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

Electrophoretic cytometry of adherent cells

Cell-matrix and cell-cell interactions influence intracellular signalling and play an important role in physiologic and pathologic processes. Detachment of cells from the surrounding microenvironment alters intracellular signalling. Here, we demonstrate and characterise an integrated microfluidic device to culture single and clustered cells in tuneable microenvironments and then directly analyse the lysate of each cell in situ, thereby eliminating the need to detach cells prior to analysis. First, we utilise microcontact printing to pattern cells in confined geometries. We then utilise a microscale isoelectric focusing (IEF) module to separate, detect, and analyse lamin A/C from substrate-adhered cells seeded and cultured at varying (500, 2000, and 9000 cells per cm2) densities. We report separation performance (minimum resolvable pI difference of 0.11) that is on par with capillary IEF and independent of cell density. Moreover, we map lamin A/C and β-tubulin protein expression to morphometric information (cell area, circumference, eccentricity, form factor, and cell area factor) of single cells and observe poor correlation with each of these parameters. By eliminating the need for cell detachment from substrates, we enhance detection of cell receptor proteins (CD44 and β-integrin) and dynamic phosphorylation events (pMLCS19) that are rendered undetectable or disrupted by enzymatic treatments. Finally, we optimise protein solubilisation and separation performance by tuning lysis and electrofocusing (EF) durations. We observe enhanced separation performance (decreased peak width) with longer EF durations by 25.1% and improved protein solubilisation with longer lysis durations. Overall, the combination of morphometric analyses of substrate-adhered cells, with minimised handling, will yield important insights into our understanding of adhesion-mediated signalling processes.

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.

Glucosamine-induced endoplasmic reticulum stress attenuates apolipoprotein B100 synthesis via PERK signaling

Glucosamine impairs hepatic apolipoprotein B100 (apoB100) production by inducing endoplasmic reticulum (ER) stress and enhancing cotranslational and posttranslational apoB100 degradation (Qiu, W., R. K. Avramoglu, A. C. Rutledge, J. Tsai, and K. Adeli. Mechanisms of glucosamine-induced suppression of the hepatic assembly and secretion of apolipoprotein B-100-containing lipoproteins. J. Lipid Res. 2006. 47: 1749-1761). Here, we report that glucosamine also regulates apoB100 protein synthesis via ER-stress-induced PERK activation. Short-term (4 h) glucosamine treatment of HepG2 cells reduced both cellular (by 62%) and secreted apoB100 (by 43%) without altering apoB100 mRNA. Treatment with proteasomal inhibitors only partially prevented the suppressive effects of glucosamine, suggesting that mechanisms other than proteasomal degradation may also be involved. Glucosamine-induced ER stress was associated with a significantly reduced apoB100 synthesis with no significant change in posttranslational decay rates, suggesting that glucosamine exerted its effect early during apoB biosynthesis. The role of PERK and its substrate, alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha), in the suppressive effects of glucosamine on apoB synthesis was then investigated. Coexpression of apoB15 (normally resistant to intracellular degradation) with wild-type double stranded (ds) RNA activated protein kinase (PKR)-like endoplasmic reticulum kinase (PERK) in COS-7 cells resulted in a dramatic reduction in the levels of newly synthesized apoB15. Interestingly, cotransfection with apoB15 and a kinase inactive PERK mutant (K618A) increased apoB15 expression. In addition, short-term glucosamine treatment stimulated an increase in phosphorylation of PERK and eIF2alpha. Taken together, these data suggest that in addition to the induction of ER-associated degradation and other degradative pathways, ER stress is associated with suppression of apoB synthesis via a PERK-dependent mechanism.

Mechanisms Targeting Apolipoprotein B100 to Proteasomal Degradation

Objectives— In lipid-poor states, the ubiquitin-proteasomal pathway rapidly degrades misfolded apolipoprotein B100 (apoB) cotranslationally, although the mechanism of delivery from the ER to cytosolic proteasomes is poorly understood. Here we demonstrate key roles of BiP, an ER luminal chaperone, and p97, a cytosolic ATPase anchored to the ER membrane, in the targeting of apoB for proteasomal degradation.

Methods and Results— Using coimmunoprecipitations, we observed associations of apoB with BiP, p97, Derlin-1, VIMP, and the E3 ubiquitin ligase Hrd1 in HepG2 cells. BiP and p97 were found to bind apoB cotranslationally. Expression of C-terminal truncated apoB molecules in COS-7 cells showed an N-terminal region outside apoB15 and a C-terminal region found in apoB72 were required for BiP and p97 binding, respectively. Interestingly, overexpression of dominant negative p97 demonstrated that the ATPase activity of p97 was essential for proteasomal degradation of apoB but not for apoB binding. However, p97 activity did not appear to affect the N terminus of apoB, which may be cleaved before degradation.

Conclusions— These data suggest that p97 and BiP play critical roles in the cotranslational delivery of apoB to proteasomes and formation of a degradative complex. Proteasomal degradation appears to selectively target apoB molecules with large C-terminal domains.

Charged amino acid residues 997-1000 of human apolipoprotein B100 are critical for the initiation of lipoprotein assembly and the formation of a stable lipidated primordial particle in McA-RH7777 cells

We previously demonstrated that a portion, or perhaps all, of the residues between 931 and 1000 of apolipoprotein (apo) B100 are required for the initiation of apoB-containing particle assembly. Based on our structural model of the first 1000 residues of apoB (designated as apoB:1000), we hypothesized that this domain folds into a three-sided lipovitellin-like "lipid pocket" via a hairpin-bridge mechanism. We proposed that salt bridges are formed between four tandem charged residues 717-720 in the turn of the hairpin bridge and four tandem complementary residues 997-1000 located at the C-terminal end of the model. To identify the specific motif within residues 931 and 1000 that is critical for apoB particle assembly, apoB:956 and apoB:986 were produced. To test the hairpin-bridge hypothesis, the following mutations were made: 1) residues 997-1000 deletion (apoB:996), 2) residues 717-720 deletion (apoB:1000Delta717-720), and 3) substitution of charged residues 997-1000 with alanines (apoB:996 + 4Ala). Characterization of particles secreted by stable transformants of McA-RH7777 cells demonstrated the following. 1) ApoB:956 did not form stable particles and was secreted as large lipid-rich aggregates. 2) ApoB:986 formed both a lipidated particle that was denser than HDL(3) and large lipid-rich aggregates. 3) Compared with wild-type apoB:1000, apoB:1000Delta717-720 displayed the following: (i) significantly diminished capacity to form intact lipidated particles and (ii) increased propensity to form large lipid-rich aggregates. 4) In striking contrast to wild-type apoB:1000, (i) apoB:996 and apoB:996 + 4Ala were highly susceptible to intracellular degradation, (ii) only a small proportion of the secreted proteins formed stable HDL(3)-like lipoproteins, and (iii) a majority of the secreted proteins formed large lipid-rich aggregates. We conclude that the first 1000 amino acid residues of human apoB100 are required for the initiation of nascent apoB-containing lipoprotein assembly, and residues 717-720 and 997-1000 play key roles in this process, perhaps via a hairpin-bridge mechanism.

Missense mutations in APOB within the betaalpha1 domain of human APOB-100 result in impaired secretion of ApoB and ApoB-containing lipoproteins in familial hypobetalipoproteinemia

Familial hypobetalipoproteinemia (FHBL) is associated with mutations in the APOB gene. We reported the first missense APOB mutation, R463W, in an FHBL kindred (Burnett, J. R., Shan, J., Miskie, B. A., Whitfield, A. J., Yuan, J., Tran, K., Mc-Knight, C. J., Hegele, R. A., and Yao, Z. (2003) J. Biol. Chem. 278, 13442-13452). Here we identified a second nonsynonymous APOB mutation, L343V, in another FHBL kindred. Heterozygotes for L343V (n = 10) had a mean plasma apoB at 0.31 g/liter as compared with 0.80 g/liter in unaffected family members (n = 22). The L343V mutation impaired secretion of apoB-100 and very low density lipoproteins. The secretion efficiency was 20% for B100wt and 10% for B100LV and B100RW. Decreased secretion of mutant apoB-100 was associated with increased endoplasmic reticulum retention and increased binding to microsomal triglyceride transfer protein and BiP. Reduced secretion efficiency was also observed with B48LV and B17LV. Biochemical and biophysical analyses of apoB domain constructs showed that L343V and R463W altered folding of the alpha-helical domain within the N terminus of apoB. Thus, proper folding of the alpha-helical domain of apoB-100 is essential for efficient secretion.

Insulin decreases the secretion of apoB-100 from hepatic HepG2 cells but does not decrease the secretion of apoB-48 from intestinal CaCo-2 cells

We compared the acute effect of insulin on the human colonic intestinal epithelial cell line CaCo-2 and the transformed human hepatic cell line HepG2. Over 24 h, 100 nM and 10 microM insulin significantly inhibited the secretion of apolipoprotein (apo) B-100 from HepG2 cells to 63 and 49% of control, respectively. Insulin had no effect on the secretion of apoB-48 from CaCo-2 cells. There was no effect of insulin on the cholesterol ester or free cholesterol concentrations in HepG2 or CaCo-2 cells. HepG2 and CaCo-2 cells bound insulin with high affinity, leading to similar stimulation of insulin receptor protein tyrosine kinase activation. Protein kinase C or mitogen-activated protein kinase activity in the presence or absence of insulin was not correlated with apoB-48 production in CaCo-2 cells. Therefore, insulin acutely decreases the secretion of apoB-100 in hepatic HepG2 cells, but does not acutely modulate the production or secretion of apoB-48 from CaCo-2 intestinal cells.

Amino acid sequences within the β1 domain of human apolipoprotein B can mediate rapid intracellular degradation

Apolipoprotein B (apoB)-48 contains a region termed the beta1 domain that is predicted to be composed of extensive amphipathic beta-strands. Analysis of truncated apoB variants revealed that sequences between the carboxyl termini of apoB-37 and apoB-42 governed the secretion efficiency and intracellular stability of apoB. Although apoB-37, apoB-34, and apoB-29 were stable and secreted efficiently, apoB-42 and apoB-100 were secreted poorly and were degraded by an acetyl-leucyl-leucyl-norleucinal (ALLN)-sensitive pathway. Amino acid sequence analysis suggested that a segment between the carboxyl termini of apoB-38 and apoB-42 was 63% homologous to fatty acid binding proteins (FABPs), which contain orthogonal beta-sheets. To test the hypothesis that sequences from the beta1 domain are involved in apoB degradation, fusion proteins were created that contained apoB-29 linked to fragments derived from the beta1 domain of apoB or to liver FABP. Fusion proteins containing the beta1 domain segments apoB-34-42 or apoB-37-42 were degraded rapidly, whereas other fusion proteins were stable and secreted efficiently. Degradation was ALLN-sensitive, and the apoB-34-42 segment increased the association of the apoB protein with the cytosolic surface of the microsomal membrane. Our data suggest that the presence of specific sequences in the beta1 domain of human apoB increases degradation by promoting the cytosolic exposure of the protein, although not all regions of the beta1 domain are functionally equivalent.

Regulation of human apolipoprotein B gene expression at multiple levels

Apolipoprotein B is a large, amphipathic protein that plays a central role in lipoprotein metabolism. Because its overproduction and deficiency leads to metabolic and pathologic disorders, much effort has been paid to investigate the mechanisms of how its homeostasis is achieved. Earlier and recent studies have showed that apoB gene locus might reside in different chromatin domains in the hepatic and intestinal cells, and two sets of very distinct regulatory elements operate to control its transcription. Posttranscriptional modification of apoB mRNA is performed by a multicomponent enzyme complex, several possible pathways regulate the editing efficiency. Understanding of the mechanism responsible for apoB mRNA editing will provide the basis for C-to-U editing in gene therapy. In addition to apoB mRNA abundance and stability, its translation can be also regulated at the steps of elongation. The translocation of apoB into the ER is an important and complicated process that is less understood. Successful transport and correct folding of apoB may lead to its final secretion, otherwise subject to intracellular degradation, which is accomplished by proteasomal and nonproteasomal pathways at multiple levels and may differ among cell types.

Dual action of neutral sphingomyelinase on rat hepatocytes: activation of cholesteryl ester metabolism and biliary cholesterol secretion and inhibition of VLDL secretion

To address the role of cell membrane neutral sphingomyelinase (EC 3.1.4.12; SMase) in the regulation of cholesterol metabolism in the liver parenchymal cell, we examined the effect of exogenous neutral SMase on the metabolism of cholesteryl esters and the secretion of VLDL and biliary lipids in isolated rat hepatocytes. We show that treatment of hepatocytes with SMase (20 mU/mL) resulted in the intracellular buildup of cholesteryl esters, increased ACAT (EC 2.3.1.26) activity without affecting the ACAT2 mRNA level, and increased cytosolic and microsomal cholesteryl ester hydrolase (EC 3.1.1.13) activity. This was accompanied by increases in the secretion of biliary bile acid, phospholipid, and cholesterol and in increased cholesterol 7alpha-hydroxylase (EC 1.14.13.17) activity and levels of mRNA, as well as decreased levels of apoB mRNA and a decreased secretion of VLDL apoB (apoB-48, approximately 45%; apoB-100, approximately 32%) and lipids (approximately 55%). Moreover, the VLDL particles secreted had an abnormal size and lipid composition; they were larger than controls, were relatively enriched in cholesteryl ester, and depleted in TG and cholesterol. Cell-permeable ceramides did not replicate any of the reported effects. These findings demonstrate that the increased cholesteryl ester turnover, oversecretion of biliary cholesterol and bile acids, and undersecretion of VLDL cholesterol and particles are concerted responses of the primary hepatocytes to exogenous neutral SMase brought about by regulation at several levels. We suggest that plasma membrane neutral SMase may have a specific, ceramide-independent effect in the regulation of cholesterol output pathways in hepatocytes.

Intracellular mechanisms mediating the inhibition of apoB-containing lipoprotein synthesis and secretion in HepG2 cells by avasimibe (CI-1011), a novel acyl-coenzyme A: cholesterol acyltransferase (ACAT) inhibitor

We have studied the cellular and molecular mechanisms involved in the suppression of apoB secretion from HepG2 cells following incubation with avasimibe (CI-1011), a novel inhibitor of acyl-coenzyme A: cholesterol acyltransferase (ACAT). Cellular lipid analysis revealed that avasimibe significantly decreased the synthesis of cholesterol and cholesteryl ester, and, at higher doses, of triglyceride. Time-course trypsin protection assays revealed that avasimibe induced the accumulation of translocationally arrested apoB intracellularly. Pulse-chase studies showed that the treatment with avasimibe induced a >75% decrease in apoB secretion relative to control, but initially enhanced the protein stability and cellular accumulation of apoB. Subcellular fractionation of microsomes further confirmed the accumulation of secretion-incompetent apoB-lipoproteins in the endoplasmic reticulum (ER) and Golgi compartments of avasimibe-treated HepG2 cells. Although incubation of drug-treated cells with carbobenzoxyl-leucinyl-leucinyl-leucinal (MG132), a potent proteasome inhibitor, increased cellular apoB (70%), it failed to increase apoB secretion. Drug treatment induced an accumulation of secretion-incompetent apoB-containing lipoprotein particles, the majority of which demonstrated a density in a range similar to that of high-density lipoprotein. However, studies in permeabilized cells demonstrated that, at longer chase times, intracellularly accumulated apoB was eventually degraded, indicating that the inhibition of degradation may be transient. Oleate treatment of avasimibe-treated cells partially restored apoB secretion but not to the levels seen in control cells. In summary, we hypothesize that avasimibe acutely blocks the secretion of apoB and its associated lipoproteins from HepG2 cells, transiently enhancing its membrane association and cellular accumulation with eventual intracellular degradation of accumulated apoB.

Increased production of apolipoprotein B-containing lipoproteins in the absence of hyperlipidemia in transgenic mice expressing cholesterol 7alpha-hydroxylase

The finding that expression of a cholesterol 7alpha-hydroxylase (CYP7A1) transgene in cultured rat hepatoma cells caused a coordinate increase in lipogenesis and secretion of apoB-containing lipoproteins led to the hypothesis that hepatic production of apoB-containing lipoproteins may be linked to the expression of CYP7A1 (Wang, S.-L., Du, E., Martin, T. D., and Davis, R. A. (1997) J. Biol. Chem. 272, 19351-19358). To examine this hypothesis in vivo, a transgene encoding CYP7A1 driven by the constitutive liver-specific enhancer of the human apoE gene was expressed in C56BL/6 mice. The expression of CYP7A1 mRNA (20-fold), protein ( approximately 10-fold), and enzyme activity (5-fold) was markedly increased in transgenic mice compared with non-transgenic littermates. The bile acid pool of CYP7A1 transgenic mice was doubled mainly due to increased hydrophobic dihydroxy bile acids. In CYP7A1 transgenic mice, livers contained approximately 3-fold more sterol response element-binding protein-2 mRNA. Hepatic expression of mRNAs encoding lipogenic enzymes (i.e. fatty-acid synthase, acetyl-CoA carboxylase, stearoyl-CoA desaturase, squalene synthase, farnesyl-pyrophosphate synthase, 3-hydroxy-3-methylglutaryl-CoA reductase, and low density lipoprotein receptor) as well as microsomal triglyceride transfer protein were elevated approximately 3-5-fold in transgenic mice. CYP7A1 transgenic mice also displayed a >2-fold increase in hepatic production and secretion of triglyceride-rich apoB-containing lipoproteins. Despite the increased hepatic secretion of apoB-containing lipoproteins in CYP7A1 mice, plasma levels of triglycerides and cholesterol were not significantly increased. These data suggest that the 5-fold increased expression of the low density lipoprotein receptor displayed by the livers of CYP7A1 transgenic mice was sufficient to compensate for the 2-fold increase production of apoB-containing lipoproteins. These findings emphasize the important homeostatic role that CYP7A1 plays in balancing the anabolic lipoprotein assembly/secretion pathway with the cholesterol catabolic bile acid synthetic pathway.

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.

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.

Short- and long-term effects of atorvastatin, lovastatin and simvastatin on the cellular metabolism of cholesteryl esters and VLDL secretion in rat hepatocytes

The short- and long-term in vitro effects of the hydroxymethylglutaryl-CoA reductase inhibitor atorvastatin, compared with lovastatin and simvastatin on VLDL secretion, and on the formation and the neutral and acid lysosomal hydrolysis of cholesteryl esters was investigated in rat liver hepatocytes maintained in suspension (2 or 4 h) or cultured in monolayers (24 h). All statins time-dependently reduced [14C]oleate incorporation into cholesteryl esters, but when exogenous cholesterol was added only atorvastatin caused an immediate transient decrease in hepatocyte ACAT activity. Activity of the lysosomal, microsomal and cytosolic CEH isoforms was unaffected by the hepatocyte treatments. Statins reduced free and esterified cholesterol mass in hepatocyte microsomes after 2 h, and this was followed by a modest decline in VLDL cholesteryl esters, whilst secretion of VLDL apoB and triglycerides was unaltered. However, after 24 h of treatment, statins caused generalized 20-40% decreases in the secretion of VLDL apoB, cholesterol and triglycerides, with the reduction in apoB48 secretion being significantly superior to that caused in apoB100. The mean diameter of secreted VLDL was not modified by either duration or drug treatment. Additional studies with subcellular fractions demonstrated that statins have a direct selective effect on the enzymes governing the cholesterol-cholesteryl ester cycle, with the exception of the microsomal CEH. Atorvastatin, lovastatin and simvastatin inhibited ACAT activity in microsomes by 50% at doses of 250, 100 and 50 microM, respectively. The cytosolic CEH elicited a biphasic profile of activity with activations up to 100 microM statin and inhibitions above 250 microM, and the lysosomal CEH was only inhibited by atorvastatin at a dose of 100 microM or more. We conclude that a prolonged, but not a short, limited availability of hepatocyte cholesterol derived from the endogenous synthesis reduces VLDL secretion, and that reactivity of statins at the cellular level are more similar than reactivity at the subcellular level as regards the cholesterol-cholesteryl ester cycle.

A targeted apolipoprotein B-38.9-producing mutation causes fatty livers in mice due to the reduced ability of apolipoprotein B-38.9 to transport triglycerides

Nonphysiological truncations of apolipoprotein (apo) B-100 cause familial hypobetalipoproteinemia (FHBL) in humans and mice. An elucidation of the mechanisms underlying the FHBL phenotypes may provide valuable information on the metabolism of apo B-containing lipoproteins and the structure-function relationship of apo B. To generate a faithful mouse model of human FHBL, a subtle mutation was introduced into the mouse apo B gene by targeting embryonic stem cells using homologous recombination followed by removal of the selection marker gene by Cre-loxP-mediated site-specific recombination. The engineered mice bear a premature stop codon at residue 1767 and a 42-base pair loxP inserted into intron 24 of the apo B gene, thus closely resembling the apo B-38.9-producing mutation in humans. Apo B-38.9 was the sole apo B protein in homozygote (apob(38.9/38.9)) plasma. In heterozygotes (apob(+/)(38. 9)), apo B-100 and apo B-48 were reduced by 75 and 40%, respectively, and apo B-38.9 represented 20% of total circulating apo B. Hepatic apo B-38.9 mRNA levels were reduced by 40%. In cultured apob(+/)(38. 9) hepatocytes, apo B-100 was produced in trace quantities, and the synthesis rate of apo B-38.9 relative to apo B-48 was reduced by 40%. However, almost equimolar amounts of apo B-38.9 and apo B-48 were secreted into the media. Pulse-chase studies revealed that apo B-38. 9 was secreted at a faster rate and more efficiently than apoB-48. Nevertheless, both apob(+/)(38.9) and apob(38.9/38.9) mice had reduced hepatic triglyceride secretion rates and fatty livers. Thus, low mRNA levels or defective secretion of apo B-38.9 may not be responsible for the FHBL phenotypes caused by the apo B-38.9 mutation. Rather, a reduced capacity of apo B-38.9 for triglyceride transport may account for the fatty livers in these mice.

Translocational status of ApoB in the presence of an inhibitor of microsomal triglyceride transfer protein

Despite numerous studies demonstrating that microsomal triglyceride transfer protein (MTP) activity is critical to apoB secretion, there is still controversy as to whether MTP directly facilitates the translocation of apoB across the membrane of the endoplasmic reticulum (ER) through either the recruitment of lipids and/or chaperone activity. In the present study, a specific inhibitor of MTP (BMS 197636) was utilized in HepG2 cells to investigate whether a direct relationship exists between the translocation of apoB across the ER membrane and the lipid-transferring activity of MTP. Inhibition of MTP (with 10 and 50 nmol/L of the inhibitor) did not significantly affect the translocation of newly synthesized apoB (P = 0.77) or the translocational efficiency of the steady-state apoB mass (P = 0.45), despite a 49% decrease in apoB secretion and increased proteosomal degradation. These results compared well with subcellular fractionation experiments which showed no significant change in the fraction of apoB accumulated in the lumen of isolated microsomes in MTP-treated cells (P = 0.35). In summary, MTP lipid transfer activity does not appear to influence translocational status of apoB, but its inhibition is associated with an increased susceptibility to proteasome-mediated degradation and reduced assembly and secretion of apoB lipoprotein particles.

Specificity of lipid incorporation is determined by sequences in the N-terminal 37 of apoB

The N-terminal 17% of apolipoprotein B (apoB-17) is secreted lipid-poor while apoB-41 particles are secreted with a triacylglycerol (TAG)-rich core. Thus, the sequence between apoB-17 and apoB-41 is necessary for the assembly of TAG-rich lipoproteins. To delineate this region, C127 cells were permanently transfected to secrete the N-terminal 29, 32.5, or 37% of apoB. Density gradient centrifugation showed that secreted apoB-29, apoB-32.5, and apoB-37 had peak densities of 1.25, 1.22, and 1.16 g/mL and percent lipid of particle weights of 30, 37, and 49%, respectively. Calculated anhydrous particle diameters were: apoB-29 = 81 A, apoB-32.5 = 88 A, and apoB-37 = 101 A. Immunoprecipitated particles labeled with [(3)H]oleate showed that, as apoB length increased from apoB-29 to apoB-32.5 and apoB-37, the number of TAG (core) molecules per apoB particle increased almost 16-fold from 8 to 32 to 124, while phospholipids and diacylglycerols (surface lipids) increased only slightly from 71 to 87 to 97 molecules, respectively. Thus, sequences in the C-terminus of apoB-29 bind phospholipids and diacylglycerols, sequences between apoB-29 and apoB-32.5 augment TAG binding and sequences between apoB-32.5 and apoB-41 account for the marked incorporation of TAG at a rate of approximately 1 TAG per 2 amino acids. Cryoelectron micrographs of isolated apoB-37 particles revealed mostly spherical particles of approximately 110 A (11.0 nm) with an electron lucent center, consistent with these particles having a TAG core. We suggest that the predicted amphipathic beta-sheets beginning at apoB-29, starts to preferentially recruit core lipids into apoB and propose that the consistent presence of DAG in the secreted particles may have a role in fission of the nascent lipoprotein particles from the endoplasmic reticulum membrane.

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.

Apolipoprotein B, a paradigm for proteins regulated by intracellular degradation, does not undergo intracellular degradation in CaCo2 cells

Studies in different liver-derived cells in culture indicate that apolipoprotein (apo) B-100 production is regulated largely by intracellular degradation and the ubiquitin-proteasome pathway is a major mechanism for the degradation. The proteasomal degradation of apoB-100 was postulated to be an intrinsic property of the protein that occurs even in the presence of optimal amounts of lipids supplied to the cell. We examined apoB-100 and apoB-48 biogenesis in CaCo2, a human colon carcinoma cell line. To our surprise, apoB-100 and apoB-48 were quantitatively secreted by CaCo2 cells; essentially none of the newly synthesized apoB was degraded before secretion in a 2-h period whether the cells were cultured on filter or on plastic. Furthermore, although ubiquitin immunoreactivity was readily detected in the intracellular apoB isolated from HepG2 cells, little or no ubiquitin was detectable in the intracellular apoB from CaCo2 cells. The amounts of free ubiquitin and total and non-apoB ubiquitinated proteins were comparable in HepG2 and CaCo2 cells, indicating that CaCo2 cells have the necessary machinery for tagging ubiquitin chains onto cellular proteins for proteasomal degradation. Incubation in lipoprotein-deficient serum did not induce apoB degradation, but the addition of a microsomal triglyceride transfer protein inhibitor led to apoB degradation in CaCo2 cells. Finally, similar proportions of apoB polypeptide in isolated microsomes from CaCo2 and HepG2 cells were accessible to exogenously added trypsin, indicating that the mere exposure of apoB nascent chains to the cytosolic compartment is insufficient to cause the proteasomal degradation. Therefore, the intracellular degradation of apoB is not an intrinsic property of the protein, and the phenomenon is neither universal nor inevitable. The unconditional use of apoB as a paradigm for intracellular protein degradation is not warranted.

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.

Studies on degradative mechanisms mediating post-translational fragmentation of apolipoprotein B and the generation of the 70-kDa fragment

It has been well established that the biogenesis of apoB is mediated co-translationally by the cytosolic proteasome. Here, however, we investigated the role of both the cytosolic proteasome as well as non-proteasome-mediated degradation systems in the post-translational degradation of apoB. In pulse-chase labeling experiments, co-translational (0-h chase) apoB degradation in both intact and permeabilized cells was sensitive to proteasome inhibitors. Interestingly, turnover of apoB in intact cells over a 2-h chase was partially inhibitable by lactacystin, thus suggesting a role for the cytosolic proteasome in the post-translational degradation of apoB. In permeabilized cells, however, there was no post-translational protection of apoB by lactacystin. Further investigations of proteasomal activity in HepG2 cells revealed that, following permeabilization, there was a dramatic loss of the 20 S proteasomal subunits, and consequently the cells exhibited no detectable lactacystin-inhibitable activity. Thus, apoB fragmentation and the generation of the 70-kDa apoB degradation fragment, characteristic of permeabilized cells, continued to occur in these cells despite the absence of functional cytosolic proteasome. Similar results were observed when we used a derivative of lactacystin, clastolactacystin beta-lactone, which represents the active species of the inhibitor. Interestingly, however, the abundance of the 70-kDa fragment could be modulated by the microsomal triglyceride transfer protein inhibitor, BMS-197636, as well as by pretreatment of the permeabilized cells with dithiothreitol. These data thus suggest that although the cytosolic proteasome appears to be involved in the post-translational turnover of apoB in intact cells, the specific post-translational fragmentation of apoB generating the 70-kDa fragment observed in permeabilized cells occurs independent of the cytosolic proteasome.