Apolipoprotein A-I regulates lipid hydrolysis by hepatic lipase

Effects of aging on fat deposition and meat quality in Sheldrake duck

Sheldrake is a duck breed widely used for its meat and eggs. In this study, the quantities of abdominal fat, sebum, intramuscular fat and liver fat, meat quality (pH, cooking loss, drip loss, and shear force), and expression and activity of several enzymes at different ages were determined. The results showed that the fat content increased (P < 0.05) during the aging process (90 d, 180 d, 270 d, and 360 d). Fatty acid synthase (FAS) and malic enzyme (ME) were chosen to represent the activity of lipid biosynthesis in Sheldrake ducks. The quantitative real-time PCR and enzymic activity data showed that the expression of both FAS and ME were generally up-regulated along with aging. Based on these results, the individual ducks were selected at 180 d and 360 d for analyzing the changes of serum lipid levels and related enzymic activities in liver. The contents of triglycerides (TG), total cholesterol (TCH), high-density lipoprotein (HDL), and low-density lipoprotein (LDL) in serum were found not significantly different (P > 0.05). However, we thought that the variation of TG/HDL (P < 0.05) might explain the increased fat deposition. Moreover, the activities of lipoprotein lipase (LPL) and hepatic lipase (HL) were both detected significantly up-regulated at 360 d (P < 0.05). The meat quality results of breast muscles indicated that pH, cooking loss, drip loss, and shear force values could all be affected by aging. Considering these results, we concluded that the best quality of Sheldrake duck meat occurs between 180 d and 270 d. These results might provide useful information for Sheldrake cultivation and research on lipid metabolism.

Binding of human apoA-I[K107del] variant to TG-rich particles: implications for mechanisms underlying hypertriglyceridemia

We found earlier that apoA-I variants that induced hypertriglyceridemia (HTG) in mice had increased affinity to TG-rich lipoproteins and thereby impaired their catabolism. Here, we tested whether a naturally occurring human apoA-I mutation, Lys107del, associated with HTG also promotes apoA-I binding to TG-rich particles. We expressed apoA-I[Lys107del] variant in Escherichia coli, studied its binding to TG-rich emulsion particles, and performed a physicochemical characterization of the protein. Compared with WT apoA-I, apoA-I[Lys107del] showed enhanced binding to TG-rich particles, lower stability, and greater exposure of hydrophobic surfaces. The crystal structure of truncated, Δ(185-243), apoA-I suggests that deletion of Lys107 disrupts helix registration and disturbs a stabilizing salt bridge network in the N-terminal helical bundle. To elucidate the structural changes responsible for the altered function of apoA-I[Lys107del], we studied another mutant, apoA-I [Lys107Ala]. Our findings suggest that the registry shift and ensuing disruption of the inter-helical salt bridges in apoA-I[Lys107del] result in destabilization of the helical bundle structure and greater exposure of hydrophobic surfaces. We conclude that the structural changes in the apoA-I[Lys107del] variant facilitate its binding to TG-rich lipoproteins and thus, may reduce their lipolysis and contribute to the development of HTG in carriers of the mutation.

Familial lipoprotein lipase deficiency: a case of compound heterozygosity of a novel duplication (R44Kfs*4) and a common mutation (N291S) in the lipoprotein lipase gene

Familial lipoprotein lipase (LPL) deficiency (FLLD) is a rare autosomal recessive genetic disorder caused by homozygous or compound heterozygous mutations in the LPL gene. FLLD individuals usually express an impaired or non-functional LPL enzyme with low or absent triglyceride (TG) hydrolysis activity causing severe hypertriglyceridaemia. Here we report a case of FLLD in a 29-year-old man, who initially presented with eruptive cutaneous xanthomata, elevated plasma TG concentration but no other co-morbidities. Subsequent genetic testing of the patient revealed compound heterozygosity of a novel duplication (p.R44Kfs*4) leading to a premature stop codon in exon 2 and a known mutation (N291S) in exon 5 of the LPL gene. Further biochemical analysis of the patient's postheparin plasma confirmed a reduction of total lipase activity compared with his heterozygous father carrying the common N291S mutation and to a healthy control. Also the patient showed increased (1.85-fold) activity of hepatic lipase (HL), indicating a functional link between HL and LPL. In summary, we report a case of FLLD caused by compound heterozygosity of a new duplication and a common mutation in the LPL gene, resulting in residual LPL activity. With such mutations, individuals may not receive a diagnosis before classical FLLD symptoms appear later in adulthood. Nevertheless, early diagnosis and lipid-lowering treatment may favour a reduced risk of premature cardiovascular disease or acute pancreatitis in such individuals.

Hepatic lipase, high density lipoproteins, and hypertriglyceridemia

Hepatic lipase (HL) is a lipolytic enzyme that contributes to the regulation of plasma triglyceride (TG) levels. Elevated TG levels may increase the risk of developing coronary heart disease, and studies suggest that mutations in the HL gene may be associated with elevated TG levels and increased risk of coronary heart disease. Hepatic lipase facilitates the clearance of TG from the very low density lipoprotein (VLDL) pool, and this function is governed by the composition and quality of high density lipoprotein (HDL) particles. In humans, HL is a liver resident enzyme regulated by factors that release it from the liver and activate it in the bloodstream. HDL regulates the release of HL from the liver and HDL structure controls HL transport and activation in the circulation. Alterations in HDL-apolipoprotein composition can perturb HL function by inhibiting the release and activation of the enzyme. HDL structure may therefore affect plasma TG levels and coronary heart disease risk.

HDL-ApoE content regulates the displacement of hepatic lipase from cell surface proteoglycans

Human hepatic lipase (HL) is an interfacial enzyme that must be liberated from cell surface proteoglycans to hydrolyze lipoprotein triglyceride. Both high-density lipoprotein (HDL) and apolipoprotein (apo)A-I can displace HL from cell surface proteoglycans, much like heparin. HL displacement is inhibited by HDL-apoE content. Postprandial HDL is approximately twofold better at displacing HL than is fasting HDL, but only has approximately one-half the apoE content. Enriching native HDL with triglyceride decreases HDL-apoE content and increases HL displacement. Incubation of HDL with the anti-apoE antibody, 6C5, also increases HL displacement. In contrast, enrichment of synthetic HDL with apoE significantly inhibits HL displacement. HDL from fasted female normolipidemic subjects displaces HL approximately twofold better than HDL from male subjects. HDL from female subjects also has significantly less apoE than HDL from males. Normolipidemic females have increased circulating HDL-bound HL. Hyperlipidemia has little effect on the HL displacement ability of HDL from men, whereas HDL from hypercholesterolemic females exhibits impaired HL displacement. HL displacement from liver heparan sulfate proteoglycans therefore appears to be linked to interlipoprotein apoE exchange. Decreased HL displacement is associated with higher HDL-apoE levels and may therefore affect vascular triglyceride hydrolysis.

The paradox of high apolipoprotein A-I levels independently predicting incident type-2 diabetes among Turks

BACKGROUND

Predictive value of apolipoprotein (apo) A-I for incident hypertension, metabolic syndrome (MetS), type 2 diabetes (DM) and coronary heart disease (CHD) needs further exploration.

METHODS

A representative sample of Turkish adults was studied with this purpose prospectively. Sex-specific apoA-I tertiles were examined regarding cardiometabolic risk.

RESULTS AND CONCLUSIONS

A total of 1044 men and 1067 women (aged 49+/-12 years at baseline) were followed up over 7.4 years. High serum apoA-I levels were significantly associated in multivariable analysis with female sex, aging, alcohol intake, (inversely) cigarette smoking and, in women, with systolic blood pressure. Risk of diabetes was predicted in logistic regression in both genders by top versus bottom apoA-I tertile (RR 1.98; [95%CI 1.31; 3.0]), additive to age, body mass index (BMI), C-reactive protein (CRP), HDL-cholesterol and lipid lowering drugs. By adding sex hormone-binding globulin to the model in a subset of the sample, the association between high apoA-I and incident diabetes was attenuated only in women. ApoA-I tertiles tended to be positively associated also with hypertension and CHD only in women but this did not reach significance. High compared with low serum apoA-I levels nearly double the risk for incident diabetes, additively to age, BMI, CRP, HDL-cholesterol among Turks. Systemic inflammation concomitant with prevailing MetS might turn apoA-I into proinflammatory particles.

HDL composition regulates displacement of cell surface-bound hepatic lipase

HDL is able to displace cell surface-bound hepatic lipase (HL) and stimulate vascular triglyceride (TG) hydrolysis, much like heparin. Displacement appears to be a result of a high-affinity association of HL and apoA-I. HDL varies in its ability to displace HL, and therefore experiments were undertaken to evaluate the impact of HDL composition and structure on HL displacement from cell surface proteoglycans. HDL apolipoprotein and lipid composition directly affect HL displacement. ApoA-II and apoC-I significantly increase HL displacement from the cell surface. While changes in HDL cholesteryl ester and fatty acid content have no effect on HL displacement, increases in HDL phospholipid and TG content significantly inhibit HL displacement. HDL fractions from hyperlipidemic patients are unable to displace HL from the cell surface. These results indicate that the structure and composition of HDL particles in plasma are central to regulation of HL displacement and the hydrolytic activity of HL.

Lipoprotein electrostatic properties regulate hepatic lipase association and activity

The effect of lipoprotein electrostatic properties on the catalytic regulation of hepatic lipase (HL) was investigated. Enrichment of serum or very low density lipoprotein (VLDL) with oleic acid increased lipoprotein negative charge and stimulated lipid hydrolysis by HL. Similarly, enrichment of serum or isolated lipoproteins with the anionic phospholipids phosphatidylinositol (PI), phosphatidic acid, or phosphatidylserine also increased lipoprotein negative charge and stimulated hydrolysis by HL. Anionic lipids had a small effect on phospholipid hydrolysis, but significantly stimulated triacylglyceride (TG) hydrolysis. High density lipoprotein (HDL) charge appears to have a specific effect on lipolysis. Enrichment of HDL with PI significantly stimulated VLDL-TG hydrolysis by HL. To determine whether HDL charge affects the association of HL with HDL and VLDL, HL-lipoprotein interactions were probed immunochemically. Under normal circumstances, HL associates with HDL particles, and only small amounts bind to VLDL. PI enrichment of HDL blocked the binding of HL with HDL. These data indicate that increasing the negative charge of HDL stimulates VLDL-TG hydrolysis by reducing the association of HL with HDL. Therefore, HDL controls the hydrolysis of VLDL by affecting the interlipoprotein association of HL. Lipoprotein electrostatic properties regulate lipase association and are an important regulator of the binding and activity of lipolytic enzymes.

Mechanism of lipoprotein retention by the extracellular matrix

PURPOSE OF REVIEW

Considerable evidence suggests that the subendothelial retention of atherogenic lipoproteins is a key early step in atherogenesis. In humans and experimental animals, elevated levels of plasma lipoproteins are associated with increased atherosclerosis, and lipoproteins with higher affinity for arterial proteoglycans are more atherogenic. Here we discuss the molecular mechanisms underlying lipoprotein retention in the arterial wall and how this interaction can be modulated.

RECENT FINDINGS

Functional proteoglycan binding sites in lipoproteins containing apolipoprotein B have been identified and shown to have atherogenic potential in vivo. In addition to apolipoprotein B, novel bridging molecules, those that can interact with both proteoglycans and lipoproteins, have been identified that mediate the retention of atherogenic particles in the vessel wall. The interaction between lipoproteins and proteoglycans can be enhanced by the modification of lipoproteins in the circulation and in the arterial wall, by alterations in the subendothelium, and by changes in proteoglycan synthesis that result in a more atherogenic profile. The retention of atherogenic lipoproteins is a potential target for therapies to reverse atherosclerosis, and in-vitro studies have identified compounds that decrease the affinity of proteoglycans for lipoproteins.

SUMMARY

Considerable progress has been made in understanding the association between lipoproteins and cardiovascular disease. This review highlights the importance of the interaction between lipoproteins and the arterial matrix.

Severe hypoalphalipoproteinemia in mice expressing human hepatic lipase deficient in binding to heparan sulfate proteoglycan

Unlike human hepatic lipase (hHL) that is mainly cell surface-anchored via binding to heparan sulfate proteoglycans (HSPG), mouse HL (mHL) has a low affinity to HSPG and thus is largely blood-borne. The reduced HSPG binding of mHL is attributable to the C-terminal amino acids. To determine the functions of HSPG binding of hHL in vivo, we created adenovirus vectors encoding hHL or a chimeric protein (designated hHLmt) in which the C-terminal HSPG-binding sequences were replaced with the corresponding mouse sequences. Injecting hHLmt-expressing virus into C57BL/6J mice (1.8 x 10(10) virus particles/mouse) resulted in a 3-fold increase in pre-heparin HL activity, whereas infection with an identical dose of hHL virus did not change pre-heparin HL activity. In hHLmt-expressing mice, the concentration of total cholesterol and phospholipids was inversely related to the hHL activity in pre-heparin plasma in a dose- and time-dependent manner, and the decrease was mainly attributable to high density lipoproteins (HDL) cholesterol and HDL phospholipids. The expression of hHL exhibited no change in plasma total cholesterol or phospholipid levels as compared with control mice infected with luciferase or injected with saline. The reduced HDL lipids in the hHLmt-expressing mice were accompanied by markedly decreased plasma and hepatic apolipoprotein (apo) A-I. In primary hepatocytes isolated from hHLmt-expressing mice, the concentration of cell-associated and secreted apoA-I was decreased by 2-3-fold as compared with hepatocytes isolated from control mice, whereas the levels of apoB and apoE were unaltered. Infection of primary hepatocytes with hHLmt virus ex vivo also resulted in reduced apoA-I secretion but had no effect on cell-associated apoA-I. These results suggest that expression of HSPG binding-deficient hHL has a profound HDL-lowering effect.

Expression profiling analysis of the metabolic and inflammatory changes following burn injury in rats

Burn injury initiates an inflammatory response as part of the healing process that is associated with extensive metabolic adjustments. While most studies have focused on understanding these changes from a biochemical perspective, not much work has been done to characterize these processes at the gene expression level. As a first step, we have comprehensively analyzed changes in gene expression in rat livers during the first 24 h after burn injury using Affymetrix GeneChips, which showed 339 genes to be differentially expressed at a statistical significance of P < 0.05 and changed at least twofold. Functional classification based on gene ontology terms indicated that two categories, metabolism (28%) and inflammation (14%), accounted for nearly 42%. Detailed analysis of the metabolism group of genes indicated that fatty acid (FA) and triglyceride (TG) biosynthesis in the liver were unchanged, whereas TG utilization, FA import, and beta-oxidation increased after burn injury. The increased FA pools after burn injury appear to serve as substrates for ATP production. Following burn injury, the cholesterol biosynthetic pathway was suppressed while cholesterol was increasingly imported and converted into bile acids. The inflammatory genes that were altered included several classic acute phase response markers, as well as genes involved in the complement, kinin, clotting, and fibrinolytic protein systems. These temporally coordinated changes in gene expression were also corroborated by biochemical measurements for FA, TG, cholesterol, and ATP. Together, these data indicate that FA are increasingly imported and oxidized in the liver to meet the enhanced energy demands arising from an inflammatory response during the first 24 h after burn injury.

Apolipoprotein A-II regulates HDL stability and affects hepatic lipase association and activity

The effect of apolipoprotein A-II (apoA-II) on the structure and stability of HDL has been investigated in reconstituted HDL particles. Purified human apoA-II was incorporated into sonicated, spherical LpA-I particles containing apoA-I, phospholipids, and various amounts of triacylglycerol (TG), diacylglycerol (DG), and/or free cholesterol. Although the addition of PC to apoA-I reduces the thermodynamic stability (free energy of denaturation) of its alpha-helices, PC has the opposite effect on apoA-II and significantly increases its helical stability. Similarly, substitution of apoA-I with various amounts of apoA-II significantly increases the thermodynamic stability of the particle alpha-helical structure. ApoA-II also increases the size and net negative charge of the lipoprotein particles. ApoA-II directly affects apoA-I conformation and increases the immunoreactivity of epitopes in the N and C termini of apoA-I but decreases the exposure of central domains in the molecule (residues 98-186). ApoA-II appears to increase HL association with HDL and inhibits lipid hydrolysis. ApoA-II mildly inhibits PC hydrolysis in TG-enriched particles but significantly inhibits DG hydrolysis in DG-rich LpA-I. In addition, apoA-II enhances the ability of reconstituted LpA-I particles to inhibit VLDL-TG hydrolysis by HL. Therefore, apoA-II affects both the structure and the dynamic behavior of HDL particles and selectively modifies lipid metabolism.

The amino acid sequences of the carboxyl termini of human and mouse hepatic lipase influence cell surface association

Human hepatic lipase (hHL) mainly exists cell surface bound, whereas mouse HL (mHL) circulates in the blood stream. Studies have suggested that the carboxyl terminus of HL mediates cell surface binding. We prepared recombinant hHL, mHL, and chimeric proteins (hHLmt and mHLht) in which the carboxyl terminal 70 amino acids of hHL were exchanged with the corresponding sequence from mHL. The hHL, mHL, and hHLmt proteins were catalytically active using triolein and tributyrin as substrates. In transfected cells, the majority of hHLs bound to the cell surface, with only 4% of total extracellular hHL released into heparin-free media, whereas under the same conditions, 61% of total extracellular mHLs were released. Like mHL, hHLmt showed decreased cell surface binding, with 68% of total extracellular hHLmt released. To determine the precise amino acid residues involved in cell surface binding, we prepared a truncated hHL mutant (hHL471) by deleting the carboxyl terminal five residues (KRKIR). The hHL471 also retained hydrolytic activity with triolein and tributyrin, and showed decreased cell surface binding, with 40% of total extracellular protein released into the heparin-free media. These data suggest that the determinants of cell surface binding exist within the carboxyl terminal 70 amino acids of hHL, of which the last five residues play an important role.

HDL regulates the displacement of hepatic lipase from cell surface proteoglycans and the hydrolysis of VLDL triacylglycerol

We have previously shown that hepatic lipase (HL) is inactive when bound to purified heparan sulfate proteoglycans and can be liberated by HDL and apolipoprotein A-I (apoA-I), but not by LDL or VLDL. In this study, we show that HDL is also able to displace HL directly from the surface of the hepatoma cell line, HepG2, and Chinese hamster ovary cells stably overexpressing human HL. ApoA-I is more efficient at displacing cell surface HL than is HDL, and different HDL classes vary in their ability to displace HL from the cell surface. HDL2s have a greater capacity to remove HL from the cell surface and intracellular compartments, as compared with the smaller HDL particles. The different HDL subclasses also uniquely affect the activity of the enzyme. HDL2 stimulates HL-mediated hydrolysis of VLDL-triacylglycerol, while HDL3 is inhibitory. Inhibition of VLDL hydrolysis appears to result from a decreased interlipoprotein shuttling of HL between VLDL and the smaller, more dense HDL particles. This study suggests that high HDL2 levels are positively related to efficient triacylglycerol hydrolysis by their ability to enhance the liberation of HL into the plasma compartment and by a direct stimulation of VLDL-triacylglycerol hydrolysis.

Overexpression of apoC-I in apoE-null mice: severe hypertriglyceridemia due to inhibition of hepatic lipase

Apolipoprotein C-I (apoC-I) has been proposed to act primarily via interference with apoE-mediated lipoprotein uptake. To define actions of apoC-I that are independent of apoE, we crossed a moderately overexpressing human apoC-I transgenic, which possesses a minimal phenotype in the WT background, with the apoE-null mouse. Surprisingly, apoE-null/C-I mice showed much more severe hyperlipidemia than apoE-null littermates in both the fasting and non-fasting states, with an almost doubling of cholesterol, primarily in IDL+LDL, and a marked increase in triglycerides; 3-fold in females to 260 +/- 80 mg/dl and 14-fold in males to 1409 +/- 594 mg/dl. HDL lipids were not significantly altered but HDL were apoC-I-enriched and apoA-II-depleted. Production rates of VLDL triglyceride were unchanged as was the clearance of post-lipolysis remnant particles. Plasma post-heparin hepatic lipase and lipoprotein lipase levels were undiminished as was the in vitro hydrolysis of apoC-I transgenic VLDL. However, HDL from apoC-I transgenic mice had a marked inhibitory effect on hepatic lipase activity, as did purified apoC-I. LPL activity was minimally affected. Atherosclerosis assay revealed significantly increased atherosclerosis in apoE-null/C-I mice assessed via the en face assay. Inhibition of hepatic lipase may be an important mechanism of the decrease in lipoprotein clearance mediated by apoC-I.

Impaired capacity of acute-phase high density lipoprotein particles to deliver cholesteryl ester to the human HUH-7 hepatoma cell line

The major role of native high density lipoprotein (HDL) is to carry cholesterol from peripheral tissues to the liver for bile excretion. As acute-phase (AP)-HDL has a decreased ability for cellular cholesterol efflux but an increased capacity for cholesteryl ester (CE) delivery to peripheral tissues, the interaction of AP-HDL with human hepatoma cells was studied. Binding studies to HUH-7 cells revealed saturable binding properties for HDL and AP-HDL at 4 degrees C. At 37 degrees C, specific cell-association of (125)I- and [1,2,6,7-(3)H]-cholesteryl palmitate ([(3)H]CE)-labeled lipoprotein particles was 2.2- and 1.6-fold higher for HDL indicating that total CE delivery was significantly (P<0.05) higher for HDL in comparison to AP-HDL. In parallel, selective CE uptake (the difference between total lipid uptake and holoparticle uptake) from AP-HDL was decreased compared with HDL. The fact that the capacity for cellular cholesterol efflux from HUH-7 cells is slightly impaired by AP-HDL (compared with HDL) is of support that scavenger receptor class B, type I (SR-BI), the only receptor so far known to mediate bi-directional lipid flux, might be involved in altered HUH-7 cholesterol hemostasis by AP-HDL. Our in vitro findings suggest that HDL and AP-HDL interact differently with cells of hepatic origin resulting in decreased hepatic cholesterol removal from the circulation during the AP reaction.