The active synthesis of phosphatidylcholine is required for very low density lipoprotein secretion from rat hepatocytes

Level of apolipoprotein B mRNA has an important effect of the synthesis and secretion of apolipoprotein B-containing lipoproteins. Studies on transfected hepatoma cell lines expressing recombinant human apolipoprotein B

The effect of apoB mRNA level on hepatic apoB production has not been studied extensively, primarily because the steady state level of apoB mRNA cannot be altered on a short-term basis. We studied the effect of vastly different apoB mRNA levels on the synthesis and secretion of apoB-containing lipoproteins using rat hepatoma (McA-RH7777) cell lines transfected with cDNA constructs encoding human apoB53 (the amino-terminal 53% of the protein; hapoB53) or apoB100 (hapoB100). Among the three hapoB53-transfected cell lines, the relative steady state levels of the hapoB53 mRNA were 10:2.5: < 0.1. Correspondingly, the relative concentration of the intracellular hapoB53 protein was 8:3:1 and of the medium hapoB53 (accumulated over a period of 18 hours) was 12:4:1, which positively correlates with the hapoB53 (d = 1.06 to 1.21 g/mL) or endogenous rat apoB100 (d < 1.06 g/mL). When cell lines containing high or intermediate hapoB53 mRNA levels were compared, there was an eightfold increase in the synthesis and a twofold increase in the secretion efficiency of hapoB53. Analysis of the synthesis and secretion of lipids revealed that in cells producing high levels of hapoB53, triglyceride synthesis (twofold) and secretion (twofold to threefold) were also increased. Furthermore, with the three hapoB100-transfected cells we also observed an increase in apoB100 synthesis (three-fold), apoB100 secretion efficiency (twofold), triglyceride synthesis (fourfold to fivefold), and triglyceride secretion (fourfold to fivefold) in the cells expressing high levels of hapoB100. In all the cell lines examined, secretion efficiency of endogenous rat apoA-I was not affected by transfection. Together these data suggest that secretion of apoB-containing triglyceride-rich lipoproteins can be influenced by the level of apoB mRNA or the rate of apoB translation.

Nascent VLDL phospholipid composition is altered when phosphatidylcholine biosynthesis is inhibited: evidence for a novel mechanism that regulates VLDL secretion

Previous work has shown that inhibition of phosphatidylcholine biosynthesis inhibits very low density lipoprotein (VLDL) secretion by causing a decrease in the number of particles in the Golgi but not in the endoplasmic reticulum of rat liver (Verkade et al. (1993) J. Biol. Chem. 268, 24,990-24,996). One explanation for this observation was that VLDL from choline deficient livers was degraded in a post-endoplasmic reticulum compartment. This hypothesis was supported by experiments in which choline deficient (CD) or choline supplemented (CS) rat hepatocytes were incubated +/- Brefeldin A. In the presence of Brefeldin A, VLDL secretion was blocked, but no difference was observed in the degradation of apolipoprotein B (apoB) within the CD or CS cells. If increased catabolism of apoB were occurring in the endoplasmic reticulum of CD hepatocytes, enhanced degradation of apoB in CD cells might have been expected. Inhibition of phosphatidylcholine biosynthesis also caused decreases in the phosphatidylcholine content of membranes of the secretory pathway. The lipids of nascent VLDLs from the lumina of endoplasmic reticulum and Golgi prepared from CD rat liver were relatively enriched in phosphatidylethanolamine and depleted of phosphatidylcholine when compared to samples from CS liver. The changes in nascent VLDL phospholipid composition mimicked that of the organelle membranes from which the VLDLs were isolated. Possibly the phospholipid composition of the organelles is a factor in determining the final phospholipid composition of VLDLs. One hypothesis is that when phosphatidylcholine biosynthesis is impaired, nascent VLDL is assembled incorrectly and degraded by a quality control protease in a post-endoplasmic reticulum compartment.

Sphingolipid biosynthesis de novo by rat hepatocytes in culture. Ceramide and sphingomyelin are associated with, but not required for, very low density lipoprotein secretion

Sphingolipids are constituents of liver and lipoproteins, but relatively little is known about their synthesis and secretion. Incubation of rat hepatocytes with [14C]- or [3H]serine labeled the long-chain base backbones of mainly ceramide and sphingomyelin. Most of the labeled sphingolipids were associated with the cells; however, 1-5% (the majority of which was ceramide) was released into the medium as part of very low density lipoproteins (VLDL). Since this is the first report that lipoproteins contain ceramide, lipoproteins were isolated from rat plasma, and the ceramide contents were (per mg of protein): 6.5 nmol for VLDL (d < 1.018), 0.6 nmol for low density lipoproteins (1.018 < d < 1.063), 0.2 nmol for high density lipoproteins (1.063 < d < 1.18), and 0.1 nmol for the albumin fraction; the lipoproteins also contained 0.1-0.4 nmol of free sphingosine/mg of protein. A number of factors affected the secretion of radiolabeled sphingolipids: 1) addition of palmitic acid, but not stearic or oleic acid, enhanced secretion due to an increase in long-chain base synthesis de novo. 2) Choline deficiency caused a 42% reduction in the secretion of radiolabeled sphingomyelin, but this was due to an effect on VLDL secretion rather than a decrease in sphingolipid synthesis. Removal of choline was examined because previous studies (Yao, Z. M., and Vance, D. E. (1988) J. Biol. Chem. 263, 2998-3004) have shown that choline deficiencies depress phosphatidylcholine synthesis and lipoprotein secretion. 3) Incubation of the cells with fumonisin B1, a mycotoxin inhibitor of sphinganine (sphingosine) N-acyltransferase, reduced overall sphingolipid synthesis and secretion by 90%, but had no effect on the secretion of apoB, phosphatidylcholine, or cholesterol. All together, these findings demonstrate that rat hepatocytes synthesize ceramide and sphingomyelin de novo and incorporate them into both cellular membranes and secreted VLDL, but normal sphingolipid synthesis is not required for lipoprotein secretion.

Role of axons in membrane phospholipid synthesis in rat sympathetic neurons

The axonal synthesis of phospholipids has been demonstrated in compartmented cultures of rat sympathetic neurons. In this model of neuron culture, metabolic events occurring in distal axons were studied independently of those occurring in cell bodies. Using radiolabeled tracers the axonal biosynthesis of the major membrane phospholipids and fatty acids but not cholesterol was detected. The capacity of axons for synthesis of phosphatidylcholine (PC), the major membrane lipid, was confirmed by the demonstration that key enzymes of PC biosynthesis were present in distal axons. A double-labeling experiment showed that at least 50% of axonal PC was synthesized locally in axons, with the remainder being made in cell bodies and transported into axons. The requirement of axonal PC synthesis for axonal elongation was investigated. When PC biosynthesis in distal axons alone was inhibited by two independent approaches (deprivation of choline or addition of the inhibitor hexadecylphosphocholine) axonal growth was markedly retarded. Our experiments demonstrated that PC synthesis in cell bodies was neither necessary nor sufficient for growth of distal axons, whereas local synthesis of PC in distal axons was required for normal axonal elongation.

Characterization of recombinant human apoB-48-containing lipoproteins in rat hepatoma McA-RH7777 cells transfected with apoB-48 cDNA. Overexpression of apoB-48 decreases synthesis of endogenous apoB-100

We studied the effect of overexpression of apolipoprotein (apo) B-48 on the synthesis and secretion of endogenous apoB-100 in rat hepatoma McA-RH7777 cell lines stably transfected with human apoB-48 cDNA under the control of the cytomegalovirus promoter. Three cell lines that secrete 40 to 60 ng human apoB.mg cell protein-1.h-1 were used. The recombinant human apoB-48 exhibited physicochemical characteristics (buoyant density, 1.06 to 1.21 g/mL; beta-electrophoretic mobility and diameters, 16 to 20 nm) indistinguishable from those of endogenous rat apoB-48. Overexpression of the recombinant human apoB-48 resulted in a 50% decrease in the secretion of endogenous apoB-100 but did not affect the secretion of apoE or apoA-I. Several possible mechanisms for the decreased secretion of apoB-100 were evaluated. First, recruitment of lipids into lipoproteins was shown to be unaffected since no major changes in the physicochemical properties of apoB-100-containing lipoproteins were observed. Second, the intracellular degradation of apoB-100 was not altered as the intracellular retention half-time and secretion efficiency remained unaffected by apoB-48 overexpression. Third, the posttranslational regulatory mechanisms for apoB-100 remained normal, as demonstrated by a twofold increase in apoB-100 secretion after supplementation with oleic acid. Unexpectedly, a 35% to 50% decrease in the steady-state synthesis of endogenous apoB-100 was observed in apoB-48-transfected cells compared with control cells. These data suggested that decreased secretion of apoB-100 was secondary to decreased synthesis. The decreased apoB-100 synthesis was not due to decreased steady-state levels of rat apoB-100 mRNA. These results suggest that overexpression of recombinant human apoB-48 may interfere with posttranscriptional events, possibly at the translation-translocation level, and decrease translational yield of apoB-100. These posttranscriptional events prior to the complete synthesis of the apoB-100 polypeptide can be important in the control of apoB-100 secretion.

Nutrients, signal transduction and carcinogenesis

Choline phospholipids play major roles in cellular regulation in addition to their essential function as structural components of membranes and lipoproteins. The unique functions of choline phospholipids as hormones (platelet activating factor, 1-alkyl, 2-acetylphosphatidylcholine, PAF) and sources (phosphatidylcholine, sphingolipids) of second messengers (sphingosine, diacylglycerol, lysophospholipids, arachidonic acid and its metabolites) may explain how dietary choline influences normal physiological processes as well as a diverse group of pathological processes, including carcinogenesis.

Intracellular triacylglycerol lipase: its role in the assembly of hepatic very-low-density lipoprotein (VLDL)

Extracellular fatty acids entering the hepatocyte are either esterified to cytosolic TAG or oxidized to ketone bodies. Very little is esterified and secreted directly in association with VLDL. Thus, even when extracellular fatty acids are available, the major, direct source of VLDL TAG is the cytosolic pool. The recruitment of cytosolic TAG for VLDL assembly involves lipolysis followed by re-esterification. At least 70% of the secreted TAG is derived via this route. Fatty acids released at this lipolytic step are utilized exclusively for VLDL TAG synthesis and are not available for ketogenesis. Substantially more cytosolic TAG undergoes lipolysis than is required to meet the needs of VLDL assembly. The remaining fatty acids are re-esterified and re-cycled to the cell cytosol. From a physiological viewpoint, the presence of this indirect route for VLDL TAG recruitment would provide a means of regulation of VLDL secretion which is independent of the plasma fatty acid concentration. In this respect, several pathophysiological conditions are known in which there is a negative association between plasma fatty acid concentration and the rate of VLDL secretion. These are: (a) insulin-dependent diabetes, (b) starvation, (c) fat-feeding. Lipolysis of cytosolic TAG and transfer of fatty acids into the ER lumen may provide a regulatory focus for the control of hepatic VLDL output.

Factors affecting the regulation of apo B secretion by liver cells

The concentration of apo B is an important risk factor for atherosclerosis, and thus its reduction is associated with a reduction in CHD mortality. In order to reduce apo B concentrations effectively, we must understand how plasma apo B concentration is regulated. Apo B is synthesized, assembled, and secreted by the liver, controlling this process will reduce the number of particles that eventually enter the plasma compartment. The assembly of apo B into a VLDL particle is a complex process which occurs through several stages: peptide synthesis, translocation, accumulation of lipid, and transport through the secretory pathway. Multiple control points regulate the synthesis and secretion of apolipoproteins. Modulation of transcription, translation and intracellular degradation represent independent regulatory mechanisms. The ability of the lipoprotein to bind cotranslationally to lipid appears to be crucial to the formation of a secreted particle. This process may be regulated solely by MTP, or may be modified by the activity of the lipid-synthesizing enzymes. A great deal of evidence supports the role of TG and CE synthesis, although the relative importance of these two lipids is a source of major controversy. In summary, all the lipoprotein components can be limiting for apo B and VLDL synthesis when their availability is substantially decreased. The rate-limiting component in vivo has still not been identified. By understanding how lipoprotein synthesis and assembly are regulated, it should become possible to design new ways of altering these processes in a beneficial manner.

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)

Nonalcoholic steatohepatitis: an expanded clinical entity

BACKGROUND/AIMS

In the past, nonalcoholic steatohepatitis has been described mostly in obese women with diabetes. The aim of this study was to describe a series of patients with nonalcoholic steatohepatitis with a different clinical profile.

METHODS

The clinical, biochemical, and histological features of 33 patients with nonalcoholic steatohepatitis seen from July 1990 to June 1993 were analyzed.

RESULTS

The mean age was 47 years. All patients were antibody to hepatitis C virus-negative. Nineteen of 33 (58%) were men, 20 of 33 (61%) were nonobese, 26 of 33 (79%) had normal glucose levels, and 26 of 33 (79%) had normal lipid levels. Fourteen of 33 (42%) had normal glucose and lipid levels and were not obese. Thirteen of 33 (39%) had pathological increases in fibrosis, 5 of whom had micronodular cirrhosis. Of these 13 with severe, progressive disease, 8 (62%) were women, 8 (62%) were obese, 4 (31%) were diabetic or had an elevated glucose level, and 3 (23%) had hyperlipidemia. Although serum iron studies (transferrin saturation and ferritin) were abnormal in 18 of 31 (58%), no patient had hemochromatosis.

CONCLUSIONS

Nonalcoholic steatohepatitis can be a severe, progressive liver disease leading to the development of cirrhosis. It should no longer be considered a disease predominantly seen in obese women with diabetes.

Inhibition of hepatic ACAT decreases ApoB secretion in miniature pigs fed a cholesterol-free diet

To test the hypothesis that hepatic cholesteryl ester is involved in the regulation of apolipoprotein (apo) B secretion into plasma, apoB kinetic studies were performed in six control miniature pigs and in six pigs after a 21-day administration of the acyl coenzyme A:cholesterol acyltransferase (ACAT) inhibitor DuP 128 (2.2 mg.kg-1.d-1 i.v.). Pigs were fed low-fat, cholesterol-free diets. Total plasma cholesterol, triglyceride, very-low-density lipoprotein (VLDL) triglyceride, and low-density lipoprotein (LDL) cholesterol decreased 18%, 29%, 40%, and 26% respectively (P < .03). 131I-VLDL and 125I-LDL were injected simultaneously into each animal, and apoB kinetics were analyzed by using multi-compartmental analysis (SAAM30). VLDL apoB pool size decreased significantly by 60% (0.32 versus 0.84 mg/kg), which was due to a 65% reduction in the VLDL apoB production or secretion rate (1.03 versus 2.94 mg.kg-1.h-1). The fractional catabolic rate was unchanged. LDL apoB pool size decreased nonsignificantly by 18% (5.61 versus 6.90 mg/kg) due entirely to a 24% decrease in production rate (0.26 versus 0.34 mg.kg-1.h-1). At necropsy, hepatic microsomal ACAT activity decreased by 68% (0.28 versus 0.88 nmol.min-1.mg-1; P < .0002). Although an increase in hepatic free cholesterol leading to a decreased LDL receptor expression might be expected, this did not occur. The concentration of hepatic cholesterol and the LDL apoB fractional catabolic rate were unaffected by DuP 128. In addition, the concentration of hepatic triglyceride and the activity of diacylglycerol acyltransferase were not altered by DuP 128, indicating a lack of effect of DuP 128 on hepatic triglyceride metabolism. We conclude that inhibition of hepatic cholesteryl ester synthesis in vivo decreases apoB secretion into plasma.

Butyrate stimulates the secretion of apolipoprotein B-100-containing lipoproteins from HepG2 cells by inhibiting the intracellular degradation

We have shown previously that sodium butyrate induces a 2-fold increase in the secretion of apo B-100 by HepG2 cells. The apo B-100 mRNA level was not changed in butyrate-treated cells, indicating regulation at the translational or co- or posttranslational level (Biochem. J. (1991) 278, 557-564). In this paper, the mechanism by which butyrate increases apo B-100 secretion was further investigated. Pulse-chase analysis showed that in control incubations only 18 +/- 4% of the total amount of labelled apo B-100, present intracellularly after a 10 min pulse period, was secreted after a 90 min chase period, indicating that the major part of newly synthesized apo B-100 is degraded intracellularly. After addition of butyrate the secreted amount increased to 32 +/- 6% of the total synthesized amount. Treatment of HepG2 cells with butyrate resulted in an enhanced intracellular concentration of triacylglycerols (+30%), with no or only a marginal effect on the cellular content of cholesterol and cholesteryl esters. Secretion of triacylglycerols (+90%) and cholesteryl esters (+78%), but not of cholesterol, was increased to the same extent as apo B-100 secretion (+102%). The total mass of triacylglycerols, i.e., the sum of triacylglycerols present intracellularly and secreted by HepG2 cells, was significantly increased upon incubation with butyrate (+32%), whereas the total mass of cholesteryl esters was not affected. Butyrate did not affect the buoyant density of apo B-100-containing lipoproteins secreted by HepG2 cells. These results suggest that an increased availability of triacylglycerols, formed after the addition of butyrate regulates the amount of apo B-100 degraded intracellularly and consequently apo B-100 secretion.

Inhibition of beta-oxidation by 3-mercaptopropionic acid produces features of Reye's syndrome in perfused rat liver

BACKGROUND/AIMS

The cause of Reye's syndrome has not been completely defined. The rate of ketogenesis in the liver is a key determinant of, and reciprocally related to, triglyceride secretion. In the present study, 3-mercaptopropionic acid (MPA), a known inhibitor of mitochondrial long-chain acyl coenzyme A (CoA) dehydrogenase, was used to investigate the relationship between ketone body production, triglyceride secretion, and triglyceride accumulation in perfused rat liver.

METHODS

Livers from fasted rats were perfused 225 minutes with or without MPA in the presence of [1-14C]oleic acid. Morphology was studied by light and electron microscopy.

RESULTS

Inhibition of fatty acid oxidation by the liver with MPA resulted in a decrease in ketone body production. Treatment with MPA caused an accumulation of small-droplet triglycerides in liver, whereas the net secretion of triglyceride ceased after an initial period of increased secretion with continued decreased ketogenesis. At the end of the perfusion period, mitochondria in the MPA group appeared to be damaged.

CONCLUSIONS

The rates of both ketogenesis and triglyceride secretion by the liver appear to be the major determinants of hepatic triglyceride content. In addition, the MPA-mediated biochemical and morphological findings are quite similar to those of Reye's syndrome.

Synthesis and secretion of hepatic apolipoprotein B-containing lipoproteins

Human apolipoprotein (apo) B-100 is required for the synthesis and secretion of hepatic triacyglycerol-rich lipoproteins. This review summarizes recent developments in understanding the interaction of cis-acting DNA sequences and trans-acting protein factors in regulation of apo B gene expression and apo B mRNA editing, and the role of structural determinants of apo B-100 in the assembly and secretion of hepatic lipoproteins. In particular, experimental results obtained from cell culture studies using techniques of molecular and cellular biology are described and discussed. The relationship between apo B length and its ability to recruit lipids is presented, and the involvement of factors other than apo B in hepatic triacylglycerol-rich lipoprotein production is discussed.

Influence of orotic acid and estrogen on hepatic lipid storage and secretion in the goose susceptible to liver steatosis

Fatty liver in the goose results from an increased hepatic lipogenesis in response to overfeeding, together with a deficient secretion of triacylglycerol as very-low-density lipoproteins (VLDL). Orotic acid and estrogen, which both modify lipid metabolism in the liver, were used in male geese as tools to understand the alterations of liver lipids and plasma lipoproteins during the induction of liver steatosis. Liver lipids were analyzed after solvent extraction and plasma lipoproteins after separation by density gradient ultracentrifugation. Contrary to what is known in the rat, orotic acid (1% in food for 2 weeks) failed to induce liver steatosis. In force-fed geese, liver weight increased from approximately 100 g to approximately 800 g in 2 weeks, as a consequence of a specific accumulation of triacylglycerol. In both groups, VLDL contained less triacylglycerol (35%) than normal. Such an uncoupling of triacylglycerol synthesis and secretion, of which the precise reason is still unknown, may facilitate their accumulation when force-feeding increases hepatic lipogenesis. As with force-feeding, triacylglycerol synthesis was enhanced by estrogen, but their secretion as VLDL was very efficient and prevented liver steatosis almost completely. Since HDL concentrations were considerably decreased by estrogen, VLDL were the main lipoprotein species, with 48 g/l and 62% triacylglycerol. Where estrogen-treated geese were force-fed concomitantly, VLDL concentration was even higher (62 g/l), but triacylglycerol secretion could not prevent liver steatosis (liver weight 640 g). The data are discussed in relation to in vitro studies showing that channelling of triacylglycerol towards secretion as VLDL or hepatic storage depends on their residence time in the different intracellular compartments.

Influence of triacylglycerol biosynthesis rate on the assembly of apoB-100-containing lipoproteins in Hep G2 cells

Apolipoprotein B-100 (apoB-100) appears in three forms in the endoplasmic reticulum of Hep G2 cells: (1) tightly bound to the membrane, ie, not extractable by sodium carbonate. This form is glycosylated but protease sensitive when present in intact microsomes, suggesting that it is only partially translocated to the microsomal lumen; (2) extractable by sodium carbonate and present on low-density lipoprotein-very-low-density lipoprotein (LDL-VLDL)-like particles. This form is glycosylated and secreted into the medium; and (3) extractable by sodium carbonate but having a higher density than the LDL-VLDL-like particles. This form, referred to as Fraction I, is glycosylated and protected against proteases when present in intact microsomal vesicles, indicating that it is completely translocated to the luminal side of the microsomal membrane. Fraction I is not secreted into the medium, but it disappears with time from the cell, suggesting that it is degraded. Oleic acid induced a 2.7-fold increase in the rate of the biosynthesis of triacylglycerol but not of phosphatidylcholine in Hep G2 cells. Incubation of the cells with oleic acid had no significant effect on the rate of initiation of the apoB-100-containing lipoproteins, nor did it influence the amount of apoB-100 that was associated with the membrane or the turnover of apoB-100 in the membrane. Instead, it increased the proportion of the nascent apoB polypeptides on initiated lipoproteins that was converted into full-length apoB-100 on LDL-VLDL-like particles, giving rise to an increased amount of these particles in the lumen of the secretory pathway. Pulse-chase experiments showed that incubation with oleic acid gave rise to an increased formation of LDL-VLDL-like particles on behalf of the formation of Fraction I. This effect of oleic acid could partially explain the protective effect of the fatty acid on apoB-100, preventing it from undergoing posttranslational degradation.

ApoA-IV is secreted on discrete HDL particles by the rat hepatoma cell line McA-RH7777 transfected with ApoA-IV cDNA

In the present study, the synthesis and secretion of transfected apolipoprotein (apo) A-IV was investigated in rat hepatoma McA-RH7777, a cell line that does not express apoA-IV mRNA or protein. An expression plasmid that contained the rat apoA-IV cDNA was transfected into the cells; five stable transformants were selected that harbor different copy numbers of the apoA-IV construct and secrete different amounts of apoA-IV. Gel filtration column chromatography and density gradient ultracentrifugation, combined with gel electrophoresis and electron microscopy techniques, demonstrated that (1) the secreted apoA-IV associated mainly with high-density lipoproteins (HDLs) and only a trace amount of apoA-IV was associated with very-low-density lipoproteins; (2) overexpression of apoA-IV resulted in an increased number of disk-shaped structures (thickness, approximately 8.0 nm and diameter, approximately 22 nm); and (3) the electrophoretic mobilities of the apoA-IV-containing particles differed from those of apoA-I-containing HDL. Expression of apoA-IV exerted no discernible effect on the density distribution or the secretion efficiency of apoB-100. Additionally, secretion of apoB-100 and apoA-IV exhibited opposite responses to serum: apoB-100 secretion was stimulated eightfold after addition of serum, whereas apoA-IV secretion was inhibited by 40%. These results suggest that synthesis of apoA-IV may lead to the formation of a subclass of HDL with a different metabolic fate than that of lipoproteins containing either apoA-I or apoB.

The hypotriglyceridemic effect of eicosapentaenoic acid in rats is reflected in increased mitochondrial fatty acid oxidation followed by diminished lipogenesis

The effect of eicosapentaenoic acid (EPA) on fatty acid oxidation and on key enzymes of triglyceride metabolism and lipogenesis was investigated in the liver of rats. Repeated administration of EPA to normolipidemic rats resulted in a time-dependent decrease in plasma triglycerides, phospholipids and cholesterol. The triglyceride-lowering effect was observed after one day of feeding whereas lowering of plasma cholesterol and phospholipids was observed after five days of treatment. The triglyceride content of liver was reduced after two-day treatment. At that time, increased mitochondrial fatty acid oxidation occurred whereas mitochondrial and microsomal glycerophosphate acyltransferase was inhibited. The phosphatidate phosphohydrolase activity was unchanged. Adenosine triphosphate:citrate lyase, acetyl-CoA carboxylase, fatty acid synthetase and glucose-6-phosphate dehydrogenase were inhibited during the 15 d of EPA treatment whereas peroxisomal beta-oxidation was increased. At one day of feeding, however, when the hypotriglyceridemic effect was established, the lipogenic enzyme activities were reduced to the same extent in palmitic acid-treated animals as in EPA-treated rats. In cultured rat hepatocytes, the oxidation of [14C]palmitic acid to carbon dioxide and acid-soluble products was stimulated in the presence of EPA. These results suggest that the instant hypolipidemia in rats given EPA could be explained at least in part by a sudden increase in mitochondrial fatty acid oxidation, thereby reducing the availability of fatty acids for lipid synthesis in the liver for export, e.g., in the form of very low density lipoproteins, even before EPA induced peroxisomal fatty acid oxidation, reduced triglyceride biosynthesis and diminished lipogenesis.

Relations of sodium-lithium countertransport kinetics to plasma and red cell membrane phospholipids in hyperlipidemia

As compared to 7 normolipidemic donors, the maximal velocity of sodium-lithium countertransport was accelerated by nearly 70% in 10 patients with elevated levels of triglyceride-rich lipoproteins and tended to be stimulated also in 5 patients with hypercholesterolemia. No significant differences were observed between normolipidemia and both hyperlipidemic groups for the apparent affinities of the transport system for intracellular sodium and extracellular lithium. Strong positive relations of the maximal activity of sodium-lithium countertransport to the percentages of red cell membrane phosphatidylcholine (r = 0.85, 2P < 0.001), the phosphatidylcholine/sphingomyelin (r = 0.82, 2P < 0.001) and the phosphatidylcholine/phosphatidylethanolamine ratio (r = 0.81, 2P < 0.001) were seen in all donors. A negative correlation was found to membrane sphingomyelin (r = -0.72, 2P < 0.001). Also plasma phosphatidylcholine and sphingomyelin exhibited positive and negative associations, respectively, to the maximal activity of sodium-lithium countertransport (r = 0.66, 2P < 0.01 and r = -0.78, 2P < 0.001). Among several plasma lipoprotein parameters investigated, total triglycerides or VLDL cholesterol levels showed independent relations to both the plasma and the membrane phosphatidylcholine/sphingomyelin ratio as well as to the maximal velocity of sodium-lithium countertransport. The results indicate that an increase in red cell membrane phosphatidylcholine and a concomitant fall in sphingomyelin are closely associated with the acceleration of sodium-lithium countertransport in hyperlipidemia.

Choline: an important nutrient in brain development, liver function and carcinogenesis

Choline is required to make certain phospholipids which are essential components of all membranes. It is a precursor for biosynthesis of the neurotransmitter acetylcholine and also is an important source of labile methyl groups. Much attention has been given to the effect of supplemental choline upon brain function, i.e., enhancement of acetylcholine synthesis and release. In addition, choline supplements administered to rats in utero or shortly after birth permanently after brain function. The mechanisms for this effect is unknown and under investigation at this time. Healthy humans fed diets deficient in choline, and humans fed parenterally have decreased plasma choline concentrations and develop liver dysfunction that is similar to that seen in choline-deficient animals. In experimental animals, fatty liver occurs in choline deficiency because phosphatidylcholine synthesis is required for very low-density lipoprotein secretion. This accumulation of lipids in liver may explain why choline-deficient rats spontaneously develop hepatocarcinoma. We found that choline deficiency was associated with the accumulation of 1,2-diacylglycerol, an activator of protein kinase C. Several lines of evidence indicate that cancers might develop secondary to abnormalities in protein kinase C-mediated signal transduction.

In vitro toxicology of fumonisins and the mechanistic implications

The effects of fumonisins B1(FB1), B2(FB2), and the backbone of fumonisin B1 remaining after hydrolysis of the tricarballylic groups with base (HFB1) on sphingolipid biosynthesis were studied in both primary rat hepatocytes and pig kidney epithelial cells (LLC-PK1). Fumonisins were potent inhibitors of sphingolipid biosynthesis in hepatocytes (IC50 of FB1 = 0.1 microM), but overt toxicity was not observed. In renal cells, fumonisins also inhibited sphingosine biosynthesis (IC50 for FB1 = 35 microM), and caused decreased cell proliferation as well. Higher doses (greater than or equal to 70 microM) killed renal cells after exposure for 3 days. The inhibition of de novo sphingolipid biosynthesis was specific, and appeared to be at the site of ceramide synthase, which catalyzes the formation of dihydroceramide or ceramide by the addition of the amide-linked fatty acid to sphinganine or sphingosine. These results may account for the ability of fumonisins to cause equine leucoencephalomalacia and to promote tumor formation.