlipidemia.

Biosynthesis and Metabolism of ApoB-Containing Lipoproteins

Recent advances in human genetics, together with a substantial body of epidemiological, preclinical and clinical trial evidence, strongly support a causal relationship between triglyceride-rich lipoproteins (TRLs) and atherosclerotic cardiovascular disease. Consequently, the secretion and metabolism of TRLs have a significant impact on cardiovascular health. This knowledge underscores the importance of understanding the molecular mechanisms and regulation of very-low-density lipoprotein (VLDL) and chylomicron biogenesis. Fortunately, there has been a resurgence of interest in the intracellular assembly, trafficking, degradation, and secretion of VLDL, leading to many ground-breaking molecular insights. Furthermore, the identification of molecular control mechanisms related to triglyceride metabolism has greatly advanced our understanding of the complex metabolism of TRLs. In this review, we explore recent advances in the assembly, secretion, and metabolism of TRLs. We also discuss available treatment strategies for hypertriglyceridemia.

Lipid droplets, autophagy, and ageing: A cell-specific tale

Lipid droplets are the essential organelle for storing lipids in a cell. Within the variety of the human body, different cells store, utilize and release lipids in different ways, depending on their intrinsic function. However, these differences are not well characterized and, especially in the context of ageing, represent a key factor for cardiometabolic diseases. Whole body lipid homeostasis is a central interest in the field of cardiometabolic diseases. In this review we characterize lipid droplets and their utilization via autophagy and describe their diverse fate in three cells types central in cardiometabolic dysfunctions: adipocytes, hepatocytes, and macrophages.

Acne Transcriptomics: Fundamentals of Acne Pathogenesis and Isotretinoin Treatment

This review on acne transcriptomics allows for deeper insights into the pathogenesis of acne and isotretinoin's mode of action. Puberty-induced insulin-like growth factor 1 (IGF-1), insulin and androgen signaling activate the kinase AKT and mechanistic target of rapamycin complex 1 (mTORC1). A Western diet (hyperglycemic carbohydrates and milk/dairy products) also co-stimulates AKT/mTORC1 signaling. The AKT-mediated phosphorylation of nuclear FoxO1 and FoxO3 results in their extrusion into the cytoplasm, a critical switch which enhances the transactivation of lipogenic and proinflammatory transcription factors, including androgen receptor (AR), sterol regulatory element-binding transcription factor 1 (SREBF1), peroxisome proliferator-activated receptor γ (PPARγ) and signal transducer and activator of transcription 3 (STAT3), but reduces the FoxO1-dependent expression of GATA binding protein 6 (GATA6), the key transcription factor for infundibular keratinocyte homeostasis. The AKT-mediated phosphorylation of the p53-binding protein MDM2 promotes the degradation of p53. In contrast, isotretinoin enhances the expression of p53, FoxO1 and FoxO3 in the sebaceous glands of acne patients. The overexpression of these proapoptotic transcription factors explains isotretinoin's desirable sebum-suppressive effect via the induction of sebocyte apoptosis and the depletion of BLIMP1(+) sebocyte progenitor cells; it also explains its adverse effects, including teratogenicity (neural crest cell apoptosis), a reduced ovarian reserve (granulosa cell apoptosis), the risk of depression (the apoptosis of hypothalamic neurons), VLDL hyperlipidemia, intracranial hypertension and dry skin.

Triglyceride-Rich Lipoprotein Metabolism: Key Regulators of Their Flux

The residual risk for arteriosclerotic cardiovascular disease after optimal statin treatment may amount to 50% and is the consequence of both immunological and lipid disturbances. Regarding the lipid disturbances, the role of triglyceride-rich lipoproteins (TRLs) and their remnants has come to the forefront in the past decade. Triglycerides (TGs) stand as markers of the remnants of the catabolism of TRLs that tend to contain twice as much cholesterol as compared to LDL. The accumulation of circulating TRLs and their partially lipolyzed derivatives, known as "remnants", is caused mainly by ineffective triglyceride catabolism. These cholesterol-enriched remnant particles are hypothesized to contribute to atherogenesis. The aim of the present narrative review is to briefly summarize the main pathways of TRL metabolism, bringing to the forefront the newly discovered role of apolipoproteins, the key physiological function of lipoprotein lipase and its main regulators, the importance of the fluxes of these particles in the post-prandial period, their catabolic rates and the role of apo CIII and angiopoietin-like proteins in the partition of TRLs during the fast-fed cycle. Finally, we provide a succinct summary of the new and old therapeutic armamentarium and the outcomes of key current trials with a final outlook on the different methodological approaches to measuring TRL remnants, still in search of the gold standard.

Distinct features of two lipid droplets types in cell nuclei from patients with liver diseases

Lipid droplets (LDs) have been observed in the nuclei of hepatocytes; however, their significance in liver disease remains unresolved. Our purpose was to explore the pathophysiological features of intranuclear LDs in liver diseases. We included 80 patients who underwent liver biopsies; the specimens were dissected and fixed for electron microscopy analysis. Depending on the presence of adjacent cytoplasmic invagination of the nuclear membrane, LDs in the nuclei were classified into two types: nucleoplasmic LDs (nLDs) and cytoplasmic LD invagination with nucleoplasmic reticulum (cLDs in NR). nLDs were found in 69% liver samples and cLDs in NR were found in 32%; no correlation was observed between the frequencies of the two LD types. nLDs were frequently found in hepatocytes of patients with nonalcoholic steatohepatitis, whereas cLDs in NR were absent from the livers of such patients. Further, cLDs in NR were often found in hepatocytes of patients with lower plasma cholesterol level. This indicates that nLDs do not directly reflect cytoplasmic lipid accumulation and that formation of cLDs in NR is inversely correlated to the secretion of very low-density lipoproteins. Positive correlations were found between the frequencies of nLDs and endoplasmic reticulum (ER) luminal expansion, suggesting that nLDs are formed in the nucleus upon ER stress. This study unveiled the presence of two distinct nuclear LDs in various liver diseases.

The Joint Link of the rs1051730 and rs1902341 Polymorphisms and Cigarette Smoking to Peripheral Artery Disease and Atherosclerotic Lesions of Different Arterial Beds

Genome-wide association studies (GWAS) have discovered numerous single nucleotide polymorphisms (SNP) contributing to peripheral artery disease (PAD), but their joint effects with risk factors like cigarette smoking (CS) on disease susceptibility have not been systematically investigated. The present study looked into whether CS mediates the effects of GWAS loci on the development of PAD and atherosclerotic lesions in different arterial beds. DNA samples from 1263 unrelated individuals of Slavic origin including 620 PAD patients and 643 healthy subjects were genotyped by the MassArray-4 system for rs1051730, rs10134584, rs1902341, rs10129758 which are known as PAD-associated GWAS loci. The rs1051730 polymorphism was strongly associated with an increased risk of PAD (p = 5.1 × 10^-6), whereas rs1902341 did not show an association with disease risk. The rs1051730 polymorphism was associated with increased plasma levels of LDL cholesterol (p = 0.001), and conferred a greater risk of PAD in cigarette smokers than in nonsmokers (p < 0.01). Interestingly, the rs1902341T allele was associated with an increased risk of PAD in smokers and a decreased disease risk in nonsmokers. SNPs and CS were both linked to unilateral and/or bilateral atherosclerotic lesions of peripheral vessels, as well as the abdominal aorta, coronary, and cerebral arteries. The studied polymorphisms exert pleiotropic and cigarette smoking-mediated effects on atherosclerotic lesions of different arterial beds.

Mechanism of the Regulation of Plasma Cholesterol Levels by PI(4,5)P2

Elevated low-density lipoprotein (LDL) cholesterol (LDLc) is one of the most well-established risk factors for cardiovascular disease, while high levels of high-density lipoprotein (HDL) cholesterol (HDLc) have been associated with protection from cardiovascular disease. Cardiovascular disease remains one of the leading causes of death worldwide; thus it is important to understand mechanisms that impact LDLc and HDLc metabolism. In this chapter, we will discuss molecular processes by which phosphatidylinositol-(4,5)-bisphosphate, PI(4,5)P₂, is thought to modulate LDLc or HDLc. Section 1 will provide an overview of cholesterol in the circulation, discussing processes that modulate the various forms of lipoproteins (LDL and HDL) carrying cholesterol. Section 2 will describe how a PI(4,5)P₂ phosphatase, transmembrane protein 55B (TMEM55B), impacts circulating LDLc levels through its ability to regulate lysosomal decay of the low-density lipoprotein receptor (LDLR), the primary receptor for hepatic LDL uptake. Section 3 will discuss how PI(4,5)P₂ interacts with apolipoprotein A-I (apoA1), the key apolipoprotein on HDL. In addition to direct mechanisms of PI(4,5)P₂ action on circulating cholesterol, Sect. 4 will review how PI(4,5)P₂ may indirectly impact LDLc and HDLc by affecting insulin action. Last, as cholesterol is controlled through intricate negative feedback loops, Sect. 5 will describe how PI(4,5)P₂ is regulated by cholesterol.

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.

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.

Regression of Liver Steatosis Following Phosphatidylcholine Administration: A Review of Molecular and Metabolic Pathways Involved

Liver steatosis is a key pathology in non-alcoholic or metabolic associated fatty liver disease. Though largely ignored for decades it is currently becoming the focus of research in hepatology. It is important to consider its origin and current opportunities in terms of pharmacotherapy. Essential phospholipids (EPLs) rich in phosphatidylcholine (PCH) is a widely used treatment option for fatty liver disease, and there is a solid amount of consistent clinical evidence for the regression of steatosis after treatment with EPLs. As knowledge of PCH (a key component of EPLs) pharmacodynamics and mode of action driving this widely observed clinical effect is currently insufficient, we aimed to explore the potential molecular and metabolic pathways involved in the positive effects of PCH on steatosis regression.

ApoB SURFs a Ride from the ER to the Golgi

Chylomicrons and very-low-density lipoproteins (VLDLs) are large, complex cargos that may require specific chaperones for efficient transport from the ER to Golgi. In this issue of Cell Metabolism, Wang et al. (2020) identify SURF4, in coordination with SAR1B, as an essential player in COPII transport of VLDLs from ER to Golgi, suggesting that SURF4 may be a target for approaches aimed at reducing secretion of triglyceride-rich, atherogenic lipoproteins from the liver.

Ezetimibe impairs transcellular lipid trafficking and induces large lipid droplet formation in intestinal absorptive epithelial cells

Ezetimibe inhibits Niemann-Pick C1-like 1 (NPC1L1) protein, which mediates intracellular cholesterol trafficking from the brush border membrane to the endoplasmic reticulum, where chylomicron assembly takes place in enterocytes or in the intestinal absorptive epithelial cells. Cholesterol is a minor lipid constituent of chylomicrons; however, whether or not a shortage of cholesterol attenuates chylomicron assembly is unknown. The aim of this study was to examine the effect of ezetimibe, a potent NPC1L1 inhibitor, on trans-epithelial lipid transport, and chylomicron assembly and secretion in enterocytes. Caco-2 cells, an absorptive epithelial model, grown onto culture inserts were given lipid micelles from the apical side, and chylomicron-like triacylglycerol-rich lipoprotein secreted basolaterally were analyzed after a 24-h incubation period in the presence of ezetimibe up to 50 μM. The secretion of lipoprotein and apolipoprotein B48 were reduced by adding ezetimibe (30% and 34%, respectively). Although ezetimibe allowed the cells to take up cholesterol normally, the esterification was abolished. Meanwhile, oleic acid esterification was unaffected. Moreover, ezetimibe activated sterol regulatory element-binding protein 2 by approximately 1.5-fold. These results suggest that ezetimibe limited cellular cholesterol mobilization required for lipoprotein assembly. In such conditions, large lipid droplet formation in Caco-2 cells and the enterocytes of mice were induced, implying that unprocessed triacylglycerol was sheltered in these compartments. Although ezetimibe did not reduce the post-prandial lipid surge appreciably in triolein-infused mice, the results of the present study indicated that pharmacological actions of ezetimibe may participate in a novel regulatory mechanism for the efficient chylomicron assembly and secretion.

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.

The unfolded protein response and hepatic lipid metabolism in non alcoholic fatty liver disease

Nonalcoholic fatty liver disease is a major public health burden. Although many features of nonalcoholic fatty liver disease pathogenesis are known, the specific mechanisms and susceptibilities that determine an individual's risk of developing nonalcoholic steatohepatitis versus isolated steatosis are not well delineated. The predominant and defining histologic and imaging characteristic of nonalcoholic fatty liver disease is the accumulation of lipids. Dysregulation of lipid homeostasis in hepatocytes leads to transient generation or accumulation of toxic lipids that result in endoplasmic reticulum (ER) stress with inflammation, hepatocellular damage, and apoptosis. ER stress activates the unfolded protein response (UPR) which is classically viewed as an adaptive pathway to maintain protein folding homeostasis. Recent studies have uncovered the contribution of the UPR sensors in the regulation of hepatic steatosis and in the cellular response to lipotoxic stress. Interestingly, the UPR sensors can be directly activated by toxic lipids, independently of the accumulation of misfolded proteins, termed lipotoxic and proteotoxic stress, respectively. The dual function of the UPR sensors in protein and lipid homeostasis suggests that these two types of stress are interconnected likely due to the central role of the ER in protein folding and trafficking and lipid biosynthesis and trafficking, such that perturbations in either impact the function of the ER and activate the UPR sensors in an effort to restore homeostasis. The precise molecular similarities and differences between proteotoxic and lipotoxic ER stress are beginning to be understood. Herein, we provide an overview of the mechanisms involved in the activation and cross-talk between the UPR sensors, hepatic lipid metabolism, and lipotoxic stress, and discuss the possible therapeutic potential of targeting the UPR in nonalcoholic fatty liver disease.

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.

Riboflavin deficiency affects lipid metabolism partly by reducing apolipoprotein B100 synthesis in rats

Lipid metabolism is dependent on riboflavin status. Apolipoprotein B100 plays an important role in lipids transportation. This study was aimed to investigate the effect of riboflavin status on lipid metabolism and explore its association with apolipoprotein B100 synthesis in vivo. Riboflavin deficiency was developed in rats by feeding riboflavin-deficient diets. Compared to the control rats, the mRNA and protein expressions of apolipoprotein B100 were significantly reduced in riboflavin-deficient rats. Endoplasmic reticulum oxidoreductin 1 (ERO1) and protein disulfide isomerase (PDI), two enzymes involved in the oxidative folding of apolipoprotein B100, were also lowered remarkably in expression at protein level. Meanwhile, total cholesterol and triglyceride levels were decreased in the plasma and increased in the liver of riboflavin-deficient rats. The plasma very low-density lipoprotein cholesterol (VLDL-c) and low-density lipoprotein cholesterol (LDL-c) were also reduced in riboflavin-deficient rats. Our findings demonstrate that riboflavin deficiency affects lipid metabolism partly by reducing apolipoprotein B100 synthesis.

Symposium review: Milk fat globule size: Practical implications and metabolic regulation

Milk fat globule (MFG) size ranges over 3 orders of magnitude, from less than 200 nm to over 15 µm. The significance of MFG size derives from its tight association with its lipidome and proteome. More specifically, small MFG have relatively higher content of membrane compared with large globules, and this membrane exerts diverse positive health effects, as reported in human and animal studies. In addition, MFG size has industrial significance, as it affects the physicochemical and sensory characteristics of dairy products. Studies on the size regulation of MFG are scarce, mainly because various confounders indirectly affect MFG size. Because MFG size is determined before and during its secretion from mammary epithelial cells, studies on the size regulation of its precursors, the intracellular lipid droplets (LD), have been used as a proxy for understanding the mechanisms controlling MFG size. In this review, we provide evidence for 2 distinct mechanisms regulating LD size in mammary epithelial cells: co-regulation of fat content and triglyceride-synthesis capacity of the cells, and fusion between LD. The latter is controlled by the membrane's polar lipid composition and involves mitochondrial enzymes. Accordingly, this review also discusses MFG size regulation in the in vivo metabolic context, as MFG morphometric features are often modulated under conditions that involve animals' altered energy status.

Changes in lipid droplets morphometric features in mammary epithelial cells upon exposure to non-esterified free fatty acids compared with VLDL

The effects of the macrostructure of long chain fatty acids on the lipid metabolism and biosynthesis of lipid droplets (LD) was studied in mammary epithelial cells (MEC). MEC were exposed to similar compositions and concentrations of fatty acids in the form of either triglycerides (Tg), as part of the very-low-density lipids (VLDL) isolated from lactating cow plasma, or as non-esterified- free fatty acids (FFA). Exposing MEC to FFA resulted in two distinct processes; each independently could increase LD size: an elevation in Tg production and alterations in phospholipid (PL) composition. In particular, the lower PC/PE ratio in the FFA treatment indicated membrane destabilization, which was concomitant with the biosynthesis of larger LD. In addition, 6 fold increase in the cellular concentration of the exogenously added linoleic acid (C18:2) was found in MEC treated with FFA, implying that long chain fatty acids administrated as FFA have higher availability to MEC, enabling greater PL synthesis, more material for the LD envelope, thereby enhancing LD formation. Availability of long chain fatty acids administrated as VLDL-Tg, is dependent on LPL which its activity can be inhibited by the hydrolysis products. Therefore, we used increasing concentrations of albumin, to reduce the allosteric inhibition on LPL by the hydrolysis products. Indeed, a combined treatment of VLDL and albumin, increased LD size and number, similar to the phenotype found in the FFA treatment. These results reveal the role played by the macrostructure of long chain fatty acids in the regulation of LD size in MEC which determine the size of the secreted MFG.

Familial Hypercholesterolemia: The Most Frequent Cholesterol Metabolism Disorder Caused Disease

Cholesterol is an essential component of cell barrier formation and signaling transduction involved in many essential physiologic processes. For this reason, cholesterol metabolism must be tightly controlled. Cell cholesterol is mainly acquired from two sources: Dietary cholesterol, which is absorbed in the intestine and, intracellularly synthesized cholesterol that is mainly synthesized in the liver. Once acquired, both are delivered to peripheral tissues in a lipoprotein dependent mechanism. Malfunctioning of cholesterol metabolism is caused by multiple hereditary diseases, including Familial Hypercholesterolemia, Sitosterolemia Type C and Niemann-Pick Type C1. Of these, familial hypercholesterolemia (FH) is a common inherited autosomal co-dominant disorder characterized by high plasma cholesterol levels. Its frequency is estimated to be 1:200 and, if untreated, increases the risk of premature cardiovascular disease. This review aims to summarize the current knowledge on cholesterol metabolism and the relation of FH to cholesterol homeostasis with special focus on the genetics, diagnosis and treatment.

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.

Membrane Protein Quantity Control at the Endoplasmic Reticulum

The canonical function of the endoplasmic reticulum-associated degradation (ERAD) system is to enforce quality control among membrane-associated proteins by targeting misfolded secreted, intra-organellar, and intramembrane proteins for degradation. However, increasing evidence suggests that ERAD additionally functions in maintaining appropriate levels of a subset of membrane-associated proteins. In this 'quantity control' capacity, ERAD responds to environmental cues to regulate the proteasomal degradation of specific ERAD substrates according to cellular need. In this review, we discuss in detail seven proteins that are targeted by the ERAD quantity control system. Not surprisingly, ERAD-mediated protein degradation is a key regulatory feature of a variety of ER-resident proteins, including HMG-CoA reductase, cytochrome P450 3A4, IP3 receptor, and type II iodothyronine deiodinase. In addition, the ERAD quantity control system plays roles in maintaining the proper stoichiometry of multi-protein complexes by mediating the degradation of components that are produced in excess of the limiting subunit. Perhaps somewhat unexpectedly, recent evidence suggests that the ERAD quantity control system also contributes to the regulation of plasma membrane-localized signaling receptors, including the ErbB3 receptor tyrosine kinase and the GABA neurotransmitter receptors. For these substrates, a proportion of the newly synthesized yet properly folded receptors are diverted for degradation at the ER, and are unable to traffic to the plasma membrane. Given that receptor abundance or concentration within the plasma membrane plays key roles in determining signaling efficiency, these observations may point to a novel mechanism for modulating receptor-mediated cellular signaling.

Impaired hepatic lipid synthesis from polyunsaturated fatty acids in TM6SF2 E167K variant carriers with NAFLD

BACKGROUND

Carriers of the transmembrane 6 superfamily member 2 E167K gene variant (TM6SF2^EK/KK) have decreased expression of the TM6SF2 gene and increased risk of NAFLD and NASH. Unlike common 'obese/metabolic' NAFLD, these subjects lack hypertriglyceridemia and have lower risk of cardiovascular disease. In animals, phosphatidylcholine (PC) deficiency results in a similar phenotype. PCs surround the core of VLDL consisting of triglycerides (TGs) and cholesteryl-esters (CEs). We determined the effect of the TM6SF2 E167K on these lipids in the human liver and serum and on hepatic gene expression and studied the effect of TM6SF2 knockdown on hepatocyte handling of these lipids.

METHODS

Liver biopsies were taken from subjects characterized with respect to the TM6SF2 genotype, serum and liver lipidome, gene expression and histology. In vitro, after TM6SF2 knockdown in HuH-7 cells, we compared incorporation of different fatty acids into TGs, CEs, and PCs.

RESULTS

The TM6SF2^EK/KK and TM6SF2^EE groups had similar age, gender, BMI and HOMA-IR. Liver TGs and CEs were higher and liver PCs lower in the TM6SF2^EK/KK than the TM6SF2^EE group (p<0.05). Polyunsaturated fatty acids (PUFA) were deficient in liver and serum TGs and liver PCs but hepatic free fatty acids were relatively enriched in PUFA (p<0.05). Incorporation of PUFA into TGs and PCs in TM6SF2 knockdown hepatocytes was decreased (p<0.05). Hepatic expression of TM6SF2 was decreased in variant carriers, and was co-expressed with genes regulated by PUFAs.

CONCLUSIONS

Hepatic lipid synthesis from PUFAs is impaired and could contribute to deficiency in PCs and increased intrahepatic TG in TM6SF2 E167K variant carriers.

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

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

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

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

Biology of proprotein convertase subtilisin kexin 9: beyond low-density lipoprotein cholesterol lowering

Proprotein convertase subtilisin kexin 9 (PCSK9) is a key regulator of low-density lipoprotein receptor levels and LDL-cholesterol levels. Loss-of-function mutations in PCSK9 gene are associated with hypocholesterolaemia and protection against cardiovascular disease, identifying PCSK9 inhibition as a valid therapeutic approach to manage hypercholesterolaemia and related diseases. Although PCSK9 is expressed mainly in the liver, it is present also in other tissues and organs with specific functions, raising the question of whether a pharmacological inhibition of PCSK9 to treat hypercholesterolaemia and associated cardiovascular diseases might be helpful or deleterious in non-hepatic tissues. For example, PCSK9 is expressed in the vascular wall, in the kidneys, and in the brain, where it was proposed to play a role in development, neurocognitive process, and neuronal apoptosis. A link between PCSK9 and immunity was also proposed as both sepsis and viral infections are differentially affected in the presence or absence of PCSK9. Despite the increasing number of observations, the debate on the exact roles of PCSK9 in extrahepatic tissues is still ongoing, and as very effective drugs that inhibit PCSK9 have become available to the clinician, a better understanding of the biological roles of PCSK9 is warranted.

The Apoe(-/-) mouse model: a suitable model to study cardiovascular and respiratory diseases in the context of cigarette smoke exposure and harm reduction

Atherosclerosis-prone apolipoprotein E-deficient (Apoe(-/-)) mice display poor lipoprotein clearance with subsequent accumulation of cholesterol ester-enriched particles in the blood, which promote the development of atherosclerotic plaques. Therefore, the Apoe(-/-) mouse model is well established for the study of human atherosclerosis. The systemic proinflammatory status of Apoe(-/-) mice also makes them good candidates for studying chronic obstructive pulmonary disease, characterized by pulmonary inflammation, airway obstruction, and emphysema, and which shares several risk factors with cardiovascular diseases, including smoking. Herein, we review the results from published studies using Apoe(-/-) mice, with a particular focus on work conducted in the context of cigarette smoke inhalation studies. The findings from these studies highlight the suitability of this animal model for researching the effects of cigarette smoking on atherosclerosis and emphysema.

TANGO1 and Mia2/cTAGE5 (TALI) cooperate to export bulky pre-chylomicrons/VLDLs from the endoplasmic reticulum

Santos et al. show that TANGO1 and a TANGO1-like protein, TALI, bind each other and function together as receptors to export bulky ApoB-containing lipid particles from the endoplasmic reticulum. However, TANGO1-mediated export of bulky collagens by the same cells is TALI independent.

Procollagens, pre-chylomicrons, and pre–very low-density lipoproteins (pre-VLDLs) are too big to fit into conventional COPII-coated vesicles, so how are these bulky cargoes exported from the endoplasmic reticulum (ER)? We have shown that TANGO1 located at the ER exit site is necessary for procollagen export. We report a role for TANGO1 and TANGO1-like (TALI), a chimeric protein resulting from fusion of MIA2 and cTAGE5 gene products, in the export of pre-chylomicrons and pre-VLDLs from the ER. TANGO1 binds TALI, and both interact with apolipoprotein B (ApoB) and are necessary for the recruitment of ApoB-containing lipid particles to ER exit sites for their subsequent export. Although export of ApoB requires the function of both TANGO1 and TALI, the export of procollagen XII by the same cells requires only TANGO1. These findings reveal a general role for TANGO1 in the export of bulky cargoes from the ER and identify a specific requirement for TALI in assisting TANGO1 to export bulky lipid particles.

Regression of Atherosclerosis: The Journey From the Liver to the Plaque and Back

Cardinal events in atherogenesis are the retention of apolipoprotein B-containing lipoproteins in the arterial wall and the reaction of macrophages to these particles. My laboratory has been interested in both the cell biological events producing apolipoprotein B-containing lipoproteins, as well as in the reversal of the damage they cause in the plaques formed in the arterial wall. In the 2013 George Lyman Duff Memorial Lecture, as summarized in this review, I covered 3 areas of my past, present, and future interests, namely, the regulation of hepatic very low density lipoprotein production by the degradation of apolipoprotein B100, the dynamic changes in macrophages in the regression of atherosclerosis, and the application of nanoparticles to both image and treat atherosclerotic plaques.

Role of Tribbles Pseudokinase 1 (TRIB1) in human hepatocyte metabolism

Genome-wide association studies for plasma triglycerides and hepatic steatosis identified a risk locus on chromosome 8q24 close to the TRIB1 gene, encoding Tribbles Pseudokinase 1 (TRIB1). In previous studies conducted in murine models, hepatic over-expression of Trib1 was shown to increase fatty acid oxidation and decrease triglyceride synthesis whereas Trib1 knockdown mice exhibited hypertriglyceridemia. Here we have examined the impact of TRIB1 suppression in human and mouse hepatocytes. Examination of a panel of lipid regulator transcripts revealed species-specific effects, prompting us to focus on human models for the remainder of the study. Acute knockdown of TRIB1 in human primary hepatocytes resulted in decreased expression of MTTP and APOB, required for very low density lipoprotein (VLDL) assembly although particle secretion was not significantly affected. A parallel analysis performed in HepG2 revealed reduced MTTP, but not APOB, protein as a result of TRIB1 suppression. Global gene expression changes of human primary hepatocytes upon TRIB1 suppression were analyzed by clustering algorithms and found to be consistent with dysregulation of several pathways fundamental to liver function, including altered CEBPA and B transcript levels and impaired glucose handling. Indeed, TRIB1 expression in HepG2 cells was found to be inversely proportional to glucose concentration. Lastly TRIB1 downregulation in primary hepatocytes was associated with suppression of the HNF4A axis. In HepG2 cells, TRIB1 suppression resulted in reduced HNF4A protein levels while HNF4A suppression increased TRIB1 expression. Taken together these studies reveal an important role for TRIB1 in human hepatocyte biology.

Treatment of Hypertriglyceridemia: a Review of Current Options

Hypertriglyceridemia is an important marker of increased levels of highly atherogenic remnant-like particles. The importance of lowering plasma levels of triglycerides (TG) has been called into question many times, but currently it is considered an integral part of residual cardiovascular risk reduction strategies. Lifestyle changes (improved diet and increased physical activity) are effective TG lowering measures. Pharmacological treatment usually starts with statins, although associated TG reductions are typically modest. Fibrates are currently the drugs of choice for hyperTG, frequently in combination with statins. Niacin and omega-3 fatty acids improve control of triglyceride levels when the above measures are inadequately effective. Some novel therapies including anti-sense oligonucleotides and inhibitors of microsomal triglyceride transfer protein have shown significant TG lowering efficacy. The current approach to the management of hypertriglyceridemia is based on lifestyle changes and, usually, drug combinations (statin and fibrate and/or omega-3 fatty acids or niacin).

Human plasma protein N-glycosylation

Glycosylation is the most abundant and complex protein modification, and can have a profound structural and functional effect on the conjugate. The oligosaccharide fraction is recognized to be involved in multiple biological processes, and to affect proteins physical properties, and has consequentially been labeled a critical quality attribute of biopharmaceuticals. Additionally, due to recent advances in analytical methods and analysis software, glycosylation is targeted in the search for disease biomarkers for early diagnosis and patient stratification. Biofluids such as saliva, serum or plasma are of great use in this regard, as they are easily accessible and can provide relevant glycosylation information. Thus, as the assessment of protein glycosylation is becoming a major element in clinical and biopharmaceutical research, this review aims to convey the current state of knowledge on the N-glycosylation of the major plasma glycoproteins alpha-1-acid glycoprotein, alpha-1-antitrypsin, alpha-1B-glycoprotein, alpha-2-HS-glycoprotein, alpha-2-macroglobulin, antithrombin-III, apolipoprotein B-100, apolipoprotein D, apolipoprotein F, beta-2-glycoprotein 1, ceruloplasmin, fibrinogen, immunoglobulin (Ig) A, IgG, IgM, haptoglobin, hemopexin, histidine-rich glycoprotein, kininogen-1, serotransferrin, vitronectin, and zinc-alpha-2-glycoprotein. In addition, the less abundant immunoglobulins D and E are included because of their major relevance in immunology and biopharmaceutical research. Where available, the glycosylation is described in a site-specific manner. In the discussion, we put the glycosylation of individual proteins into perspective and speculate how the individual proteins may contribute to a total plasma N-glycosylation profile determined at the released glycan level.

Non-alcoholic fatty liver disease as a consequence of autonomic imbalance and circadian desynchronization

The circadian system, headed by the suprachiasmatic nucleus, synchronizes behaviour and metabolism according to the external light-dark cycle through neuroendocrine and autonomic signals. Metabolic diseases, such as steatosis, obesity and glucose intolerance, have been associated with conditions of circadian misalignment wherein the feeding schedule has been moved to the resting phase. Here we describe the physiological processes involved in liver lipid accumulation and show how they follow a circadian pattern importantly regulated by both the autonomic nervous system and the feeding-fasting cycle. We propose that an unbalanced activity of the sympathetic-parasympathetic branches between organs induced by circadian misalignment provides the conditions for the development and progression of non-alcoholic fatty liver disease.

A comparative study of four independent methods to measure LDL particle concentration

BACKGROUND

Low-density lipoprotein particle concentration (LDL-P) is generally more predictive of clinical cardiovascular endpoints than LDL cholesterol (LDL-C). Few studies have directly compared multiple LDL-P methods, particularly with ultracentrifugation.

OBJECTIVE

Examine comparability and precision of 4 LDL-P methods.

METHODS

We divided serum from 48 subjects into blinded triplicates and measured LDL-P in 3 separate laboratories by 4 methods: ultracentrifugation (reference method), a novel electrophoretic method, and nuclear magnetic resonance spectroscopy (NMR) by 2 independent methods: a 400 MHz Vantera(®) instrument supplied by Liposcience (LS-NMR) and operated at ARUP Laboratories, and a 600 MHz Bruker instrument (ASCEND 600) operated at Health Diagnostic Laboratory (HD-NMR).

RESULTS

Of the 4 methods, ultracentrifugation was the most precise and LS-NMR the least; the latter had a significantly greater CV (p < 0.0001) as compared with all 3 of the other methods, although all CVs were clinically acceptable. The electrophoretic method showed similar precision to ultracentrifugation, while HD-NMR was intermediate. The HD-NMR had the slope closest to 1 (0.90, 95% CI 0.71 to 1.09) and the intercept closest to 0 (-48, -353 to 256) compared to the ultracentrifugation method in Deming regression models. While the two NMR methods correlated well (r = 0.95) with each other and had a slope equivalent to 1 (1.08, 0.98 to 1.19), their intercept in Deming regression excluded 0 (194, 53 to 335) indicating a vertical shift between the two methods.

CONCLUSIONS

This LDL-P method comparison may prove useful for future research and clinical applications.

Hyperlipoproteinemia type 3: the forgotten phenotype

Hyperlipoproteinemia type 3 (HLP3) is caused by impaired removal of triglyceride-rich lipoproteins (TGRL) leading to accumulation of TGRL remnants with abnormal composition. High levels of these remnants, called β-VLDL, promote lipid deposition in tuberous xanthomas, atherosclerosis, premature coronary artery disease, and early myocardial infarction. Recent genetic and molecular studies suggest more genes than previously appreciated may contribute to the expression of HLP3, both through impaired hepatic TGRL processing or removal and increased TGRL production. HLP3 is often highly amenable to appropriate treatment. Nevertheless, most HLP3 probably goes undiagnosed, in part because of lack of awareness of the relatively high prevalence (about 0.2% in women and 0.4-0.5% in men older than 20 years) and largely because of infrequent use of definitive diagnostic methods.

Hepatocyte-Specific Depletion of UBXD8 Induces Periportal Steatosis in Mice Fed a High-Fat Diet

We showed previously that UBXD8 plays a key role in proteasomal degradation of lipidated ApoB in hepatocarcinoma cell lines. In the present study, we aimed to investigate the functions of UBXD8 in liver in vivo. For this purpose, hepatocyte-specific UBXD8 knockout (UBXD8-LKO) mice were generated. They were fed with a normal or high-fat diet, and the phenotypes were compared with those of littermate control mice. Hepatocytes obtained from UBXD8-LKO and control mice were analyzed in culture. After 26 wk of a high-fat diet, UBXD8-LKO mice exhibited macrovesicular steatosis in the periportal area and microvesicular steatosis in the perivenular area, whereas control mice exhibited steatosis only in the perivenular area. Furthermore, UBXD8-LKO mice on a high-fat diet had significantly lower concentrations of serum triglyceride and VLDL than control mice. A Triton WR-1339 injection study revealed that VLDL secretion from hepatocytes was reduced in UBXD8-LKO mice. The decrease of ApoB secretion upon UBXD8 depletion was recapitulated in cultured primary hepatocytes. Accumulation of lipidated ApoB in lipid droplets was observed only in UBXD8-null hepatocytes. The results showed that depletion of UBXD8 in hepatocytes suppresses VLDL secretion, and could lead to periportal steatosis when mice are fed a high-fat diet. This is the first demonstration that an abnormality in the intracellular ApoB degradation mechanism can cause steatosis, and provides a useful model for periportal steatosis, which occurs in several human diseases.

New insights into the pathophysiology of dyslipidemia in type 2 diabetes

Cardiovascular disease (CVD) remains the leading cause of morbidity and mortality for patients with type 2 diabetes, despite recent significant advances in management strategies to lessen CVD risk factors. A major cause is the atherogenic dyslipidemia, which consists of elevated plasma concentrations of both fasting and postprandial triglyceride-rich lipoproteins (TRLs), small dense low-density lipoprotein (LDL) and low high-density lipoprotein (HDL) cholesterol. The different components of diabetic dyslipidemia are not isolated abnormalities but closely linked to each other metabolically. The underlying disturbances are hepatic overproduction and delayed clearance of TRLs. Recent results have unequivocally shown that triglyceride-rich lipoproteins and their remnants are atherogenic. To develop novel strategies for the prevention and treatment of dyslipidaemia, it is essential to understand the pathophysiology of dyslipoproteinaemia in humans. Here, we review recent advances in our understanding of the pathophysiology of diabetic dyslipidemia.

On the function and homeostasis of PCSK9: reciprocal interaction with LDLR and additional lipid effects

Proprotein convertase subtilisin kexin type 9 (PCSK9) is a circulatory ligand that terminates the lifecycle of the low-density lipoprotein (LDL) receptor (LDLR) thus affecting plasma LDL-cholesterol (LDL-C) levels. Recent evidence shows that in addition to the straightforward mechanism of action, there are more complex interactions between PCSK9, LDLR and plasma lipoprotein levels, including: (a) the presence of both parallel and reciprocal regulation of surface LDLR and plasma PCSK9; (b) a correlation between PCSK9 and LDL-C levels dependent not only on the fact that PCSK9 removes hepatic LDLR, but also due to the fact that up to 40% of plasma PCSK9 is physically associated with LDL; and (c) an association between plasma PCSK9 production and the assembly and secretion of triglyceride-rich lipoproteins. The effect of PCSK9 on LDLR is being successfully utilized toward the development of anti-PCSK9 therapies to reduce plasma LDL-C levels. Current biochemical research has uncovered additional mechanisms of action and interacting partners for PCSK9, and this opens the way for a more thorough understanding of the regulation, metabolism, and effects of this interesting protein.

Mature VLDL triggers the biogenesis of a distinct vesicle from the trans-Golgi network for its export to the plasma membrane

Post-Golgi trafficking of mature VLDL (very-low-density lipoprotein) is crucial in maintaining normal TAG (triacylglycerol) homoeostasis of hepatocytes; however, the mechanism that regulates the exit of mature VLDL from the TGN (trans-Golgi network) is not known. We developed an in vitro TGN-budding assay that allowed us to examine the formation of secretory vesicles from the TGN in primary rat hepatocytes. We isolated TAG-rich PG-VTVs (post-TGN VLDL transport vesicles) using a continuous sucrose density gradient. PG-VTVs were distributed in low-density fractions, whereas protein transport vesicles were present in relatively higher-density fractions of the same sucrose gradient. EM revealed large intact PG-VTVs ranging 300-350 nm in size. The biogenesis of PG-VTVs from the TGN required cytosol, ATP, GTP hydrolysis and incubation at 37°C. PG-VTVs concentrated the VLDL proteins: apolipoproteins apoB100, apoAIV, apoAI and apoE, but did not contain either albumin or transferrin. Proteinase K treatment did not degrade VLDL core proteins, suggesting that PG-VTVs were sealed. PG-VTVs were able to fuse with and deliver VLDL to the PM (plasma membrane) in a vectorial manner. We conclude that we have identified a new TGN-derived vesicle, the PG-VTV, which specifically transports mature VLDL from the TGN to the PM.

Control of very low-density lipoprotein secretion by N-ethylmaleimide-sensitive factor and miR-33

RATIONALE

Several reports suggest that antisense oligonucleotides against miR-33 might reduce cardiovascular risk in patients by accelerating the reverse cholesterol transport pathway. However, conflicting reports exist about the impact of anti-miR-33 therapy on the levels of very low-density lipoprotein-triglycerides (VLDL-TAG).

OBJECTIVE

We test the hypothesis that miR-33 controls hepatic VLDL-TAG secretion.

METHODS AND RESULTS

Using therapeutic silencing of miR-33 and adenoviral overexpression of miR-33, we show that miR-33 limits hepatic secretion of VLDL-TAG by targeting N-ethylmaleimide-sensitive factor (NSF), both in vivo and in primary hepatocytes. We identify conserved sequences in the 3'UTR of NSF as miR-33 responsive elements and show that Nsf is specifically recruited to the RNA-induced silencing complex following induction of miR-33. In pulse-chase experiments, either miR-33 overexpression or knock-down of Nsf lead to decreased secretion of apolipoproteins and TAG in primary hepatocytes, compared with control cells. Importantly, Nsf rescues miR-33-dependent reduced secretion. Finally, we show that overexpression of Nsf in vivo increases global hepatic secretion and raises plasma VLDL-TAG.

CONCLUSIONS

Together, our data reveal key roles for the miR-33-NSF axis during hepatic secretion and suggest that caution should be taken with anti-miR-33-based therapies because they might raise proatherogenic VLDL-TAG levels.

How does protein misfolding in the endoplasmic reticulum affect lipid metabolism in the liver?

PURPOSE OF REVIEW

The endoplasmic reticulum (ER) maintains cellular metabolic homeostasis by coordinating protein synthesis, secretion activities, lipid biosynthesis and calcium (Ca²⁺) storage. In this review, we will discuss how altered ER homeostasis contributes to dysregulation of hepatic lipid metabolism and contributes to liver-associated metabolic diseases.

RECENT FINDINGS

Perturbed ER functions or accumulation of unfolded protein in the ER leads to the activation of the unfolded protein response (UPR) to protect the cell from ER stress. Recent findings pinpoint the key regulatory role of the UPR in hepatic lipid metabolism and demonstrate the potential causal mechanism of ER stress in metabolic dysregulation including diabetes and obesity.

SUMMARY

A wide range of factors can alter the protein-folding environment in the ER of hepatocytes and contribute to dysregulation of hepatic lipid metabolism and liver disease. The UPR constitutes a series of adaptive programs that preserve ER protein-folding environment and maintain hepatic lipid homeostasis. Signaling components of the UPR are emerging as potential targets for intervention and treatment of human liver-associated metabolic diseases.

Apolipoprotein B100 quality control and the regulation of hepatic very low density lipoprotein secretion

Apolipoprotein B (apoB) is the main protein component of very low density lipoprotein (VLDL) and is necessary for the assembly and secretion of these triglyceride (TG)-rich particles. Following release from the liver, VLDL is converted to low density lipoprotein (LDL) in the plasma and increased production of VLDL can therefore play a detrimental role in cardiovascular disease. Increasing evidence has helped to establish VLDL assembly as a target for the treatment of dyslipidemias. Multiple factors are involved in the folding of the apoB protein and the formation of a secretion-competent VLDL particle. Failed VLDL assembly can initiate quality control mechanisms in the hepatocyte that target apoB for degradation. ApoB is a substrate for endoplasmic reticulum associated degradation (ERAD) by the ubiquitin proteasome system and for autophagy. Efficient targeting and disposal of apoB is a regulated process that modulates VLDL secretion and partitioning of TG. Emerging evidence suggests that significant overlap exists between these degradative pathways. For example, the insulin-mediated targeting of apoB to autophagy and postprandial activation of the unfolded protein response (UPR) may employ the same cellular machinery and regulatory cues. Changes in the quality control mechanisms for apoB impact hepatic physiology and pathology states, including insulin resistance and fatty liver. Insulin signaling, lipid metabolism and the hepatic UPR may impact VLDL production, particularly during the postprandial state. In this review we summarize our current understanding of VLDL assembly, apoB degradation, quality control mechanisms and the role of these processes in liver physiology and in pathologic states.

Postprandial inflammation: targeting glucose and lipids

Many risk factors have been identified as being responsible for the process of atherogenesis. Several of these risk factors are related to inflammation, which is an obligatory feature of the atherosclerotic plaque. Increasing evidence suggests that postprandial lipoproteins and glucose may be involved in the inflammatory process preceding the development of atherosclerosis. During the postprandial situation, remnants of chylomicrons and very low-density lipoproteins bind to circulating leukocytes and endothelial cells, leading to a state of acute activation with the expression of integrins on different cells, the generation of oxidative stress, production of cytokines and complement activation. Elevated plasma glucose levels may also induce leukocyte activation in humans. In addition, advanced glycation end products, formed during hyperglycemia, cause inflammation and endothelial damage. This chain of events results in a situation of acute inflammation causing endothelial dysfunction, which may be one of the earliest defects in atherogenesis. Interestingly, while this may occur several times each day after each meal, there is only limited information on the contribution of different nutrients on the postprandial inflammatory processes. In this review, we will focus on the available evidence and we will discuss the role of lifestyle and pharmaceutical interventions in modulating postprandial inflammation.