Mechanisms Targeting Apolipoprotein B100 to Proteasomal Degradation

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.

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.

Effects of Early Life Oral Arsenic Exposure on Intestinal Tract Development and Lipid Homeostasis in Neonatal Mice: Implications for NAFLD Development

BACKGROUND

Newborns can be exposed to inorganic arsenic (iAs) through contaminated drinking water, formula, and other infant foods. Epidemiological studies have demonstrated a positive association between urinary iAs levels and the risk of developing nonalcoholic fatty liver disease (NAFLD) among U.S. adolescents and adults.

OBJECTIVES

The present study examined how oral iAs administration to neonatal mice impacts the intestinal tract, which acts as an early mediator for NAFLD.

METHODS

Neonatal mice were treated with a single dose of iAs via oral gavage. Effects on the small intestine were determined by histological examination, RNA sequencing, and biochemical analysis. Serum lipid profiling was analyzed by fast protein liquid chromatography (FPLC), and hepatosteatosis was characterized histologically and biochemically. Liver X receptor-alpha (LXR α ) knockout (L x r α - / - ) mice and liver-specific activating transcription factor 4 (ATF4)-deficient (A t f 4 Δ H e p ) mice were used to define their roles in iAs-induced effects during the neonatal stage.

RESULTS

Neonatal mice exposed to iAs via oral gavage exhibited accumulation of dietary fat in enterocytes, with higher levels of enterocyte triglycerides and free fatty acids. These mice also showed accelerated enterocyte maturation and a longer small intestine. This was accompanied by higher levels of liver-derived very low-density lipoprotein and low-density lipoprotein triglycerides, and a lower level of high-density lipoprotein cholesterol in the serum. Mice exposed during the neonatal period to oral iAs also developed hepatosteatosis. Compared with the control group, iAs-induced fat accumulation in enterocytes became more significant in neonatal L x r α - / - mice, accompanied by accelerated intestinal growth, hypertriglyceridemia, and hepatosteatosis. In contrast, regardless of enterocyte fat accumulation, hepatosteatosis was largely reduced in iAs-treated neonatal A t f 4 Δ H e p mice.

CONCLUSION

Exposure to iAs in neonatal mice resulted in excessive accumulation of fat in enterocytes, disrupting lipid homeostasis in the serum and liver, revealing the importance of the gut-liver axis and endoplasmic reticulum stress in mediating iAs-induced NAFLD at an early age. https://doi.org/10.1289/EHP12381.

Hepatic MDM2 Causes Metabolic Associated Fatty Liver Disease by Blocking Triglyceride-VLDL Secretion via ApoB Degradation

Dysfunctional triglyceride-very low-density lipoprotein (TG-VLDL) metabolism is linked to metabolic-associated fatty liver disease (MAFLD); however, the underlying cause remains unclear. The study shows that hepatic E3 ubiquitin ligase murine double minute 2 (MDM2) controls MAFLD by blocking TG-VLDL secretion. A remarkable upregulation of MDM2 is observed in the livers of human and mouse models with different levels of severity of MAFLD. Hepatocyte-specific deletion of MDM2 protects against high-fat high-cholesterol diet-induced hepatic steatosis and inflammation, accompanied by a significant elevation in TG-VLDL secretion. As an E3 ubiquitin ligase, MDM2 targets apolipoprotein B (ApoB) for proteasomal degradation through direct protein-protein interaction, which leads to reduced TG-VLDL secretion in hepatocytes. Pharmacological blockage of the MDM2-ApoB interaction alleviates dietary-induced hepatic steatohepatitis and fibrosis by inducing hepatic ApoB expression and subsequent TG-VLDL secretion. The effect of MDM2 on VLDL metabolism is p53-independent. Collectively, these findings suggest that MDM2 acts as a negative regulator of hepatic ApoB levels and TG-VLDL secretion in MAFLD. Inhibition of the MDM2-ApoB interaction may represent a potential therapeutic approach for MAFLD treatment.

Hsp40s play distinct roles during the initial stages of apolipoprotein B biogenesis

Apolipoprotein B (ApoB) is the primary component of atherogenic lipoproteins, which transport serum fats and cholesterol. Therefore, elevated levels of circulating ApoB are a primary risk factor for cardiovascular disease. During ApoB biosynthesis in the liver and small intestine under nutrient-rich conditions, ApoB cotranslationally translocates into the endoplasmic reticulum (ER) and is lipidated and ultimately secreted. Under lipid-poor conditions, ApoB is targeted for ER Associated Degradation (ERAD). Although prior work identified select chaperones that regulate ApoB biogenesis, the contributions of cytoplasmic Hsp40s are undefined. To this end, we screened ApoB-expressing yeast and determined that a class A ER-associated Hsp40, Ydj1, associates with and facilitates the ERAD of ApoB. Consistent with these results, a homologous Hsp40, DNAJA1, functioned similarly in rat hepatoma cells. DNAJA1 deficient cells also secreted hyperlipidated lipoproteins, in accordance with attenuated ERAD. In contrast to the role of DNAJA1 during ERAD, DNAJB1-a class B Hsp40-helped stabilize ApoB. Depletion of DNAJA1 and DNAJB1 also led to opposing effects on ApoB ubiquitination. These data represent the first example in which different Hsp40s exhibit disparate effects during regulated protein biogenesis in the ER, and highlight distinct roles that chaperones can play on a single ERAD substrate.

Apolipoprotein B and Cardiovascular Disease: Biomarker and Potential Therapeutic Target

Apolipoprotein (apo) B, the critical structural protein of the atherogenic lipoproteins, has two major isoforms: apoB48 and apoB100. ApoB48 is found in chylomicrons and chylomicron remnants with one apoB48 molecule per chylomicron particle. Similarly, a single apoB100 molecule is contained per particle of very-low-density lipoprotein (VLDL), intermediate density lipoprotein, LDL and lipoprotein(a). This unique one apoB per particle ratio makes plasma apoB concentration a direct measure of the number of circulating atherogenic lipoproteins. ApoB levels indicate the atherogenic particle concentration independent of the particle cholesterol content, which is variable. While LDL, the major cholesterol-carrying serum lipoprotein, is the primary therapeutic target for management and prevention of atherosclerotic cardiovascular disease, there is strong evidence that apoB is a more accurate indicator of cardiovascular risk than either total cholesterol or LDL cholesterol. This review examines multiple aspects of apoB structure and function, with a focus on the controversy over use of apoB as a therapeutic target in clinical practice. Ongoing coronary artery disease residual risk, despite lipid-lowering treatment, has left patients and clinicians with unsatisfactory options for monitoring cardiovascular health. At the present time, the substitution of apoB for LDL-C in cardiovascular disease prevention guidelines has been deemed unjustified, but discussions continue.

Hepatitis C virus induces oxidation and degradation of apolipoprotein B to enhance lipid accumulation and promote viral production

Hepatitis C virus (HCV) infection induces the degradation and decreases the secretion of apolipoprotein B (ApoB). Impaired production and secretion of ApoB-containing lipoprotein is associated with an increase in hepatic steatosis. Therefore, HCV infection-induced degradation of ApoB may contribute to hepatic steatosis and decreased lipoprotein secretion, but the mechanism of HCV infection-induced ApoB degradation has not been completely elucidated. In this study, we found that the ApoB level in HCV-infected cells was regulated by proteasome-associated degradation but not autophagic degradation. ApoB was degraded by the 20S proteasome in a ubiquitin-independent manner. HCV induced the oxidation of ApoB via oxidative stress, and oxidized ApoB was recognized by the PSMA5 and PSMA6 subunits of the 20S proteasome for degradation. Further study showed that ApoB was degraded at endoplasmic reticulum (ER)-associated lipid droplets (LDs) and that the retrotranslocation and degradation of ApoB required Derlin-1 but not gp78 or p97. Moreover, we found that knockdown of ApoB before infection increased the cellular lipid content and enhanced HCV assembly. Overexpression of ApoB-50 inhibited lipid accumulation and repressed viral assembly in HCV-infected cells. Our study reveals a novel mechanism of ApoB degradation and lipid accumulation during HCV infection and might suggest new therapeutic strategies for hepatic steatosis.

Hepatitis C virus (HCV) infection induces the degradation of apolipoprotein B (ApoB), which is the primary apolipoprotein in low-density lipoprotein (LDL) and very low-density lipoprotein (VLDL). Impaired production and secretion of ApoB-containing lipoprotein is associated with an increase in hepatic steatosis. Thus, ApoB degradation might contribute to HCV infection-induced fatty liver. Here, we found that ApoB was not degraded through endoplasmic reticulum-associated degradation (ERAD) or autophagy, as reported previously. Instead, HCV infection induced ApoB oxidation through oxidative stress, and oxidatively damaged ApoB could be recognized and directly degraded by the 20S proteasome. We also found that ApoB was retrotranslocated from the endoplasmic reticulum (ER) to lipid droplets (LDs) for degradation. Through overexpression of ApoB-50, which can mediate the assembly and secretion of LDL and VLDL, we confirmed that ApoB degradation contributed to hepatocellular lipid accumulation induced by HCV infection. Additionally, expression of ApoB-50 impaired HCV production due to the observed decrease in lipid accumulation. In this study, we identified new mechanisms of ApoB degradation and HCV-induced lipid accumulation, and our findings might facilitate the development of novel therapeutic strategies for HCV infection-induced fatty liver.

Endoplasmic reticulum quality control in lipoprotein metabolism

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

N-Glycosylation Defects in Humans Lower Low-Density Lipoprotein Cholesterol Through Increased Low-Density Lipoprotein Receptor Expression

BACKGROUND

The importance of protein glycosylation in regulating lipid metabolism is becoming increasingly apparent. We set out to further investigate this by studying patients with type I congenital disorders of glycosylation (CDGs) with defective N-glycosylation.

METHODS

We studied 29 patients with the 2 most prevalent types of type I CDG, ALG6 (asparagine-linked glycosylation protein 6)-deficiency CDG and PMM2 (phosphomannomutase 2)-deficiency CDG, and 23 first- and second-degree relatives with a heterozygous mutation and measured plasma cholesterol levels. Low-density lipoprotein (LDL) metabolism was studied in 3 cell models-gene silencing in HepG2 cells, patient fibroblasts, and patient hepatocyte-like cells derived from induced pluripotent stem cells-by measuring apolipoprotein B production and secretion, LDL receptor expression and membrane abundance, and LDL particle uptake. Furthermore, SREBP2 (sterol regulatory element-binding protein 2) protein expression and activation and endoplasmic reticulum stress markers were studied.

RESULTS

We report hypobetalipoproteinemia (LDL cholesterol [LDL-C] and apolipoprotein B below the fifth percentile) in a large cohort of patients with type I CDG (mean age, 9 years), together with reduced LDL-C and apolipoprotein B in clinically unaffected heterozygous relatives (mean age, 46 years), compared with 2 separate sets of age- and sex-matched control subjects. ALG6 and PMM2 deficiency led to markedly increased LDL uptake as a result of increased cell surface LDL receptor abundance. Mechanistically, this outcome was driven by increased SREBP2 protein expression accompanied by amplified target gene expression, resulting in higher LDL receptor protein levels. Endoplasmic reticulum stress was not found to be a major mediator.

CONCLUSIONS

Our study establishes N-glycosylation as an important regulator of LDL metabolism. Given that LDL-C was also reduced in a group of clinically unaffected heterozygotes, we propose that increasing LDL receptor-mediated cholesterol clearance by targeting N-glycosylation in the LDL pathway may represent a novel therapeutic strategy to reduce LDL-C and cardiovascular disease.

Acetylation of N-terminus and two internal amino acids is dispensable for degradation of a protein that aberrantly engages the endoplasmic reticulum translocon

Conserved homologues of the Hrd1 ubiquitin ligase target for degradation proteins that persistently or aberrantly engage the endoplasmic reticulum translocon, including mammalian apolipoprotein B (apoB; the major protein component of low-density lipoproteins) and the artificial yeast protein Deg1-Sec62. A complete understanding of the molecular mechanism by which translocon-associated proteins are recognized and degraded may inform the development of therapeutic strategies for cholesterol-related pathologies. Both apoB and Deg1-Sec62 are extensively post-translationally modified. Mass spectrometry of a variant of Deg1-Sec62 revealed that the protein is acetylated at the N-terminal methionine and two internal lysine residues. N-terminal and internal acetylation regulates the degradation of a variety of unstable proteins. However, preventing N-terminal and internal acetylation had no detectable consequence for Hrd1-mediated proteolysis of Deg1-Sec62. Our data highlight the importance of empirically validating the role of post-translational modifications and sequence motifs on protein degradation, even when such elements have previously been demonstrated sufficient to destine other proteins for destruction.

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.

Functional characterization of the chaperon-like protein Cdc48 in cryptogein-induced immune response in tobacco

Cdc48, a molecular chaperone conserved in different kingdoms, is a member of the AAA+ family contributing to numerous processes in mammals including proteins quality control and degradation, vesicular trafficking, autophagy and immunity. The functions of Cdc48 plant orthologues are less understood. We previously reported that Cdc48 is regulated by S-nitrosylation in tobacco cells undergoing an immune response triggered by cryptogein, an elicitin produced by the oomycete Phytophthora cryptogea. Here, we inv estigated the function of NtCdc48 in cryptogein signalling and induced hypersensitive-like cell death. NtCdc48 was found to accumulate in elicited cells at both the protein and transcript levels. Interestingly, only a small proportion of the overall NtCdc48 population appeared to be S-nitrosylated. Using gel filtration in native conditions, we confirmed that NtCdc48 was present in its hexameric active form. An immunoprecipitation-based strategy following my mass spectrometry analysis led to the identification of about a hundred NtCdc48 partners and underlined its contribution in cellular processes including targeting of ubiquitylated proteins for proteasome-dependent degradation, subcellular trafficking and redox regulation. Finally, the analysis of cryptogein-induced events in NtCdc48-overexpressing cells highlighted a correlation between NtCdc48 expression and hypersensitive cell death. Altogether, this study identified NtCdc48 as a component of cryptogein signalling and plant immunity.

AUP1 (Ancient Ubiquitous Protein 1) Is a Key Determinant of Hepatic Very-Low-Density Lipoprotein Assembly and Secretion

OBJECTIVE

AUP1 (ancient ubiquitous protein 1) is an endoplasmic reticulum-associated protein that also localizes to the surface of lipid droplets (LDs), with dual role in protein quality control and LD regulation. Here, we investigated the role of AUP1 in hepatic lipid mobilization and demonstrate critical roles in intracellular biogenesis of apoB100 (apolipoprotein B-100), LD mobilization, and very-low-density lipoprotein (VLDL) assembly and secretion. APPROACH AND RESULTS: siRNA (short/small interfering RNA) knockdown of AUP1 significantly increased secretion of VLDL-sized apoB100-containing particles from HepG2 cells, correcting a key metabolic defect in these cells that normally do not secrete much VLDL. Secreted particles contained higher levels of metabolically labeled triglyceride, and AUP1-deficient cells displayed a larger average size of LDs, suggesting a role for AUP1 in lipid mobilization. Importantly, AUP1 was also found to directly interact with apoB100, and this interaction was enhanced with proteasomal inhibition. Knockdown of AUP1 reduced apoB100 ubiquitination, decreased intracellular degradation of newly synthesized apoB100, and enhanced extracellular apoB100 secretion. Interestingly, the stimulatory effect of AUP1 knockdown on VLDL assembly was reminiscent of the effect previously observed after MEK-ERK (mitogen-activated protein kinase kinase-extracellular signal-regulated kinase) inhibition; however, further studies indicated that the AUP1 effect was independent of MEK-ERK signaling.

CONCLUSIONS

In summary, our findings reveal an important role for AUP1 as a regulator of apoB100 stability, hepatic LD metabolism, and intracellular lipidation of VLDL particles. AUP1 may be a crucial factor in apoB100 quality control, determining the rate at which apoB100 is degraded or lipidated to enable VLDL particle assembly and secretion.

Endoplasmic Reticulum-Associated Degradation and Lipid Homeostasis

The endoplasmic reticulum is the port of entry for proteins into the secretory pathway and the site of synthesis for several important lipids, including cholesterol, triacylglycerol, and phospholipids. Protein production within the endoplasmic reticulum is tightly regulated by a cohort of resident machinery that coordinates the folding, modification, and deployment of secreted and integral membrane proteins. Proteins failing to attain their native conformation are degraded through the endoplasmic reticulum-associated degradation (ERAD) pathway via a series of tightly coupled steps: substrate recognition, dislocation, and ubiquitin-dependent proteasomal destruction. The same ERAD machinery also controls the flux through various metabolic pathways by coupling the turnover of metabolic enzymes to the levels of key metabolites. We review the current understanding and biological significance of ERAD-mediated regulation of lipid metabolism in mammalian cells.

The VCP/p97 and YOD1 Proteins Have Different Substrate-dependent Activities in Endoplasmic Reticulum-associated Degradation (ERAD)

Endoplasmic reticulum-associated degradation (ERAD) is an essential quality control mechanism of the folding state of proteins in the secretory pathway that targets unfolded/misfolded polypeptides for proteasomal degradation. The cytosolic p97/valosin-containing protein is an essential ATPase for degradation of ERAD substrates. It has been considered necessary during retro-translocation to extract proteins from the endoplasmic reticulum that are otherwise supposed to accumulate in the endoplasmic reticulum lumen. The activity of the p97-associated deubiquitinylase YOD1 is also required for substrate disposal. We used the in vivo biotinylation retro-translocation assay in mammalian cells under conditions of impaired p97 or YOD1 activity to directly discriminate their requirements and diverse functions in ERAD. Using different ERAD substrates, we found that both proteins participate in two distinct retro-translocation steps. For CD4 and MHC-Iα, which are induced to degradation by the HIV-1 protein Vpu and by the CMV immunoevasins US2 and US11, respectively, p97 and YOD1 have a retro-translocation-triggering role. In contrast, for three other spontaneous ERAD model substrates (NS1, NHK-α1AT, and BST-2/Tetherin), p97 and YOD1 are required in the downstream events of substrate deglycosylation and proteasomal degradation.

Recent advances in physiological lipoprotein metabolism

Research into lipoprotein metabolism has developed because understanding lipoprotein metabolism has important clinical indications. Lipoproteins are risk factors for cardiovascular disease. Recent advances include the identification of factors in the synthesis and secretion of triglyceride rich lipoproteins, chylomicrons (CM) and very low density lipoproteins (VLDL). These included the identification of microsomal transfer protein, the cotranslational targeting of apoproteinB (apoB) for degradation regulated by the availability of lipids, and the characterization of transport vesicles transporting primordial apoB containing particles to the Golgi. The lipase maturation factor 1, glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 and an angiopoietin-like protein play a role in lipoprotein lipase (LPL)-mediated hydrolysis of secreted CMs and VLDL so that the right amount of fatty acid is delivered to the right tissue at the right time. Expression of the low density lipoprotein (LDL) receptor is regulated at both transcriptional and post-transcriptional level. Proprotein convertase subtilisin/kexin type 9 (PCSK9) has a pivotal role in the degradation of LDL receptor. Plasma remnant lipoproteins bind to specific receptors in the liver, the LDL receptor, VLDL receptor and LDL receptor-like proteins prior to removal from the plasma. Reverse cholesterol transport occurs when lipid free apoAI recruits cholesterol and phospholipid to assemble high density lipoprotein (HDL) particles. The discovery of ABC transporters (ABCA1 and ABCG1) and scavenger receptor class B type I (SR-BI) provided further information on the biogenesis of HDL. In humans HDL-cholesterol can be returned to the liver either by direct uptake by SR-BI or through cholesteryl ester transfer protein exchange of cholesteryl ester for triglycerides in apoB lipoproteins, followed by hepatic uptake of apoB containing particles. Cholesterol content in cells is regulated by several transcription factors, including the liver X receptor and sterol regulatory element binding protein. This review summarizes recent advances in knowledge of the molecular mechanisms regulating lipoprotein metabolism.

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.

Role of cysteine-protease CGHC motifs of ER-60, a protein disulfide isomerase, in hepatic apolipoprotein B100 degradation

Apolipoprotein B100 (apoB), the structural component of very low density lipoproteins (VLDL), is susceptible to misfolding and subsequent degradation by several intracellular pathways. ER-60, which has been implicated in apoB degradation, is a protein disulfide isomerase (PDI) that forms or rearranges disulfide bonds in substrate proteins and also possesses cysteine protease activity. To determine which ER-60 function is important for apoB degradation, adenoviruses encoding wild-type human ER-60 or a mutant form of human ER-60 (C60A, C409A) that lacked cysteine protease activity were overexpressed in HepG2 cells. Overexpression of wild-type ER-60 in HepG2 cells promoted apoB degradation and impaired apoB secretion, but mutant ER-60 overexpression did not. In McArdle RH-7777 cells, VLDL secretion was markedly inhibited following overexpression of wild-type but not mutant ER-60, an effect that could be blocked by oleate treatment. Mutant ER-60 was not trapped on apoB as it was with the control substrate tapasin, suggesting that ER-60's role in apoB degradation is likely unrelated to its protein disulfide isomerase activity. Thus, ER-60 may participate in apoB degradation by acting as a cysteine protease. We postulate that apoB cleavage by ER-60 within the ER lumen could facilitate proteasomal degradation of the C-terminus of translocationally-arrested apoB.

Purinergic signaling, dyslipidemia and inflammatory disease

Metabolic syndrome is a compound obesity disorder, wherein the abnormal metabolism of glucose and lipid is associated with the development of chronic inflammatory diseases. The prevalence of this disease is increasing in the developed world, but the causative linkage between these metabolic disorders has remained obscure. Metabolic disease may be associated with chronic nucleotide secretion, purinergic signaling and activation of inflammatory pathways. Purinergic signaling has been implicated in impaired glucose metabolism and inflammatory disease and may contribute to dyslipidemia. Our research shows that purinergic signaling disrupts hepatic lipoprotein metabolism by blocking insulin receptor signaling and by activating cellular autophagic pathways. Chronic stimulation of purinergic signaling may therefore be causative to glucose and lipid metabolic disorders and associated with the development of cardiovascular disease.

Aberrant substrate engagement of the ER translocon triggers degradation by the Hrd1 ubiquitin ligase

The Hrd1 ubiquitin ligase plays a role in quality control of two substrates associated with the Sec61 translocon.

Little is known about quality control of proteins that aberrantly or persistently engage the endoplasmic reticulum (ER)-localized translocon en route to membrane localization or the secretory pathway. Hrd1 and Doa10, the primary ubiquitin ligases that function in ER-associated degradation (ERAD) in yeast, target distinct subsets of misfolded or otherwise abnormal proteins based primarily on degradation signal (degron) location. We report the surprising observation that fusing Deg1, a cytoplasmic degron normally recognized by Doa10, to the Sec62 membrane protein rendered the protein a Hrd1 substrate. Hrd1-dependent degradation occurred when Deg1-Sec62 aberrantly engaged the Sec61 translocon channel and underwent topological rearrangement. Mutations that prevent translocon engagement caused a reversion to Doa10-dependent degradation. Similarly, a variant of apolipoprotein B, a protein known to be cotranslocationally targeted for proteasomal degradation, was also a Hrd1 substrate. Hrd1 therefore likely plays a general role in targeting proteins that persistently associate with and potentially obstruct the translocon.

Selective hepatic insulin resistance, VLDL overproduction, and hypertriglyceridemia

Insulin plays a central role in regulating energy metabolism, including hepatic transport of very low-density lipoprotein (VLDL)-associated triglyceride. Hepatic hypersecretion of VLDL and consequent hypertriglyceridemia leads to lower circulating high-density lipoprotein levels and generation of small dense low-density lipoproteins characteristic of the dyslipidemia commonly observed in metabolic syndrome and type 2 diabetes mellitus. Physiological fluctuations of insulin modulate VLDL secretion, and insulin inhibition of VLDL secretion upon feeding may be the first pathway to become resistant in obesity that leads to VLDL hypersecretion. This review summarizes the role of insulin-related signaling pathways that determine hepatic VLDL production. Disruption in signaling pathways that reduce generation of the second messenger phosphatidylinositide (3,4,5) triphosphate downstream of activated phosphatidylinositide 3-kinase underlies the development of VLDL hypersecretion. As insulin resistance progresses, a number of pathways are altered that further augment VLDL hypersecretion, including hepatic inflammatory pathways. Insulin plays a complex role in regulating glucose metabolism, and it is not surprising that the role of insulin in VLDL and lipid metabolism will prove equally complex.

Extracellular nucleotides inhibit insulin receptor signaling, stimulate autophagy and control lipoprotein secretion

Hyperglycemia is associated with abnormal plasma lipoprotein metabolism and with an elevation in circulating nucleotide levels. We evaluated how extracellular nucleotides may act to perturb hepatic lipoprotein secretion. Adenosine diphosphate (ADP) (>10 µM) acts like a proteasomal inhibitor to stimulate apoB100 secretion and inhibit apoA-I secretion from human liver cells at 4 h and 24 h. ADP blocks apoA-I secretion by stimulating autophagy. The nucleotide increases cellular levels of the autophagosome marker, LC3-II, and increases co-localization of LC3 with apoA-I in punctate autophagosomes. ADP affects autophagy and apoA-I secretion through P2Y₁₃. Overexpression of P2Y₁₃ increases cellular LC3-II levels by ∼50% and blocks induction of apoA-I secretion. Conversely, a siRNA-induced reduction in P2Y₁₃ protein expression of 50% causes a similar reduction in cellular LC3-II levels and a 3-fold stimulation in apoA-I secretion. P2Y₁₃ gene silencing blocks the effects of ADP on autophagy and apoA-I secretion. A reduction in P2Y₁₃ expression suppresses ERK1/2 phosphorylation, increases the phosphorylation of IR-β and protein kinase B (Akt) >3-fold, and blocks the inhibition of Akt phosphorylation by TNFα and ADP. Conversely, increasing P2Y₁₃ expression significantly inhibits insulin-induced phosphorylation of insulin receptor (IR-β) and Akt, similar to that observed after treatment with ADP. Nucleotides therefore act through P2Y₁₃, ERK1/2 and insulin receptor signaling to stimulate autophagy and affect hepatic lipoprotein secretion.

Proteasome inhibitor treatment reduced fatty acid, triacylglycerol and cholesterol synthesis

In the present study, the beneficial effects of proteasome inhibitor treatment in reducing ethanol-induced steatosis were investigated. A microarray analysis was performed on the liver of rats injected with PS-341 (Bortezomib, Velcade), and the results showed that proteasome inhibitor treatment significantly reduced the mRNA expression of SREBP-1c, and the downstream lipogenic enzymes, such as fatty acid synthase (FAS) and acetyl-CoA carboxylase (ACC), which catalyzes the carboxylation of acetyl-CoA to malonyl-CoA, the rate-limiting step in fatty acid synthesis. ELOVL6, which is responsible for fatty acids long chain elongation, was also significantly downregulated by proteasome inhibitor treatment. Moreover, PS-341 administration significantly reduced the expression of acyl-glycerol-3-phosphate acyltransferase (AGPAT), and diacylglycerol acyltransferase (DGAT), enzyme involved in triacylglycerol (TAG) synthesis. Finally, PS-341 was found to downregulate the enzyme 3-hydroxy-3-methylglutaryl-CoenzymeA synthase (HMG-CoA synthase) that is responsible for cholesterol synthesis. Proteasome inhibitor was also found to play a role in intestinal lipid adsorption because apolipoproteins A (apoA-I, apoAII, apoA-IV and ApoCIII) were downregulated by proteasome inhibitor treatment, especially ApoA-II that is known to be a marker of alcohol consumption. Proteasome inhibitor treatment also decreased apobec-1 complementation factor (ACF) leading to lower level of editing and production of ApoB protein. Moreover apolipoprotein C-III, a major component of chylomicrons was significantly downregulated. However, lipoprotein lipase (Lpl) and High density lipoprotein binding protein (Hdlbp) mRNA levels were increased by proteasome inhibitor treatment. These results suggested that proteasome inhibitor treatment could be used to reduce the alcohol-enhanced lipogenesis and alcohol-induced liver steatosis. A morphologic analysis, performed on the liver of rats fed ethanol for one month and treated with PS-341, showed that proteasome inhibitor treatment significantly decreased ethanol-induced liver steatosis. SREBP-1c, FAS and ACC were increased by ethanol feeding alone, but were significantly decreased when proteasome inhibitor was administered to rats fed ethanol. Our results also show that both mRNA and protein levels of these lipogenic enzymes, up regulated by ethanol, were then downregulated when proteasome inhibitor was administered to rats fed ethanol. It was also confirmed that alcohol feeding caused an increase in AGPAT and DGAT, which was prevented by proteasome inhibitor treatment of the animal fed ethanol. Chronic alcohol feeding did not affect the gene expression of HMG-CoA synthase. However, PS341 administration significantly reduced the HMG-CoA synthase mRNA levels, confirming the results obtained with the microarray analysis. C/EBP transcription factors alpha (CCAAT/enhancer-binding protein alpha) has been shown to positively regulate SREBP-1c mRNA expression, thus regulating lipogenesis. Proteasome inhibition caused a decrease in C/EBP alpha mRNA expression, indicating that C/EBP downregulation may be the mechanism by which proteasome inhibitor treatment reduced lipogenesis. In conclusion, our results indicate that proteasome activity is not only involved in downregulating fatty acid synthesis and triacylglycerol synthesis, but also cholesterol synthesis and intestinal lipid adsorption. Proteasome inhibitor, administrated at a non-toxic low dose, played a beneficial role in reducing lipogenesis caused by chronic ethanol feeding and these beneficial effects are obtained because of the specificity and reversibility of the proteasome inhibitor used.

The degradation of apolipoprotein B100: multiple opportunities to regulate VLDL triglyceride production by different proteolytic pathways

Very low density lipoproteins (VLDL) are a major secretory product of the liver. They serve to transport endogenously synthesized lipids, mainly triglycerides (but also some cholesterol and cholesteryl esters) to peripheral tissues. VLDL is also the precursor of LDL. ApoB100 is absolutely required for VLDL assembly and secretion. The amount of VLDL triglycerides secreted by the liver depends on the amount loaded onto each lipoprotein particle, as well as the number of particles. Each VLDL has one apoB100 molecule, making apoB100 availability a key determinant of the number of VLDL particles, and hence, triglycerides, that can be secreted by hepatic cells. Surprisingly, the pool of apoB100 in the liver is typically regulated not by its level of synthesis, which is relatively constant, but by its level of degradation. It is now recognized that there are multiple opportunities for the hepatic cell to intercept apoB100 molecules and to direct them to distinct degradative processes. This mini-review will summarize progress in understanding these processes, with an emphasis on autophagy, the most recently described pathway of apoB100 degradation, and the one with possibly the most physiologic relevance to common metabolic perturbations affecting VLDL production. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

The Gp78 ubiquitin ligase: probing endoplasmic reticulum complexity

The endoplasmic reticulum (ER) has been classically divided, based on electron microscopy analysis, into parallel ribosome-studded rough ER sheets and a tubular smooth ER network. Recent studies have identified molecular constituents of the ER, the reticulons and DP1, that drive ER tubule formation and whose expression determines expression of ER sheets and tubules and thereby rough and smooth ER. However, segregation of the ER into only two domains remains simplistic and multiple functionally distinct ER domains necessarily exist. In this review, we will discuss the sub-organization of the ER in different domains focusing on the localization and role of the gp78 ubiquitin ligase in the mitochondria-associated smooth ER and on the evidence for a quality control ERAD domain.

Derlin-1 and UBXD8 are engaged in dislocation and degradation of lipidated ApoB-100 at lipid droplets

Apolipoprotein B-100 after lipidation is dislocated from the ER lumen to the cytoplasmic surface of lipid droplets for proteasomal degradation. UBXD8 in lipid droplets and Derlin-1 in the ER membrane interact with each other and with ApoB and are engaged in the pre- and postdislocation steps, respectively.

Apolipoprotein B-100 (ApoB) is the principal component of very low density lipoprotein. Poorly lipidated nascent ApoB is extracted from the Sec61 translocon and degraded by proteasomes. ApoB lipidated in the endoplasmic reticulum (ER) lumen is also subjected to proteasomal degradation, but where and how it dislocates to the cytoplasm remain unknown. In the present study, we demonstrate that ApoB after lipidation is dislocated to the cytoplasmic surface of lipid droplets (LDs) and accumulates as ubiquitinated ApoB in Huh7 cells. Depletion of UBXD8, which is almost confined to LDs in this cell type, decreases recruitment of p97 to LDs and causes an increase of both ubiquitinated ApoB on the LD surface and lipidated ApoB in the ER lumen. In contrast, abrogation of Derlin-1 function induces an accumulation of lipidated ApoB in the ER lumen but does not increase ubiquitinated ApoB on the LD surface. UBXD8 and Derlin-1 bind with each other and with lipidated ApoB and show colocalization around LDs. These results indicate that ApoB after lipidation is dislocated from the ER lumen to the LD surface for proteasomal degradation and that Derlin-1 and UBXD8 are engaged in the predislocation and postdislocation steps, respectively.

Gut-liver interaction in triglyceride-rich lipoprotein metabolism

The liver and intestine have complementary and coordinated roles in lipoprotein metabolism. Despite their highly specialized functions, assembly and secretion of triglyceride-rich lipoproteins (TRL; apoB-100-containing VLDL in the liver and apoB-48-containing chylomicrons in the intestine) are regulated by many of the same hormonal, inflammatory, nutritional, and metabolic factors. Furthermore, lipoprotein metabolism in these two organs may be affected in a similar fashion by certain disorders. In insulin resistance, for example, overproduction of TRL by both liver and intestine is a prominent component of and underlies other features of a complex dyslipidemia and increased risk of atherosclerosis. The intestine is gaining increasing recognition for its importance in affecting whole body lipid homeostasis, in part through its interaction with the liver. This review aims to integrate recent advances in our understanding of these processes and attempts to provide insight into the factors that coordinate lipid homeostasis in these two organs in health and disease.

Ubiquitination regulates the assembly of VLDL in HepG2 cells and is the committing step of the apoB-100 ERAD pathway

Apolipoprotein B-100 (apoB-100) is degraded by endoplasmic reticulum-associated degradation (ERAD) when lipid availability limits assembly of VLDLs. The ubiquitin ligase gp78 and the AAA-ATPase p97 have been implicated in the proteasomal degradation of apoB-100. To study the relationship between ERAD and VLDL assembly, we used small interfering RNA (siRNA) to reduce gp78 expression in HepG2 cells. Reduction of gp78 decreased apoB-100 ubiquitination and cytosolic apoB-ubiquitin conjugates. Radiolabeling studies revealed that gp78 knockdown increased secretion of newly synthesized apoB-100 and, unexpectedly, enhanced VLDL assembly, as the shift in apoB-100 density in gp78-reduced cells was accompanied by increased triacylglycerol (TG) secretion. To explore the mechanisms by which gp78 reduction might enhance VLDL assembly, we compared the effects of gp78 knockdown with those of U0126, a mitogen-activated protein kinase/ERK kinase1/2 inhibitor that enhances apoB-100 secretion in HepG2 cells. U0126 treatment increased secretion of both apoB100 and TG and decreased the ubiquitination and cellular accumu-lation of apoB-100. Furthermore, p97 knockdown caused apoB-100 to accumulate in the cell, but if gp78 was concomitantly reduced or assembly was enhanced by U0126 treatment, cellular apoB-100 returned toward baseline. This indicates that ubiquitination commits apoB-100 to p97-mediated retrotranslocation during ERAD. Thus, decreasing ubiquitination of apoB-100 enhances VLDL assembly, whereas improving apoB-100 lipidation decreases its ubiquitination, suggesting that ubiquitination has a regulatory role in VLDL assembly.

The unfolded protein response transducer IRE1α prevents ER stress-induced hepatic steatosis

The endoplasmic reticulum (ER) is the cellular organelle responsible for protein folding and assembly, lipid and sterol biosynthesis, and calcium storage. The unfolded protein response (UPR) is an adaptive intracellular stress response to accumulation of unfolded or misfolded proteins in the ER. In this study, we show that the most conserved UPR sensor inositol-requiring enzyme 1 α (IRE1α), an ER transmembrane protein kinase/endoribonuclease, is required to maintain hepatic lipid homeostasis under ER stress conditions through repressing hepatic lipid accumulation and maintaining lipoprotein secretion. To elucidate physiological roles of IRE1α-mediated signalling in the liver, we generated hepatocyte-specific Ire1α-null mice by utilizing an albumin promoter-controlled Cre recombinase-mediated deletion. Deletion of Ire1α caused defective induction of genes encoding functions in ER-to-Golgi protein transport, oxidative protein folding, and ER-associated degradation (ERAD) of misfolded proteins, and led to selective induction of pro-apoptotic UPR trans-activators. We show that IRE1α is required to maintain the secretion efficiency of selective proteins. In the absence of ER stress, mice with hepatocyte-specific Ire1α deletion displayed modest hepatosteatosis that became profound after induction of ER stress. Further investigation revealed that IRE1α represses expression of key metabolic transcriptional regulators, including CCAAT/enhancer-binding protein (C/EBP) β, C/EBPδ, peroxisome proliferator-activated receptor γ (PPARγ), and enzymes involved in triglyceride biosynthesis. IRE1α was also found to be required for efficient secretion of apolipoproteins upon disruption of ER homeostasis. Consistent with a role for IRE1α in preventing intracellular lipid accumulation, mice with hepatocyte-specific deletion of Ire1α developed severe hepatic steatosis after treatment with an ER stress-inducing anti-cancer drug Bortezomib, upon expression of a misfolding-prone human blood clotting factor VIII, or after partial hepatectomy. The identification of IRE1α as a key regulator to prevent hepatic steatosis provides novel insights into ER stress mechanisms in fatty liver diseases associated with toxic liver injuries.

Apolipoprotein B100 biogenesis: a complex array of intracellular mechanisms regulating folding, stability, and lipoprotein assemblyThis paper is one of a selection of papers published in this special issue entitled “Canadian Society of Biochemistry, Molecular & Cellular Biology 52nd Annual Meeting — Protein Folding: Principles and Diseases” and has undergone the Journal's usual peer review process

Apolipoprotein B100 (apoB) is a large amphipathic lipid-binding protein that is synthesized by hepatocytes and used to assemble and stabilize very low density lipoproteins (VLDL). It may have been derived through evolution from other lipid-associating proteins such as microsomal triglyceride transfer protein or vitellogenin. The correct folding of apoB requires assistance from chaperone proteins in co-translational lipidation, disulfide bond formation, and glycosylation. Any impairment in these processes results in co-translational targeting of the misfolded apoB molecule for proteasomal degradation. In fact, most of the regulation of apoB production is mediated by intracellular degradation. ApoB that misfolds post-translationally, perhaps as a result of oxidative stress, may be eliminated through autophagy. This review focuses on the proposed pentapartite domain structure of apoB, the role that each domain plays in the binding of lipid species and regulation of apoB synthesis, and the process of VLDL assembly. The factors involved in the recognition, ubiquitination, and proteasomal delivery of defective apoB molecules are also discussed.

Stringent requirement for HRD1, SEL1L, and OS-9/XTP3-B for disposal of ERAD-LS substrates

Soluble ERAD substrates require the Hrd1 E3 ligase for degradation compared with membrane-anchored peptides that use GP78.

Sophisticated quality control mechanisms prolong retention of protein-folding intermediates in the endoplasmic reticulum (ER) until maturation while sorting out terminally misfolded polypeptides for ER-associated degradation (ERAD). The presence of structural lesions in the luminal, transmembrane, or cytosolic domains determines the classification of misfolded polypeptides as ERAD-L, -M, or -C substrates and results in selection of distinct degradation pathways. In this study, we show that disposal of soluble (nontransmembrane) polypeptides with luminal lesions (ERAD-L_S substrates) is strictly dependent on the E3 ubiquitin ligase HRD1, the associated cargo receptor SEL1L, and two interchangeable ERAD lectins, OS-9 and XTP3-B. These ERAD factors become dispensable for degradation of the same polypeptides when membrane tethered (ERAD-L_M substrates). Our data reveal that, in contrast to budding yeast, tethering of mammalian ERAD-L substrates to the membrane changes selection of the degradation pathway.