Lipoprotein Metabolism1 1In preparation of this study, Dr. Eisenberg was supported in part through the Special Foreign Currency Program of the National Library of Medicine, National Institutes of Health, Public Health Service, U. S. Department of Health, Education, and Welfare, Bethesda, Maryland, under an agreement with the Israel Journal of Medical Sciences, Jerusalem, Israel

Epidemiologic, Genetic, Pathogenic, Metabolic, Epigenetic Aspects Involved in NASH-HCC: Current Therapeutic Strategies

Hepatocellular carcinoma (HCC) is the most common primary liver cancer and is the sixth most frequent cancer in the world, being the third cause of cancer-related deaths. Nonalcoholic steatohepatitis (NASH) is characterized by fatty infiltration, oxidative stress and necroinflammation of the liver, with or without fibrosis, which can progress to advanced liver fibrosis, cirrhosis and HCC. Obesity, metabolic syndrome, insulin resistance, and diabetes exacerbates the course of NASH, which elevate the risk of HCC. The growing prevalence of obesity are related with increasing incidence of NASH, which may play a growing role in HCC epidemiology worldwide. In addition, HCC initiation and progression is driven by reprogramming of metabolism, which indicates growing appreciation of metabolism in the pathogenesis of this disease. Although no specific preventive pharmacological treatments have recommended for NASH, dietary restriction and exercise are recommended. This review focuses on the molecular connections between HCC and NASH, including genetic and risk factors, highlighting the metabolic reprogramming and aberrant epigenetic alterations in the development of HCC in NASH. Current therapeutic aspects of NASH/HCC are also reviewed.

Metabolomics profiles in acute-on-chronic liver failure: Unveiling pathogenesis and predicting progression

Acute-on-chronic liver failure (ACLF) usually develops based on acute decompensation (AD) of cirrhosis and is characterized by intense systemic inflammation, multiple organ failure, and high short-term mortality. Validated biomarkers for the diagnosis and prognosis of ACLF remain to be clarified. Metabolomics is an emerging method used to measure low-molecular-weight metabolites and is currently frequently implemented to understand pathophysiological processes involved in disease progression, as well as to search for new diagnostic or prognostic biomarkers of various disorders. The characterization of metabolites in ACLF has recently been described via metabolomics. The role of metabolites in the pathogenesis of ACLF deserves further investigation and improvement and could be the basis for the development of new diagnostic and therapeutic strategies. In this review, we focused on the contributions of metabolomics on uncovering metabolic profiles in patients with ACLF, the key metabolic pathways that are involved in the progression of ACLF, and the potential metabolite-associated therapeutic targets for ACLF.

Untargeted lipidomics uncovers lipid signatures that distinguish severe from moderate forms of acutely decompensated cirrhosis

BACKGROUND & AIMS

Acute decompensation (AD) of cirrhosis is a heterogeneous clinical entity associated with moderate mortality. In some patients, this condition develops quickly into the more deadly acute-on-chronic liver failure (ACLF), in which other organs such as the kidneys or brain fail. The aim of this study was to characterize the blood lipidome in a large series of patients with cirrhosis and identify specific signatures associated with AD and ACLF development.

METHODS

Serum untargeted lipidomics was performed in 561 patients with AD (518 without and 43 with ACLF) (discovery cohort) and in 265 patients with AD (128 without and 137 with ACLF) in whom serum samples were available to perform repeated measurements during the 28-day follow-up (validation cohort). Analyses were also performed in 78 patients with AD included in a therapeutic albumin trial (43 patients with compensated cirrhosis and 29 healthy individuals).

RESULTS

The circulating lipid landscape associated with cirrhosis was characterized by a generalized suppression, which was more manifest during AD and in non-surviving patients. By computing discriminating accuracy and the variable importance projection score for each of the 223 annotated lipids, we identified a sphingomyelin fingerprint specific for AD of cirrhosis and a distinct cholesteryl ester and lysophosphatidylcholine fingerprint for ACLF. Liver dysfunction and infections were the principal net contributors to these fingerprints, which were dynamic and interchangeable between patients with AD whose condition worsened to ACLF and those who improved. Notably, blood lysophosphatidylcholine levels increased in these patients after albumin therapy.

CONCLUSIONS

Our findings provide insights into the lipid landscape associated with decompensation of cirrhosis and ACLF progression and identify unique non-invasive diagnostic biomarkers of advanced cirrhosis.

LAY SUMMARY

Analysis of lipids in blood from patients with advanced cirrhosis reveals a general suppression of their levels in the circulation of these patients. A specific group of lipids known as sphingomyelins are useful to distinguish between patients with compensated and decompensated cirrhosis. Another group of lipids designated cholesteryl esters further distinguishes patients with decompensated cirrhosis who are at risk of developing organ failures.

Association between Lipid Profiles and the Incidence of Hepatocellular Carcinoma: A Nationwide Population-Based Study

Simple Summary

Cholesterol plays an important role in cell structure and cell proliferation. Altered lipid metabolism have been implicated in the development of hepatocellular carcinoma (HCC). This study investigated the relationships between lipid profiles and HCC development using large-scale, nationally representative data from the Korean National Health Insurance Service. During a median of 7.3 years follow-up, 26,891 incident HCCs were identified. The incidence of HCC gradually decreased according to the increase of total-cholesterol and LDL-cholesterol. This inverse association was consistent across subgroups stratified by the presence of liver cirrhosis or viral hepatitis. This large nationwide population-based study suggests that low lipid profile is an independent risk factor and preclinical marker for HCC.

Abstract

Background and Aims: Altered lipid metabolism has been implicated in the development of hepatocellular carcinoma (HCC). This study investigated the relationships between lipid profiles and HCC development. Methods: Data were obtained from the Korean National Health Insurance Service from 2009 to 2017. Cox regression analysis was used to examine the hazard ratios of HCC in 8,528,790 individuals who had undergone health check-ups in 2009. Results: During a median of 7.3 years follow-up, 26,891 incidents of HCCs were identified. The incidence of HCC (per 100,000 person-years) gradually decreased according to the increase in total-cholesterol and LDL-cholesterol; the incidence of HCC was 69.2, 44.0, 33.9, and 25.8 in quartile-1 (Q1), Q2, Q3, and Q4 population of total-cholesterol, and 63.6, 44.5, 37.2, and 28.3 in Q1, Q2, Q3, and Q4 population of LDL-cholesterol, respectively. Compared to Q1 of total-cholesterol, subjects in higher total-cholesterol levels were associated with a lower incidence of HCC (multiple covariates-adjusted hazard ratio (aHR): Q2 0.61; Q3 0.46; Q4 0.36). These associations were consistently observed in stratified subgroup analysis by the presence of liver cirrhosis or viral hepatitis. Conclusions: Low serum lipid levels were significantly associated with the increased risk of developing HCC. A low lipid profile might be an independent risk factor and preclinical marker for HCC.

Tumor Resection Induces Alterations on Serum Phospholipidome of Liver Cancer Patients

Hepatocellular carcinoma and cholangiocarcinoma are the most common primary malignant liver tumors. Since the liver plays a key role in lipid metabolism, the study of serum phospholipid (PL) profiles may provide a better understanding of alterations in hepatic lipid metabolism. In this study, we used a high-resolution HILIC-LC-MS lipidomic approach to establish the serum phospholipidome profile of patients with liver cancer before (T0) and after tumor resection (T1) and a control group (CT) of healthy individuals. After the analysis of PL profiles, we observed that the phospholipidome of patients with liver cancer was significantly modified after the tumor resection procedure. We observed an upregulation of some phosphatidylcholine (PtdCho) species, namely, PtdCho(36:6), PtdCho(42:6), PtdCho(38:5), PtdCho(36:5), PtdCho(38:6) and choline plasmalogens (PlsCho), and/or 1-O-alkyl-2-acyl-glycerophosphocholine (PakCho) in patients with liver cancer at T0 compared to the CT group, and a downregulation after tumor resection (T1) when compared to T0. These results show that LC-MS can detect different serum PL profiles in patients with liver cancer, before and after tumor resection, by defining a specific PL fingerprint that was used to determine the effect of tumor and tumor resection on lipid metabolism.

Lipidomics in the study of lipid metabolism: Current perspectives in the omic sciences

The advances in systems biology and in the development of new technological tools in analysis, as well as in the omic sciences, among which, metabolomics, and more specifically, lipidomics, have made it possible to investigate the structural and functional complexity of lipids in biological systems. Liquid chromatography and mass spectrometry are the analytical approaches most used in lipid research. Biomedical research, with the development of specific markers for lipids, together with new software development, have both enabled the early diagnosis of several illnesses, besides the evaluation of drug activity and treatment efficacy.

Proteomic analysis of HCV cirrhosis and HCV-induced HCC: identifying biomarkers for monitoring HCV-cirrhotic patients awaiting liver transplantation

BACKGROUND

Progression from chronic hepatitis C virus (HCV) infection to cirrhosis and hepatocellular carcinoma (HCC) results in protein changes in the peripheral blood. We evaluated global protein expression in plasma samples of HCV-cirrhotic and HCV-cirrhotic-HCC patients.

PATIENTS AND METHODS

Plasma samples from 25 HCV-cirrhotic-HCC and 10 HCV-cirrhotic patients were quantitatively evaluated for protein expression. Tryptic peptides were analyzed using Thermo linear ion-trap mass spectrometer (LTQ) coupled with a Surveyor HPLC system (Thermo). SEQUEST and X!Tandem database search algorithms were used for peptide sequence identification. Protein relative quantification was performed using the area under the curve from the select ion chromatogram. A significant fold change between groups was based on controlling the false discovery rate (FDR) at less than 5%.

RESULTS

We identified and quantified 2320 proteins from the analysis of the different protein pattern between HCV-cirrhosis and HCV-HCC samples. Gene ontology terms classified the more important biologic process related to these proteins as signal transduction, regulation of transcription DNA-dependent, protein amino acid phosphorylation, cell adhesion, transport, and immune response. Seven proteins showed significant expression changes with a FDR less than 5% between cirrhosis and tumor groups. Moreover, 18 proteins showed significant expression changes (FDR <5%) when plasma samples from HCV-cirrhosis were compared with early HCV-HCC.

CONCLUSIONS

Differential protein expression was observed between samples from HCV patients with cirrhosis with and without HCC. Also, differences were observed between early and advanced HCV-HCC samples. This study provides important information for discovery of potential biomarkers for early HCC diagnosis in HCV cirrhotic patients.

Lipids changes in liver cancer

Liver is one of the most important organs in energy metabolism. Most plasma apolipoproteins and endogenous lipids and lipoproteins are synthesized in the liver. It depends on the integrity of liver cellular function, which ensures homeostasis of lipid and lipoprotein metabolism. When liver cancer occurs, these processes are impaired and the plasma lipid and lipoprotein patterns may be changed. Liver cancer is the fifth common malignant tumor worldwide, and is closely related to the infections of hepatitis B virus (HBV) and hepatitis C virus (HCV). HBV and HCV infections are quite common in China and other Southeast Asian countries. In addition, liver cancer is often followed by a procession of chronic hepatitis or cirrhosis, so that hepatic function is damaged obviously on these bases, which may significantly influence lipid and lipoprotein metabolism in vivo. In this review we summarize the clinical significance of lipid and lipoprotein metabolism under liver cancer.

Influence of liver cancer on lipid and lipoprotein metabolism

Liver plays a key role in the metabolism of plasma apolipoproteins, endogenous lipids and lipoproteins. Hepatocellular carcinoma (HCC) is one of the most common fatal malignant tumors in China and in other Southeast Asian countries. This has been attributed to the high incidence of hepatitis B infection. Hepatitis B proteins, such as the hepatitis B X protein (HBx) that is large hepatitis B surface protein could regulate transcription of many candidate genes for liver carcinogenesis. It has known that patients who suffered from acute hepatitis B could have lipid disorders such as decreased plasma level of high-density lipoproteins (HDL). Furthermore, aberrations of lipid metabolism are often seen in the chronic hepatitis B infection. Plasma lipid profiles could be changed under HCC. In majority of the reports in HCC, plasma levels of triglycerides (TG), cholesterol, free fatty acids (FFA), HDL, low-density lipoproteins (LDL), lipoprotein (a) (Lp(a)), apolipoprotein AI (apoAI) and apoB were slight to significantly decreased, however, in some cases plasma levels of TG and Lp(a) might be increased. It has been suggested that analysis of plasma levels of lipids, lipoproteins and apolipoproteins in the patients suffered from HCC reflects on the hepatic cellular impairment status. Studies revealed that alterations seen in the plasma levels of lipids, lipoproteins and apolipoproteins reflecting patients' pathologic conditions. Decreased serum levels of cholesterol and apoAI may indicate a poor prognosis. Human leukaemic cells and certain tumor tissues have a higher receptor-mediated uptake of HDL and LDL than the corresponding normal cells or tissues. LDL and HDL have therefore been proposed as a carrier for the water-insoluble anti-cancer agents.

Measurement of antioxidant activity in lipoproteins using enhanced chemiluminescence

We describe a new assay for antioxidant activity (AOA) in lipoprotein solutions based upon their potential to quench light emission from a glowing horseradish peroxidase-catalyzed enhanced chemiluminescent reaction. By comparison with the quenching activity of the tocopherol-analogue trolox the AOA can be quantified. These measurements suggest that all lipoprotein fractions have significant AOA and that this has a non-linear relationship with lipoprotein concentration increasing significantly on a per particle basis at higher concentrations. Mean AOA in very low density, low density and high density lipoprotein fractions were 39.9 +/- 5.3, 20.3 +/- 4.0 and 5.3 +/- 1.0 mumol of trolox equivalents per litre, respectively, when measured at 1 mg protein/ml. Using known values for the protein content of the lipoprotein fractions, these values correspond to 79.8 +/- 10.7, 10.3 +/- 2.0 and 0.84 +/- 0.15 equivalents per particle. Parallel measurements of light emission and conjugated diene formation suggest that the oxidative stress imposed by the peroxidase-catalyzed reaction leads to lipid peroxidation but only after all AOA has been exhausted. AOA was significantly correlated with the alpha-tocopherol content in 30 lipoprotein samples (r = 0.764). This assay offers a rapid and simple method for investigating the effects of diseases, drugs or dietary manipulation on lipoprotein AOA.

Regulatory effects of multifunctional cytokines and steroid hormones on apolipoprotein B production by human fetal hepatocytes

OBJECTIVE

The purpose of the present investigation was to determine the effects of certain multifunctional cytokines (tumor necrosis factor-alpha [TNF-alpha], interleukin-1 beta [IL-1 beta], and IL-6) and steroid hormones (estradiol, testosterone, progesterone, and cortisol) on the production of apolipoprotein B (Apo B) by cultured human fetal hepatocytes. We conducted these experiments because of our recent observations that purified human fetal Kupffer cells produce TNF-alpha, IL-1 beta, and IL-6.

METHODS

Human fetal hepatocytes, specifically depleted of hematopoietic precursors and Kupffer cells, were cultured in defined medium. The amounts of Apo B released into the culture medium were measured by a sensitive and specific enzyme-linked immunosorbent assay.

RESULTS

Hepatocytes (10(6) cells) produced 82 +/- 16 ng Apo B per 24 hours during days 4-7 in culture. Results demonstrated that treatment of cultured hepatocytes with TNF-alpha maximally inhibited Apo B production by 50% at a half-maximal concentration of 100 pg/mL, whereas IL-1 beta maximally inhibited Apo B production by 80% at a half-maximal dose of 200 pg/mL. Cells exposed to IL-6 produced increased amounts of Apo B, but only after IL-6 was removed from the culture medium. The addition of TNF-alpha, IL-1 beta, or IL-6 did not significantly affect hepatocyte viability. At physiologic concentrations (1 mumol/L), estrogens were able to increase the production of Apo B by 25-65%; however, no positive or negative effect could be demonstrated with dexamethasone, cortisol, testosterone, or progesterone. When using synthetic estrogens such as ethinyl estradiol and mestranol, the stimulatory effect was most pronounced.

CONCLUSION

Tumor necrosis factor-alpha and IL-1 beta have an inhibitory effect and estrogens have a stimulatory effect on Apo B production by human fetal hepatocytes in culture. These studies suggest that fetal hepatocytes can produce Apo B and that the synthesis of Apo B is under the control of multifunctional cytokines and steroid hormones.

Regulation of hepatic and non-hepatic apolipoprotein A-I and E gene expression during liver regeneration

In this study, we have determined what tissues other than liver express apolipoprotein (apo) A-I and apo E genes during liver regeneration at the level of the specific mRNAs, and have compared these findings with the serum values of high-density lipoprotein (HDL), low-density lipoprotein (LDL) and very-low-density lipoprotein (VLDL). Our results show that liver and intestine express most of the apo A-I mRNA during liver regeneration. Although apo E mRNA is expressed principally by the liver, its expression is reduced in liver during regeneration but is increased markedly in non-hepatic tissues, such as in intestine, kidney, lung and brain. These results suggest that humoral or circulating factors released during liver regeneration influence apolipoprotein E gene expression, not only in hepatic but also in non-hepatic tissue.

Mechanisms of hypertriglyceridemia in the coconut oil/cholesterol-fed rabbit. Increased secretion and decreased catabolism of very low density lipoprotein

Rabbits fed a 14% coconut oil/0.5% cholesterol (CNO/Chol) diet develop mild to severe hypertriglyceridemia compared with rabbits fed a 14% olive oil/0.5% cholesterol (OO/Chol) diet. Lipids and apolipoprotein (apo) B were significantly higher in the very low density lipoprotein (VLDL) and intermediate density lipoprotein fractions from CNO/Chol than from OO/Chol rabbits. Yet, the particle diameters of these lipoproteins were similar in both diet groups, indicating that CNO/Chol rabbits had a much larger number of VLDL and intermediate density lipoprotein particles in plasma. Although the composition of CNO/Chol VLDL differed from that of OO/Chol VLDL, the rates of triglyceride hydrolysis of CNO/Chol VLDL and OO/Chol VLDL by postheparin lipoprotein lipase in vitro were the same, suggesting that VLDLs from the two diet groups were equally good substrates for lipoprotein lipase. To determine the mechanisms of hypertriglyceridemia in the CNO/Chol rabbit, triglyceride and apo B of CNO/Chol VLDL and OO/Chol VLDL were labeled with tritium-containing triolein and iodine-131 and injected intravenously into CNO/Chol and OO/Chol rabbits. The fractional clearance rate for triglyceride in OO/Chol rabbits was twice that of CNO/Chol rabbits, which parallels the previously observed differences in postheparin lipoprotein lipase activity. Although the average fractional removal of apo B did not differ between diet groups, there was a significant inverse relation between plasma cholesterol and apo B fractional clearance rate. We conclude that the hypertriglyceridemia and the enhanced hypercholesterolemia in the CNO/Chol rabbit results primarily from increased hepatic secretion of VLDL and a modest decrease in VLDL triglyceride clearance capacity.

The surface properties of apolipoproteins A-I and A-II at the lipid/water interface

The monolayer system was employed to investigate the relative affinities of apolipoproteins A-I and A-II for the lipid/water interface. The adsorption of reductively 14C-methylated apolipoproteins to phospholipid monolayers spread at the air/water interface was determined by monitoring the surface pressure of the mixed monolayer and the surface concentration of the apoprotein. ApoA-II has a higher affinity than apoA-I for lipid monolayers; for a given initial surface pressure, apoA-II adsorbs more than apoA-I to monolayers of egg phosphatidylcholine (PC), distearoyl-PC and human high-density lipoprotein (HDL3) surface lipids. Comparison of the molecular packing of apolipoproteins A-I and A-II suggests that apoA-II adopts a more condensed conformation at the lipid/water interface compared to apoA-I. The ability of apoA-II to displace apoA-I from egg PC and HDL3 surface lipid monolayers was studied by following the adsorption and desorption of the reductively 14C-methylated apolipoproteins. At saturating subphase concentrations of the apoproteins (3.10(-5) g/100 ml), two molecules of apoA-II absorbed for each molecule of apoA-I displaced. This displacement was accompanied by an increase in surface pressure. An identical stoichiometry for the displacement of apoA-I from HDL particles by apoA-II has been reported by others. At low subphase concentrations of apoproteins (5.10(-6) g/100 ml), the apoA-I/lipid monolayer was not fully compressed and could accommodate the adsorbing apoA-II molecules without displacement of apoA-I molecules. ApoA-I molecules were unable to displace apoA-II from the lipid/water interface. The average residue hydrophobicity of apoA-II is higher than that of apoA-I; this may contribute to the higher affinity of apoA-II for lipids compared to apoA-I. The probable helical regions in apolipoproteins A-I and A-II were located using a secondary structure prediction algorithm. The analysis suggests that the amphiphilic properties of the alpha-helical regions of apoA-I and apoA-II are probably not significantly different. Further understanding of the differences in surface activity of these apolipoproteins will require more knowledge of their secondary and tertiary structures.

Human lipoprotein influence on the partition of 2,2',4,4',5,5'-hexabromobiphenyl between 3T3L1 adipocytes and culture medium

A previous study established that 2,2',4,4',5,5'-hexabromobiphenyl (HBB) entered 3T3L1 adipocytes in culture by passive diffusion from the surrounding medium. The extent to which HBB accumulated within the cell was mediated by the level of triglyceride in the cell. The present study was concerned with the conditions that would facilitate HBB removal from adipocytes as part of a continuing effort to establish an effective and safe technology for reducing body burdens of lipophilic xenobiotics. Addition of human lipoprotein to the culture medium increased HBB removal from preloaded adipocytes 18 to 80 times more than did the addition of other blood proteins. Lipoproteins also decreased equilibrium deposition of HBB in the cells. The order of effect was low-density lipoprotein (LDL) much greater than high-density lipoproteins (HLD) greater than very-low-density lipoproteins (VLDL). These results are consistent with the hypothesis that lipoproteins act as a depot by binding HBB to immobilize the xenobiotic in the medium. The rate of removal of HBB was correlated with concentrations of lipoprotein cholesterol, cholesterol ester, and phospholipid in the culture medium (r greater than .95). Total lipoprotein fractions from individuals with high levels of serum cholesterol significantly increased HBB removal from preloaded adipocytes when compared with lipoproteins from normal human serum. Decreased removal was observed with lipoproteins from individuals with low serum cholesterol or triglyceride. These results suggest that cholesterol and/or cholesterol esters in the blood play an important role in both delivery and removal of HBB from the adipose tissue. Evidence has been presented that supports the hypothesis that HBB moves freely across the adipocyte membrane and is sequestered in either the cell or pseudoblood according to its relative solubility in these compartments.

Apolipoprotein measurements in clinical biochemistry and their utility vis-a-vis conventional assays

Qualitative and quantitative measurements of apolipoproteins such as Apo CII and Apo E, as well as Apo AI and Apo B can aid in the diagnosis of specific lipoprotein metabolic defects. When used in this way, their usefulness will probably remain more or less confined to specialized research centers which have the expertise to perform such analyses and interpret the measurements. In addition, however, Apo AI and Apo B also promise to be useful for the estimation of cardiovascular risk both in individuals and in populations, and both of these apolipoprotein measurements will probably supplement rather than replace conventional lipoprotein measurements. As with the lipoproteins, the usefulness of Apo AI and Apo B measurements will depend on the accuracy and precision with which they can be measured. The current standardization efforts should lead to the development of reference methods for the measurement of total plasma levels of Apo AI and Apo B, the availability of reference materials for these apolipoproteins, and eventually, to the establishment of methods that can specifically measure Apo AI and Apo B in particular lipoprotein subfractions.

Relationship of serum triglyceride concentration to lipoprotein composition and concentration in normolipidemic and hyperlipidemic subjects

In a series of 438 subjects (184 normolipidemics and 254 hyperlipidemics) the relationship among serum concentration of triglycerides, lipoprotein lipids and apoproteins A-I and B has been evaluated. The results show that as serum triglyceride level increases, VLDL rise and become enriched in triglycerides. The increase of VLDL is associated with a reduction of serum levels of LDL and HDL which appear to be rich in triglycerides and poor in cholesterol. The decrease in serum HDL level is mainly due to a reduction in serum concentration of the HDL2 subfraction. The triglyceride content of HDL2 and HDL3 rises with increasing serum triglycerides. The increase in serum triglyceride concentration seems then to be associated with a complex metabolic derangement which involves all the lipoprotein fractions.

Incorporation of cholesterol into apolipoprotein A-I-dimyristoylphosphatidylcholine recombinants

Apolipoprotein A-I (apoA-I) spontaneously associates with dimyristoylphosphatidylcholine (DMPC) liposomes to form discoidal high-density lipoprotein (HDL) recombinants. The uptake of cholesterol by this model HDL was studied by incubation with Celite-dispersed cholesterol. Separation of the resulting complexes by gradient centrifugation and gel filtration showed a heterogeneous distribution of particle size and composition as a consequence of the disruption and rearrangement of the recombinants. Quantitation of the amount of cholesterol taken up gave values between about 28 and 40 mol% cholesterol for the fractions within the protein peaks; the fractions with the lowest DMPC/apoA-I ratios had the lowest cholesterol contents. In another set of experiments, the association of apoA-I with DMPC-cholesterol liposomes was shown to result in complexes with characteristics similar to those obtained by the cholesterol-uptake experiments. Low concentrations of cholesterol in the liposomes enhanced the rate of lipid-protein association, but larger amounts decreased the yield of complexes by making the process thermodynamically and kinetically unfavorable. The enthalpy of recombinant formation increased with decreasing lipid/protein ratio and increasing cholesterol content, and became endothermic at about 23 mol% cholesterol. The effect of cholesterol on the thermal properties of HDL recombinants suggests that cholesterol is partially excluded from the boundary region adjacent to apoA-I. It is concluded that discoidal HDL recombinants, as a model for 'nascent' HDL, can acquire substantial amounts of cholesterol, which may be of great physiological importance for the reverse cholesterol transport and prevention of atherosclerosis.

Probucol reduces the rate of association of apolipoprotein C-III with dimyristoylphosphatidylcholine

The effect of low concentrations of probucol and cholesterol on the association of dimyristoylphosphatidylcholine with human plasma apolipoprotein C-III was studied. Liposomes of dimyristoylphosphatidylcholine with or without probucol or cholesterol were prepared by swelling the lipids in buffer at 37 degrees C. The association of apolipoprotein C-III with the liposomes was determined at 24 degrees C by measuring the rate of clearing of turbidity at 400 nm following addition of protein. At a weight ratio of probucol/dimyristoylphosphatidylcholine of 1:25 (5 mol% probucol), the rate of clearing of liposomes was decreased by 60%; 5 mol% cholesterol had no effect on the clearing rate. Liposomes were then added to the preformed apolipoprotein C-III/lipid micelles. In the absence of probucol, the added liposomes cleared rapidly regardless of the presence or absence of cholesterol. With 5 mol% probucol, almost no decrease in absorbance was noted on addition of liposomes to the micelles. These data show that probucol reduces the rate of association of an apolipoprotein with lipid and suggests that the interaction of probucol with lipid may modify the assembly and/or metabolism of lipoproteins.

A comparison of the surface activities of human apolipoproteins A-I and A-II at the air/water interface

Surface pressure (pi) and adsorption isotherms for human apolipoproteins A-I and A-II at the air/water interface have been determined and used to deduce the probable molecular structures of the monomolecular films. The surface concentrations were measured using the surface radioactivity method to monitor the adsorption of reductively [14C]methylated apoproteins. Apolipoprotein A-I and apolipoprotein A-II are extremely surface-active proteins and adsorb to exert maximal pi values of 22 and 24 mN.m-1 respectively, at a steady-state subphase concentration of about 3.10(-5) g/100 ml (equivalent to 11 and 17 nM for apolipoprotein A-I and apolipoprotein A-II, respectively). At saturation monolayer coverage, the average molecular areas for apolipoprotein A-I and apolipoprotein A-II are 15 and 13 A2/residue, respectively. These packing densities are consistent with monolayers consisting largely of alpha-helical protein molecules lying with the long axes of the helical segments in the plane of the interface. Comparison of the molecular packings of spread and adsorbed monolayers of these proteins indicates that at low pi values, the adsorbed films are more expanded, but at high pi values, the molecular packing in both types of film is the same.

The interaction of prostaglandins with high-density lipoproteins: a non-equilibrium model of ligand-receptor interaction

Using high-density lipoproteins (HDL) labeled with a fluorescent phospholipid probe (an anthrylvinyl-labeled analogue of sphingomyelin) it was found that low amounts (10(-12) M) of the prostaglandins E1 and F2 alpha induced different structural changes of the HDL surface, whereas prostaglandin E2 had no effect. The effects of prostaglandin E1 on HDL were largely paralleled by those of this prostaglandin on synthetic recombinants prepared from apolipoprotein A1, phospholipids and cholesterol. The prostaglandin E1-HDL interaction resembled that of a ligand with a receptor site because it was specific, reversible, concentration- and temperature-dependent and saturable. However, the maximal HDL retaining capacity for prostaglandin E1 as determined by equilibrium dialysis was very low, and a single prostaglandin E1 molecule was able to induce structural changes in a large number of discrete lipoprotein particles. To explain this remarkable fact, a non-equilibrium model of ligand-receptor interaction is proposed. According to this model in open systems characterized by a short life-time of the ligand-receptor complex, high diffusion rates of the ligand and long relaxation times which exceed the interval between two successive ligand-receptor occupations, the ligand-induced changes will accumulate, resulting in amplification of the primary biological signal. It is emphasized that the low mobility of lipids constituting the environment of the receptor protein plays a critical role in this type of signal amplification.

Characterization and specificity of lipoprotein binding to term human placental membranes

The binding characteristics of very-low-density (VLDL), low-density (LDL) and high-density (HDL) lipoprotein fractions to a purified human term placental microvillous membrane preparation were determined. Binding of LDL was saturable with a maximal binding capacity of 270 ng LDL protein per mg of membrane protein. Scatchard analysis revealed the presence of a single population of 3.4 X 10(11) sites per mg of membrane protein and a mean affinity constant of 5.8 X 10(-9) M. Binding of VLDL was also saturable but the maximal capacity was 4.5-times greater than that of LDL. The Scatchard analysis revealed the presence of 2.1 X 10(11) binding sites and an affinity constant nearly one order of magnitude greater than that of LDL. Binding of HDL showed less tendency to saturate. Scatchard analysis showed a similar number of receptor sites to that calculated for VLDL and LDL but the affinity constant for HDL was over 100-fold less than that of VLDL. Self- and cross-inhibition studies of VLDL and LDL binding revealed that VLDL was better at blocking the binding of LDL than was LDL itself. This preferential binding of VLDL suggests that this lipoprotein fraction could be an important source of cholesterol for placental progesterone production.

Lipoproteins and atherosclerosis

The plasma lipoproteins are the primary means of transport of cholesterol among tissues. In particular, the apo B-containing lipoproteins (VLDL, IDL and LDL) are important for the delivery of cholesterol from the liver to peripheral tissues, while HDL appear to mediate the reverse process of movement of cholesterol from tissues back to the liver. Both of these transport processes are necessary for efficient whole body cholesterol homeostasis, because the liver is the major site of both the production and excretion of cholesterol. However, deviations from a proper balance of transport of cholesterol, either increases in LDL levels or decreases in HDL cholesterol flux, may result in accumulation of cholesterol in extrahepatic tissues. Increased risk of atherosclerosis and CHD may be associated with elevation in the number of LDL particles, increase or decrease in LDL particle size, or changes in the composition of plasma LDL. These modifications of plasma LDL may be brought about following perturbation of one of several aspects of LDL metabolism. These include decreased LDL receptor activity, increased VLDL production and cholesterol enrichment of the liver-derived VLDL. The events in the arterial wall that make some LDL particles apparently atherogenic are not well understood. In the case of nonhuman primates, large-size LDL are associated with an increased risk of CHD. One characteristic of these LDL is that their core lipids are rich in saturated cholesteryl esters and their transition temperatures are frequently above body temperature. The liquid crystalline cholesteryl ester cores of such LDL may modulate the conformation of apo B on the surface and thereby affect the interaction of these LDL with cellular receptors or connective tissue matrix proteoglycans. It is likely, though, that changes in LDL particle number, LDL particle size and LDL particle composition may each contribute to progression of atherosclerosis. The presumed metabolic events that make HDL protective against atherosclerosis have been termed reverse cholesterol transport, and suggest that small HDL that are deficient in free cholesterol acquire this lipid from cell membranes. The HDL cholesterol is esterified by LCAT in the circulation, forming large HDL that can then deliver the cholesteryl ester to the liver by both direct and indirect means. In most circumstances, it is assumed that an increase in plasma HDL cholesterol concentration reflects an increase in the rate at which HDL is removing cholesterol from tissues and, consequently, a decrease in atherosclerosis.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of AY 9944 and chlorpromazine on Concanavalin A-induced stimulation of human lymphocytes

Amphiphilic molecules AY 9944 and chlorpromazine (CPZ) inhibited DNA synthesis in Concanavalin A-stimulated lymphocytes in a dose-dependent manner. While AY 9944 strongly decreased 7-dehydrocholesterol conversion to cholesterol, CPZ did not significantly affect this reaction. Moreover, the inhibitory effect of AY 9944 and CPZ on DNA synthesis took place in the presence of cholesterol in the culture medium. These findings suggest that the mechanism of inhibition of DNA synthesis by AY 9944 or CPZ is not related to endogenous cholesterol synthesis or exogenous cholesterol supply. Results are discussed in relation to the amphiphilic properties of AY 9944 and CPZ and to the interaction of these drugs with membranes or other intracellular targets such as calmodulin.

Apolipoprotein composition of bovine lipoproteins isolated by gel filtration chromatography

1. Bovine lipoproteins were isolated from plasma by gel filtration and apolipoprotein composition determined by SDS-polyacrylamide gel electrophoresis. 2. Bovine triglyceride-rich lipoproteins contained a novel low mol. wt protein Mr = 22,000 and low mol. wt proteins that may be analogous to non-ruminant apolipoproteins A-I, A-IV, and E. 3. Apolipoprotein C appeared to be a minor constituent of bovine triglyceride-rich lipoproteins. 4. Triglyceride-rich lipoproteins contained two high mol. wt proteins of approx. Mr = 220,000 and 290,000. 5. The predominant bovine low density lipoprotein apolipoprotein was approx. Mr = 290,000, however, greater then 25 proteins were often observed between Mr = 110,000 and 370,000. 6. Bovine high density lipoprotein contained proteins analogous to apolipoprotein A-I and C apolipoproteins. 7. Differences in apolipoprotein profiles between non-lactating and lactating cows were not apparent.

Changes in lipids in liver and serum of rats fed a histidine-excess diet or cholesterol-supplemented diets

The influence of dietary excess (5%) L-histidine on serum and liver lipids was examined in rats. Feeding a histidine-excess diet for 3, 6, 14 or 30 days caused growth retardation, hepatomegaly and decreased liver lipids throughout the period of the experiment. Hypercholesterolemia was observed after feeding a histidine-excess diet for 6 days; then serum cholesterol continuously increased for 30 days. Serum triglyceride on day 30 in rats fed the histidine-excess diet showed a significant decrease compared to rats fed the basal diet. Serum phospholipids of rats fed the histidine-excess diet for 7 or 14 days showed a significant increase compared to rats fed the basal diet. When rats were fed a basal, histidine-excess or cholesterol-supplemented diet (0.5% and 1.0% cholesterol) for 6 days, the distribution of serum high density (HDL), low density (LDL) and very low density lipoprotein cholesterol in rats fed the histidine-excess diet was similar to that of rats fed the basal diet, whereas LDL-cholesterol increased and HDL-cholesterol decreased in rats fed the cholesterol-supplemented diet.

Increased fatty acid uptake, a factor in increased hepatic triacylglycerol synthesis in aging rats

The mechanisms responsible for the increased hepatic triacylglycerol synthesis in aging are not established. We studied [1-14C] palmitate uptake and its esterification to triacylglycerols in the isolated hepatocytes of 2-month, 10-month and 20-month-old normal rats. In all hepatocytes, palmitate uptake and its esterification were linearly related to medium palmitate concentration, but palmitate uptake and triacylglycerol synthesis by the hepatocytes of 10-month and 20-month-old rats were nearly double that observed with the cells of 2-month-old rats. These results suggest that increased fatty acid uptake by the liver cells was a contributory factor in the increased triacylglycerol synthesis observed in the liver of senescent rats. The changes in the hepatocyte leading to increased fatty acid uptake and hence increased triacylglycerol synthesis are detected as early as middle age of the rat.

Apolipoproteins B, C-III and E in two major subpopulations of low-density lipoproteins

To determine the concentration and distribution of apolipoproteins C-III and E in low density lipoproteins (LDL) of d 1.025-1.043 g/ml, fresh human plasma was fractionated by single-spin density gradient ultracentrifugation into five layers. Two major subpopulations including layer 2 (d 1.025-1.029 g/ml) and layer 3 (d 1.032-1.043 g/ml) were isolated and characterized by determination of flotation coefficient, neutral lipids and apolipoproteins B, C-III and E. The apolipoprotein B/C-III/E ratio of layer 2 was 100/(3.3 +/- 2.0)/(5.1 +/- 2.9) (wt/wt) and that of layer 3 was 100/(0.61 +/- 0.32)/(0.58 +/- 0.29) (wt/wt). These weight ratios corresponded to molar ratios of 1.0/(1.90 +/- 1.16)/(0.74 +/- 0.42) and 1.0/(0.34 +/- 0.18)/(0.08 +/- 0.04), respectively. Layer 2 contained 6-23% of the total plasma apolipoprotein B or 7-27% of total LDL2 (d 1.019-1.063 g/ml) apolipoprotein B. Layer 3 contained 41-65% of plasma apolipoprotein B or 62-86% of LDL2 apolipoprotein B. About 5-17% of apolipoprotein C-III and 8-30% of apolipoprotein E in plasma are distributed in layers 2 and 3 with the majority present in layer 2. These results show an evident apolipoprotein heterogeneity of LDL2 isolated from normolipidemic subjects. Moreover, they show that the relatively small amounts of apolipoprotein C-III and apolipoprotein E in lower-density segments of LDL2 take on a greater significance when presented in molar rather than weight concentrations. The existence of different ratios of apolipoprotein C-III/apolipoprotein E in layer 2 and layer 3 suggest the presence in LDL2 of varying amounts of several discrete apolipoprotein B- and/or apolipoprotein C-III- and apolipoprotein E-containing lipoprotein particles.

Complete protein sequence and identification of structural domains of human apolipoprotein B

Epidemiological, pathological and genetic studies show a strong positive correlation between elevated plasma concentrations of low-density lipoprotein (LDL) cholesterol and the risk of premature coronary heart disease. Apolipoprotein (apo) B-100 is the sole protein component of LDL and is the ligand responsible for the receptor-mediated uptake and clearance of LDL from the circulation. Apo B-100 is made by the liver and is essential for the assembly of triglyceride-rich very low-density lipoproteins (VLDL) in the cisternae of the endoplasmic reticulum and for their secretion into the plasma. VLDL transports triglyceride to peripheral muscle and adipose tissue, where the triglyceride is hydrolysed by lipoprotein lipase. The resultant particle, relatively enriched in cholesteryl ester, constitutes LDL. LDL delivers cholesterol to peripheral tissues where it is used for membrane and steroid hormone biosynthesis and to the liver, the only organ which can catabolize and excrete cholesterol. Plasma LDL levels are therefore determined by the balance between their rate of production from VLDL and clearance by the hepatic LDL (apo B/E) receptor pathway. Here we report the complete 4,563-amino-acid sequence of apo B-100 precursor (relative molecular mass (Mr) 514,000 (514K] determined from complementary DNA clones. Numerous lipid-binding structures are distributed throughout the extraordinary length of apo B-100 and must underlie its special functions as a nucleus for lipoprotein assembly and maintenance of plasma lipoprotein integrity. A domain enriched in basic amino-acid residues has been identified as important for the cellular uptake of cholesterol by the LDL receptor pathway.