The molecular mechanism for the assembly and secretion of ApoB-100-containing lipoproteins

Spectrum of Rare and Common Genetic Variants in Arrhythmogenic Cardiomyopathy Patients

Arrhythmogenic cardiomyopathy (ACM) is a rare inherited disorder, whose genetic cause is elusive in about 50–70% of cases. ACM presents a variable disease course which could be influenced by genetics. We performed next-generation sequencing on a panel of 174 genes associated with inherited cardiovascular diseases on 82 ACM probands (i) to describe and classify the pathogenicity of rare variants according to the American College of Medical Genetics and Genomics both for ACM-associated genes and for genes linked to other cardiovascular genetic conditions; (ii) to assess, for the first time, the impact of common variants on the ACM clinical disease severity by genotype-phenotype correlation and survival analysis. We identified 15 (likely) pathogenic variants and 66 variants of uncertain significance in ACM-genes and 4 high-impact variants in genes never associated with ACM (ABCC9, APOB, DPP6, MIB1), which deserve future consideration. In addition, we found 69 significant genotype-phenotype associations between common variants and clinical parameters. Arrhythmia-associated polymorphisms resulted in an increased risk of arrhythmic events during patients’ follow-up. The description of the genetic framework of our population and the observed genotype-phenotype correlation constitutes the starting point to address the current lack of knowledge in the genetics of ACM.

Differential distribution of gene polymorphisms associated with hypercholesterolemia, hypertriglyceridemia, and hypoalphalipoproteinemia among Native American and Mestizo Mexicans

BACKGROUND

Dyslipidemias are metabolic abnormalities associated with chronic diseases caused by genetic and environmental factors. The Mexican population displays regional differences according to ethnicity with an impact on the type of dyslipidemia.

AIM

To define the main dyslipidemias, the frequency of lipid-related risk alleles, and their association with hyperlipidemic states among different ethnic groups in West Mexico.

METHODS

In a retrospective study, 1324 adults were selected to compare dyslipidemias and lipid-related gene polymorphisms. Demographic, clinical, and laboratory data were collected. A subgroup of 196 normal weight subjects without impaired glucose was selected for the association analyses. Genotyping was determined by allelic discrimination assay.

RESULTS

Hypercholesterolemia was the most prevalent dyslipidemia (42.3%). The frequency of the risk alleles associated with hypoalphalipoproteinemia (ABCA1) and hypercholesterolemia (APOE, LDLR) was higher in the Native Americans (P = 0.047). In contrast, the Mestizos with European ancestry showed a higher frequency of the risk alleles for hypertriglyceridemia (APOE2, MTTP) (P = 0.045). In normal weight Mestizo subjects, the APOB TT and LDLR GG genotypes were associated risk factors for hypercholesterolemia (OR = 5.33, 95%CI: 1.537-18.502, P = 0.008 and OR = 3.90, 95%CI: 1.042-14.583, P = 0.043, respectively), and displayed an increase in low-density lipoprotein cholesterol levels (APOB: β = 40.39, 95%CI: 14.415-66.366, P = 0.004; LDLR: β = 20.77, 95%CI: 5.763-35.784, P = 0.007).

CONCLUSION

Gene polymorphisms and dyslipidemias showed a differential distribution. Regional primary health care strategies are required to mitigate their prevalence considering the genetic and environmental features which could have important implications for personalized medicine within the new era of precision medicine.

The Perilipins: Major Cytosolic Lipid Droplet-Associated Proteins and Their Roles in Cellular Lipid Storage, Mobilization, and Systemic Homeostasis

The discovery by Dr. Constantine Londos of perilipin 1, the major scaffold protein at the surface of cytosolic lipid droplets in adipocytes, marked a fundamental conceptual change in the understanding of lipolytic regulation. Focus then shifted from the enzymatic activation of lipases to substrate accessibility, mediated by perilipin-dependent protein sequestration and recruitment. Consequently, the lipid droplet became recognized as a unique, metabolically active cellular organelle and its surface as the active site for novel protein-protein interactions. A new area of investigation emerged, centered on lipid droplets' biology and their role in energy homeostasis. The perilipin family is of ancient origin and has expanded to include five mammalian genes and a growing list of evolutionarily conserved members. Universally, the perilipins modulate cellular lipid storage. This review provides a summary that connects the perilipins to both cellular and whole-body homeostasis.

What is the contribution of obesity to the metabolic syndrome?

Many prospective studies show that obesity is accompanied by increased risk for cardiovascular disease. Obesity affects metabolism of lipids and glucose, regulation of blood pressure, thrombotic and fibrinolytic processes, and inflammatory reactions. Multiple aberrations exist in each of these systems; obesity acting alone probably is not sufficient to produce full-blown metabolic syndrome. There must be other factors, including genetic and aging factors. It is difficult to sort out all pathogenic factors that link obesity to cardiovascular disease risk. It is worthwhile to investigate individually the components of the obesity-induced metabolic syndrome for their atherogenic potential, because out of that investigation likely will come new targets for clinical or public health intervention to reduce the risk for cardiovascular disease.

RNA editing: cytidine to uridine conversion in apolipoprotein B mRNA

RNA editing is a post-transcriptional process that changes the informational capacity within the RNA. These processes include alterations made by nucleotide deletion, insertion and base conversion. A to I and C to U conversion occurs in mammals and these editing events are catalysed by RNA binding deaminases. C to U editing of apoB mRNA was the first mammalian editing event to be identified. The minimal protein complex necessary for apoB mRNA editing has been determined and consists of APOBEC-1 and ACF. Overexpression of APOBEC-1 in transgenic animals caused liver dysplasia and APOBEC-1 has been identified in neurofibromatosis type 1 tumours, suggesting that RNA editing may be another mechanism for tumourigenesis. Several APOBEC-1-like proteins have been identified, including a family of APOBEC-1-related proteins with unknown function on chromosome 22. This review summarises the different types of RNA editing and discusses the current status of C to U apoB mRNA editing. This knowledge is very important in understanding the structure and function of these related proteins and their role in biology.

Metabolic complications of obesity

The rising prevalence of obesity is accompanied by an increasing number of patients with the metabolic complications of obesity. The major complications come under the heading of the metabolic syndrome. This syndrome is characterized by plasma lipid disorders (atherogenic dyslipidemia), raised blood pressure, elevated plasma glucose, and a prothrombotic state. The clinical consequences of the metabolic syndrome are coronary heart disease and stroke, type 2 diabetes and its complications, fatty liver, cholesterol gallstones, and possibly some forms of cancer. At the heart of the metabolic syndrome is insulin resistance, which represents a generalized derangement in metabolic processes. Obesity is the predominant factor leading to insulin resistance, although other factors play a role. The mechanistic link between insulin resistance and the metabolic syndrome is complex. The relationship is modulated by yet other factors, such as physical activity, body fat distribution, hormones, and a person's genetic polymorphic architecture. A better understanding of the molecular basis of this relationship is needed to suggest new targets for prevention and treatment of the complications of obesity. In addition, understanding at the clinical level will lead to improved management of these complications.

Enhanced secretion of ApoB by transfected HepG2 cells overexpressing fibrinogen

HepG2 cells stably transfected with cDNA-encoding single fibrinogen chains overexpress fibrinogen and have increased (4-fold) secretion of apolipoprotein B. Overexpression of fibrinogen does not affect the secretion of three representative acute-phase proteins but causes a small increase in albumin secretion. Enhanced apolipoprotein B secretion is due to less intracellular degradation and not to increased expression. The increased secretion of apolipoprotein B is independent of the acute-phase response, since stimulation of fibrinogen gene expression by interleukin 6 did not affect secretion. HepG2 cells overexpressing fibrinogen chains had increased 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA levels, enhanced cholesterol production but normal levels of triglyceride and phospholipid synthesis and of sterol response binding proteins. These results, that associate overexpression of fibrinogen with enhance apolipoprotein B secretion, may be significant since epidemiological studies indicate that elevated levels of fibrinogen and lipids are independent risk factors in coronary artery disease.

A proposed model for the assembly of chylomicrons

The intestine synthesizes very low density lipoproteins (VLDL) and chylomicrons (CM) to transport fat and fat-soluble vitamins into the blood. VLDL assembly occurs constitutively whereas CM assembly is a characteristic property of the enterocytes during the postprandial state. The secretion of CM is specifically inhibited by Pluronic L81. CM are very heterogeneously-sized particles that consist of a core of triglycerides (TG) and cholesterol esters and a monolayer of phospholipids (PL), cholesterol and proteins. The fatty acid composition of TG, but not PL, in CM mirrors the fatty acid composition of fat in the diet. CM assembly is deficient in abetalipoproteinemia and CM retention disease. Abetalipoproteinemia results due to mutation in the mttp gene and is characterized by the virtual absence of apoB-containing lipoproteins in the plasma. Patients suffer from neurologic disorders, visual impairment, and exhibit acanthocytosis. CM retention disease, an inherited recessive disorder, is characterized by chronic diarrhea with steatorrhea in infancy, abdominal distention and failure to thrive. It is caused by a specific defect in the secretion of intestinal lipoproteins; secretion of lipoproteins by the liver is not affected. Besides human disorders, mice that do not assemble intestinal lipoproteins have been developed. These mice are normal at birth, but defective in fat and fat-soluble vitamin absorption, and fail to thrive. Thus, fat and fat-soluble vitamin transport by the intestinal lipoproteins is essential for proper growth and development of neonates. Recently, differentiated Caco-2 cells and rabbit primary enterocytes have been described that synthesize and secrete CM. These cells can be valuable in distinguishing between the two different models proposed for the assembly of CM. In the first model, the assembly of VLDL and CM is proposed to occur by two 'independent' pathways. Second, CM assembly is proposed to be a product of 'core expansion' that results in the synthesis of lipoproteins of different sizes. According to this model, intestinal lipoprotein assembly begins with the synthesis of 'primordial' lipoprotein particles and involves release of the nascent apoB with PL derived from the endoplasmic reticulum (ER) membrane. In addition, TG-rich 'lipid droplets' of different sizes are formed independent of apoB synthesis. The fusion of lipid droplets and primordial lipoproteins results in the formation of different size lipoproteins due to the 'core expansion' of the primordial lipoproteins.

Acyltransferase activities in the yolk sac membrane of the chick embryo

The activities of some enzymes of glycerolipid synthesis and fatty acid oxidation were measured in subcellular fractions of the yolk sac membrane (YSM), an extra-embryonic tissue that mediates the transfer of lipid from the yolk to the circulation of the chick embryo. The activities of monoacylglycerol acyltransferase and carnitine palmitoyl transferase-1 in the YSM (respectively, 284.8+/-13.2 nmol/min/mg microsomal protein and 145.6+/-9.1 nmol/min/mg mitochondrial protein; mean +/- SE; n = 4) at day 12 of development appear to be the highest yet reported for any animal tissue. Also, the carnitine palmitoyl transferase-1 of the YSM was very insensitive to inhibition by malonyl CoA. The maximal activities of glycerol-3-phosphate acyltransferase and diacylglycerol acyltransferase in the YSM (respectively, 26.7+/-2.2 and 36.1+/-2.1 nmol/min/mg microsomal protein) were also high compared with the reported values for various animal tissues. The very high enzymic capacity for glycerolipid synthesis supports the hypothesis that the yolk-derived lipids are subjected to hydrolysis followed by reesterification during transit across the YSM. The monoacylglycerol pathway appears to be the main route for glycerolipid resynthesis in the YSM. The results also suggest that the YSM has the capacity to perform simultaneously beta-oxidation at a high rate in order to provide energy for the lipid transfer process.

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.

Use of cyclodextrin to deliver lipids and to modulate apolipoprotein B-100 production in HepG2 cells

2-Hydroxypropyl-beta-cyclodextrin (cyclodextrin), cyclodextrin-solubilized oleate, and cyclodextrin-solubilized cholesterol were used to modulate proteolysis and secretion of newly-synthesized apolipoprotein B-100 (apoB) in HepG2 cells. Following cyclodextrin and lipid treatments, cells were pulse-labeled with [3H] leucine, and quantitative immunoprecipitation was used to measure apoB synthesis, apoB secreted into the medium, and the cellular content of undegraded apoB that was not secreted. Three-hour treatment with cyclodextrin-solubilized oleate (0.2 mM) increased secreted apoB from 4% (control cells) to 32% and cellular undegraded apoB from 15% (control cells to 64% of apoB synthesized, which is consistent with earlier studies using bovine serum albumin to complex exogenous oleate. Prolonged daily (4 d or more) administration of 0.5% (3.5 mM) cyclodextrin with medium containing 10% fetal bovine serum increased the secretion of nascent apoB from 5-10% (control) to 17-28% and cellular undegraded apoB from 15-20% (control) to 25-31% of apoB synthesized, respectively. Subsequent administration of cyclodextrin solubilized cholesterol (10-40 micrograms) for only 3 h reversed the cyclodextrin-mediated increase in apoB secretion. The application of 0.5% cyclodextrin to HepG2 cells can rapidly (within minutes) stimulate cholesterol efflux, and transiently (over a 1-2 d period) increase cholesterol synthesis. In the current studies, the cyclodextrin-mediated increase in cholesterol synthesis was not concurrent with the increase in apoB secretion. However, prolonged (15 d) administration of cyclodextrin was shown to increase the cellular free cholesterol concentration by 25-41%, reduce the cellular triglyceride concentration by 59%, and increase apoB secretion 3- to 4-fold, without affecting the cellular cholesteryl ester concentration. In comparison, 14-d treatment with cyclodextrin-solubilized cholesterol (20 micrograms/mL) followed by 1-d equilibration without cholesterol was shown to increase the cellular free cholesterol and cholesteryl ester concentrations by 76% and 10-fold, respectively, although apoB secretion was not affected. It is hypothesized that chronic daily administration of 0.5% cyclodextrin increased the cellular cholesterol concentration and flux in discrete putative regulatory compartments, which "shielded" nascent apoB from rapid proteolysis and facilitated apoB secretion. In conclusion, cyclodextrin was used independently and in combination with cholesterol or oleate to modulate apoB proteolysis and secretion. We speculate that subcellular changes in cholesterol concentration and flux may modulate apoB production in HepG2 cells.

Chylomicron assembly and catabolism: role of apolipoproteins and receptors

Chylomicrons are lipoproteins synthesized exclusively by the intestine to transport dietary fat and fat-soluble vitamins. Synthesis of apoB48, a translational product of the apob gene, is required for the assembly of chylomicrons. The apob gene transcription in the intestine results in 14 and 7 kb mRNAs. These mRNAs are post-transcriptionally edited creating a stop codon. The edited mRNAs chylomicrons from the shorter apoB48 peptide remains to be elucidated. In addition, the roles of proteins involved in the assembly pathway, e.g. apobec-1, MTP and apoA-IV, needs to be studied. Cloning of enzymes involved in the intestinal biosynthesis of triglycerides will be crucial to fully appreciate the assembly of chylomicrons. There is a need for cell culture and transgenic animal models that can be used for intestinal lipoprotein assembly. The catabolism of chylomicrons is far more complex and efficient than the catabolism of VLDL. Even though the major steps involved in the catabolism of chylomicrons are now known, the determinants for apolipoprotein exchange, processing of remnants in the space of Disse, as well as the mechanism of uptake of these particles by extra-hepatic tissue needs further exploration.

Involvement of molecular chaperones in intracellular protein breakdown

In all cells and organelles, there exist multiple molecular chaperones, which not only can facilitate the proper folding, transport and assembly of multimeric structures, but also appear to function in intracellular protein degradation. Recent findings in E. coli indicate that the major chaperones of the Hsp70 (DnaK) and Hsp60 (GroEL) families and their cofactors (DnaJ, GrpE or GroEL and Trigger Factor) associate with certain short-lived proteins (e.g. mutant polypeptides or regulatory proteins) and promote their degradation by the ATP-dependent proteases, La (lon or ClpP). Moreover, ATPases of ClpA/B family not only function in ATP-dependent proteolysis in association with the Clp protease, but by themselves can facilitate or act as chaperones in protein assembly. In eukaryotes, Hsp70 and their cofactors, the DnaJ homologs, are essential for the ubiquitination of certain abnormal and regulatory proteins and in the breakdown of certain polyubiquitinated polypeptides by 26S proteasome. It is likely that the chaperones function in proteolysis either as elements that faciliate the recognition of unfolded proteins or that the chaperones partially unfold substrates to make them more susceptible to proteases or ubiquitinating enzymes.

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.

Regulation of apolipoprotein B secretion in hepatocytes from Watanabe heritable hyperlipidemic rabbit, an animal model for familial hypercholesterolemia

Regulation of apolipoprotein B-100 (apo B) secretion in the liver in familial hypercholesterolemia (FH) remains largely unknown. In a previous study, we developed a rabbit hepatocyte culture system and investigated a cellular mechanism which regulates apo B secretion from hepatocytes in response to cellular cholesteryl ester contents [18]. Using this system, in the present study, we investigated regulation of apo B secretion in hepatocytes from the Watanabe heritable hyperlipidemic (WHHL) rabbit, an animal model for FH. Incubation with low density lipoproteins (LDL) at concentrations of 50 or 200 micrograms/ml, which increased both cellular cholesteryl ester and apo B secretion significantly in normal rabbit hepatocytes, did not cause such increases in WHHL rabbit hepatocytes. However, when WHHL rabbit hepatocytes were incubated with LDL at a concentration of 500 micrograms/ml, a significant increase in cellular cholesteryl ester and apo B secretion was observed. The effect of the increase in cellular level of cholesteryl ester upon apo B secretion in WHHL rabbit hepatocytes was compatible with that in normal rabbit hepatocytes. Indeed, when WHHL rabbit hepatocytes were incubated with LDL at 1680 micrograms/ml, a concentration comparable to that of WHHL rabbit plasma, the amount of LDL degradation, cellular cholesteryl ester level, and level of apo B secretion were the same as those in normal rabbit hepatocytes that were incubated with LDL at 160 micrograms/ml, a concentration comparable to that of normal rabbit plasma. In summary, our present study suggests that, at a steady state with such a high plasma concentration of LDL, the hepatic cholesterol content in WHHL rabbits could be set at the same level as in normal rabbits. It was also shown that cellular cholesteryl ester contents had the same regulatory effect on apo B secretion in WHHL rabbit hepatocytes as in normal rabbit hepatocytes. Therefore, we conclude that in the presence of the genetic defect of the LDL receptor, plasma cholesterol in WHHL rabbit is maintained at a concentration such that apo B secretion is similar to that in normal rabbit.

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.

Microsomal triglyceride transfer protein. Specificity of lipid binding and transport

Microsomal triglyceride transfer protein (MTP) is a lipid transfer protein that is required for the assembly and secretion of very low density lipoproteins by the liver and chylomicrons by the intestine. To further elucidate the nature of the lipid molecule binding and transport site on MTP, we have studied the relative rates at which MTP transports different lipid species. Assay conditions were chosen in which there were minimal changes in the physical properties of the substrate membranes so that transfer rates would reflect MTP-lipid interactions at a membrane surface. Lipid transport rates decreased in order of triglyceride > cholesteryl ester > diglyceride > cholesterol > phosphatidylcholine. Changes in the hydrophobic nature of a lipid molecule by the addition of a fatty acid, modulated the ability of MTP to transport it. Addition of one acyl chain from diglyceride to triglyceride, lysophosphatidylcholine to phosphatidylcholine, or cholesterol to cholesteryl ester increased the rate of MTP-mediated transport 10-fold. In contrast, the lipid transport rate was insensitive to the changes in the structure or charge of the polar head group on phospholipid substrates. Zwitterionic, net negative, or net positive charged phospholipid molecules were all transported at a comparable rate. The ability of MTP to transport lipids is strongly correlated to the binding of these lipids to MTP. Thus, MTP has a specific preference for binding and transporting nonpolar lipid compared with phospholipids, and within a class of lipid molecules, a decrease in polarity increases its tendency to be transported.

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)

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.

Effects of albumin on lipid synthesis, apo B-100 secretion, and LDL catabolism in HepG2 cells

This study examines the effects of extracellular albumin on hepatic apo B-100 metabolism. To do so, a transformed human liver cell line, HepG2, was used as a hepatocyte model and the concentration of albumin in the medium was varied between 0 and 5 g%. Apo B-100 and apo A1 concentrations in the medium were determined by specific enzyme-linked immunoassay (ELISA) and intracellular synthesis of cholesterol ester and triglyceride were determined by addition of appropriate radiolabels to the medium. The data demonstrate that the reduction of extracellular albumin concentration resulted in increased apo B-100 concentration in the medium. Apo A1 secretion, however, was unaffected. While the differences in apo B-100 concentration in the medium were statistically significant (33% +/- 7%, P < 0.0025, 0 g% albumin compared to 5 g% albumin in the medium), the absolute magnitude of the effect under these conditions was relatively modest. Nevertheless, the changes were consistent and evident over incubation periods as long as 8 days. Of interest, although triglyceride synthesis was unaffected, cholesterol ester synthesis changed such that as albumin concentration decreased, synthesis of cholesterol ester increased paralleling the changes in apo B-100 (170% +/- 9%, P < 0.005). These findings were extended by studying interventions which altered cholesterol ester synthesis. Addition of the compound 58-035 (5 micrograms/ml, a specific inhibitor of acylcholesterol acyltransferase activity) resulted in substantial inhibition of cholesterol ester synthesis (39% to 66%, P < 0.025 and P < 0.005, respectively) and apo B-100 concentrations in the medium which decreased by 20% to 28%, P < 0.025. Triglyceride synthesis, in contrast, increased significantly by 32% P < 0.025. Therefore, addition of 58-035 confirmed the previous findings of a parallel relation between cholesterol ester synthesis and apo B-100 concentration in the medium. Nonetheless, albumin still had an additional inhibitory effect on cholesterol ester and apo B-100 secretion. Of interest, when chylomicron remnants (25 micrograms/ml cholesterol), which cause apo B-100 secretion to increase by more than threefold, were added to the medium, albumin now had a more pronounced absolute effect on apo B-100 secretion with a 48% inhibition observed as albumin was increased from 0 to 5 g% in the medium (P < 0.0125). The effect of extracellular albumin on the low density lipoprotein (LDL) pathway was also examined. No differences in non-specific cell association component were detected.(ABSTRACT TRUNCATED AT 400 WORDS)

Cotranslational inhibition of apoB-100 synthesis by cyclosporin A in the human hepatoma cell line HepG2

Treatment of patients with cyclosporin A (CsA) increases low-density lipoprotein (LDL) cholesterol levels. We investigated whether an elevated hepatic secretion of apolipoprotein (apo) B-100-containing lipoproteins is responsible for the increase of LDL by using the human hepatoma cell line HepG2. Addition of CsA to the culture medium of HepG2 cells resulted in a dose- and time-dependent decrease in the secretion of apoB-100. Maximal inhibition (-50%), which was obtained at 5 mumol/L CsA, was achieved within 8 hours. The secretion of apoA-I, albumin, and [35S]methionine-labeled proteins was not affected by CsA. The reduced accumulation of apoB-100 in the culture medium could not be explained by changes in the uptake and degradation of LDL by HepG2 cells treated with CsA. In addition, [35S]methionine incorporation studies indicated that synthesis and/or secretion of newly synthesized apoB-100 decreased in the presence of CsA. CsA did not affect the apoB-100 mRNA level, indicating that CsA regulates the secretion of apoB-100 at the cotranslational or posttranslational level. The decreased secretion of apoB-100 was accompanied by a diminished secretion of triglycerides (-47%), cholesterol (-18%), and cholesteryl esters (-27%) in the presence of CsA. In contrast, the intracellular concentrations and the total amount of these lipids present in the culture medium and cells were not changed. This indicates that a possible limited availability of one of these lipids was not responsible for the decreased secretion of apoB-100 by CsA. Pulse-chase experiments showed that the amount of intracellular apoB-100 was already decreased by 50% after the 10-minute pulse period and that CsA did not affect the intracellular processing of apoB-100 once it was fully synthesized. Short pulse incubations in the presence of [35S]methionine showed a decrease in the intracellular amount of labeled apoB-100 after an incubation of only 2 through 4 minutes, indicating that the translation was not affected but that inhibition of the apoB-100 secretion by CsA occurred at the cotranslational level. Our results suggest that the elevated plasma LDL levels observed in patients treated with CsA are not caused by hepatic overproduction of apoB-100-containing lipoproteins.