Apoprotein B100, an inefficiently translocated secretory protein, is bound to the cytosolic chaperone, heat shock protein 70

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.

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.

The Targeting of Native Proteins to the Endoplasmic Reticulum-Associated Degradation (ERAD) Pathway: An Expanding Repertoire of Regulated Substrates

All proteins are subject to quality control processes during or soon after their synthesis, and these cellular quality control pathways play critical roles in maintaining homeostasis in the cell and in organism health. Protein quality control is particularly vital for those polypeptides that enter the endoplasmic reticulum (ER). Approximately one-quarter to one-third of all proteins synthesized in eukaryotic cells access the ER because they are destined for transport to the extracellular space, because they represent integral membrane proteins, or because they reside within one of the many compartments of the secretory pathway. However, proteins that mature inefficiently are subject to ER-associated degradation (ERAD), a multi-step pathway involving the chaperone-mediated selection, ubiquitination, and extraction (or “retrotranslocation”) of protein substrates from the ER. Ultimately, these substrates are degraded by the cytosolic proteasome. Interestingly, there is an increasing number of native enzymes and metabolite and solute transporters that are also targeted for ERAD. While some of these proteins may transiently misfold, the ERAD pathway also provides a route to rapidly and quantitatively downregulate the levels and thus the activities of a variety of proteins that mature or reside in the ER.

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.

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.

Impaired response of VLDL lipid and apoB secretion to endotoxin in the fasted rat liver

Bacterial infection elicits hypertriglyceridemia attributed to increased hepatic production of very low-density lipoprotein (VLDL) particles and decreased peripheral metabolism. The mechanisms underlying VLDL overproduction in sepsis are as yet unclear, but seem to be fed/fasted state-dependent. To learn more about this, we investigated hepatocytes isolated from fasted rats, made endotoxic by 1 mg/kg lipopolysaccharide (LPS) injection, for their ability to secrete the VLDL protein and lipid components. The results were then related to lipogenesis markers and expression of genes critical to VLDL biogenesis. Endotoxic rats showed increased levels of serum VLDL-apoB (10-fold), -triglyceride (2-fold), and -cholesterol (2-fold), whereby circulating VLDL were lipid-poor particles. Similarly, VLDL-apoB secretion by isolated endotoxic hepatocytes was ~85% above control, whereas marginal changes in the output of VLDL-lipid classes occurred. This was accompanied by a substantial rise in apoB and a moderate rise in MTP mRNA levels, but with basal de novo formation and efficiency of secretion of triglycerides, cholesterol and cholesteryl esters. These results indicate that during periods of food restriction, endotoxin does not enhance lipid provision to accomplish normal lipidation of overproduced apoB molecules, though this does occur to a sufficient extent to pass the proteasome checkpoint and secretion of lipid-poor, type 2 VLDL takes place.

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.

Isoforms of Hsp70-binding human LDL in adult Schistosoma mansoni worms

Schistosoma mansoni is one of the most common parasites infecting humans. They are well adapted to the host, and this parasite's longevity is a consequence of effective escape from the host immune system. In the blood circulation, lipoproteins not only help to conceal the worm from attack by host antibodies but also act as a source of lipids for S. mansoni. Previous SEM studies showed that the low-density lipoprotein (LDL) particles present on the surface of adult S. mansoni worms decreased in size when the incubation time increased. In this study, immunocytochemical and proteomic analyses were used to locate and identify S. mansoni binding proteins to human plasma LDL. Ultrathin sections of adult worms were cut transversely from the anterior, medial and posterior regions of the parasite. Immunocytochemical experiments revealed particles of gold in the tegument, muscle region and spine in male worms and around vitelline cells in females. Immunoblotting and 2D-electrophoresis using incubations with human serum, anti-LDL antibodies and anti-chicken IgG peroxidase conjugate were performed to identify LDL-binding proteins in S. mansoni. Analysis of the binding proteins using LC-MS identified two isoforms of the Hsp70 chaperone in S. mansoni. Hsp70 is involved in the interaction with apoB in the cytoplasm and its transport to the endoplasmic reticulum. However, further studies are needed to clarify the functional role of Hsp70 in S. mansoni, mainly related to the interaction with human LDL.

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.

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.

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.

The assembly of triacylglycerol-rich lipoproteins: an essential role for the microsomal triacylglycerol transfer protein

Raised plasma triacylglycerol is an independent risk factor for cardiovascular disease, and an understanding of factors which regulate the synthesis and degradation of lipoproteins which carry triacylglycerol in the blood may lead to novel approaches to the treatment of hypertriacylglycerolaemia. An active microsomal triacylglycerol transfer protein (MTP) is essential for the assembly of particles which transport triacylglycerol through the circulation. After absorption in the intestine, dietary fat and fat-soluble vitamins are incorporated into chylomicrons in the intestinal epithelial cells, and these lipoproteins reach the bloodstream via the lymphatic system. Patients with the rare genetic disorder, abetalipoproteinaemia, in which MTP activity is absent, present clinically with fat-soluble vitamin and essential fatty acid deficiency, indicating a key role for MTP in the movement of fat into the body. The triacylglycerol-rich lipoprotein found in fasting blood, VLDL, is assembled in the liver by an MTP-dependent process similar to chylomicron assembly, and transports triacylglycerol to extra-hepatic tissues such as adipose tissue and heart. In the absence of MTP activity, VLDL are not synthesized and only extremely low levels of triacylglycerol are present in the blood. Dietary components, including fat, cholesterol and ethanol, can modify the expression of the MTP gene and, hence, MTP activity. The present review summarizes current knowledge of the role of MTP in the assembly and secretion of triacylglycerol-rich lipoproteins, and the regulation of its activity in both animal and cell systems.

Translocation Efficiency of Apolipoprotein B Is Determined by the Presence of β-Sheet Domains, Not Pause Transfer Sequences

Cotranslational translocation of apoB100 across the endoplasmic reticulum (ER) membrane is inefficient, resulting in exposure of nascent apoB on the cytosolic surface of the ER. This predisposes apoB100 to ubiquitinylation and targeting for proteasomal degradation. It has been suggested that pause transfer sequences (PTS) present throughout apoB cause inefficient translocation. On the other hand, our previous study demonstrated that the translocation efficiency of apoB100 is dependent on the presence of a beta-sheet domain between 29 and 34% of full-length apoB100 (Liang, J.-S., Wu, X., Jiang, H., Zhou, M., Yang, H., Angkeow, P., Huang, L.-S., Sturley, S. L., and Ginsberg, H. N. (1998) J. Biol. Chem. 273, 35216-35221); this region of apoB has no PTS. However, the effects of the beta-sheet domain may require the presence of PTS elsewhere in the N-terminal region of apoB100. To further investigate the roles of PTS and beta-sheet domains in the translocation of apoB100 across the ER, we transfected McArdle RH7777, HepG2, or Chinese hamster ovary cells with human albumin (ALB)/human apoB chimeric cDNA constructs: ALB/B12-17 (two PTS but no beta-sheet), ALB/B29-34 (beta-sheet but no PTS), ALB/B36-41 (two PTS and a beta-sheet), and ALB/B49-54 (neither PTS nor a beta-sheet). ALB/ALB1-40 served as a control. Compared with ALB/ALB1-40, secretion rates of ALB/B12-17, ALB/B29-34, and ALB/B36-41 were reduced. Secretion of ALB/B49-54 was similar to that of ALB/ALB1-40. However, only ALB/B29-34 and ALB/B36-41 had increased proteinase K sensitivity, ubiquitinylation, and increased physical interaction with Sec61alpha. These results indicate that the translocation efficiency of apoB100 is determined mainly by the presence of beta-sheet domains. PTS do not appear to affect translocation, but may affect secretion by other mechanisms.

Inhibition of the synthesis of apolipoprotein B-containing lipoproteins

Increased serum concentrations of low density lipoproteins represent a major cardiovascular risk factor. Low-density lipoproteins are derived from very low density lipoproteins secreted by the liver. Apolipoprotein (apo)B that constitutes the essential structural protein of these lipoproteins exists in two forms, the full length form apoB-100 and the carboxy-terminal truncated apoB-48. The generation of apoB-48 is due to editing of the apoB mRNA which generates a premature stop translation codon. The editing of apoB mRNA is an important regulatory event because apoB-48-containing lipoproteins cannot be converted into the atherogenic low density lipoproteins. The apoB gene is constitutively expressed in liver and intestine, and the rate of apoB secretion is regulated post-transcriptionally. The translocation of apoB into the endoplasmic reticulum is complicated by the hydrophobicity of the nascent polypeptide. The assembly and secretion of apoB-containing lipoproteins within the endoplasmic reticulum is strictly dependent on the microsomal tricylceride transfer protein which shuttles triglycerides onto the nascent lipoprotein particle. The overall synthesis of apoB lipoproteins is regulated by proteosomal and nonproteosomal degradation and is dependent on triglyceride availability. Noninsulin dependent diabetes mellitus, obesity and the metabolic syndrome are characterized by an increased hepatic synthesis of apoB-containing lipoproteins. Interventions aimed to reduce the hepatic secretion of apoB-containing lipoproteins are therefore of great clinical importance. Lead targets in these pathways are discussed.

Fatty acids increase presenilin-1 levels and γ-secretase activity in PSwt-1 cells

Presenilin-1 (PS1) is an important determinant of the gamma-secretase activity necessary for the generation of beta-amyloid (Abeta), likely the central pathogenic molecule in Alzheimer's disease. Most presenilin is rapidly degraded, and determinants of the level of the active cleaved form are unknown. We examined the influence of fatty acids on PS1 levels and gamma-secretase activity using stably transfected CHO cells that express human PS1 and the human amyloid precursor protein. Cells cultured with 0.4 mM oleic acid (OA), with 0.1 mM linoleic acid, or with a triglyceride emulsion expressed increased PS1 and Abeta. This effect was independent of any secondary increase in cellular cholesterol. Cells cultured in 0.4 mM OA also exhibited significantly increased gamma-secretase activity. PS1 mRNA levels were unchanged, and pulse-chase experiments indicated that OA slowed presenilin holoprotein degradation. Nontransfected human neuroblastoma cells also showed increased presenilin when cultured in 0.4 mM OA. Lipids may be important biological determinants of PS1 level and gamma-secretase activity.

Overexpression of the tumor autocrine motility factor receptor Gp78, a ubiquitin protein ligase, results in increased ubiquitinylation and decreased secretion of apolipoprotein B100 in HepG2 cells

Apolipoprotein B100 (apoB) is a large (520-kDa) complex secretory protein; its secretion is regulated posttranscriptionally by several degradation pathways. The best described of these degradative processes is co-translational ubiquitinylation and proteasomal degradation of nascent apoB, involving the 70- and 90-kDa heat shock proteins and the multiple components of the proteasomal pathway. Ubiquitinylation involves several proteins, including ligases called E3s, that coordinate the covalent binding of ubiquitin to target proteins. The recent discovery that tumor autocrine motility factor receptor, also known as gp78, is an endoplasmic reticulum (ER)-associated E3, raised the possibility that this E3 might be involved in the ER-associated degradation of nascent apoB. In a series of experiments in HepG2 cells, we demonstrated that overexpression of gp78 was sufficient for increased ubiquitinylation and proteasomal degradation of apoB, with reduced secretion of apoB-lipoproteins. This action of gp78 was specific: overexpression of the protein did not affect secretion of either albumin or apolipoprotein AI. Furthermore, overexpression of a cytosolic E3, Itch, had no effect on apoB secretion. Finally, using an in vitro translation system, we demonstrated that gp78 led to increased ubiquitinylation and proteasomal degradation of apoB48. Together, these results indicate that an ER-associated protein, gp78, is a bona fide E3 ligase in the apoB ER-associated degradation pathway.

The late addition of core lipids to nascent apolipoprotein B100, resulting in the assembly and secretion of triglyceride-rich lipoproteins, is independent of both microsomal triglyceride transfer protein activity and new triglyceride synthesis

Although microsomal triglyceride transfer protein (MTP) and newly synthesized triglyceride (TG) are critical for co-translational targeting of apolipoprotein B (apoB100) to lipoprotein assembly in hepatoma cell lines, their roles in the later stages of lipoprotein assembly remain unclear. Using N-acetyl-Leu-Leu-norleucinal to prevent proteasomal degradation, HepG2 cells were radiolabeled and chased for 0-90 min (chase I). The medium was changed and cells chased for another 150 min (chase II) in the absence (control) or presence of Pfizer MTP inhibitor CP-10447 (CP). As chase I was extended, inhibition of apoB100 secretion by CP during chase II decreased from 75.9% to only 15% of control (no CP during chase II). Additional studies were conducted in which chase I was either 0 or 90 min, and chase II was in the presence of [(3)H]glycerol and either BSA (control), CP (inhibits both MTP activity and TG synthesis),BMS-1976360-1) (BMS) (inhibits only MTP activity), or triacsin C (TC) (inhibits only TG synthesis). When chase I was 0 min, CP, BMS, and TC reduced apoB100 secretion during chase II by 75.3, 73.9, and 53.9%. However, when chase I was 90 min, those agents reduced apoB100 secretion during chase II by only 16.0, 19.2, and 13.9%. Of note, all three inhibited secretion of newly synthesized TG during chase II by 80, 80, and 40%, whether chase I was 0 or 90 min. In both HepG2 cells and McA-RH7777 cells, if chase I was at least 60 min, inhibition of TG synthesis and/or MTP activity did not affect the density of secreted apoB100-lipoproteins under basal conditions. Oleic acid increased secretion of TG-enriched apoB100-lipoproteins similarly in the absence or presence of either of CP, BMS, or TC. We conclude that neither MTP nor newly synthesized TG is necessary for the later stages of apoB100-lipoprotein assembly and secretion in either HepG2 or McA-RH7777 cells.

Differential degradation of the three fibrinogen chains by proteasomes: involvement of Sec61p and cytosolic Hsp70

HepG2 cells, which synthesize and secrete fibrinogen, accumulate surplus Aalpha and gamma chains. The nonsecreted fibrinogen chains are degraded both by proteasomes and lysosomes, with unassembled chains primarily degraded by proteasomes and an Aalpha-gamma complex by lysosomes. To further determine the mechanisms by which unassembled fibrinogen chains are degraded, and to explain the pools of Aalpha and gamma chains that occur in HepG2 cells, the association of fibrinogen chains with Sec61beta, a component of the translocon, and with a cytosol chaperone, Hsp70, was studied in both HepG2 cells and COS cells expressing single fibrinogen chains. Retrotranslocation from the lumen of the endoplasmic reticulum was shown by treatment with MG132, a proteasome inhibitor. MG132 caused glycosylated Bbeta to accumulate on Sec61beta in COS cells expressing Bbeta and acted similarly with all three fibrinogen chains in HepG2 cells. In HepG2 cells, Bbeta was associated with Sec61beta ahead of Aalpha and gamma chains, suggesting that pools of Aalpha and gamma chains may be caused by unequal rates of retrotranslocation. In COS cells, retrotranslocation into the cytoplasm was demonstrated by the ATP-sensitive association of ubiquitinylated Aalpha, Bbeta, and gamma chains bound to Hsp70. More Aalpha and gamma than Bbeta accumulated on Hsp70 of HepG2 cells, consistent with more rapid degradation of Bbeta. Overexpression of Hsp70 in HepG2 cells resulted in decreased secretion, but not synthesis, of fibrinogen. Decreased secretion may be due to enhanced degradation of unassembled fibrinogen chains, indicating that proteolysis by proteasomes might regulate both the intracellular pools of fibrinogen chains and fibrinogen secretion.

Variation in the human ApoB signal peptide modulates ApoB17 translocation

The functional effects of the common 27- or 24-amino-acid (aa) variants in the human apoB signal peptide (SP) on intracellular and secreted apoB17 were investigated in vitro. Only in the presence of oleate was a significant difference in intracellular and secreted SP27-B17 compared to SP24-B17 observed (P = 0.01 and P < 0.0007, respectively), although in the presence or absence of oleate mRNA levels from the two constructs were similar. After fractionation, oleate treatment enhanced microsomal SP27-B17 by 150% (P < 0.0005) with a modest but significant effect on SP24-B17 (32% P = 0.007). Oleate stimulated SP24-B17 accumulation in the nonmicrosomal fraction. The data suggest that the presence of oleate leads to inefficient translocation of the 24-amino-acid signal peptide, possibly resulting in increased retrograde translocation into the cytoplasm and reduced intracellular and secreted levels compared to the "wildtype" 27 aa SP. This implies a direct role of the SP variants in the regulation of apoB intracellular metabolism.

APOLIPOPROTEIN B: mRNA editing, lipoprotein assembly, and presecretory degradation

Apolipoprotein (apo)B circulates in two distinct forms, apoB100 and apoB48. Human liver secretes apoB100, the product of a large mRNA encoding 4536 residues. The small intestine of all mammals secretes apoB48, which arises following C-to-U deamination of a single cytidine base in the nuclear apoB transcript, introducing a translational stop codon. This process, referred to as apoB RNA editing, operates through a multicomponent enzyme complex that contains a single catalytic subunit, apobec-1, in addition to other protein factors that have yet to be cloned. ApoB RNA editing also exhibits stringent cis-acting requirements that include both structural and sequence-specific elements-specifically efficiency elements that flank the minimal cassette, an AU-rich RNA context, and an 11-nucleotide mooring sequence-located in proximity to a suitably positioned (usually upstream) cytidine. C-to-U RNA editing may become unconstrained under circumstances where apobec-1 is overexpressed, in which case multiple cytidines in apoB RNA, as well as in other transcripts, undergo C-to-U editing. ApoB RNA editing is eliminated following targeting of apobec-1, establishing that there is no genetic redundancy in this function. Under physiological circumstances, apoB RNA editing exhibits developmental, hormonal, and nutritional regulation, in some cases related to transcriptional regulation of apobec-1 mRNA. ApoB and the microsomal triglyceride transfer protein (MTP) are essential for the assembly and secretion of apoB-containing lipoproteins. MTP functions by transferring lipid to apoB during its translation and by transporting triglycerides into the endoplasmic reticulum to form apoB-free lipid droplets. These droplets fuse with nascent apoB-containing particles to form mature, very low-density lipoproteins or chylomicrons. In cultured hepatic cells, lipid availability dictates the rate of apoB production. Unlipidated or underlipidated forms of apoB are subjected to presecretory degradation, a process mediated by retrograde transport from the lumen of the endoplasmic reticulum to the cytosol, coupled with multiubquitination and proteasomal degradation. Although control of lipid secretion in vivo is primarily achieved at the level of lipoprotein particle size, regulation of apoB production by presecretory degradation may be relevant in some dyslipidemic states.

Characterization of the biosynthesis of human immunodeficiency virus type 1 Env from infected T-cells and the effects of glucose trimming of Env on virion infectivity

HIV (human immunodeficiency virus)-1 Env is displayed on the surface of infected cells and subsequently incorporated into virions, which is necessary for the initiation of a viral infection by recognition of the CD4 and the chemokine receptors (such as CCR5 or CXCR4) on the surface of new target cells. As a type 1 integral membrane glycoprotein, Env is cotranslationally translocated into the endoplasmic reticulum. In this report, we characterized the synthesis of Env, which did not occur at a constant rate but by translational/translocational pausing that has not previously been shown with a viral encoded glycoprotein. Overall translation was not impeded by the presence of the reducing agent dithiothreitol in vivo, although this did influence the cleavage of the precursor gp160 into its mature form, gp120. Env interacts transiently with resident components of the endoplasmic reticulum such as calnexin, which had maximal association at a 10-min post-translation. Addition of the glucosidase inhibitor, castanospermine, failed to significantly influence the association of Env with calnexin, consistent with the notion that calnexin recognizes components other than alpha-terminal glucose. Moreover, castanospermine treatment failed to affect the infectivity of virions. Taken together, this report demonstrates the existence of translational/translocational pausing for a viral glycoprotein and suggests that trimming of glucose from HIV-1 Env is not essential for the initiation of virus infection.

Co-translational interactions of apoprotein B with the ribosome and translocon during lipoprotein assembly or targeting to the proteasome

Hepatic lipoprotein assembly and secretion can be regulated by proteasomal degradation of newly synthesized apoB, especially if lipid synthesis or lipid transfer is low. Our previous studies in HepG2 cells showed that, under these conditions, newly synthesized apoB remains stably associated with the endoplasmic reticulum (ER) membrane (Mitchell, D. M., Zhou, M., Pariyarath, R., Wang, H., Aitchison, J. D., Ginsberg, H. N., and Fisher, E. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 14733-14738). We now show that independent of lipid synthesis, apoB chains that appear full-length are, in fact, incompletely translated polypeptides still engaged by the ribosome and associated with the ER translocon. In the presence of active lipid synthesis and transfer, translation and lipoprotein assembly are completed, and the complexes exit the ER. Upon omitting fatty acids from, or adding a microsomal triglyceride transfer protein inhibitor to, culture media to reduce lipid synthesis or transfer, respectively, apoB was degraded while it remained associated with the ER and complexed with cytosolic hsp70 and proteasomes. Thus, unlike other ER substrates of the proteasome, such as major histocompatibility complex class I molecules, apoB does not fully retrotranslocate to the cytosol before entering the ubiquitin-proteasome pathway. Although, upon immunofluorescence, apoB in proteasome-inhibited cells accumulated in punctate structures similar in appearance to aggresomes (cytosolic structures containing molecules irreversibly lost from the secretory pathway), these apoB molecules could be secreted when lipid synthesis was stimulated. The results suggest a model in which 1) apoB translation does not complete until lipoprotein assembly terminates, and 2) assembly with lipids or entry into the ubiquitin-proteasome pathway occurs while apoB polypeptides remain associated with the translocon and attached to the ribosome.

The Hsc70 co-chaperone CHIP targets immature CFTR for proteasomal degradation

The folding of both wild-type and mutant forms of the cystic-fibrosis transmembrane-conductance regulator (CFTR), a plasma-membrane chloride-ion channel, is inefficient. Most nascent CFTR is retained in the endoplasmic reticulum and degraded by the ubiquitin proteasome pathway. Aberrant folding and defective trafficking of CFTRDeltaF508 is the principal cause of cystic fibrosis, but how the endoplasmic-reticulum quality-control system targets CFTR for degradation remains unknown. CHIP is a cytosolic U-box protein that interacts with Hsc70 through a set of tetratricorepeat motifs. The U-box represents a modified form of the ring-finger motif that is found in ubiquitin ligases and that defines the E4 family of polyubiquitination factors. Here we show that CHIP functions with Hsc70 to sense the folded state of CFTR and targets aberrant forms for proteasomal degradation by promoting their ubiquitination. The U-box appeared essential for this process because overexpresion of CHIPDeltaU-box inhibited the action of endogenous CHIP and blocked CFTR ubiquitination and degradation. CHIP is a co-chaperone that converts Hsc70 from a protein-folding machine into a degradation factor that functions in endoplasmic-reticulum quality control.

Intracellular Assembly of VLDL Two Major Steps in Separate Cell Compartments

The assembly of very low-density lipoproteins (VLDL) occurs in two major steps. The first step is the co-and post-translational lipidation of apoB, forming pre-VLDL in the rough endoplasmic reticulum. The microsomal triglyceride transfer protein catalyzes this step. In the second step pre-VLDL is converted to bona fide VLDL in a smooth membrane compartment. This step depends on ADP-ribosylation factor 1 and its activation of phospholipase D.

B48 is preferentially translated over B100 in cells with increased endogenous apo B mRNA

We recently demonstrated that expression of BHMT in McArdle RH-7777 (McA-BHMT) cells increases apo B mRNA abundance, leading to parallel increases in apo B secretion. The ratio of unedited to edited apo B mRNA was unchanged by BHMT expression. Based on the observation that secretion of B48 is increased relative to B100 in McA-BHMT cells, current studies now include comparison of B48 and B100 synthesis and intracellular degradation. Minor differences in co- and posttranslational degradation were unable to account for relative increase in B48 secretion, and the disappearance kinetics of B48 were similar in McA-BHMT and control cells. Consistent with the increase in endogenous apo B mRNA in McA-BHMT cells, B48 synthesis is increased significantly. In contrast, synthesis of B100 was not significantly increased. We conclude that B48 is preferentially translated compared to B100 when endogenous apo B mRNA is increased.

Progress towards understanding the role of microsomal triglyceride transfer protein in apolipoprotein-B lipoprotein assembly

The microsomal triglyceride transfer protein (MTP) is necessary for the proper assembly of the apolipoprotein B containing lipoproteins, very low density lipoprotein and chylomicrons. Recent research has significantly advanced our understanding of the role of MTP in these pathways at the molecular and cellular level. Biochemical studies suggest that initiation of lipidation of the nascent apolipoprotein B polypeptide may occur through a direct association with MTP. This early lipidation may be required to allow the nascent polypeptide to fold properly and therefore avoid ubiquitination and degradation. Concerning the addition of core neutral lipids in the later stages of lipoprotein assembly, cell culture studies show that MTP lipid transfer activity is not required for this to occur for apolipoprotein B-100 containing lipoproteins. Likewise, MTP does not appear to directly mediate addition of core neutral lipid to nascent apoB-48 particles. However, new data indicate that MTP is required to produce triglyceride rich droplets in the smooth endoplasmic reticulum which may supply the core lipids for conversion of nascent, dense apoB-48 particles to mature VLDL. In addition, assembly of dense apolipoprotein B-48 containing lipoproteins has been observed in mouse liver in the absence of MTP. As a result of these new data, an updated model for the role of MTP in lipoprotein assembly is proposed.

The assembly and secretion of apolipoprotein B-48-containing very low density lipoproteins in McA-RH7777 cells

We have used an extraction procedure, which released membrane-bound apoB-100, to study the assembly of apoB-48 VLDL (very low density lipoproteins). This procedure released apoB-48, but not integral membrane proteins, from microsomes of McA-RH7777 cells. Upon gradient ultracentrifugation, the extracted apoB-48 migrated in the same position as the dense apoB-48-containing lipoprotein (apoB-48 HDL (high density lipoprotein)) secreted into the medium. Labeling studies with [(3)H]glycerol demonstrated that the HDL-like particle extracted from the microsomes contains both triglycerides and phosphatidylcholine. The estimated molar ratio between triglyceride and phosphatidylcholine was 0.70 +/- 0.09, supporting the possibility that the particle has a neutral lipid core. Pulse-chase experiments indicated that microsomal apoB-48 HDL can either be secreted as apoB-48 HDL or converted to apoB-48 VLDL. These results support the two-step model of VLDL assembly. To determine the size of apoB required to assemble HDL and VLDL, we produced apoB polypeptides of various lengths and followed their ability to assemble VLDL. Small amounts of apoB-40 were associated with VLDL, but most of the nascent chains associated with VLDL ranged from apoB-48 to apoB-100. Thus, efficient VLDL assembly requires apoB chains of at least apoB-48 size. Nascent polypeptides as small as apoB-20 were associated with particles in the HDL density range. Thus, the structural requirements of apoB to form HDL-like first-step particles differ from those to form second-step VLDL. Analysis of proteins in the d < 1.006 g/ml fraction after ultracentrifugation of the luminal content of the cells identified five chaperone proteins: binding protein, protein disulfide isomerase, calcium-binding protein 2, calreticulin, and glucose regulatory protein 94. Thus, intracellular VLDL is associated with a network of chaperones involved in protein folding. Pulse-chase and subcellular fractionation studies showed that apoB-48 VLDL did not accumulate in the rough endoplasmic reticulum. This finding indicates either that the two steps of apoB lipoprotein assembly occur in different compartment or that the assembled VLDL is transferred rapidly out of the rough endoplasmic reticulum.

Cell and molecular biology of the assembly and secretion of apolipoprotein B-containing lipoproteins by the liver

Triglycerides are one of the most efficient storage forms of free energy. Because of their insolubility in biological fluids, their transport between cells and tissues requires that they be assembled into lipoprotein particles. Genetic disruption of the lipoprotein assembly/secretion pathway leads to several human disorders associated with malnutrition and developmental abnormalities. In contrast, patients displaying inappropriately high rates of lipoprotein production display increased risk for the development of atherosclerotic cardiovascular disease. Insights provided by diverse experimental approaches describe an elegant biological adaptation of basic chemical interactions required to overcome the thermodynamic dilemma of producing a stable emulsion vehicle for the transport and tissue targeting of triglycerides. The mammalian lipoprotein assembly/secretion pathway shows an absolute requirement for: (1) the unique amphipathic protein: apolipoprotein B, in a form that is sufficiently large to assemble a lipoprotein particle containing a neutral lipid core; and, (2) a lipid transfer protein (microsomal triglyceride transfer protein-MTP). In the endoplasmic reticulum apolipoprotein B has two distinct metabolic fates: (1) entrance into the lipoprotein assembly pathway within the lumen of the endoplasmic reticulum; or, (2) degradation in the cytoplasm by the ubiquitin-dependent proteasome. The destiny of apolipoprotein B is determined by the relative availability of individual lipids and level of expression of MTP. The dynamically varied expression of cholesterol-7alpha-hydroxylase indirectly influences the rate of lipid biosynthesis and the assembly and secretion lipoprotein particles by the liver.

The degradation of nascent fibrinogen chains is mediated by the ubiquitin proteasome pathway

Recent studies have shown that ubiquitin-dependent proteolysis by proteasomes plays an essential role in the degradation of ER-retained proteins. We investigated the degradation of individual fibrinogen chains in transfected COS cells which express but do not secrete single chains. In transfected COS cells, the degradation of fibrinogen Bbeta and gamma chain was markedly inhibited by the proteasome inhibitors lactacystin and MG132. These specific proteasome inhibitors also partially affected the degradation of Aalpha chain. In HepG2 cells, which synthesize and secrete fibrinogen, the degradation of intracellular free gamma chain was also inhibited by MG132. We also detected high molecular weight polyubiquitinated forms of fibrinogen chains in transfected COS cells and in HepG2 cells by sequential immunoprecipitation. These results implicate proteasomes in the degradation of fibrinogen chains. In COS cells, gamma chains have a longer half-life than Bbeta chains and Aalpha chains, suggesting that the presence of surplus gamma chains in fibrinogen-producing cells is due to the unequal degradation rate of fibrinogen chains. These results indicate that the ubiquitin-proteasome pathway may be a major system for the degradation of unassembled fibrinogen chains.

The assembly and secretion of apolipoprotein B-containing lipoproteins

The assembly of lipoproteins containing apolipoprotein B is a complex process that occurs in the lumen of the secretory pathway. The process consists of two relatively well-identified steps. In the first step, two VLDL precursors are formed simultaneously and independently: an apolipoprotein B-containing VLDL precursor (a partially lipidated apolipoprotein B) and a VLDL-sized lipid droplet that lacks apolipoprotein B. In the second step, these two precursors fuse to form a mature VLDL particle. The apolipoprotein B-containing VLDL precursor is formed during the translation and concomitant translocation of the protein to the lumen of the endoplasmic reticulum. The VLDL precursor is completed shortly after the protein is fully synthesized. The process is dependent on the microsomal triglyceride transfer protein (MTP). Although the mechanism by which the lipid droplets are formed is unknown, recent observations indicate that the process is dependent on MTP. The fusion of the two precursors is not dependent on MTP, but the mechanism remains to be elucidated. The conversion of the apolipoprotein B-containing precursor to VLDL seems to be dependent on the ADP ribosylation factor 1 (ARF 1) and its activation of phospholipase D. During their assembly, nascent apolipoprotein B chains undergo quality control and are sorted to degradation. Such sorting, which occurs cotranslationally during the formation of the apolipoprotein B-containing precursor, involves cytosolic chaperons and ubiquitination that targets apolipoprotein B to proteasomal degradation. Other levels of sorting occur in the secretory pathway. Thus, lysosomal enzymes are involved as well as the LDL receptor.

Ubiquitin-proteasome-dependent degradation of apolipoprotein B100 in vitro

Apolipoprotein B100 (apoB) is a large secretory protein that forms very low density lipoprotein in liver. An in vitro degradation assay was developed using rabbit reticulocyte (RR) lysate in order to investigate the mechanism of intracellular degradation of newly synthesized apoB by the ubiquitin-proteasome pathway. [3H]apoB, isolated from [3H]leucine pulsed/chased Hep G2 cells, was degraded 51% when incubated for 2 h at 37 degreesC in an assay mixture that included RR lysate (source of the ubiquitin conjugation system and proteasome) and an exogenous ATP regenerating system. ApoB degradation was ATP-dependent and degradation fragments were not observed suggesting that the very large apoB molecule was extensively degraded. ApoB degradation was decreased to 50% when potent proteasome inhibitors, clasto-lactacystin beta-lactone (10 microM) or MG-132 (50 microM), were added to the reaction mixture, but was not affected by the cysteine protease inhibitor, E-64, or the serine protease inhibitor, phenylmethylsulfonyl fluoride. ApoB degradation was inhibited by the mutant ubiquitin protein K48R and by ubiquitin aldehyde, an inhibitor of ubiquitin-protein isopeptidases. During incubation ubiquitination of apoB increased even as apoB was being degraded. These results suggest that in vitro degradation of apoB, a large secretory protein that is normally found in the endoplasmic reticulum (ER) lumen or associated with the ER membrane, was proteasome-dependent and involved both ubiquitination and deubiquitination steps.

Translocation efficiency, susceptibility to proteasomal degradation, and lipid responsiveness of apolipoprotein B are determined by the presence of beta sheet domains

Apolipoprotein (apo) B100 is an atypical secretory protein in that its translocation across the endoplasmic reticulum membrane is inefficient, resulting in the partial translocation and exposure of apoB100 on the cytoplasmic surface of the endoplasmic reticulum. Cytosolic exposure leads to the association of nascent apoB with heat shock protein 70 and to its predisposition to ubiquitination and proteasomal degradation. The basis for the inefficient translocation of apoB100 remains unclear and controversial. To test the hypothesis that beta sheet domains present in apoB100 contribute to its inefficient translocation, we created human apoB chimeric constructs apoB13,16 and apoB13,13,16, which contain amino-terminal alpha globular domains but no beta sheet domains, and apoB13,16,beta, which has an amphipathic beta sheet domain of apoB100 inserted into apoB13,16. These constructs, along with carboxyl-terminal truncations of apoB100, apoB34 and apoB42, were used to transfect HepG2 and Chinese hamster ovary cells. In contrast to the lack of effect of proteinase K on apoB13,16 and apoB13,13,16, the levels of apoB34, apoB42, and apoB13,16,beta were decreased by 70-85% after proteinase K-induced proteolysis in both HepG2 and Chinese hamster ovary cells. Either oleic acid or proteasomal inhibitors (N-acetyl-leucinyl-leucinyl-norleucinal and lactacystin) significantly increased the cell levels of apoB13,16,beta, apoB34, apoB42, and full-length apoB100 but had no effect on the cell levels of apoB13,16 and apoB13,13,16. When HepG2 cells were incubated with a microsomal triglyceride transfer protein inhibitor, the cellular levels of apoB13,16,beta, apoB34, and apoB42 were decreased by 70-80%, whereas the levels of apoB13,16 and apoB13,13,16 were unaffected. The effects of microsomal triglyceride transfer protein inhibition were reversed by lactacystin. Our results clearly demonstrate that the translocation efficiency, susceptibility to proteasomal degradation, and lipid responsiveness of apoB were determined by the presence of a lipid binding beta sheet domain. It is possible that beta sheet domains may at least transiently facilitate the interaction of apoB with the lipid bilayer surrounding the translocation channel.

Intracellular translocation and stability of apolipoprotein B are inversely proportional to the length of the nascent polypeptide

We have studied the relationship between the length of apolipoprotein B (apoB) and its intracellular translocation and stability using McArdle RH7777 (McA-RH7777) cells expressing recombinant human apoB variants, ranging in size from B15 to B100. The translocational status of apoB was assessed based on trypsin sensitivity of apoB using isolated microsomes as well as permeabilized cells. In isolated microsomes, shorter apoB variants (</=B48) were 75-100% resistant to exogenous trypsin digestion, whereas apoB variants larger than B48 were less than 40% trypsin-resistant. Experiments with hepatic microsomes isolated from rat or transgenic mice expressing human B48 and B100 also confirmed the high trypsin accessibility of B100 compared with B48. In permeabilized cells, apoB variants shorter than B48 were relatively resistant to exogenous trypsin (percentage of trypsin-resistant apoB greater than 70%) in contrast to recombinant human B72 and B100, which were only 55 and 42% trypsin-resistant, respectively. The trypsin sensitivity of human B100 was comparable with that of endogenous rat B100 in McA-RH7777 cells as well as endogenous B100 in HepG2 cells (percentages of trypsin-resistant cells were as follows: for human B100 construct, 42 +/- 7.5%; for endogenous McA-RH7777 B100, 52 +/- 2.9%; and for endogenous HepG2 B100, 46 +/- 6.3%). Overall, an inverse correlation between the length of apoB and its resistance to exogenous trypsin was evident irrespective of the model system examined. An inverse relationship was also observed between the size of apoB and its co-translational resistance to proteasomal degradation. Truncated apoB constructs were relatively insensitive to proteasome inhibition by MG132 co-translationally (during the pulse) compared with the full-length B100, which was highly sensitive (apoB recovered in the presence of MG132 as a percentage of control was as follows: B15, 127%; B29, 94%; B48, 110%; B72, 140%; B100, 282%). Post-translationally (over a 2-h chase), a similar inverse relationship was found, with B100 being the least stable in comparison with truncated apoB variants. In summary, as the size of the nascent apoB chain increases, there appears to be a greater cytosolic exposure of the polypeptide, leading to a higher sensitivity to proteasomal degradation.

Apoprotein B100 has a prolonged interaction with the translocon during which its lipidation and translocation change from dependence on the microsomal triglyceride transfer protein to independence

When lipid synthesis is limited in HepG2 cells, apoprotein B100 (apoB100) is not secreted but rapidly degraded by the ubiquitin-proteasome pathway. To investigate apoB100 biosynthesis and secretion further, the physical and functional states of apoB100 destined for either degradation or lipoprotein assembly were studied under conditions in which lipid synthesis, proteasomal activity, and microsomal triglyceride transfer protein (MTP) lipid-transfer activity were varied. Cells were pretreated with a proteasomal inhibitor (which remained with the cells throughout the experiment) and radiolabeled for 15 min. During the chase period, labeled apoB100 remained associated with the microsomes. Furthermore, by crosslinking sec61beta to apoB100, we showed that apoB100 remained close to the translocon at the same time apoB100-ubiquitin conjugates could be detected. When lipid synthesis and lipoprotein assembly/secretion were stimulated by adding oleic acid (OA) to the chase medium, apoB100 was deubiquitinated, and its interaction with sec61beta was disrupted, signifying completion of translocation concomitant with the formation of lipoprotein particles. MTP participates in apoB100 translocation and lipoprotein assembly. In the presence of OA, when MTP lipid-transfer activity was inhibited at the end of pulse labeling, apoB100 secretion was abolished. In contrast, when the labeled apoB100 was allowed to accumulate in the cell for 60 min before adding OA and the inhibitor, apoB100 lipidation and secretion were no longer impaired. Overall, the data imply that during most of its association with the endoplasmic reticulum, apoB100 is close to or within the translocon and is accessible to both the ubiquitin-proteasome and lipoprotein-assembly pathways. Furthermore, MTP lipid-transfer activity seems to be necessary only for early translocation and lipidation events.

Regulated Co-translational ubiquitination of apolipoprotein B100. A new paradigm for proteasomal degradation of a secretory protein

Presentation of a wild-type secretory protein, apolipoprotein B100 (apoB), to the cytosol for ubiquitin-proteasome proteolysis has been observed in HepG2 cells. A currently accepted model for proteasomal degradation of secretory proteins is retrograde translocation of the substrate polypeptides from the lumen of endoplasmic reticulum (ER) back to the cytosol. In this report, we present evidence that newly synthesized apoB becomes exposed to the cytosol and targeted to the proteasomes in a co-translational manner. Thus, after protein translation was synchronized with puromycin, partially synthesized apoB polypeptides were found to be conjugated to ubiquitin. The magnitude of co-translational ubiquitination and subsequent degradation of apoB was increased when cells were pretreated with either herbimycin A to induce cytosolic Hsp70 or with an inhibitor of microsomal triglyceride transfer protein; both treatments impede translocation of nascent apoB across the ER membrane. These treatments also decreased secretion of apoB and increased its degradation via the ubiquitin-proteasome pathway. We suggest that translocation arrest with subsequent co-translational exposure to the cytosol provides an alternative model to explain how mammalian secretory proteins can overcome topological segregation by the ER membrane and undergo degradation by the ubiquitin-proteasome pathway.

Multiple molecular chaperones interact with apolipoprotein B during its maturation. The network of endoplasmic reticulum-resident chaperones (ERp72, GRP94, calreticulin, and BiP) interacts with apolipoprotein b regardless of its lipidation state

The present study was undertaken to identify and characterize molecular chaperones that assist in the folding of apolipoprotein (apo) B, a secretory protein that requires assembly with lipids (lipidation) for its secretion. Both HepG2 cells, normally secreting full-length apoB (apoB-100), and C127 cells transfected to secrete truncated forms of apoB, apoB-41, apoB-29, and apoB-17, respectively, were employed. C127 cells were used to determine whether chaperone binding is dependent on apoB lipidation as they secrete both unlipidated and lipidated apoB forms despite their lack of microsomal triglyceride transfer protein (MTP), which mediates lipidation of apoB in HepG2 cells. The endoplasmic reticulum (ER)-resident molecular chaperones GRP94, calreticulin, and ERp72 were co-immunoprecipitated with apoB-100 from HepG2 cell lysates following cross-linking of proteins in living cells. The same chaperones including BiP/GRP78 were also associated with all truncated forms of apoB. Sequential immunoprecipitation with antibodies to MTP and apoB revealed the presence of ternary complexes containing apoB-100, MTP, and ERp72. However, MTP is not obligatory for the binding of ERp72 as it was associated with all truncated forms of apoB in C127 cells that lack MTP. The interactions between apoB-100 and ERp72 or GRP94 persisted for at least 2 h following a 30-min pulse. Thus, BiP/GRP78, calreticulin, ERp72, and GRP94 may participate in critical steps in the folding of apoB before any substantial lipidation occurs. ERp72 and GRP94 may also mediate the folding of more advanced folding intermediates and/or target the misfolded underlipidated pool of apoB for degradation.

S-phase DNA damage checkpoint in budding yeast

Eukaryotic cells must be able to coordinate DNA repair, replication and cell cycle progression in response to DNA damage. A failure to activate the checkpoints which delay the cell cycle in response to internal and external cues and to repair the DNA lesions results in an increase in genetic instability and cancer predisposition. The use of the yeast Saccharomyces cerevisiae has been invaluable in isolating many of the genes required for the DNA damage response, although the molecular mechanisms which couple this regulatory pathway to different DNA transactions are still largely unknown. In analogy with prokaryotes, we propose that DNA strand breaks, caused by genotoxic agents or by replication-related lesions, trigger a replication coupled repair mechanism, dependent upon recombination, which is induced by the checkpoint acting during S-phase.

Apolipoprotein(a) synthesis and secretion from hepatoma cells is coupled to triglyceride synthesis and secretion

Apolipoprotein(a) (apo(a)) is synthesized and secreted from liver cells and represents one of the two major protein components of the atherogenic lipoprotein, Lp(a). Little is known, however, of the factors that regulate the secretion of this protein. We have undertaken an analysis of the response to oleate supplementation in stable clones of HepG2 and McA-RH7777 cells expressing either a 6 K-IV or 17 K-IV isoform of apo(a). These cell lines were examined by pulse-chase analysis and each demonstrated an increase (range 2-6-fold) in apo(a) secretion following supplementation with 0.8 mM oleate. Microsomal membranes, prepared from HepG2 cells expressing a 6 K-IV apo(a) isoform, demonstrated that oleate supplementation increased the apparent protection of apo(a) from protease digestion, suggesting that alterations in the translocation efficiency of apo(a) may accompany the addition of oleate. Cells incubated with brefeldin A demonstrated increased recovery of the precursor form of apo(a) with oleate supplementation, suggesting that alterations in post-translational degradation may also contribute to the observed increase in apo(a) secretion following oleate addition. To further characterize the oleate-dependent increase in apo(a) secretion, cells were incubated with an inhibitor of the microsomal triglyceride transfer protein. These experiments demonstrated a dose-dependent decrease in apo(a) secretion from both cell lines. Furthermore, addition of either the microsomal triglyceride transfer protein inhibitor or triacsin C, an inhibitor of acyl-CoA synthase, completely abrogated the oleate-dependent increase in apo(a) secretion. Taken together, these data provide evidence that apo(a) secretion from hepatoma cells may be linked to elements of cellular triglyceride assembly and secretion.

Calnexin and other factors that alter translocation affect the rapid binding of ubiquitin to apoB in the Sec61 complex

Several secretory proteins, including apolipoprotein B, have been shown to undergo degradation by proteasomes. We found that the rapid degradation of nascent apolipoprotein B in HepG2 cells was diminished but not abolished by the addition of any of three different inhibitors of proteasomes. Ubiquitin is conjugated to apolipoprotein B that is not assembled with sufficient lipids either during or soon after synthesis. In addition, we found that apolipoprotein B that has entered the endoplasmic reticulum sufficiently to become glycosylated can be degraded by proteasomes. Furthermore, we detected ubiquitin-apolipoprotein B that is associated with the Sec61 complex, the major constituent of the translocational channel. Treatment of cells with monomethylethanolamine or dithiothreitol decreased the translocation of apolipoprotein B and increased the proportion of ubiquitin-conjugated molecules associated with Sec61. Conversely, treatment of cells with oleic acid, which increased the proportion of translocated apolipoprotein B, decreased the amount of ubiquitin-apolipoprotein B in the Sec61 complex. Finally, we found that inhibition of the interaction between calnexin and apolipoprotein B decreases the translocation of apolipoprotein B, increases the ubiquitin-apolipoprotein B in the Sec61 complex, and increases the proteasomal degradation of glycosylated apolipoprotein B. Thus, ubiquitin can be attached to unassembled apolipoprotein B in the Sec61 complex, and this process is affected by factors including calnexin that alter the translocation of apolipoprotein B.

Identification of two regions in apolipoprotein B100 that are exposed on the cytosolic side of the endoplasmic reticulum membrane

Protease protection assays of apolipoprotein B100 (apoB) in digitonin-permeabilized HepG2 cells indicated that multiple domains of apoB are exposed to the cytosol through an extensive portion of the secretory pathway. The intracellular orientation of apoB in the secretory pathway was confirmed by immunocytochemistry using antibodies recognizing specific domains of apoB in streptolysin-O (STP-O)- and saponin-permeabilized HepG2 cells. Lumenal epitopes on marker proteins in secretory pathway compartments (p63, p53, and galactosyltransferase) were not stained by antibodies in STP-O-treated cells, but were brightly stained in saponin-treated cells, confirming that internal membranes were not perforated in STP-O-treated cells. An anti-apoB peptide antibody (B4) recognizing amino acids 3221-3240 caused intense staining in close proximity to the nuclear membrane, and less intensely throughout the secretory pathway in STP-O-permeabilized cells. Staining with this antibody was similar in STP-O- and saponin-treated cells, indicating that this epitope in apoB is exposed to the cytosol at the site of apoB synthesis and throughout most of the remaining secretory pathway. Similar results indicating a cytosolic orientation were obtained with monoclonal antibody CC3.4, which recognizes amino acids 690-797 (79-91 kD) in apoB. Two polyclonal antibodies made to human LDL and two monoclonal antibodies recognizing amino acids 1878-2148 (D7.2) and 3214-3506 (B1B6) in apoB did not produce a strong reticular signal for apoB in STP-O-treated cells. The anti-LDL and B1B6 antibodies produced almost identical punctate patterns in STP-O-treated cells that overlapped with LAMP-1, a membrane marker for lysosomes. These observations suggest that the B1B6 epitope of apoB is exposed on the surface of the lysosome. The results identify two specific regions in apoB that are exposed to the cytosol in the secretory pathway.

Co-translational degradation of apolipoprotein B100 by the proteasome is prevented by microsomal triglyceride transfer protein. Synchronized translation studies on HepG2 cells treated with an inhibitor of microsomal triglyceride transfer protein

We studied the effect of inhibition of microsomal triglyceride transfer protein (MTP) on apolipoprotein (apo) B100 translation and secretion using HepG2 cells. The MTP-mediated lipid transfer activity was reduced using a specific MTP inhibitor. ApoB100 translation was synchronized by treatment with puromycin prior to L-[35S]methionine pulse-chase labeling. During the first 4 min of chase, synthesis of apoB polypeptides the size of 100-200 kDa was insensitive to the inhibitor, suggesting that inhibition of MTP did not affect the initiation of apoB100 translation. After 15 min of chase, the 100-200-kDa species were chased into polypeptides larger than 320 kDa (i.e. apoB65 or 65% of full-length apoB100) in both control and inhibitor-treated cells. However, the amount of these polypeptides decreased (by 36% for apoB65-75, by 64% for apoB75-85, by 76% for apoB85-95, and by 77% for apoB100) upon MTP inhibition. No accumulation of smaller polypeptides was observed, but total immunoprecipitable apoB radioactivity was decreased suggesting that apoB could undergo co-translational degradation when MTP activity was reduced. Inhibitors of the multicatalytic proteinase complex (proteasome) such as lactacystin or MG-115 could prevent apoB co-translational degradation. Nevertheless, MG-115 could not avoid the MTP inhibitor decreasing apoB100 secretion but rather induced the accumulation of secretion-incompetent apoB100 in the cell. These results indicate that MTP activity is required during the elongation of apoB100 polypeptides, particularly at the sequences downstream of carboxyl terminus of apoB65. Co-translational degradation might constitute a more general mechanism of early quality control for large or complex proteins.

The degradation of apolipoprotein B100 is mediated by the ubiquitin-proteasome pathway and involves heat shock protein 70

Apolipoprotein B (apoB) is the major protein component of atherogenic lipoproteins of hepatic origin. In HepG2 cells, the standard cell culture model of human hepatic lipoprotein metabolism, there is a limited availability of core lipids in the endoplasmic reticulum for association with nascent apoB. Under these conditions, apoB is partially translocated, interacts with cytosolic Hsp70, and undergoes rapid degradation. We show that increasing the expression of Hsp70 in HepG2 cells promotes apoB degradation. In addition, apoB is polyubiquitinated and its degradation both normally and after Hsp70 induction is blocked by inhibitors of the proteasome. The apoB that accumulates after proteasome inhibition is endoplasmic reticulum-associated and can be assembled into lipoproteins and secreted if new lipid synthesis is stimulated. Thus, apoB is the first example of a wild-type mammalian protein whose secretion is regulated by degradation in the cytosol via the ubiquitin-proteasome pathway. Furthermore, targeting of this secretory protein to the proteasome is regulated by the molecular chaperone Hsp70 and the availability of apoB's lipid-ligands.

Role of lipid synthesis, chaperone proteins and proteasomes in the assembly and secretion of apoprotein B-containing lipoproteins from cultured liver cells

1. Apolipoprotein B (apoB) is necessary for the assembly and secretion of both chylomicrons from the small intestine and very low-density lipoproteins (VLDL) from the liver. ApoB is also the major protein in low-density lipoproteins (LDL) and is the ligand for the LDL receptor. Studies in humans suggest that increased production of apoB-containing lipoproteins, particularly VLDL, is a common abnormality in dyslipidaemias. 2. Studies in primary and long-term cultures of hepatocytes and hepatoma cells indicate that a significant proportion of newly synthesized apoB is rapidly degraded and that this is the major mechanism for regulation of apoB secretion. The availability of newly synthesized lipids, particularly triglyceride and cholesteryl ester, appears to be a critical factor in targeting apoB for secretion rather than degradation. 3. ApoB is an atypical secretory protein in that cotranslational translocation across the endoplasmic reticulum membrane, a feature of all secretory proteins, seems to slow or stop in the absence of adequate lipid availability (or in the absence of microsomal triglyceride transfer protein), allowing for rapid degradation of apoB. 4. The degradation of apoB seems to be facilitated by the association of nascent apoB with the major cytosolic chaperone protein, heat shock protein 70. Additionally, degradation of nascent apoB appears to occur, to a large degree, via the proteasomal pathway for degradation of cytosolic proteins.