Oleate-mediated stimulation of apolipoprotein B secretion from rat hepatoma cells. A function of the ability of apolipoprotein B to direct lipoprotein assembly and escape presecretory degradation

Intracellular tPA-PAI-1 interaction determines VLDL assembly in hepatocytes

Apolipoprotein B (apoB)-lipoproteins initiate and promote atherosclerotic cardiovascular disease. Plasma tissue plasminogen activator (tPA) activity is negatively associated with atherogenic apoB-lipoprotein cholesterol levels in humans, but the mechanisms are unknown. We found that tPA, partially through the lysine-binding site on its Kringle 2 domain, binds to the N terminus of apoB, blocking the interaction between apoB and microsomal triglyceride transfer protein (MTP) in hepatocytes, thereby reducing very-low-density lipoprotein (VLDL) assembly and plasma apoB-lipoprotein cholesterol levels. Plasminogen activator inhibitor 1 (PAI-1) sequesters tPA away from apoB and increases VLDL assembly. Humans with PAI-1 deficiency have smaller VLDL particles and lower plasma levels of apoB-lipoprotein cholesterol. These results suggest a mechanism that fine-tunes VLDL assembly by intracellular interactions among tPA, PAI-1, and apoB in hepatocytes.

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

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

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.

Inhibition of apoB secretion from HepG2 cells by insulin is amplified by naringenin, independent of the insulin receptor

Hepatic overproduction of apolipoprotein B (apoB)-containing lipoproteins is characteristic of the dyslipidemia associated with insulin resistance. Recently, we demonstrated that the flavonoid naringenin, like insulin, decreased apoB secretion from HepG2 cells by activation of both the phosphoinositide-3-kinase (PI3-K) pathway and the mitogen-activated protein kinase/extracellular-regulated kinase (MAPK(erk)) pathway. In the present study, we determined whether naringenin-induced signaling required the insulin receptor (IR) and sensitized the cell to the effects of insulin, and whether the kinetics of apoB assembly and secretion in cells exposed to naringenin were similar to those of insulin. Immunoblot analysis revealed that insulin stimulated maximal phosphorylation of IR and IR substrate-1 after 10 min, whereas naringenin did not affect either at any time point up to 60 min. The combination of naringenin and submaximal concentrations of insulin potentiated extracellular-regulated kinase 1/2 activation and enhanced upregulation of the LDL receptor, downregulation of microsomal triglyceride transfer protein expression, and inhibition of apoB-100 secretion. Multicompartmental modeling of apoB pulse-chase studies revealed that attenuation of secreted radiolabeled apoB in naringenin- or insulin-treated cells was similar under lipoprotein-deficient or oleate-stimulated conditions. Naringenin and insulin both stimulated intracellular apoB degradation via a kinetically defined rapid pathway. Therefore, naringenin, like insulin, inhibits apoB secretion through activation of both PI3-K and MAPK(erk) signaling, resulting in similar kinetics of apoB secretion. However, the mechanism for naringenin-induced signaling is independent of the IR. Naringenin represents a possible strategy for reduction of hepatic apoB secretion, particularly in the setting of insulin resistance.

Both intestinal and hepatic lipoprotein production are stimulated by an acute elevation of plasma free fatty acids in humans

BACKGROUND

Hepatic lipoprotein production has been shown previously to be regulated by free fatty acid (FFA) flux to the liver, whereas intestinal lipoprotein production is stimulated mainly by ingested fat absorbed from the intestinal lumen. Emerging evidence indicates that intestinal lipoprotein production is increased in insulin resistance and type 2 diabetes mellitus, conditions that are associated with increased levels of circulating FFAs. Here we investigated whether short-term elevation of plasma FFAs stimulates intestinal apolipoprotein (apo) B-48- and hepatic apoB-100-containing triglyceride-rich lipoprotein (TRL) production in humans in the fed state.

METHODS AND RESULTS

TRL apoB-48 and apoB-100 metabolism were examined in 12 healthy men during a constant fed state. The studies were as follows, respectively: (1) Intralipid/heparin was infused intravenously immediately before and during the kinetics study to induce an approximately 3-fold difference in plasma FFA compared with the saline study; (2) saline was infused intravenously as a control. ApoB-48- and apoB-100-containing TRL production and clearance were determined with a 12-hour primed constant infusion of [D3]L-leucine and multicompartmental kinetic modeling. TRL apoB-48 production rate was 69% higher in the Intralipid/heparin study than in the saline control (5.95+/-1.13 versus 3.53+/-0.58 mg/kg per day; P=0.027), and there was no significant difference in TRL apoB-48 clearance. TRL apoB-100 concentrations were also increased (P<0.001) and TRL apoB-100 production rate was 35% higher in the Intralipid/heparin study compared with saline (28+/-4 versus 21+/-3 mg/kg per day; P=0.020).

CONCLUSIONS

This is the first study to demonstrate that intestinal TRL apoB-48 production is increased after short-term elevation of plasma FFAs in humans in the fed state, similar to the well-described stimulation of hepatic TRL apoB100-containing particles by FFAs.

Impact of 5 years of growth hormone replacement therapy on cardiovascular risk factors in growth hormone-deficient adults

The benefits of long-term effects of growth hormone (GH) substitution on carbohydrate and lipid metabolism in GH-deficient (GHD) adults are still controversial. The purpose of this study was to evaluate the effects of 5 years of GH substitution on body composition, glucose and lipid metabolism, and carotid artery intima-media thickness (IMT) in GHD adults. Fourteen patients were clinically assessed every 3 months for 5 years. Serum insulin-like growth factor 1 levels, lipid profile, oral glucose tolerance test, and ultrasonography of the carotid arteries were performed at baseline, 6 months, and every year during replacement. Visceral fat was measured by computed tomographic scan at baseline and at 6, 12, 24, and 60 months. The waist circumference was reduced after 6 months but increased during the next months toward baseline values. Visceral fat decreased during the study. Fasting glucose and insulin levels did not change, as well as the homeostasis model assessment of insulin resistance index. Despite an initial increase in frequency of abnormal glucose tolerance, mean 2-hour oral glucose tolerance test glucose levels decreased during the last 2 years. There was an increase in apolipoprotein A-1 levels during the treatment. Apolipoprotein B levels were reduced after 6 months and remained stable thereafter. A reduction in carotid artery IMT was observed during replacement. We concluded that 5 years of GH replacement therapy promoted positive effects on visceral fat, lipid profile, and carotid artery IMT in GHD adults. Long-term therapy improves insulin sensitivity through a reduction in visceral fat, and continuing monitoring is mandatory in terms of glucose metabolism.

Metabolic syndrome: clinical concept and molecular basis

The metabolic syndrome is a cluster of insulin resistance, elevated blood pressure, and atherogenic dyslipidemia and is a common basis of cardiovascular diseases (CVD). Although the precise mechanism remains to be elucidated, a practical definition is needed. A worldwide definition that considers increased waist circumference as an essential component has been settled. Visceral fat locates upstream of the liver. Free fatty acids and glycerol derived from visceral fat reach the liver and stimulate lipoprotein synthesis and gluconeogenesis, respectively. The adipose tissue produces a variety of bioactive substances conceptualized as 'adipocytokines'. Overproduction of plasminogen activator inhibitor-1 and tumor necrosis factor- seems to relate to the thrombotic and inflammatory tendency. On the other hand, adiponectin, which has antiatherogenic and antidiabetic activities, is reduced in subjects with metabolic syndrome. In Japan, the waist circumference criterion based on visceral fat accumulation has been adopted. The concept of this syndrome has been widely publicized, and health promotion programs based on the concept have commenced in various areas of the country. Such 'Adipo-Do-It' movement is an incentive to encourage physical exercise to reduce visceral fat and is a big challenge to prevent life-style-related diseases and CVD.

Lipid and lipoprotein dysregulation in insulin resistant states

Insulin resistant states are commonly associated with an atherogenic dyslipidemia that contributes to significantly higher risk of atherosclerosis and cardiovascular disease. Indeed, disorders of carbohydrate and lipid metabolism co-exist in the majority of subjects with the "metabolic syndrome" and form the basis for the definition and diagnosis of this complex syndrome. The most fundamental defect in these patients is resistance to cellular actions of insulin, particularly resistance to insulin-stimulated glucose uptake. Insulin insensitivity appears to cause hyperinsulinemia, enhanced hepatic gluconeogenesis and glucose output, reduced suppression of lipolysis in adipose tissue leading to a high free fatty acid flux, and increased hepatic very low density lipoprotein (VLDL) secretion causing hypertriglyceridemia and reduced plasma levels of high density lipoprotein (HDL) cholesterol. Although the link between insulin resistance and dysregulation of lipoprotein metabolism is well established, a significant gap of knowledge exists regarding the underlying cellular and molecular mechanisms. Emerging evidence suggests that insulin resistance and its associated metabolic dyslipidemia result from perturbations in key molecules of the insulin signaling pathway, including overexpression of key phosphatases, downregulation and/or activation of key protein kinase cascades, leading to a state of mixed hepatic insulin resistance and sensitivity. These signaling changes in turn cause an increased expression of sterol regulatory element binding protein (SREBP) 1c, induction of de novo lipogensis and higher activity of microsomal triglyceride transfer protein (MTP), which together with high exogenous free fatty acid (FFA) flux collectively stimulate the hepatic production of apolipoprotein B (apoB)-containing VLDL particles. VLDL overproduction underlies the high triglyceride/low HDL-cholesterol lipid profile commonly observed in insulin resistant subjects.

Mutation of F417 but not of L418 or L420 in the lipid binding domain decreases the activity of triacylglycerol hydrolase

Human triacylglycerol hydrolase (hTGH) has been shown to play a role in hepatic lipid metabolism. Triacylglycerol hydrolase (TGH) hydrolyzes insoluble carboxylic esters at lipid/water interfaces, although the mechanism by which the enzyme adsorbs to lipid droplets is unclear. Three-dimensional modeling of hTGH predicts that catalytic residues are adjacent to an alpha-helix that may mediate TGH/lipid interaction. The helix contains a putative neutral lipid binding domain consisting of the octapeptide FLDLIADV (amino acid residues 417-424) with the consensus sequence FLXLXXXn (where n is a nonpolar residue and X is any amino acid except proline) identified in several other proteins that bind or metabolize neutral lipids. Deletion of this alpha-helix abolished the lipolytic activity of hTGH. Replacement of F417 with alanine reduced activity by 40% toward both insoluble and soluble esters, whereas replacement of L418 and L420 with alanine did not. Another potential mechanism of increasing TGH affinity for lipid is via reversible acylation. Molecular modeling predicts that C390 is available for covalent acylation. However, neither chemical modification of C390 nor mutation to alanine affected activity. Our findings indicate that F417 but not L418, L420, or C390 participates in substrate hydrolysis by hTGH.

Insulin, glucagon and fatty acid treatment of hepatocytes does not result in phosphorylation or changes in activity of triacylglycerol hydrolase

It is recognized that the majority of very low density lipoprotein (VLDL) associated triacylglycerol (TG) is synthesized from fatty acids and partial acylglycerols generated by lipolysis of intra-hepatic storage rather than made de novo. Triacylglycerol hydrolase (TGH) is involved in mobilizing stored TG. Modulating the ability of TGH to hydrolyze stored lipids represents a potentially regulated and rate limiting step in VLDL assembly. Phosphorylation of lipases and carboxylesterases trigger diverse but functionally significant events. We explored the potential for regulating the mobilization of hepatic TG through phosphorylation of TGH. Insulin is known to suppress VLDL secretion from liver, and glucagon can be considered an opposing hormone. However, neither insulin nor glucagon treatment of hepatocytes led to phosphorylation of TGH or changes in its activity. Augmenting intracellular TG stores by incubations with oleic acid also did not lead to changes in TGH activity. Therefore, changes in phosphorylation state are not a mechanism for regulating TGH activity, access to TG substrate pools or for TGH-mediated contributions to VLDL assembly and secretion.

Triacylglycerol hydrolase is localized to the endoplasmic reticulum by an unusual retrieval sequence where it participates in VLDL assembly without utilizing VLDL lipids as substrates

The majority of hepatic intracellular triacylglycerol (TG) is mobilized by lipolysis followed by reesterification to reassemble TG before incorporation into a very-low-density lipoprotein (VLDL) particle. Triacylglycerol hydrolase (TGH) is a lipase that hydrolyzes TG within hepatocytes. Immunogold electron microscopy in transfected cells revealed a disparate distribution of this enzyme within the endoplasmic reticulum (ER), with particularly intense localization in regions surrounding mitochondria. TGH is localized to the lumen of the ER by the C-terminal tetrapeptide sequence HIEL functioning as an ER retention signal. Deletion of HIEL resulted in secretion of catalytically active TGH. Mutation of HIEL to KDEL, which is the consensus ER retrieval sequence in animal cells, also resulted in ER retention and conservation of lipolytic activity. However, KDEL-TGH was not as efficient at mobilizing lipids for VLDL secretion and exhibited an altered distribution within the ER. TGH is a glycoprotein, but glycosylation is not required for catalytic activity. TGH does not hydrolyze apolipoprotein B-associated lipids. This suggests a mechanism for vectored movement of TGs onto developing VLDL in the ER as TGH may mobilize TG for VLDL assembly, but will not access this lipid once it is associated with VLDL.

Intestinal lipoprotein production is stimulated by an acute elevation of plasma free fatty acids in the fasting state: studies in insulin-resistant and insulin-sensitized Syrian golden hamsters

It is not known whether intestinal lipoprotein production is stimulated by an acute elevation of plasma free fatty acids (FFA). We examined the effect of an intralipid and heparin infusion on the intestinal lipoprotein production rate (PR) in insulin-sensitive [chow-fed (CHOW)], insulin-resistant [60% fructose (FRUC) or 60% fat-fed (FAT)], and insulin-sensitized [FRUC or FAT plus rosiglitazone (RSG)-treated] Syrian Golden hamsters. After 5 wk of treatment, overnight-fasted hamsters underwent in vivo Triton WR-1339 studies for measurement of apolipoprotein B48 (apoB48) PR in large (Svedberg unit, >400) and small (Svedberg unit, 100-400) lipoprotein fractions, with an antecedent 90-min infusion of 20% intralipid and heparin (IH) to raise plasma FFA levels approximately 5- to 8-fold vs. those in the saline control study. IH markedly increased apoB48 PR in CHOW by 3- to 5-fold, which was confirmed ex vivo in pulse-chase experiments in primary cultured hamster enterocytes. Oleate, but not glycerol, infusion was associated with a similar elevation of apoB48 PR as IH. In FRUC and FAT, basal (saline control) apoB48 PR was approximately 4-fold greater than that in CHOW; there was no additional stimulation with IH in vivo and only minimal additional stimulation ex vivo. RSG partially normalized basal apoB48 PR in FAT and FRUC, and PR was markedly stimulated with IH. We conclude that intestinal lipoprotein production is markedly stimulated by an acute elevation of plasma FFAs in insulin-sensitive hamsters, in which basal production is low, but minimally in insulin-resistant hamsters, in which basal production is already elevated. With RSG treatment, basal PR is partially normalized, and they become more susceptible to the acute FFA stimulatory effect.

Regulation of Hepatic Apolipoprotein B-lipoprotein Assembly and Secretion by the Availability of Fatty Acids

The in vivo effects of increased delivery of fatty acids (FA) to the liver are poorly defined. Therefore, we compared the effects of infusing either 6 mM oleic acid (OA) bound to albumin, 0.5-20% Intralipid, or saline for 3 or 6 h into male C57BL/6J mice. Infusions were followed by studies of triglyceride (TG) and apoB secretion. Although plasma FA levels increased similarly after either 20% Intralipid or 6 mM OA, TG secretion increased only after infusion of 4-20% Intralipid; TG secretion was unchanged by 6 mM OA. By contrast, 6-h infusions of either 6 mM OA or 4-20% Intralipid increased apoB secretion. 6 mM OA and 20% Intralipid each increased secretion of apoB from primary hepatocytes ex vivo. Importantly, 0.5-2% Intralipid, which delivered more FA to the liver than 6 mM OA, did not stimulate apoB secretion. Hepatic apoB mRNA levels were unaffected by either 6 mM OA or 20% Intralipid, but microsomal triglyceride transfer protein mRNA was significantly lower after 6-h infusions with 6 mM OA versus either saline or 20% Intralipid. Lower microsomal triglyceride transfer protein mRNA levels were associated with reduced hepatic TG mass after 6-h infusions of 6 mM OA. We conclude that 1) increased FA delivery to the liver in vivo increases secretion of apoB-lipoproteins via post-transcriptional mechanisms, 2) OA-induced apoB-lipoprotein secretion occurred at least in part via mechanisms other than by providing substrate for TG synthesis, and 3) the route of delivery of FA is important for its effects on apoB secretion.

Microsomal triglyceride transfer protein: a protein complex required for the assembly of lipoprotein particles

The mechanism of assembly of lipoprotein particles in the lumen of the endoplasmic reticulum is an important but poorly understood biological problem. A knowledge of this process is of great practical importance because possession of elevated levels of lipoproteins is one of the major risk factors for the development of atherosclerosis. This review describes a major advance in the delineation of the mechanisms involved in the assembly and secretion of apolipoprotein-B-containing lipoproteins: the demonstration of a requirement for microsomal triglyceride transfer protein.

Myristic acid increases dense lipoprotein secretion by inhibiting apoB degradation and triglyceride recruitment

Fatty acids of varying lengths and saturation differentially affect plasma apolipoprotein B-100 (apoB-100) levels. To identify mechanisms at the level of production, rat hepatoma cells, McA-RH7777, were incubated with [(35)S]methionine and either fatty acid-BSA complexes or BSA alone. There were increases in labeled apoB-100 secretion with saturated fatty acids palmitic and myristic (MA) (153 +/- 20% and 165 +/- 11%, respectively, relative to BSA). Incubation with polyunsaturated docosahexaenoic acid (DHA) decreased secretion to 26 +/- 2.0%, while monounsaturated oleic acid (OA) did not change it. In pulse-chase studies, MA treatment resulted in reduced apoB-100 degradation, in agreement with its promotion of secretion. In triglyceride (TG) studies, synthesis was stimulated equally by OA, MA, and DHA, but TG secretion was relatively decreased with MA and DHA. With OA, the majority of newly secreted apoB100-lipoproteins was d < or = 1.006, but with MA, they were much denser (1.063 < d). Furthermore, the relative recruitment of newly synthesized TG to lipoproteins was impaired with MA. We conclude that mechanisms for effects of specific dietary fatty acids on plasma lipoprotein levels may include changes in hepatic production. In turn, hepatic production may be regulated by specific fatty acids at the steps of apoB-100 degradation and the recruitment of nascent TG to lipoprotein 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.

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.

Disordered fat storage and mobilization in the pathogenesis of insulin resistance and type 2 diabetes

The primary genetic, environmental, and metabolic factors responsible for causing insulin resistance and pancreatic beta-cell failure and the precise sequence of events leading to the development of type 2 diabetes are not yet fully understood. Abnormalities of triglyceride storage and lipolysis in insulin-sensitive tissues are an early manifestation of conditions characterized by insulin resistance and are detectable before the development of postprandial or fasting hyperglycemia. Increased free fatty acid (FFA) flux from adipose tissue to nonadipose tissue, resulting from abnormalities of fat metabolism, participates in and amplifies many of the fundamental metabolic derangements that are characteristic of the insulin resistance syndrome and type 2 diabetes. It is also likely to play an important role in the progression from normal glucose tolerance to fasting hyperglycemia and conversion to frank type 2 diabetes in insulin resistant individuals. Adverse metabolic consequences of increased FFA flux, to be discussed in this review, are extremely wide ranging and include, but are not limited to: 1) dyslipidemia and hepatic steatosis, 2) impaired glucose metabolism and insulin sensitivity in muscle and liver, 3) diminished insulin clearance, aggravating peripheral tissue hyperinsulinemia, and 4) impaired pancreatic beta-cell function. The precise biochemical mechanisms whereby fatty acids and cytosolic triglycerides exert their effects remain poorly understood. Recent studies, however, suggest that the sequence of events may be the following: in states of positive net energy balance, triglyceride accumulation in "fat-buffering" adipose tissue is limited by the development of adipose tissue insulin resistance. This results in diversion of energy substrates to nonadipose tissue, which in turn leads to a complex array of metabolic abnormalities characteristic of insulin-resistant states and type 2 diabetes. Recent evidence suggests that some of the biochemical mechanisms whereby glucose and fat exert adverse effects in insulin-sensitive and insulin-producing tissues are shared, thus implicating a diabetogenic role for energy excess as a whole. Although there is now evidence that weight loss through reduction of caloric intake and increase in physical activity can prevent the development of diabetes, it remains an open question as to whether specific modulation of fat metabolism will result in improvement in some or all of the above metabolic derangements or will prevent progression from insulin resistance syndrome to type 2 diabetes.

Post-transcriptional stimulation of the assembly and secretion of triglyceride-rich apolipoprotein B lipoproteins in a mouse with selective deficiency of brown adipose tissue, obesity, and insulin resistance

A mouse model of insulin resistance and its associated dyslipidemia was generated by crossing mice expressing human apolipoprotein B (apoB) with mice lacking only brown adipose tissue (BATless). On a high fat diet, male apoB/BATless mice became obese, hypercholesterolemic, hypertriglyceridemic, and hyperinsulinemic compared with control apoB mice. Fast performance liquid chromatography revealed increased triglyceride concentrations in intermediate density lipoprotein/low density lipoprotein (LDL) and reduced high density lipoprotein cholesterol concentrations. Inhibition of lipolysis by the drug, tetrahydrolipostatin, demonstrated that very low density lipoprotein-sized particles were initially secreted. Metabolic studies employing Triton WR-1339 and either [(3)H]glycerol or [(3)H]palmitate showed that the hypertriglyceridemia in apoB/BATless mice was due to the increased synthesis and secretion of triglyceride. Furthermore, lipoprotein lipase and hepatic lipase activities were not defective. ApoB was also secreted at increased rates in the apoB/BATless mice. Similar levels of apoB mRNA in apoB and apoB/BATless mice indicated that apoB secretion was regulated post-transcriptionally. LDL receptor mRNA was increased in the apoB/BATless mice, indicating that the observed increase in apoB-lipoprotein secretion was not due to their decreased reuptake. Finally, mRNA levels of the large subunit of microsomal triglyceride transfer protein, a required component for very low density protein assembly, were not different between apoB and apoB/BATless mice. This rodent model should prove useful in exploring mechanisms underlying the regulation of apoB secretion in the context of insulin resistance.

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.

Variation in the human ApoB signal peptide modulates ApoB17 translocation

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

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

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

A common apolipoprotein B signal peptide polymorphism modifies the relation between plasma non-esterified fatty acids and triglyceride concentration in men

Insulin and non-esterified fatty acids (NEFA) are important regulators of triglyceride metabolism. The relations between these compounds and the effect of a common 3 amino acid deletion in the apolipoprotein B (ApoB) signal peptide (SP) following an oral glucose challenge have been investigated. The frequency of the shorter SP-24 allele was 32% (95% C.I. 29.5-36.5) in 725 subjects undergoing an oral glucose tolerance test (OGTT). Fasting plasma triglyceride concentration was positively correlated with fasting plasma insulin concentration and negatively with the degree of plasma NEFA suppression following the glucose challenge. Linear regression analysis showed the relation between triglyceride concentration and NEFA suppression, but not the relation between triglyceride concentration and fasting insulin, to be altered by the SP polymorphism in men but not in women. The strength of the association was dependent on the number of SP-24 alleles, with SP-24 homozygotes showing the greatest dependence (men P=0.031, women P=0. 914). It was proposed that the complex regulation of very low density lipoprotein (VLDL) output by NEFA and by insulin may explain, at least in part, the conflicting reports concerning the presence of the ApoB SP polymorphism, fasting serum lipids and ischaemic heart disease (IHD).

A deficiency of microsomal triglyceride transfer protein reduces apolipoprotein B secretion

Microsomal triglyceride transfer protein (MTP) transfers lipids to apolipoprotein B (apoB) within the endoplasmic reticulum, a process that involves direct interactions between apoB and the large subunit of MTP. Recent studies with heterozygous MTP knockout mice have suggested that half-normal levels of MTP in the liver reduce apoB secretion. We hypothesized that reduced apoB secretion in the setting of half-normal MTP levels might be caused by a reduced MTP:apoB ratio in the endoplasmic reticulum, which would reduce the number of apoB-MTP interactions. If this hypothesis were true, half-normal levels of MTP might have little impact on lipoprotein secretion in the setting of half-normal levels of apoB synthesis (since the ratio of MTP to apoB would not be abnormally low) and might cause an exaggerated reduction in lipoprotein secretion in the setting of apoB overexpression (since the MTP:apoB ratio would be even lower). To test this hypothesis, we examined the effects of heterozygous MTP deficiency on apoB metabolism in the setting of normal levels of apoB synthesis, half-normal levels of apoB synthesis (heterozygous Apob deficiency), and increased levels of apoB synthesis (transgenic overexpression of human apoB). Contrary to our expectations, half-normal levels of MTP reduced the plasma apoB100 levels to the same extent ( approximately 25-35%) at each level of apoB synthesis. In addition, apoB secretion from primary hepatocytes was reduced to a comparable extent at each level of apoB synthesis. Thus, these results indicate that the concentration of MTP within the endoplasmic reticulum rather than the MTP:apoB ratio is the critical determinant of lipoprotein secretion. Finally, we found that heterozygosity for an apoB knockout mutation lowered plasma apoB100 levels more than heterozygosity for an MTP knockout allele. Consistent with that result, hepatic triglyceride accumulation was greater in heterozygous apoB knockout mice than in heterozygous MTP knockout mice.

Caco-2 cells secrete two independent classes of lipoproteins with distinct density: effect of the ratio of unsaturated to saturated fatty acid

Polarized Caco-2 cells can synthesize two distinct density classes of lipoproteins, i.e. chylomicron/VLDL (d<1.006 g/ml) or IDL/LDL density (1.009<d<1.068 g/ml). When saturated fatty acid in the incubation medium is replaced with unsaturated fatty acid, this results in an increase in the basolateral secretion of triglycerides from 18.6+/-3.6 nmol/filter (with 0.5 mmol/l 16:0) to 21.4+/-6.2, 27. 5+/-4.8 and 28.9+/-5.3 nmol/filter when 10, 20 or 30% of 16:0 were substituted by 18:1. The secretion of IDL/LDL-sized lipoproteins diminished and chylomicron/VLDL secretion increased in proportion to the increase of unsaturated fatty acid in the medium. To gain insight into the relationship between these lipoprotein classes, we determined their secretion at several time intervals (0-4, 4-8 and 22-26 h) after incubation with a fatty acid mixture containing 16:0 and 18:1 in a 9:1 molar ratio (total fatty acid concentration was 0.5 mmol/l). Chylomicron/VLDL secretion was detectable immediately upon the start of the incubation and persisted during all intervals. In contrast, IDL/LDL density lipoproteins were first detectable in the 4-8 h time interval and their secretion was highest in the final phase of the incubation (22-26 h). We conclude that Caco-2 cells secrete two distinct density classes of lipoproteins that show no precursor-product relation.

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.

Human apolipoprotein (Apo) B-48 and ApoB-100 kinetics with stable isotopes

The kinetics of apolipoprotein (apo) B-100 and apoB-48 within triglyceride-rich lipoproteins (TRLs) and of apoB-100 within IDL and LDL were examined with a primed-constant infusion of (5,5,5-(2)H(3)) leucine in the fed state (hourly feeding) in 19 subjects after consumption of an average American diet (36% fat). Lipoproteins were isolated by ultracentrifugation and apolipoproteins by SDS gels, and isotope enrichment was assessed by gas chromatography/mass spectrometry. Kinetic parameters were calculated by multicompartmental modeling of the data with SAAM II. The pool sizes (PS) of TRL apoB-48, VLDL apoB-100, and LDL apoB-100 were 17+/-10, 273+/-167, and 3325+/-1146 mg, respectively. There was a trend toward a faster fractional catabolic rate (FCR) for VLDL apoB-100 than for TRL apoB-48 (6.73+/-3.48 versus 5.02+/-2.07 pools/d, respectively, P=0.06). The mean FCRs for IDL and LDL apoB-100 were 10.07+/-7.28 and 0.27+/-0.08 pools/d, respectively. The mean production rate (PR) of TRL apoB-48 was 6.5% of VLDL apoB-100 (1. 3+/-0.90 versus 20.06+/-6.53 mg. kg(-1). d(-1), P<0.0001). TRL apoB-48 PS was correlated with apoB-48 PR (r=0.780, P<0.0001) but not FCR (r=-0.1810, P=0.458). VLDL apoB-100 PS was correlated with both PR (r=0.713, P=0.0006) and FCR (r=-0.692, P=0.001) of VLDL apoB-100 and by apoB-48 PR (r=0.728, P=0.0004). LDL apoB-100 PS was correlated with FCR (r=-0.549, P=0.015). These data indicate that (1) the FCRs of TRL apoB-48 and VLDL apoB-100 are similar in the fed state, (2) TRL apoB-48 PS is correlated with TRL apoB-48 PR, (3) VLDL apoB-100 PS is correlated with both PR and FCR of VLDL apoB-100 and PR of TRL apoB-48, and (4) LDL apoB-100 PS is correlated with LDL FCR.

NK-104, a potent 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, decreases apolipoprotein B-100 secretion from Hep G2 cells

Intracellular cholesterol biosynthesis may play a key role in supplying cholesterol (as cholesteryl ester) for the neutral core of very low density lipoprotein (VLDL), thus modulating the secretion of apolipoprotein B-100 (apo B-100) from hepatocytes. The effect of compound NK-104 was studied, a new competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA-reductase), on apo B-100 synthesis and secretion from the human hepatoma cell line Hep G2. Cells were preincubated with NK-104 (0.01-5 microM) in the presence or absence of oleate (0.8 mM). Apo B-100 in the medium was determined by an enzyme-linked immunosorbent assay (ELISA). Incubation of Hep G2 with NK-104 resulted in a marked inhibition of cholesterogenesis (up to 95%), determined as incorporation of [14C]acetate into sterols, and decreased in a dose-dependent manner apo B-100 secretion, both in basal conditions (from 110 to 82 ng/mg cell protein, P < 0.01) and after incubation with oleate (from 227 to 165 ng/mg cell protein, P < 0.01). Density gradient for distribution of apo B-100 secreted, showed that this decrease was essentially due to a reduction of apo B-100 associated with lipoproteins in the density range of low density lipoproteins (LDL). Pulse chase experiment demonstrated that NK-104 did not affect the synthetic rate of apo B-100 but increased intracellular degradation of newly synthesized protein. The compound had only marginal effect on the mass of intracellular triglyceride but significantly decreased intracellular mass of free cholesterol and cholesteryl ester (P < 0.01). It is speculated that the ability of compound NK-104 to decrease apo B-100 secretion from Hep G2 cells is due to a decreased intracellular cholesterol availability.

Regulation of HepG2 cell apolipoprotein B metabolism by the citrus flavanones hesperetin and naringenin

Our previous studies showed that replacing the drinking water of rabbits fed a casein-containing diet with either orange juice or grapefruit juice reduced serum low density lipoprotein cholesterol and hepatic cholesteryl ester concentrations. To determine whether the changes observed in rabbits were due to flavonoids present in the juices acting directly on the liver, the effects of hesperetin and naringenin on net apolipoprotein B (apoB) secretion by HepG2 cells were investigated. These flavanones dose-dependently reduced net apoB secretion by up to 81% after a 24 h incubation, while doses of 60 micrograms/mL reduced net apoB secretion by 50% after 4 h. Coincubation with the proteasome inhibitor, MG-132, did not alter the ability of the flavonoids to reduce net apoB secretion over 4 h, suggesting that the flavonoid-induced changes in apoB metabolism were not due to a direct increase in proteasomal activity. However, the flavonoids were unable to reduce net apoB secretion after 4 h in the presence of oleate, suggesting that these compounds may interfere with the availability of neutral lipids for lipoprotein assembly. Furthermore, our 14C-acetate-labeling studies showed a 50% reduction in cholesteryl ester synthesis in the presence of either flavonoid, which could account for the reduction in net apoB secretion caused by incubation with these compounds. These in vitro studies suggest that hesperetin and naringenin may, in part, reduce net apoB secretion by HepG2 cells by inhibiting cholesteryl ester synthesis and that these compounds are good candidates for further in vivo studies to determine whether they are responsible for the cholesterol-lowering properties of dietary citrus juices.

Role of the enterohepatic circulation of bile salts in lipoprotein metabolism

The enterohepatic circulation of bile salts and cholesterol plays a central role in maintaining whole body cholesterol homeostasis. Hepatic lipoprotein metabolism is reviewed and the role of disturbances in bile salt metabolism in the pathogenesis of dyslipidemias is discussed. Further, the manipulation of bile salt metabolism to treat dyslipidemia is reviewed.

Ubiquitin-proteasome-dependent degradation of apolipoprotein B100 in vitro

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

The structure of vitellogenin provides a molecular model for the assembly and secretion of atherogenic lipoproteins

The assembly of atherogenic lipoproteins requires the formation in the endoplasmic reticulum of a complex between apolipoprotein (apo)B, a microsomal triglyceride transfer protein (MTP) and protein disulphide isomerase (PDI). Here we show by molecular modelling and mutagenesis that the globular amino-terminal regions of apoB and MTP are closely related in structure to the ancient egg yolk storage protein, vitellogenin (VTG). In the MTP complex, conserved structural motifs that form the reciprocal homodimerization interfaces in VTG are re-utilized by MTP to form a stable heterodimer with PDI, which anchors MTP at the site of apoB translocation, and to associate with apoB and initiate lipid transfer. The structural and functional evolution of the VTGs provides a unifying scheme for the invertebrate origins of the major vertebrate lipid transport system.

Cholesterol synthesis is increased in mixed hyperlipidaemia

We showed previously that hypertriglyceridaemia, but not hypercholesterolaemia, is correlated with increases in cholesterol synthesis and apolipoprotein B secretion in patients with secondary hypertriglyceridaemia. The aim of the present study was to compare the rate of cholesterol synthesis, using fasting plasma mevalonic acid (MVA) as an index, in patients with primary mixed hyperlipidaemia (type IIb phenotype, n=45) and primary hypercholesterolaemia (type IIa phenotype, n=92). LDL cholesterol was significantly higher in types IIa (6.38+/-0.18 mmol/l) and IIb (5.89+/-0.25 mmol/l) compared to 40 normolipidaemic controls (2. 99+/-0.1 mmol/l, P<0.0001), whereas serum triglyceride was higher in type IIb (2.62 (range 2.2-3.0) mmol/l) than type IIa (1.22 (range 0. 85-1.60) mmol/l, P<0.001) and controls (0.90 (range 0.68-1.24) mmol/l, P<0.001). Similarly, MVA was higher in type IIb (7.0+/-0.46 ng/ml) than IIa (5.6+/-0.23 ng/ml, P<0.0) and controls (5.6+/-0.36 ng/ml, P<0.05). Plasma MVA correlated positively with serum triglyceride (r=0.22, P=0.004) and negatively with LDL cholesterol (r=-0.21, P=0.014). These results are in accordance with previous observations that VLDL-apolipoprotein B secretion and cholesterol synthesis are linked and demonstrate that the latter is increased in mixed hyperlipidaemia.

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.

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

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

Effects of atorvastatin on the intracellular stability and secretion of apolipoprotein B in HepG2 cells

We investigated the effects of atorvastatin, a new 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, on the biogenesis of apolipoprotein B (apoB) in intact and permeabilized HepG2 cells. Intact cells were pretreated either with single or multiple doses of atorvastatin (0.1 to 20 micromol/L) for periods of 6 to 20 hours and pulsed with [35S]methionine. In some cases the cells were permeabilized with digitonin. Experiments were performed to investigate the effects of atorvastatin on (1) the rates of lipid synthesis and secretion, (2) the synthesis and accumulation of apoB, (3) the intracellular stability of apoB, (4) the amount of apoB-containing lipoprotein particles assembled in HepG2 microsomes, and (5) the secretion and accumulation of apoB into the culture medium. ApoB synthesis, degradation, and secretion were measured by pulse-chase experiments with [35S]methionine in both intact and permeabilized HepG2 cells. Lipid synthesis was assessed by pulse-labeling experiments with [3H]acetate or [3H]oleate bound to bovine serum albumin. Comparisons were made under basal conditions and in the presence of oleate (0.36 micromol/L). Atorvastatin acutely inhibited the synthesis of cholesterol and cholesterol ester but did not have a significant effect on triglyceride or phospholipid synthesis. Atorvastatin did not affect the uptake of [35S]methionine by the cells nor did it influence the synthesis of apoB or a control protein, albumin. However, atorvastatin reduced the secretion of apoB into the culture medium, apparently by enhancing the degradation of apoB in the cell under basal and induced conditions with oleate. The stability of apoB associated with the lipoprotein particles was also significantly lowered by atorvastatin. The stimulated degradation of apoB in atorvastatin-treated cells was sensitive to MG132, a proteasome inhibitor. The net effect of atorvastatin was a reduction in the number of apoB-containing lipoprotein particles of different sizes isolated from microsomes and a reduction in apoB secretion into the culture medium. The data suggest that atorvastatin may impair the translocation of apoB into the lumen of the endoplasmic reticulum, thus increasing the amount of apoB degraded intracellularly. It is hypothesized that atorvastatin alters these parameters primarily as a result of inhibiting cholesterol synthesis and limiting the availability of cholesterol and/or cholesterol ester for the normal assembly of apoB-containing lipoprotein particles.

Lysophosphatidylcholine increases apolipoprotein B secretion by enhancing lipid synthesis and decreasing its intracellular degradation in HepG2 cells

Free fatty acids and lysophosphatidylcholine (lysoPC) are the major lipids bound to human plasma albumin. The effects of fatty acids on the hepatic production of Apolipoprotein B (apo B) have been studied but those of lysoPC have not. In HepG2 cells, lysoPC increased apo B secretion in different experiments by 50-120%, but did not affect the flotation properties of secreted lipoproteins. LysoPC affected neither the cellular protein levels nor apo A-I secretion suggesting that its effect was specific to apo B. Apo B secretion was maximum after incubating cells for 6 h with 0.2 mM lysoPC as equimolar fatty acid free bovine serum albumin (BSA) complexes. LysoPC was metabolized by cells and its fatty acids were used for the synthesis of phosphatidylcholine and triglycerides (TG). Experiments were performed to understand the mechanism of lysoPC action. LysoPC increased the incorporation of 3H-glycerol into newly synthesized cellular (3-fold) and secreted (4-fold) triglycerides, and increased the synthesis (40%) and secretion (4-fold) of phospholipids. LysoPC did not affect apo B synthesis, but inhibited the intracellular degradation of apo B and increased its secretion. Triacsin C (5 microM), an inhibitor of long chain acyl-CoA synthase, completely inhibited the induction of lipid synthesis and abolished the effect of lysoPC on apo B secretion. These studies indicated that lysoPC increased apo B secretion by inducing lipid synthesis; newly synthesized lipids probably protected apo B from intracellular degradation and enhanced secretion. These studies are consistent with the hypothesis that physiologic concentrations of lysoPC can be an important modulator for hepatic apo B secretion.