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

Insights into mechanisms responsible for mesangial alterations associated with fibrogenic glomerulopathic light chains

Our previous studies have shown that human mesangial cells (HMCs) incubated with fibrogenic glomerulopathic monoclonal light chains (G-LCs) obtained from the urine of patients with light chain deposition disease produce increased extracellular matrix (ECM) when compared with HMCs not exposed to fibrogenic LCs. This overproduction of ECM proteins is regulated by transforming growth factor-beta (TGF-beta); blocking TGF-beta normalizes the production of ECM proteins. All ECM proteins, after synthesis, have to go through the secretory pathway in the endoplasmic reticulum (ER) and Golgi complex for final maturation and secretion. Blocking the secretory pathway may reduce the accumulation of ECM proteins. We tested the effect of tunicamycin, a specific inhibitor of N-linked glycosylation in the ER which inhibited glycosylation and brefeldin A, an inhibitor of vesicle transport between the endoplasmic reticulum and the Golgi complex, on ECM protein production, both resulting in subsequent upregulation of glucose-regulated protein 78. Overproduction of fibronectin and tenascin by HMCs was normalized by tunicamycin and brefeldin A. Similarly, when HMCs were exposed to exogenous TGF-beta, the increase in fibronectin was reversed by tunicamycin and brefeldin A. Exogenous platelet-derived growth factor-beta (PDGF-beta) did not induce fibronectin overproduction but significantly stimulated proliferation of HMCs. In summary, this study further supports the notion that fibrogenic G-LCs promote the accumulation of ECM proteins, through the actions of TGF-beta. Importantly, the data indicate that altering protein trafficking in the ER results in impairment of secretion of proteins into the ECM. Furthermore, the data also reveal that PDGF-beta and TGF-beta act independently and that PDGF-beta activation by itself cannot increase ECM proteins directly, but only by increasing the number of HMCs.

Ubiquitin-dependent and -independent proteasomal degradation of apoB associated with endoplasmic reticulum and Golgi apparatus, respectively, in HepG2 cells

Studies in hepatocyte cultures indicate that apolipoprotein (apo) B-100 production is regulated largely by intracellular degradation and the proteasome pathway is a major mechanism for the degradation. In the present study, we have examined the detailed itinerary of apoB degradation through its secretory pathway in HepG2 cells. We found that ubiquitin-dependent proteasomal degradation of apoB largely occurred on the cytosolic surface of rough and smooth endoplasmic reticulum (ER) and that a small proportion of apoB was dislodged from the secretory organelles into the cytosolic compartment where it underwent ubiquitination for proteasomal degradation. The transmembrane conformation of apoB persisted as the protein was transported through the Golgi apparatus. We further demonstrated that proteasomal degradation of apoB was associated the Golgi apparatus but Golgi-associated apoB was not ubiquitinated, indicating an ubiquitin-independent proteasomal degradation of apoB is associated with this organelle. We conclude that apoB undergoes proteasomal degradation while going through different compartments of the secretory pathway; further, ER-associated proteasomal degradation of apoB in the ER is ubiquitin-dependent whereas that occurring in the Golgi is ubiquitin-independent.

Relaxin increases ubiquitin-dependent degradation of fibronectin in vitro and ameliorates renal fibrosis in vivo

Fibronectin, a large adhesive glycoprotein, is a prominent constituent of the extracellular matrix. Abnormalities in fibronectin homeostasis occur in numerous disease states, ranging from primary fibrosing conditions to neoplastic transformation. We demonstrate that fibronectin is a target protein substrate for ubiquitin-dependent degradation. Coimmunoprecipitation experiments and confocal microscopy demonstrated ubiquitin-fibronectin interaction. In an in vitro model of renal fibrosis, relaxin, an insulin-like growth factor, increased ubiquitin-dependent fibronectin degradation. Relaxin also was evaluated in an anti-glomerular basement membrane model of renal fibrosis. Animals treated with relaxin experienced renoprotection, manifested by decreased serum creatinine and proteinuria. Histological evaluation of kidney sections from animals treated with relaxin showed decreased glomerulosclerosis and interstitial fibrosis. We conclude that relaxin might be developed as a useful agent for the treatment of renal fibrosis.

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.

Transmembrane lipid transfer is crucial for providing neutral lipids during very low density lipoprotein assembly in endoplasmic reticulum

Very low density lipoprotein (VLDL), a large particle containing apolipoprotein B (apoB) and large amounts of neutral lipids, is formed in the luminal space within the endoplasmic reticulum (ER) of hepatic cells. The assembly mechanism of VLDL particles is a tightly regulated process where apoB, associated with an insufficient amount of lipids, is selectively degraded intracellularly. In this study we found that treatment of HuH-7 human hepatoma cells with verapamil inhibited secretion of apoB-containing lipoprotein particles through increasing degradation of apoB. Addition of N-acetylleucyl-leucyl-norleucinal, an inhibitor of proteasome and other cysteinyl proteases that are responsible for apoB degradation, restored apoB recovery from verapamil-treated cells. De novo synthesis of lipids from [14C]acetate was increased in the presence of verapamil, suggesting that verapamil decreases lipid availability for apoB thus leading to the secretion of apoB-containing lipoprotein. We prepared cytosolic fractions from cells preincubated with [14C]acetate and used as a donor of radioactive lipids. When this cytosolic fraction was incubated with microsomes isolated separately, radioactive triglyceride (TG) accumulated in the luminal space of the microsomes. The transfer of radioactive TG from the cytosolic fraction to the microsomal lumen was inhibited in the presence of verapamil, suggesting that there is a verapamil-sensitive mechanism for TG transfer across ER membranes that is involved in formation of apoB-containing lipoprotein particles in ER. Verapamil showed no inhibitory effect on microsomal TG transfer protein, a well known lipid transfer protein in ER. We propose from these results that there is novel machinery for transmembrane movement of neutral lipids, which is involved in providing TG for apoB during VLDL assembly in ER.

Proteolysis and sterol regulation

The mammalian cell continuously adjusts its sterol content by regulating levels of key sterol synthetic enzymes and levels of LDL receptors that mediate uptake of cholesterol-laden particles. Control is brought about by sterol-regulated transcription of relevant genes and by regulated degradation of the committed step enzyme HMG-CoA reductase (HMGR). Current work has revealed that proteolysis is at the heart of each of these mechanistically distinct axes. Transcriptional control is effected by regulated cleavage of the membrane-bound transcription factor sterol regulatory element binding protein (SREBP), and HMGR degradation is brought about by ubiquitin-mediated degradation. In each case, ongoing cell biological processes are being harnessed to bring about regulation. The secretory pathway plays a central role in allowing sterol-mediated control of transcription. The constitutively active endoplasmic reticulum (ER) quality control apparatus is employed to bring about regulated destruction of HMGR. This review describes the methods and results of various studies to understand the mechanisms and molecules involved in these distinct but interrelated aspects of sterol regulation and the intriguing similarities that appear to exist at the levels of protein sequence and cell biology.

Blocking microsomal triglyceride transfer protein interferes with apoB secretion without causing retention or stress in the ER

Microsomal triglyceride transfer protein (MTP) is an intraluminal protein in the endoplasmic reticulum (ER) that is essential for the assembly of apolipoprotein B (apoB)-containing lipoproteins. In this study, we examine how the livers of mice respond to two distinct methods of blocking MTP function: Cre-mediated disruption of the gene for MTP and chemical inhibition of MTP activity. Blocking MTP significantly reduced plasma levels of triglycerides, cholesterol, and apoB-containing lipoproteins in both wild-type C57BL/6 and LDL receptor-deficient mice. While treating LDL receptor-deficient mice with an MTP inhibitor for 7 days lowered plasma lipids to control levels, liver triglyceride levels were increased by only 4-fold. Plasma levels of apoB-100 and apoB-48 fell by >90% and 65%, respectively, but neither apoB isoform accumulated in hepatic microsomes. Surprisingly, loss of MTP expression was associated with a nearly complete absence of apoB-100 in hepatic microsomes. Levels of microsomal luminal chaperone proteins [e.g., protein disulfide isomerase, glucose-regulated protein 78 (GRP78), and GRP94] and cytosolic heat shock proteins (HSPs) (e.g., HSP60, HSC, HSP70, and HSP90) were unaffected by MTP inhibition. These findings show that the liver responds rapidly to inhibition of MTP by degrading apoB and preventing its accumulation in the ER. The rapid degradation of secretion-incompetent apoB in the ER may block the induction of proteins associated with unfolded protein and heat shock responses.

Functional characterization of bacterial signal peptide OmpA in a baculovirus-mediated expression system

Signal sequences are evolutionarily conserved and are often functionally interchangeable between prokaryotes and eukaryotes. However, we have found that the bacterial signal peptide, OmpA, functions incompletely in insect cells. Upon baculovirus-mediated expression of chloramphenicol acetyltransferase (CAT) in insect cells, OmpA signal peptide led to the cytosolic accumulation of the CAT molecules in an aglycosylated, signal-peptide cleaved form, in addition to the secretion of the glycosylated CAT. When green fluorescent protein (GFP) was used as another reporter, the GFP molecules expressed from the OmpA-GFP construct was distributed primarily in the cytosol as aggresome-like structures. These results together suggest that, subsequent to the cleavage of OmpA signal peptide in the ER, some of the processed proteins are returned to the cytoplasm. Since the prototypical insect signal peptide, melittin, did not result in this ER-to-cytosol dislocation of the reporter proteins, we proposed a model explaining the dislocation process in insect cells, apparently selective to the OmpA-directed secretory pathway bypassing the co-translational transport.

Effects of HIV protease inhibitor therapy on lipid metabolism

Highly active antiretroviral therapy, which includes a combination of protease inhibitors, is highly successful in controlling human immunodeficiency virus (HIV) infection and reducing the morbidity and mortality of autoimmune deficiency syndrome (AIDS). However, the benefits of HIV protease inhibitors are compromised by numerous undesirable side effects. These include peripheral fat wasting and excessive central fat deposition (lipodystrophy), overt hyperlipidemia, and insulin resistance. The mechanism associated with protease inhibitor-induced metabolic abnormalities is multifactorial. One major effect of the protease inhibitor is its suppression of the breakdown of the nuclear form of sterol regulatory element binding proteins (nSREBP) in the liver and adipose tissues. Hepatic accumulation of nSREBP results in increased fatty acid and cholesterol biosynthesis, whereas nSREBP accumulation in adipose tissue causes lipodystrophy, reduces leptin expression, and promotes insulin resistance. The HIV protease inhibitors also suppress proteasome-mediated breakdown of nascent apolipoprotein (apo) B, thus resulting in the overproduction and secretion of triglyceride-rich lipoproteins. Finally, protease inhibitor also suppresses the inhibition of the glucose transporter GLUT-4 activity in adipose and muscle. This latter effect also contributes directly to insulin resistance and diabetes. These adverse effects need to be alleviated for long-term use of protease inhibitor therapy in treatment of HIV infection.

A tripartite motif protein TRIM11 binds and destabilizes Humanin, a neuroprotective peptide against Alzheimer's disease-relevant insults

Humanin (HN) is a newly identified neuroprotective peptide that specifically suppresses Alzheimer's disease (AD)-related neurotoxicity. HN peptide has been detected in the human AD brain as well as in mouse testis and colon by immunoblot and immunohistochemical analyses. By means of yeast two-hybrid screening, we identified TRIM11 as a novel HN-interacting protein. TRIM11, which is a member of protein family containing a tripartite motif (TRIM), is composed of a RING finger domain, which is a putative E3 ubiquitin ligase, a B-box domain, a coiled-coil domain and a B30.2 domain. Deletion of the B30.2 domain in TRIM11 abolished the interaction with HN, whereas the B30.2 domain alone did not interact with HN. For their interaction, at least the coiled-coil domain was indispensable together with the B30.2 domain. The intracellular level of glutathione S-transferase-fused or EGFP-fused HN peptides or plain HN was drastically reduced by the coexpression of TRIM11. Disruption of the RING finger domain by deleting the first consensus cysteine or proteasome inhibitor treatment significantly diminished the effect of TRIM11 on the intracellular level of HN. These results suggest that TRIM11 plays a role in the regulation of intracellular HN level through ubiquitin-mediated protein degradation pathways.

Nascent lipidated apolipoprotein B is transported to the Golgi as an incompletely folded intermediate as probed by its association with network of endoplasmic reticulum molecular chaperones, GRP94, ERp72, BiP, calreticulin, and cyclophilin B

We have previously demonstrated that endoplasmic reticulum (ER)-resident molecular chaperones interact with apolipoprotein B-100 (apoB) during its maturation. The initial stages of apoB folding occur while it is bound to the ER membrane, where it becomes partially lipidated to form a primordial intermediate. We determined whether this intermediate is dependent on the assistance of molecular chaperones for its subsequent folding steps. To that end, microsomes were prepared from HepG2 cells and luminal contents were subjected to KBr density gradient centrifugation. Immunoprecipitation of apoB followed by Western blotting showed that the luminal pool floated at a density of 1.12 g/ml and, like the membrane-bound pool, was associated with GRP94, ERp72, BiP, calreticulin, and cyclophilin B. Except for calreticulin, chaperone/apoB ratio in the lumen was severalfold higher than that in the membrane, suggesting a role for these chaperones both in facilitating the release of the primordial intermediate into the ER lumen and in providing stability. Subcellular fractionation on sucrose gradients showed that apoB in the Golgi was associated with the same array of chaperones as the pool of apoB recovered from heavy microsomes containing the ER, except that chaperone/apoB ratio was lower. KBr density gradient fractionation showed that the major pool of luminal apoB in the Golgi was recovered from 1.02 < d < 1.08 g/ml, whereas apoB in ER was recovered primarily from 1.08 < d < 1.2 g/ml. Both fractions were associated with the same spectrum of chaperones. Together with the finding that GRP94 was found associated with sialylated apoB, we conclude that correct folding of apoB is dependent on the assistance of molecular chaperone, which play multiple roles in its maturation throughout the secretory pathway including distal compartments such as the trans-Golgi network.

Microsomal triglyceride transfer protein and its role in apoB-lipoprotein assembly

Apolipoprotein B (apoB) and microsomal triglyceride transfer protein (MTP) are necessary for lipoprotein assembly. ApoB consists of five structural domains, betaalpha(1)-beta(1)-alpha(2)-beta(2)-alpha(3). We propose that MTP contains three structural motifs (N-terminal beta-barrel, central alpha-helix, and C-terminal lipid cavity) and three functional domains (lipid transfer, membrane associating, and apoB binding). MTP's lipid transfer activity is required for the assembly of lipoproteins. This activity renders nascent apoB secretion-competent and may be involved in the import of triglycerides into the lumen of endoplasmic reticulum. In addition, MTP binds to apoB with high affinity involving ionic interactions. MTP interacts at multiple sites in the N-terminal betaalpha(1) structural domain of apoB. A novel antagonist that inhibits apoB-MTP binding decreases apoB secretion. Furthermore, site-directed mutagenesis and deletion analyses that inhibit apoB-MTP binding decrease apoB secretion. Lipids modulate protein-protein interactions between apoB and MTP. Lipids associated with MTP increase apoB-MTP binding whereas lipids associated with apoB decrease this binding. Thus, specific antagonist, site-directed mutagenesis, deletion analyses, and modulation studies support the notion that apoB-MTP binding plays a role in lipoprotein biogenesis. However, specific steps in lipoprotein assembly that require apoB-MTP binding have not been identified. ApoB-MTP binding may be important for the prevention of degradation and lipidation of nascent apoB.

The HCMV gene products US2 and US11 target MHC class I molecules for degradation in the cytosol

Over millions of years of coevolution with their hosts, viruses have developed highly effective strategies to elude the host immune system. The degradation of major histocompatibility complex (MHC) class I heavy chains by human cytomegalovirus (HCMV) is an example of this. Two HCMV proteins, US2 and US11, target newly synthesized MHC class I heavy chains for destruction via a pathway that involves ubiquitin-dependent retrograde transport, or "dislocation", of the heavy chains from the ER to the cytosol, where the proteins are degraded by proteasomes. In this review, US2- and US11-mediated degradation of MHC class I heavy chains is discussed in relation to data concerning the degradation of other ER luminal proteins. A new, unified model for translocon-facilitated dislocation and degradation of MHC class I heavy chains is presented.

Nrdp1/FLRF is a ubiquitin ligase promoting ubiquitination and degradation of the epidermal growth factor receptor family member, ErbB3

The epidermal growth factor receptor (EGFR/ErbB) family of receptor tyrosine kinases plays fundamental roles in the regulation of cell survival, proliferation, and differentiation. Here, we present evidence that ErbB3 is degraded by proteasomes, and that Nrdp1 (referred to as FLRF in mice) associates with ErbB3 and stimulates its ubiquitination and degradation by proteasomes. Nrdp1 mRNAs are expressed in a variety of human tissues. The N-terminal half of Nrdp1 possesses an atypical RING finger domain, which is required for enhancing ErbB3 degradation. Its C-terminal half by itself associates with ErbB3 and raises ErbB3 levels in cells, probably by acting as a dominant-negative form of Nrdp1. In cell-free systems, Nrdp1 has ubiquitin ligase (E3) activity and ubiquitinates ErbB3, as well as itself, in the presence of the ubiquitin-carrier protein (E2), UbcH5. These data indicate that Nrdp1 is a RING finger-type of ubiquitin ligase, which promotes degradation of ErbB3 by proteasomes and, thus, may be an important factor influencing cell growth.

Stimulation of lipogenesis by pharmacological activation of the liver X receptor leads to production of large, triglyceride-rich very low density lipoprotein particles

The oxysterol-activated liver X receptor (LXR) provides a link between sterol and fatty acid metabolism; activation of LXR induces transcription of lipogenic genes. This study shows that induction of the lipogenic genes Srebp-1c, Fas, and Acc1 upon administration of the synthetic LXR agonist T0901317 to C57BL/6J mice (10 mg/kg/day, 4 days) is associated with massive hepatic steatosis along the entire liver lobule and a 2.5-fold increase in very low density lipoprotein-triglyceride (VLDL-TG) secretion. The increased VLDL-TG secretion was fully accounted for by formation of larger (129 +/- 9 nm versus 94 +/- 12 nm, a 2.5-fold increase of particle volume) TG-rich particles. Stimulation of VLDL-TG secretion did not lead to elevated plasma TG levels in C57BL/6J mice, indicating efficient particle metabolism and clearance. However, T0901317 treatment did lead to severe hypertriglyceridemia in mouse models of defective TG-rich lipoprotein clearance, i.e. APOE*3-Leiden transgenic mice (3.2-fold increase) and apoE-/- LDLr-/- double knockouts (12-fold increase). Incubation of rat hepatoma McA-RH7777 cells with T0901317 also resulted in intracellular TG accumulation and enhanced TG secretion. We conclude that, in addition to raising high density lipoprotein cholesterol concentrations, pharmacological LXR activation in mice leads to development of hepatic steatosis and secretion of atherogenic, large TG-rich VLDL particles.

Inhibition of hepatocyte apoB secretion by naringenin: enhanced rapid intracellular degradation independent of reduced microsomal cholesteryl esters

The grapefruit flavonoid, naringenin, is hypocholesterolemic in vivo, and inhibits basal apolipoprotein B (apoB) secretion and the expression and activities of both ACAT and microsomal triglyceride transfer protein (MTP) in human hepatoma cells (HepG2). In this report, we examined the effects of naringenin on apoB kinetics in oleate-stimulated HepG2 cells and determined the contribution of microsomal lumen cholesteryl ester (CE) availability to apoB secretion. Pulse-chase studies of apoB secretion and intracellular degradation were analyzed by multicompartmental modeling. The model for apoB metabolism in HepG2 cells includes an intracellular compartment from which apoB can be either secreted or degraded by both rapid and slow pathways. In the presence of 0.1 mM oleic acid, naringenin (200 micro M) reduced the secretion of newly synthesized apoB by 52%, due to a 56% reduction in the rate constant for secretion. Intracellular degradation was significantly increased due to a selective increase in rapid degradation, while slow degradation was unaffected. Incubation with either N-acetyl-leucinyl-leucinyl-norleucinal (ALLN) or lactacystin showed that degradation via the rapid pathway was largely proteasomal. Although these changes in apoB metabolism were accompanied by significant reductions in CE synthesis and mass, subcellular fractionation experiments comparing naringenin to specific ACAT and HMG-CoA reductase inhibitors revealed that reduced accumulation of newly synthesized CE in the microsomal lumen is not consistently associated with reduced apoB secretion. However, naringenin, unlike the ACAT and HMG-CoA reductase inhibitors, significantly reduced lumenal TG accumulation. We conclude that naringenin inhibits apoB secretion in oleate-stimulated HepG2 cells and selectively increases intracellular degradation via a largely proteasomal, rapid kinetic pathway. Although naringenin inhibits ACAT, CE availability in the endoplasmic reticulum (ER) lumen does not appear to regulate apoB secretion in HepG2 cells. Rather, inhibition of TG accumulation in the ER lumen via inhibition of MTP is the primary mechanism blocking apoB secretion.

Clastosome: a subtype of nuclear body enriched in 19S and 20S proteasomes, ubiquitin, and protein substrates of proteasome

Nuclear bodies represent a heterogeneous class of nuclear structures. Herein, we describe that a subset of nuclear bodies is highly enriched in components of the ubiquitin-proteasome pathway of proteolysis. We coined the term clastosome (from the Greek klastos, broken and soma, body) to refer to this type of nuclear body. Clastosomes contain a high concentration of 1) ubiquitin conjugates, 2) the proteolytically active 20S core and the 19S regulatory complexes of the 26S proteasome, and 3) protein substrates of the proteasome. Although detected in a variety of cell types, clastosomes are scarce under normal conditions; however, they become more abundant when proteasomal activity is stimulated. In contrast, clastosomes disappear when cells are treated with proteasome inhibitors. Protein substrates of the proteasome that are found concentrated in clastosomes include the short-lived transcription factors c-Fos and c-Jun, adenovirus E1A proteins, and the PML protein. We propose that clastosomes are sites where proteolysis of a variety of protein substrates is taking place.

An HRD/DER-independent ER quality control mechanism involves Rsp5p-dependent ubiquitination and ER-Golgi transport

We have identified a new pathway of ER-associated degradation in Saccharomyces cerevisiae that functions separately from the HRD/DER pathway comprised of Hrd1p, Hrd3p, Der1p, and Ubc7p. This pathway, termed Hrd1p independent-proteolysis (HIP), is capable of recognizing and degrading both lumenal (CPY* and PrA*), and integral membrane proteins (Sec61-2p) that misfold in the ER. CPY* overexpression likely saturates the HRD/DER pathway and activates the HIP pathway, so the slowed degradation kinetics of CPY* in a hrd1 Delta strain is restored to a wild-type rate when CPY* is overexpressed. Substrates of HIP require vesicular trafficking between the ER and Golgi apparatus before degradation by the ubiquitin-proteasome system. Ubiquitination of HIP substrates does not involve the HRD/DER pathway ubiquitin ligase Hrd1p, but instead uses another ubiquitin ligase, Rsp5p. HIP is regulated by the unfolded protein response as Ire1p is necessary for the degradation of CPY* when overexpressed, but not when CPY* is expressed at normal levels. Both the HIP and HRD/DER pathways contribute to the degradation of CPY*, and only by eliminating both is CPY* degradation completely blocked.

Influence of peroxisome proliferator-activated receptor alpha agonists on the intracellular turnover and secretion of apolipoprotein (Apo) B-100 and ApoB-48

The peroxisome proliferator-activated receptor (PPAR) alpha agonist WY 14,643 increased the secretion of apolipoprotein (apo) B-100, but not that of apoB-48, and decreased triglyceride biosynthesis and secretion from primary rat hepatocytes. These effects resulted in decreased secretion of apoB-100-very low density lipoprotein (VLDL) and an increased secretion of apoB-100 on low density lipoproteins/intermediate density lipoproteins. ApoB-48-VLDL was also replaced by more dense particles. The proteasomal inhibitor lactacystin did not influence the recovery of apoB-100 or apoB-48 in primary rat hepatocytes, indicating that co-translational (proteasomal) degradation is of less importance in these cells. Treatment with WY 14,643 made the recovery of apoB-100 sensitive to lactacystin, most likely reflecting the decreased biosynthesis of triglycerides. The PPAR alpha agonist induced a significant increase in the accumulation of pulse-labeled apoB-100 even after a short pulse (2-5 min). There was also an increase in apoB-100 nascent polypeptides, indicating that the co-translational degradation of apoB-100 was inhibited. However, a minor influence on an early posttranslation degradation cannot be excluded. This decreased co-translational degradation of apoB-100 explained the increased secretion of the protein. The levels of apoB-48 remained unchanged during these pulse-chase experiments, and albumin production was not affected, indicating a specific effect of PPAR alpha agonists on the co-translational degradation of apoB-100. These findings explain the difference in the rate of secretion of the two apoB proteins seen after PPAR alpha activation. PPAR alpha agonists increased the expression and biosynthesis of liver fatty acid-binding protein (LFABP). Increased expression of LFABP by transfection of McA-RH7777 cells increased the secretion of apoB-100, decreased triglyceride biosynthesis and secretion, and increased PPAR alpha mRNA levels. These findings suggest that PPAR alpha and LFABP could interact to amplify the effect of endogenous PPAR alpha agonists on the assembly of VLDL.

A proteomic approach identifies proteins in hepatocytes that bind nascent apolipoprotein B

The biogenesis of apolipoprotein B is quite complex in view of its huge size, hydrophobicity, obligate association with lipids such as cholesterol and triglycerides prior to secretion, and intracellular degradation of a substantial proportion of newly synthesized molecules. Multiple proteins likely serve roles as molecular chaperones to assist in folding, assembly with lipids, and regulation of the secretion of apolipoprotein B. In these studies, we developed a strategy to isolate proteins associated with apolipoprotein B in rat livers. The purification consisted of two stages: first, microsomes were prepared from rat liver and treated with chemical cross-linkers, and second, the solubilized proteins were co-immunoprecipitated with antibody against apolipoprotein B. We found that several proteins were cross-linked to apolipoprotein B. The proteins were digested with trypsin, and the released peptides were sequenced by tandem mass spectrometry. The sequences precisely matched 377 peptides in 99 unique proteins. We show that at least two of the identified proteins, ferritin heavy and light chains, can directly bind apolipoprotein B. These and possibly other proteins identified by this proteomic approach are novel candidates for proteins that affect apolipoprotein B during its biogenesis.

S-propyl cysteine reduces the secretion of apolipoprotein B100 and triacylglycerol by HepG2 cells

A number of sulfur-containing amino acids and peptides are found in allium plants such as onion and garlic that have physiologic functions. In HepG2 cells, S-propyl cysteine decreased the secretion of apolipoprotein B100. The compound reduced the secretion of newly synthesized triacylglycerol and cholesterols from radiolabeled acetate. We associated the decrease of apolipoprotein B100 secretion to the length of the acyl-chain of the sulfur-containing amino acids. The present study suggests that foods containing S-propyl cysteine including onions have beneficial effects.

Biosynthesis and secretion of parathyroid hormone are sensitive to proteasome inhibitors in dispersed bovine parathyroid cells

Preproparathyroid hormone (prepro-PTH) is one of the proteins abundantly synthesized by parathyroid chief cells; yet under normal growth conditions, little or no prepro-PTH can be detected in these cells. Although this may be attributed to effective cotranslational translocation and proteolytic processing, proteasome-mediated degradation of PTH precursors may be important in the regulation of the levels of these precursors and hence PTH secretion. The effects of N-acetyl-Leu-Leu-norleucinal, N-acetyl-Leu-Leu-methional, carbobenzoxy-Leu-Leu-leucinal (MG132), benzyloxycarbonyl-Ile-Glu(t-butyl)-Ala-leucinal (proteasome inhibitor I), and lactacystin on the biosynthesis and secretion of PTH were examined in dispersed bovine parathyroid cells. We demonstrate that treatment of these cells with proteasome inhibitors caused the accumulation of prepro-PTH and pro-PTH. Compared with mock-treated cells, the processing of pro-PTH to PTH was delayed, and the secretion of intact PTH decreased in proteasome inhibitor-treated cells. Relieving the inhibition of the proteasome by chasing MG132-treated cells in medium without the inhibitor led to the rapid disappearance of the accumulated prepro-PTH, and the rate of PTH secretion was restored to levels comparable to those in mock-treated cells. Furthermore, overexpression of the Hsp70 family of molecular chaperones was observed in proteasome inhibitor-treated cells, and we show that PTH/PTH precursors interact with these molecular chaperones. These data suggest the involvement of parathyroid cell proteasomes in the quality control of PTH biosynthesis.

Apolipoprotein B is synthesized in selected human non-hepatic cell lines but not processed into mature lipoprotein

We studied apolipoprotein B100 (apoB) metabolism in a series of non-hepatic cell lines (HT29 colon adenocarcinoma, HeLa cervical epithelioid carcinoma, and 1321N1J astrocytoma human cell lines) and in the human hepatoma cell line HepG2. ApoB mRNA was detected by reverse transcription polymerase chain reaction in each non-hepatic cell line. ApoB was detected in HepG2 cells by immunoprecipitation, Western blotting, and immunocytochemistry using a polyclonal anti-human low-density lipoprotein (LDL) antibody, an anti-human apoB peptide antibody, and several monoclonal anti-apoB antibodies. ApoB was identified in the three non-hepatic cell lines by each method using the anti-apoB peptide and monoclonal antibodies, but not with the anti-LDL antibody. Immunocytochemistry indicated that epitopes of apoB were evident throughout the endoplasmic reticulum, and gel mobility of newly labeled apoB and immunoblot with anti-ubiquitin showed that apoB was highly ubiquinated in non-hepatic cells. The observations that apoB is synthesized in non-hepatic cell lines but never recognized by the anti-LDL antibody suggests that apoB is not processed into a nascent lipoprotein in these cells. Immunocytochemical localization of apoB epitopes at many locations throughout non-hepatic cells raises the exciting possibility that apoB can be used for other purposes in these cells.

Apolipoprotein B metabolism: tracer kinetics, models, and metabolic studies

The study of apolipoprotein (apo) B metabolism is central to our understanding of lipoprotein metabolism. However, the assembly and secretion of apoB-containing lipoproteins is a complex process. Specialized techniques, developed and applied to in vitro and in vivo studies of apoB metabolism, have provided insights into the mechanisms involved in the regulation of this process. Moreover, these studies have important implications for understanding both the pathophysiology as well as the therapeutic options for the dyslipidemias. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the applications of kinetic analysis and multicompartmental modeling to the study of apoB metabolism. New developments and significant advances over the last decade are discussed.

Inhibition of the ubiquitin-proteasome pathway induces differential heat-shock protein response in cardiomyocytes and renders early cardiac protection

The effects of proteasome inhibition (PI) on heat-shock protein (HSP) expression in cardiomyocytes were investigated. Neonatal rat cardiomyocytes were incubated with MG132 (0.1-10 microM) for 1 h. Induction of various HSPs was determined by real-time PCR and Western blotting. PI induced a 2- to 3-fold increase in HSP27, HSP60, and HSP90, and a 18-fold increase in HSP70 mRNA expression, whereas HSP40 levels were unaffected. Western blotting revealed increased protein expression for HSP70 after PI. Similar results were obtained with MG262. HSP induction correlated with enhanced survival of neonatal cardiomyocytes after sublethal heat stress in XTT testing. In papillary muscles, pretreatment with MG132 (10 microM, 90 min) was associated with enhanced recovery of the contractile parameters after a 40-min hypoxia. In these proof-of-principle experiments, we show that PI induces differential heat-shock response in cardiomyocytes, accompanied by enhanced cell survival and functional recovery after various forms of stress.

Hepatic very low density lipoprotein-ApoB overproduction is associated with attenuated hepatic insulin signaling and overexpression of protein-tyrosine phosphatase 1B in a fructose-fed hamster model of insulin resistance

A fructose-fed hamster model of insulin resistance was previously documented to exhibit marked hepatic very low density lipoprotein (VLDL) overproduction. Here, we investigated whether VLDL overproduction was associated with down-regulation of hepatic insulin signaling and insulin resistance. Hepatocytes isolated from fructose-fed hamsters exhibited significantly reduced tyrosine phosphorylation of the insulin receptor and insulin receptor substrates 1 and 2. Phosphatidylinositol 3-kinase activity as well as insulin-stimulated Akt-Ser473 and Akt-Thr308 phosphorylation were also significantly reduced with fructose feeding. Interestingly, the protein mass and activity of protein-tyrosine phosphatase-1B (PTP-1B) were significantly higher in fructose-fed hamster hepatocytes. Chronic ex vivo exposure of control hamster hepatocytes to high insulin also appeared to attenuate insulin signaling and increase PTP-1B. Elevation in PTP-1B coincided with marked suppression of ER-60, a cysteine protease postulated to play a role in intracellular apoB degradation, and an increase in the synthesis and secretion of apoB. Sodium orthovanadate, a general phosphatase inhibitor, partially restored insulin receptor phosphorylation and significantly reduced apoB secretion. In summary, we hypothesize that fructose feeding induces hepatic insulin resistance at least in part via an increase in expression of PTP-1B. Induction of hepatic insulin resistance may then contribute to reduced apoB degradation and enhanced VLDL particle assembly and secretion.

HIV protease inhibitors protect apolipoprotein B from degradation by the proteasome: a potential mechanism for protease inhibitor-induced hyperlipidemia

Highly active anti-retroviral therapies, which incorporate HIV protease inhibitors, resolve many AIDS-defining illnesses. However, patients receiving protease inhibitors develop a marked lipodystrophy and hyperlipidemia. Using cultured human and rat hepatoma cells and primary hepatocytes from transgenic mice, we demonstrate that protease inhibitor treatment inhibits proteasomal degradation of nascent apolipoprotein B, the principal protein component of triglyceride and cholesterol-rich plasma lipoproteins. Unexpectedly, protease inhibitors also inhibited the secretion of apolipoprotein B. This was associated with inhibition of cholesteryl-ester synthesis and microsomal triglyceride transfer-protein activity. However, in the presence of oleic acid, which stimulates neutral-lipid biosynthesis, protease-inhibitor treatment increased secretion of apolipoprotein B-lipoproteins above controls. These findings suggest a molecular basis for protease-inhibitor-associated hyperlipidemia, a serious adverse effect of an otherwise efficacious treatment for HIV infection.

The polymorphism at codon 54 of the FABP2 gene increases fat absorption in human intestinal explants

Based on titration microcalorimetry and Caco-2 cell line transfection studies, it has been suggested that the A54T of the FABP2 gene plays a significant role in the assimilation of dietary fatty acids. However, reports were divergent with regard to the in vivo interaction between this polymorphism and postprandial lipemia. We therefore determined the influence of this intestinal fatty acid-binding protein polymorphism on intestinal fat transport using the human jejunal organ culture model, thus avoiding the interference of various circulating factors capable of metabolizing in vivo postprandial lipids. Analysis of DNA samples from 32 fetal intestines revealed 22 homozygotes for the wild-type Ala-54/Ala-54 genotype (0.83) and 10 heterozygotes for the polymorphic Thr-54/Ala-54 genotype (0.17). The Thr-encoding allele was associated with increased secretion of newly esterified triglycerides, augmented de novo apolipoprotein B synthesis, and elevated chylomicron output. On the other hand, no alterations were found in very low density lipoprotein and high density lipoprotein production, apolipoprotein A-I biogenesis, or microsomal triglyceride transfer protein mass and activity. Similarly, the alanine to threonine substitution at residue 54 did not result in changes in brush border hydrolytic activities (sucrase, glucoamylase, lactase, and alkaline phosphatase) or in glucose uptake or oxidation. Our data clearly document that the A54T polymorphism of FABP2 specifically influences small intestinal lipid absorption without modifying glucose uptake or metabolism. It is proposed that, in the absence of confounding factors such as environmental and genetic variables, the FABP2 polymorphism has an important effect on postprandial lipids in vivo, potentially influencing plasma levels of lipids and atherogenesis.

Ubiquitination is essential for human cytomegalovirus US11-mediated dislocation of MHC class I molecules from the endoplasmic reticulum to the cytosol

Human cytomegalovirus encodes two glycoproteins, US2 and US11, which cause rapid degradation of MHC class I molecules, thus preventing recognition of virus-infected cells by the immune system. This degradation process involves retrograde transport or 'dislocation' of MHC class I molecules from the endoplasmic reticulum (ER) to the cytosol, where they are deglycosylated by an N-glycanase and degraded by the proteasome. At present it is unknown whether ubiquitination is required for US2- and US11-mediated dislocation and degradation of MHC class I molecules. Here, we show that in E36ts20 hamster cells, which contain a temperature-sensitive mutation in the E1 ubiquitin-activating enzyme, US11-mediated degradation of MHC class I molecules is strongly impaired at the non-permissive temperature, indicating the necessity for ubiquitination in this process. We next addressed the question of whether ubiquitination is a condition for the retrograde movement of MHC class I molecules from the ER to the cytosol, or whether ubiquitination is merely required for recognition of dislocated MHC class I molecules by the proteasome. In the absence of a functional ubiquitin system, complexes of US11 and MHC class I molecules accumulate in the ER. In this state the membrane topology of MHC class I molecules does not significantly change, as judged from proteinase K digestions. Thus the results indicate that a functional ubiquitin system is essential for dislocation of MHC class I molecules from the ER to the cytosol.

The triple threat to nascent apolipoprotein B. Evidence for multiple, distinct degradative pathways

We previously showed that Omega-3 fatty acids reduce secretion of apolipoprotein B (apoB) from cultured hepatocytes by stimulating post-translational degradation. In this report, we now characterize this process, particularly in regard to the two known processes that degrade newly synthesized apoB, endoplasmic reticulum (ER)-associated degradation and re-uptake from the cell surface. First, we found that Omega-3-induced degradation preferentially reduces the secretion of large, assembled apoB-lipoprotein particles, and apoB polypeptide length is not a determinant. Second, based on several experimental approaches, ER-associated degradation is not involved. Third, re-uptake, the only process known to destroy fully assembled nascent lipoproteins, was clearly active in primary hepatocytes, but Omega-3-induced degradation of apoB continued even when re-uptake was blocked. Cell fractionation showed that Omega-3 fatty acids induced a striking loss of apoB100 from the Golgi, while sparing apoB100 in the ER, indicating a post-ER process. To determine the signaling involved, we used wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, which blocked most, if not all, of the Omega-3 fatty acid effect. Therefore, nascent apoB is subject to ER-associated degradation, re-uptake, and a third distinct degradative pathway that appears to target lipoproteins after considerable assembly and involves a post-ER compartment and PI3K signaling. Physiologic, pathophysiologic, and pharmacologic regulation of net apoB secretion may involve alterations in any of these three degradative steps.

Liver alcohol dehydrogenase is degraded by the ubiquitin-proteasome pathway

Dihydrotestosterone (DHT) decreases rat liver alcohol dehydrogenase (ADH) due principally to an increased rate of degradation of the enzyme. The pathway of degradation of ADH was investigated. Exposure of hepatocytes in culture to lactacystin or to MG132, which are inhibitors of the ubiquitin-proteasome pathway of protein degradation, resulted in higher ADH. Furthermore, both lactacystin and MG132 prevented the decrease in ADH caused by DHT. By contrast, the lysosomal proteolytic inhibitors 3-methyladenine and leupeptin as well as inhibitors of the calcium-activated neutral protease calpain system had no effect on ADH in the absence or presence of DHT. ADH isolated by immunoprecipitation from hepatocytes exposed to DHT reacted specifically with anti-ubiquitin antibody. Ubiquitinated ADH was also demonstrated in hepatocytes exposed to MG132. The combination of DHT and MG132 resulted in more ubiquitinated ADH than exposure to either compound alone. These results suggest that the ubiquitin-proteasome pathway plays a role in the degradation of ADH and in the enhanced degradation of this enzyme by DHT.

Apoprotein B degradation is promoted by the molecular chaperones hsp90 and hsp70

Apoprotein B (apoB) is the major protein of liver-derived atherogenic lipoproteins. The net production of apoB can be regulated by presecretory degradation mediated by the ubiquitin-proteasome pathway and cytosolic hsp70. To further explore the mechanisms of apoB degradation, we have established a cell-free system in which degradation can be faithfully recapitulated. Human apoB48 synthesized in vitro was translocated into microsomes, glycosylated, and ubiquitinylated. Subsequent incubation with rat hepatic cytosol led to proteasome-mediated degradation. To explore whether hsp90 is required for apoB degradation, geldanamycin (GA) was added during the degradation assay. GA increased the recovery of microsomal apoB48 approximately 3-fold and disrupted the interaction between hsp90 and apoB48. Confirming the hsp90 effect in the cell-free system, we also found that transfection of hsp90 cDNA into rat hepatoma cells enhanced apoB48 degradation. Finally, apoB48 degradation was reconstituted in vitro using cytosol prepared from wild type yeast. Notably, degradation was attenuated when apoB48-containing microsomes were incubated with cytosol supplemented with GA or with cytosol prepared from yeast strains with mutations in the homologues of mammalian hsp70 and hsp90. Overall, our data suggest that hsp90 facilitates the interaction between endoplasmic reticulum-associated apoB and components of the proteasomal pathway, perhaps in cooperation with hsp70.

Characterization and regulation of the major histocompatibility complex-encoded proteins Hsp70-Hom and Hsp70-1/2

Vertebrate cells contain at least 12 different genes for Hsp70 proteins, 3 of which are encoded in the major histocompatibility complex (MHC) class III region. In the human MHC, these are named Hsp70-1, -2, and -Hom. To characterize these proteins, we have determined their substrate binding specificity, their cellular and tissue distribution, and the regulation of their expression. We show for the first time (1) peptide binding specificity of Hsp70-Hom; (2) endogenous expression of Hsp70-Hom in human cell lines; (3) cytoplasmic location of Hsp70-Hom protein under basal conditions and concentration in the nucleus after heat shock; (4) unique RNA expression profiles in human tissues for each of the MHC-encoded Hsp70s, significantly different from that for the constitutive Hsc70; (5) a relative increase in levels of Hsp70-Hom protein, compared with other Hsp70s, in response to interferon gamma; and (6) a specific increase on lipopolysaccharide (LPS) treatment of in vivo messenger RNA levels for the MHC-encoded Hsp70s and the DnaJ homologue, hdj2, relative to other chaperones. The unique tissue distributions and specific up-regulation by LPS of the MHC-encoded Hsp70s suggest some specialization of functions for these members of the Hsp70 family, possibly in the inflammatory response.

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.

A novel quality control compartment derived from the endoplasmic reticulum

Degradation of proteins that, because of improper or suboptimal processing, are retained in the endoplasmic reticulum (ER) involves retrotranslocation to reach the cytosolic ubiquitin-proteasome machinery. We found that substrates of this pathway, the precursor of human asialoglycoprotein receptor H2a and free heavy chains of murine class I major histocompatibility complex (MHC), accumulate in a novel preGolgi compartment that is adjacent to but not overlapping with the centrosome, the Golgi complex, and the ER-to-Golgi intermediate compartment (ERGIC). On its way to degradation, H2a associated increasingly after synthesis with the ER translocon Sec61. Nevertheless, it remained in the secretory pathway upon proteasomal inhibition, suggesting that its retrotranslocation must be tightly coupled to the degradation process. In the presence of proteasomal inhibitors, the ER chaperones calreticulin and calnexin, but not BiP, PDI, or glycoprotein glucosyltransferase, concentrate in the subcellular region of the novel compartment. The "quality control" compartment is possibly a subcompartment of the ER. It depends on microtubules but is insensitive to brefeldin A. We discuss the possibility that it is also the site for concentration and retrotranslocation of proteins that, like the mutant cystic fibrosis transmembrane conductance regulator, are transported to the cytosol, where they form large aggregates, the "aggresomes."

2000 George Lyman Duff Memorial Lecture: atherosclerosis is a liver disease of the heart

The production of apolipoprotein B (apoB)-containing lipoproteins by the liver is regulated by a complex series of processes involving apoB being cotranslationally translocated across the endoplasmic reticulum and assembled into a lipoprotein particle. The translocation of apoB across the endoplasmic reticulum is facilitated by the intraluminal chaperone, microsomal triglyceride transfer protein (MTP). MTP facilitates the translocation and folding of apoB, as well as the addition of lipid to lipid-binding domains (which consist of amphipathic beta sheets and alpha helices). In the absence of MTP or sufficient lipid, apoB exhibits translocation arrest. Thus, apoB translation, translocation, and assembly with lipids to form a core-containing lipoprotein particle occur as concerted processes. Abrogation of >/=1 of these processes diverts apoB into a degradation pathway that is dependent on conjugation with ubiquitin and proteolysis by the proteasome. The nascent core-containing lipoprotein particle that forms within the lumen of the endoplasmic reticulum can be "enlarged" to form a mature very low density lipoprotein particle. Additional studies show that the assembly and secretion of apoB-containing lipoproteins are linked to the cholesterol/bile acid synthetic pathway controlled by cholesterol 7alpha-hydroxylase. Studies in cultured cells and transgenic mice indicate that the expression of cholesterol 7alpha-hydroxylase indirectly regulates the expression of lipogenic enzymes through changes in the cellular content of mature sterol response element binding proteins. Oxysterols and bile acids may also act via the ligand-activated nuclear receptors LXR and FXR to link the metabolic pathways controlling energy balance and lipid metabolism to nutritional state.

Microsomal triglyceride transfer protein binding and lipid transfer activities are independent of each other, but both are required for secretion of apolipoprotein B lipoproteins from liver cells

Recent studies indicate that microsomal triglyceride transfer protein (MTP) and apolipoprotein B (apoB) interact physically via two specific binding sites located within the amino-terminal globular region of apoB100. The first site is thought to be within the first 5.8% of the amino-terminal sequence, and the second site is between 9 and 16% of the amino-terminal sequence. It is not clear from prior studies whether these sites have unique or overlapping functions. Furthermore, there are no data differentiating between lipid transfer and potential chaperone functions of MTP. In the present study we have attempted to further characterize the physiologic interaction between apoB and MTP and to determine the relationship between the binding and lipid transfer aspects of the interaction. HepG2 cells were transiently transfected with apoB cDNAs, and MTP binding to apoB polypeptides was determined by two-step immunoprecipitation. MTP bound equally well to apoB polypeptides with (apoB13, 16,beta, apoB34, and apoB42) or without (apoB16, apoB13, and 16 or apoB13, 13, and 16) beta sheet domains. When proteasomal degradation of newly synthesized apoB polypeptides was blocked, MTP binding to all of the apoB polypeptides was only modestly affected by lipid availability and was independent of MTP-associated lipid transfer. Furthermore, MTP did not bind directly to a portion of the first beta sheet domain. We created two apoB constructs (apoB16del and apoB34del) by deleting the first 210 amino acids of apoB16 and apoB34. These apoB polypeptides, therefore, lacked the putative first MTP binding site. MTP binding to apoB16del and apoB34del was decreased significantly. However, the secretion of apoB16del was not different from apoB16, whereas the secretion of apoB34del was impaired significantly. Our results indicate that the interaction between MTP and apoB involves independent binding and lipid transfer activities but that both activities are required for the secretion of apolipoprotein B from liver cells.

Hsp70 molecular chaperone facilitates endoplasmic reticulum-associated protein degradation of cystic fibrosis transmembrane conductance regulator in yeast

Membrane and secretory proteins fold in the endoplasmic reticulum (ER), and misfolded proteins may be retained and targeted for ER-associated protein degradation (ERAD). To elucidate the mechanism by which an integral membrane protein in the ER is degraded, we studied the fate of the cystic fibrosis transmembrane conductance regulator (CFTR) in the yeast Saccharomyces cerevisiae. Our data indicate that CFTR resides in the ER and is stabilized in strains defective for proteasome activity or deleted for the ubiquitin-conjugating enzymes Ubc6p and Ubc7p, thus demonstrating that CFTR is a bona fide ERAD substrate in yeast. We also found that heat shock protein 70 (Hsp70), although not required for the degradation of soluble lumenal ERAD substrates, is required to facilitate CFTR turnover. Conversely, calnexin and binding protein (BiP), which are required for the proteolysis of ER lumenal proteins in both yeast and mammals, are dispensable for the degradation of CFTR, suggesting unique mechanisms for the disposal of at least some soluble and integral membrane ERAD substrates in yeast.

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 E in macrophages and hepatocytes is eegraded via the proteasomal pathway

Macrophage-derived apolipoprotein E (apoE) influences the susceptibility of the arterial wall to atherosclerosis. Previous studies have shown that production of apoE in these cells is regulated at a posttranscriptional level and is increased by inhibitors of proteasomal degradation. To further investigate this mechanism, we stably transfected RAW 264.7 macrophages and HepG2 cells with a construct overexpressing ubiquitin, the peptide targeting proteins to the proteasome, fused to an influenza virus hemagglutinin epitope tag. Ubiquitination of apoE was investigated by immunoprecipitation and Western blot analysis. In both cell types, apoE was ubiquitinated, and inhibition of proteasome function by lactacystin led to accumulation of ubiquitinated apoE. These studies provide strong evidence for proteasomal degradation of apoE in the two main cell types responsible for its production and indicate a possible new level of regulation of this important protein.

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.

Newly synthesized human delta opioid receptors retained in the endoplasmic reticulum are retrotranslocated to the cytosol, deglycosylated, ubiquitinated, and degraded by the proteasome

We have previously shown that only a fraction of the newly synthesized human delta opioid receptors is able to leave the endoplasmic reticulum (ER) and reach the cell surface (Petäjä-Repo, U. E, Hogue, M., Laperrière, A., Walker, P., and Bouvier, M. (2000) J. Biol. Chem. 275, 13727-13736). In the present study, we investigated the fate of those receptors that are retained intracellularly. Pulse-chase experiments revealed that the disappearance of the receptor precursor form (M(r) 45,000) and of two smaller species (M(r) 42,000 and 39,000) is inhibited by the proteasome blocker, lactacystin. The treatment also promoted accumulation of the mature receptor form (M(r) 55,000), indicating that the ER quality control actively routes a significant proportion of rescuable receptors for proteasome degradation. In addition, degradation intermediates that included full-length deglycosylated (M(r) 39,000) and ubiquitinated forms of the receptor were found to accumulate in the cytosol upon inhibition of proteasome function. Finally, coimmunoprecipitation experiments with the beta-subunit of the Sec61 translocon complex revealed that the receptor precursor and its deglycosylated degradation intermediates interact with the translocon. Taken together, these results support a model in which misfolded or incompletely folded receptors are transported to the cytoplasmic side of the ER membrane via the Sec61 translocon, deglycosylated and conjugated with ubiquitin prior to degradation by the cytoplasmic 26 S proteasomes.

Tunicamycin induces ubiquitination and degradation of apolipoprotein B in HepG2 cells

Apolipoprotein (apo) B-100 is an essential component of atherogenic plasma lipoproteins. Previous studies have demonstrated that the production of apoB-100 is regulated largely by intracellular degradation at both the co-translational and post-translational levels and that proteasome-mediated and non-proteasome-mediated pathways are involved in this process. ApoB-100 is a glycoprotein. The present study was undertaken to address the question of whether the inhibition of N-linked glycosylation with tunicamycin would interfere with apoB-100 production. We demonstrated that the treatment of HepG2 cells with tunicamycin decreased the net production of apoB-100 by enhancing co-translational degradation of the protein. This effect of tunicamycin was partly prevented by lactacystin, a specific proteasome inhibitor. Because lactacystin only partly reversed the effects of tunicamycin on apoB biogenesis, tunicamycin seemed also to induce apoB co-translational degradation in HepG2 cells by one or more non-proteasomal pathways. Furthermore, tunicamycin increased apoB ubiquitination approx. 4-fold. The proportion of the newly synthesized apoB-100 that was secreted and incorporated into the nascent lipoprotein particles was unaffected by tunicamycin. Thus the tunicamycin-mediated inhibition of N-linked glycosylation interferes with the production of apoB-100 that is mediated by both proteasomal and non-proteasomal pathways.

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.