Determination of the intracellular distribution and pool sizes of apolipoprotein B in rabbit liver

A role for smooth endoplasmic reticulum membrane cholesterol ester in determining the intracellular location and regulation of sterol-regulatory-element-binding protein-2

Cellular cholesterol homoeostasis is regulated through proteolysis of the membrane-bound precursor sterol-regulatory-element-binding protein (SREBP) that releases the mature transcription factor form, which regulates gene expression. Our aim was to identify the nature and intracellular site of the putative sterol-regulatory pool which regulates SREBP proteolysis in hamster liver. Cholesterol metabolism was modulated by feeding hamsters control chow, or a cholesterol-enriched diet, or by treatment with simvastatin or with the oral acyl-CoA:cholesterol acyltransferase inhibitor C1-1011 plus cholesterol. The effects of the different treatments on SREBP activation were confirmed by determination of the mRNAs for the low-density lipoprotein receptor and hydroxymethylglutaryl-CoA (HMG-CoA) reductase and by measurement of HMG-CoA reductase activity. The endoplasmic reticulum was isolated from livers and separated into subfractions by centrifugation in self-generating iodixanol gradients. Immunodetectable SREBP-2 accumulated in the smooth endoplasmic reticulum of cholesterol-fed animals. Cholesterol ester levels of the smooth endoplasmic reticulum membrane (but not the cholesterol levels) increased after cholesterol feeding and fell after treatment with simvastatin or C1-1011. The results suggest that an increased cellular cholesterol load causes accumulation of SREBP-2 in the smooth endoplasmic reticulum and, therefore, that membrane cholesterol ester may be one signal allowing exit of the SREBP-2/SREBP-cleavage-regulating protein complex to the Golgi.

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.

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.

Dietary fish oils modify the assembly of VLDL and expression of the LDL receptor in rabbit liver

Supplementation of the diet of rabbits with fish oil or sunflower oil resulted in significant changes in the lipoproteins and lipids in serum. Compared with chow-fed rabbits, dietary fish oils decreased very low density lipoprotein (VLDL), increased low density lipoprotein (LDL), and shifted the peak of the LDL to denser fractions, whereas sunflower oil increased high density lipoprotein and shifted LDL to the lighter fractions. The amount of LDL receptors in fish oil-fed rabbit liver decreased by > 70% while there was only a small fall in these levels in sunflower oil-fed rabbit liver. The concentrations of apolipoprotein (apo) B in the subcellular organelles of the secretory compartment (rough and smooth endoplasmic reticula and Golgi fractions) were also changed by dietary lipids. In both sunflower oil- and fish oil-fed liver, apo B was increased in the lumen of the rough endoplasmic reticulum compared with fractions from chow-fed rabbit liver. The apo B in the trans-Golgi lumen from fish oil-fed livers was reduced and occurred in particles of d approximately 1.21 g/mL. In contrast, apo B in the trans-Golgi lumen from livers of sunflower oil-fed rabbits was increased and occurred in particles of d < 1.21 g/mL. These results suggests that feeding of fish oils causes an interruption in the intracellular transfer of apo B and hence assembly of VLDL. This leads to an enrichment of the rough endoplasmic reticulum membranes with cholesterol, thus downregulating the expression of the LDL receptor.

S-phase DNA damage checkpoint in budding yeast

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

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

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

A novel method for the rapid separation of plasma lipoproteins using self-generating gradients of iodixanol

We describe a new method for the rapid fractionation of plasma lipoproteins, which makes use of a new non-ionic, iodinated, density gradient medium, iodixanol, commercially available as Optiprep(TM). The method is simple: plasma or serum is mixed with iodixanol followed by centrifugation in a vertical or near vertical rotor. Separation of VLDL, LDL and HDL can be achieved in 3 h and the lipoprotein fractions are comparable in density and composition with those prepared using conventional salt based gradients. Each class of lipoprotein can be removed in a single fraction, or a profile of lipoprotein distribution can be obtained using a gradient fractionator. Because the medium is inert, fractions from the gradient can be analysed by agarose gel electrophoresis or assayed for lipid content or apolipoprotein composition by SDS-PAGE without removing the iodixanol. Small differences in electrophoretic mobility of HDL and LDL across several gradient fractions suggest that subfractionation of these classes may occur. The new method is simple, rapid and versatile with potential application for preparation of lipoproteins and for analysis of lipoprotein profiles in the research or clinical laboratory.

Intracellular degradation in the regulation of secretion of apolipoprotein B-100 by rabbit hepatocytes

Isolated rabbit hepatocytes were incubated with [35S]methionine to label intracellular pools of apolipoprotein B (apo-B). The cells were then reincubated with an excess of unlabelled methionine in the presence of oleate or protease inhibitors and the intracellular sites of accumulation of radiolabelled apo-B and the mass of apo-B were determined by isolation and analysis of subcellular fractions. Oleate or inhibitors of metalloproteases (o-phenanthroline), serine proteases (aprotinin), serine/cysteine proteases (leupeptin) or cysteins proteases (calpain inhibitor I; ALLN) but not aspartate proteases (pepstatin) resulted in inhibition of the cellular degradation of apo-B. The effect of o-phenanthroline was reversed by the addition of zinc ions. Oleate, o-phenanthroline and leupeptin also stimulated secretion of radiolabelled apo-B; the effects of the inhibitors and oleate were additive, suggesting that they could act via different mechanisms. o-Phenanthroline caused accumulation of apo-B in the rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER) membranes; leupeptin caused accumulation of apo-B in the SER and cis-Golgi membranes, and ALLN and aprotinin caused accumulation of apo-B in the trans-Golgi membranes. These results suggest that intracellular degradation of apo-B occurs in the endoplasmic reticulum and in the trans-Golgi membranes and involves different proteases. Apo-B that accumulates in the ER membrane can be diverted into the lumen for secretion; however, apo-B that accumulates in the trans-Golgi membrane is irretrievably diverted from secretion.

Degradation of apolipoprotein B in cultured rat hepatocytes occurs in a post-endoplasmic reticulum compartment

The site of apolipoprotein B (apoB) degradation was investigated in cultured rat hepatocytes. Brefeldin A plus nocodazole completely blocked apoB degradation suggesting the involvement of a post-endoplasmic reticulum (ER) compartment. Monensin inhibited apoB degradation by 40% implying that a post-Golgi compartment could be involved in degradation of apoB. Ammonium chloride or chloroquine inhibited partially the degradation of apoB100 and apoB48, indicating some degradation in lysosomes, or in an acidic compartment such as trans-Golgi or endosomes. The degradations of apoB100 and apoB48 were blocked completely by (2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methylbutane ethyl ester (EST) during a chase of 90 min demonstrating that a cysteine protease was responsible for apoB degradation. Chymostatin, leupeptin, pepstatin, phenylmethylsulfonyl fluoride, and aprotinin had no significant effect on the degradation of apoB48. However, leupeptin and pepstatin decreased the degradation of apoB100 by 20-30%. Degradation of apoB100 and apoB48 occurred in isolated Golgi fractions with little degradation in heavy or light ER. Degradation of apoB in Golgi fractions was inhibited by EST and by preincubating hepatocytes with 10 nM dexamethasone. Immunofluorescent microscopy revealed that apoB accumulated in the Golgi region after EST treatment. It is concluded that a major part of apoB degradation in rat hepatocytes occurs in a post-ER compartment via the action of a cysteine protease that is regulated by glucocorticoids.

Intracellular events in the assembly of very-low-density-lipoprotein lipids with apolipoprotein B in isolated rabbit hepatocytes

Isolated rabbit hepatocytes incorporated [35S]methionine into cellular and secreted apolipoprotein B (apo-B), and [3H]glycerol into cellular and secreted triacylglycerol and phospholipids. Newly synthesized apo-B was incorporated into rough endoplasmic reticulum (RER), smooth endoplasmic reticulum (SER), cis-Golgi and trans-Golgi membranes and was preferentially transferred into the lumen of the RER with specific radioactivities ten times those in the membrane. Radiolabelled apo-B did not equilibrate with pre-existing unlabelled apo-B, and pools of different specific radioactivities were established in different subcellular fractions. Only a small fraction of the newly synthesized apo-B was transferred to the Golgi lumen. In pulse-chase experiments, most of the newly synthesized apo-B in the RER membrane and the RER lumen was degraded. [3H]Glycerol was incorporated into triacylglycerol and phospholipids in the lumen of the RER, SER, cis-Golgi and trans-Golgi. However, in contrast with apo-B, all of the radiolabelled lipids in the lumen of the RER, SER and cis-Golgi were transferred to the trans-Golgi lumen or secreted. Analysis of the lipid composition of the lumenal content fractions suggests that, although very-low-density-lipoprotein (VLDL) lipids are present in the endoplasmic reticulum lumen, a large fraction of these is not associated with apo-B. Collectively these observations suggest that assembly of apo-B into complete VLDL is not cotranslational, that most lipids become associated with apo-B late in the endoplasmic reticulum compartment and that the lipids are further modified in the Golgi lumen.

Murine mammary-derived cells secrete the N-terminal 41% of human apolipoprotein B on high density lipoprotein-sized lipoproteins containing a triacylglycerol-rich core

The cDNA encoding the N-terminal 41% of human apolipoprotein B (apoB), apoB-41, was transfected into nonhepatic, nonintestinal, mammary-derived mouse cells (C127) to generate stably transfected cells expressing human apoB-41 (C127B-41). As determined by centrifugation, apoB-41 is secreted exclusively on lipoproteins (LPs) having a peak density of 1.13 g/ml. Electron microscopy of apoB-41-containing LPs purified by immunoaffinity chromatography showed round particles about 12 nm in diameter. No discoidal particles were observed. Characterization of apoB-41-associated lipids after labeling C127B-41 cells with [3H]oleate and immunoprecipitating the secreted LPs with antibodies to apoB showed that 3H-labeled triacylglycerols were a major lipid class and accounted for about 54% of the total labeled lipids. Cholesterol esters and phospholipids accounted for about 6% and 22%, respectively. Incubation of cells with 0.4 mM oleate resulted in an increased incorporation of the added oleate into lipids associated with secreted apoB-41, along with a 2- to 3-fold increased secretion of apoB-41. The newly formed LPs appear to be transported through the Golgi complex, as brefeldin A (1 microgram/ml) and monensin (1 microM) greatly reduced (> 90%) the secretion of labeled apoB-41 and the amount of triacylglycerol and phospholipid associated with it. Microsomal triacylglycerol transfer protein (MTP) was not detected in these cells. Taken together, the data presented demonstrate that apoB-41 can direct the assembly and secretion of LPs that contain a triacylglycerol-rich core in nonhepatic cells that apparently lack MTP. These cells, therefore, represent an important model for studying LP assembly and may offer some advantages over cultured hepatic or intestinal cells that express their endogenous apoB gene.

Insulin regulation of triacylglycerol-rich lipoprotein synthesis and secretion

This review has considered a number of observations obtained from studies of insulin in perfused liver, hepatocytes, transformed liver cells and in vivo and each of the experimental systems offers advantages. The evaluation of insulin effects on component lipid synthesis suggests that overall, lipid synthesis is positively influenced by insulin. Short-term high levels of insulin through stimulation of intracellular degradation of freshly translated apo B and effects on synthesis limit the ability of hepatocytes to form and secrete TRL. The intracellular site of apo B degradation may involve membrane-bound apo B, cytoplasmic apo B and apo B which has entered the ER lumen. How insulin favors intracellular apo B degradation is not known. An area of recent investigation is in insulin-stimulated phosphorylation of intracellular substrates such as IRS-1 which activates insulin specific cellular signaling molecules [245]. Candidate molecules to study insulin action on apo B include IRS-1 and SH2-containing signaling molecules. Insulin dysregulation in carbohydrate metabolism occurs in non-insulin-dependent diabetes mellitus due to an imbalance between insulin sensitivity of tissue and pancreatic insulin secretion (reviewed in Refs. [307,308]). Insulin resistance in the liver results in the inability to suppress hepatic glucose production; in muscle, in impaired glucose uptake and oxidation and in adipose tissue, in the inability to suppress release of free FA. This lack of appropriate sensitivity towards insulin action leads to hyperglycemia which in turn stimulates compensatory insulin secretion by the pancreas leading to hyperinsulinemia. Ultimately, there may be failure of the pancreas to fully compensate, hyperglycemia worsens and diabetes develops. The etiology of insulin resistance is being intensively studied for the primary defect may be over secretion of insulin by the pancreas or tissue insulin resistance and both of these defects may be genetically predetermined. We suggest that, in addition to effects in carbohydrate metabolism, insulin resistance in liver results in the inability of first phase insulin to suppress hepatic TRL production which results in hypertriglyceridemia leading to high levels of plasma FA which accentuate insulin resistance in other target organs. As recently reviewed [17,254] the role of insulin as a stimulator of hepatic lipogenesis and TRL production has been long established. Several lines of evidence support that insulin is stimulatory to the production of hepatic TRL in vivo. First, population based studies support a positive relationship between plasma insulin and total TG and VLDL [253]. Second, there is a strong association between chronic hyperinsulinemia and VLDL overproduction [309].(ABSTRACT TRUNCATED AT 400 WORDS)