Alterations in erythrocyte membrane lipid composition and fluidity in primary lipoprotein lipase deficiency

Metabolomics profiling reveals novel markers for leukocyte telomere length

Leukocyte telomere length (LTL) is considered one of the most predictive markers of biological aging. The aim of this study was to identify novel pathways regulating LTL using a metabolomics approach. To this end, we tested associations between 280 blood metabolites and LTL in 3511 females from TwinsUK and replicated our results in the KORA cohort. We furthermore tested significant metabolites for associations with several aging-related phenotypes, gene expression markers and epigenetic markers to investigate potential underlying pathways. Five metabolites were associated with LTL: Two lysolipids, 1-stearoylglycerophosphoinositol (P=1.6×10(-5)) and 1-palmitoylglycerophosphoinositol (P=1.6×10(-5)), were found to be negatively associated with LTL and positively associated with phospholipase A2 expression levels suggesting an involvement of fatty acid metabolism and particularly membrane composition in biological aging. Moreover, two gamma-glutamyl amino acids, gamma-glutamyltyrosine (P=2.5×10(-6)) and gamma-glutamylphenylalanine (P=1.7×10(-5)), were negatively correlated with LTL. Both are products of the glutathione cycle and markers for increased oxidative stress. Metabolites were also correlated with functional measures of aging, i.e. higher blood pressure and HDL cholesterol levels and poorer lung, liver and kidney function. Our results suggest an involvement of altered fatty acid metabolism and increased oxidative stress in human biological aging, reflected by LTL and age-related phenotypes of vital organ systems.

Sphingolipids and cardiovascular diseases: lipoprotein metabolism, atherosclerosis and cardiomyopathy

Heart disease is widely believed to develop from two pathological processes. Circulating lipoproteins containing the nondegradable lipid, cholesterol, accumulate within the arterial wall and perhaps are oxidized to more toxic lipids. Both lipid accumulation and vascular reaction to the lipids lead to the gradual thickening of the vascular wall. A second major process that in some circumstances is a primary event is the development of a local inflammatory reaction. This might be a reaction to vessel wall injury that accompanies infections, immune disease, and perhaps diabetes and renal failure. In this chapter, we will focus on the relationship between de novo synthesis of sphingolipids and lipid metabolism, atherosclerosis, and cardiomyopathy.

Lipoprotein lipase deficiency is associated with elevated acylation stimulating protein plasma levels

Acylation stimulating protein (ASP, C3adesArg) is an adipose tissue derived hormone that stimulates triglyceride (TG) synthesis. ASP stimulates lipoprotein lipase (LPL) activity by relieving feedback inhibition caused by fatty acids (FA). The present study examines plasma ASP and lipids in male and female LPL-deficient subjects primarily with the P207L mutation, common in the population of Quebec, Canada. We evaluated the fasting and postprandial states of LPL heterozygotes and fasting levels in LPL homozygotes. Homozygotes displayed increased ASP (58-175% increase, P < 0.05-0.01), reduced HDL-cholesterol (64-75% decrease, P < 0.0001), and elevated levels of TG (19-38-fold, P < 0.0001) versus control (CTL) subjects. LPL heterozygotes with normal fasting TG (1.3-1.9 mmol/l) displayed increased ASP (101-137% increase, P < 0.05-0.01) and delayed TG clearance after a fatload; glucose levels remained similar to controls. Hypertriglyceridemics with no known LPL mutation also had increased ASP levels (63-192% increase, P < 0.001). High-TG LPL heterozygotes were administered a fatload before and after fibrate treatment. The treatment reduced fasting and postprandial plasma ASP, TG, and FA levels without changing insulin or glucose levels. ASP enhances adipose tissue fatty-acid trapping following a meal; however in LPL deficiency, high ASP levels are coupled with delayed lipid clearance.

Endothelial lipase promotes apolipoprotein AI-mediated cholesterol efflux in THP-1 macrophages

OBJECTIVE

Endothelial lipase (EL) is expressed by macrophages within atherosclerotic lesions. We investigated the influence of EL expression on cholesterol efflux in macrophages.

METHODS AND RESULTS

The present study used lentivirus to introduce either EL shRNA for loss-of-function studies or EL cDNA for gain-of-function studies to investigate the role of EL in apoAI-mediated cholesterol efflux. ApoAI-mediated cholesterol efflux was decreased after EL suppression, but increased with EL overexpression in free cholesterol labeled and acLDL loaded THP-1 macrophages. Similar findings were observed in THP-1 macrophages after exogenous EL addition and in transfected 293 cells. EL-related apoAI-mediated cholesterol efflux decreased after treatment with heparin or catalytic inactivation (S149A mutation or tetrahydrolipstatin) alone, and completely inhibited in combination. Furthermore, EL expression did not change ABCA1 expression, but was positively correlated with apoAI binding to macrophages and 293 cells. This effect was mitigated after heparin treatment but not influenced by catalytic inactivation via tetrahydrolipstatin or the S149A mutation. Moreover, EL expression was positively associated with lysophosphatidylcholine production and inversely with phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin levels. Lysophosphatidylcholine treatment dose-dependently stimulated apoAI-mediated cholesterol efflux in THP-1 macrophages.

CONCLUSIONS

EL appears to promote apoAI-mediated cholesterol efflux through catalytic and noncatalytic-dependent mechanisms.

Absorption and lipoprotein transport of sphingomyelin

Dietary sphingomyelin (SM) is hydrolyzed by intestinal alkaline sphingomyelinase and neutral ceramidase to sphingosine, which is absorbed and converted to palmitic acid and acylated into chylomicron triglycerides (TGs). SM digestion is slow and is affected by luminal factors such as bile salt, cholesterol, and other lipids. In the gut, SM and its metabolites may influence TG hydrolysis, cholesterol absorption, lipoprotein formation, and mucosal growth. SM accounts for approximately 20% of the phospholipids in human plasma lipoproteins, of which two-thirds are in LDL and VLDL. It is secreted in chylomicrons and VLDL and transferred into HDL via the ABCA1 transporter. Plasma SM increases after periods of large lipid loads, during suckling, and in type II hypercholesterolemia, cholesterol-fed animals, and apolipoprotein E-deficient mice. SM is thus an important amphiphilic component when plasma lipoprotein pools expand in response to large lipid loads or metabolic abnormalities. It inhibits lipoprotein lipase and LCAT as well as the interaction of lipoproteins with receptors and counteracts LDL oxidation. The turnover of plasma SM is greater than can be accounted for by the turnover of LDL and HDL particles. Some SM must be degraded via receptor-mediated catabolism of chylomicron and VLDL remnants and by scavenger receptor class B type I receptor-mediated transfer into cells.

Lipoprotein lipase affects the survival and differentiation of neural cells exposed to very low density lipoprotein

Lipoprotein lipase (LPL) is a key enzyme involved in the metabolism of lipoproteins, providing tissues like adipose tissue or skeletal muscle with fatty acids. LPL is also expressed in the brain, fulfilling yet unknown functions. Using a neuroblastoma cell line transfected with a NEO- or a LPL-expression vector, we have developed a model to study the function of LPL in neurons exposed to native or copper-oxidized lipoproteins. The addition to the culture media of VLDL with 10 microm copper sulfate led to a significant reduction in the viability of NEO transfectants whereas LPL-transfectants were protected from this injury. In the presence of VLDL and CuSO(4), LPL transfectants were even able to display significant neurite extension. This neuritogenic effect was also observed in LPL transfectants exposed to native lipoproteins. However, addition of VLDL particles oxidized with CuSO(4) prior to their addition to the culture media resulted in neurotoxic effects on LPL transfectants. These findings suggest that the presence of LPL in cultured neuronal cells modulates the physiological response of neurons following exposure to native or oxidized lipoproteins. LPL could thus play a key role in the differentiation of Neuro-2A cells and in the pathophysiological effects of oxidative stress in several neurodegenerative disorders.

Combined effects of sphingomyelin and cholesterol on the hydrolysis of emulsion particle triolein by lipoprotein lipase

Sphingomyelin (SM) is one of the major lipids in lipoproteins. However, its function in lipoprotein metabolism is unknown. In an attempt to understand the role that this lipid plays in modulation of lipoprotein lipase (LPL)-mediated hydrolysis, triolein-based emulsion particles containing 15% (physiological concentration) and 30% of the phospholipid content as SM together with phosphatidyl choline were used as substrate for the enzyme. Using a continuous fluorescence displacement assay to measure triglyceride (triolein) hydrolysis, it is shown that LPL activity was not modified by physiological concentrations of SM. However, under these assay conditions the presence of 30% SM inhibited LPL hydrolysis. SM and cholesterol (a normal component of the lipoprotein surface monolayer) become closely associated in phospholipid monolayers and bilayers. Incorporation of cholesterol into emulsion particles containing only PC increased LPL activity, but this increase was reduced by the additional presence of a physiological concentration (15%) of SM. These model studies suggest that the ratio, cholesterol:SM, in the monolayer may regulate the hydrolytic activity of the LPL. The production of ceramide by sphingomyelinase pre-treatment of emulsion particles containing SM leads to a two- to three-fold increase in LPL activity. This effect was dependent on sphingomyelinase concentration and time of pre-incubation and was not seen with cholesterol containing substrates. The ability of apolipoprotein CII to enhance LPL-catalysed triolein hydrolysis was not affected by the presence of SM; however, the stimulatory effect of this apolipoprotein was attenuated by pre-treatment of emulsion particles with sphingomyelinase. In summary, physiological concentrations of SM can inhibit the hydrolysis of cholesterol-containing emulsion particles; while pre-treatment of SM containing emulsion particles with sphingomyelinase in the absence of cholesterol can increase LPL-mediated triglyceride hydrolysis.

Lipoprotein lipase gene variants and risk of coronary disease: a quantitative analysis of population-based studies

The purpose of this study is to quantify the magnitude of the association between common variants in the lipoprotein lipase gene and coronary disease, based on published population-based studies. Fourteen studies, representing 15,708 subjects, report allelic distribution for lipoprotein lipase gene variants among coronary disease patients and control subjects. Patient outcomes included clinical coronary disease events and documented coronary disease based on angiography. Allele frequencies are estimated for disease and non-disease groups within each study. A 2 x 2 contingency table is used to compute individual study odds ratios and 95% confidence intervals, relating the presence of the rare allele to disease status. Mantel-Haenszel-stratified analysis of each allelic variant results in a summary odds ratio and 95% confidence interval for the association between each rare allele in the lipoprotein lipase gene and coronary disease. The lipoprotein lipase D9N allele has a summary odds ratio of 1.59 (95% confidence interval 1.03-2.55), indicating a 59% increase in risk of coronary disease for carriers with this allelic variant. The lipoprotein lipase N291S allele showed no association with coronary disease (summary odds ratio 0.93, 95% confidence interval 0.73-1.19). The summary odds ratio for lipoprotein lipase S447Ter allele is 0.81 (95% confidence interval 0.65-1.0), indicating a marginal negative association between this variant and coronary disease. The common lipoprotein lipase Pvu II polymorphism shows no relation to coronary disease (summary odds ratio 0.90, 95% confidence interval 0.80-1.01). The rare allele of the lipoprotein lipase HindIII polymorphism is negatively associated with coronary disease (summary odds ratio 0.84, 95% confidence interval 0.73-0.96). The lipoprotein lipase D9N allele is associated with high levels of triglyceride and low levels of high-density lipoprotein. Similar atherogenic lipid levels are observed in subjects with structural mutations lipoprotein lipase C188E and P207L. Carriers of the S447Ter allele have low levels of triglyceride. The lipoprotein, lipase gene variants which decrease lipoprotein lipase catalytic activity are associated with familial combined hyperlipidemia, but not the elevation of apolipoprotein B seen in this disorder. In conclusion, allelic variants in the lipoprotein lipase gene are associated with altered lipid levels and differential coronary disease risk.

Correction of erythrocyte shape abnormalities in familial hypercholesterolemia after LDL-apheresis: does it influence cerebral hemodynamics?

It is well known that red blood cells incubated in low-density lipoprotein (LDL)-rich medium show shape abnormalities that revert to normal after reincubation in normal plasma. Patients with homozygous familial hypercholesterolemia (HFH) have an increased percentage of abnormally-shaped erythrocytes (mostly stomatocytes, knisocytes, and crenated cells) compared to normocholesterolemic controls: 7.73+/-0.96 versus 3.52+/-0.52 (mean+/-SEM; P = 0.001). To confirm the role of high LDL concentration in inducing red cell shape abnormalities we determined the percentage of abnormally shaped erythrocytes in seven HFH patients 1 day after the procedure of LDL-apheresis with a 40% cholesterol decrease. A reduction in kniscocytes, stomatocytes, and crenated cells was observed in the patients treated by LDL-apheresis (P < 0.01). To investigate the possible benefit of a reduction in erythrocyte shape abnormality on cerebral hemodynamics, cerebral flow velocity, as evaluated by transcranial Doppler, was evaluated concomitantly and found to be remarkably increased after apheresis (P < 0.01). No significant change in hematocrit, plasma viscosity, blood viscosity, mean pressure, or cardiac output was detected, 1 day after apheresis. An inverse correlation was demonstrated (r = 0.55; P = 0.04) between changes in the percentage of knisocytes+stomatocytes +crenated cells and percent changes in middle cerebral artery peak systolic velocity. The correction of erythrocyte shape abnormalities after LDL-apheresis might be related to dramatic changes in plasma phospholipid concentration and proportion occurring after this procedure in HFH patients. The reduction of erythrocyte shape abnormalities could contribute, together with other hemorheological factors, to the improvement of cerebral hemodynamics after LDL-apheresis.

Molecular pathobiology of the human lipoprotein lipase gene

Lipoprotein lipase (LPL; E.C. 3.1.1.34) is a key enzyme in the metabolism of lipids. Many diseases, including obesity, coronary heart disease, chylomicronemia (pancreatitis), and atherosclerosis, appear to be directly or indirectly related to abnormalities in LPL function. Human LPL is a member of a superfamily of lipases that includes hepatic lipase and pancreatic lipase. These lipases are characterized by extensive homology, both at the level of the gene and the mature protein, suggesting that they have a common evolutionary origin. A large number of natural mutations have been discovered in the human LPL gene, which are located at different sites in the gene and affect different functions of the mature protein. There is a high prevalence of two of these mutations (207 and 188) in the Province of Québec, and one of them (207) is almost exclusive to the French-Canadian population. A study of these and other naturally occurring mutant LPL molecules, as well as those created in vitro by site-directed mutagenesis, indicate that the sequence of LPL is organized into multiple structural and functional units that act in concert in the normal enzyme. In this review, we discuss the interrelationships of LPL structure and its function, the molecular etiology of abnormal LPL in humans, and the clinical and therapeutic aspects of LPL deficiency.

Anemia in copper-deficient rats: role of alterations in erythrocyte membrane fluidity and oxidative damage

This study was designed to make precise the nature and the mechanism of the anemia induced by dietary copper (Cu) deficiency. Male Wistar rats were pair fed from weanling for 6 wk either a Cu-deficient or a control diet. The reduced red blood cell (RBC) 51Cr survival indicates an increased destruction of RBC during Cu deficiency. 1,6-Diphenyl-1,3,5-hexatriene fluorescence polarization studies revealed an increase in the fluidity of erythrocyte membranes from deficient rats. The reduced cholesterol-to-phospholipid ratio was consistent with the increased fluidity. Other results indicate an increased vulnerability of RBC to hemolysis in dilute hydrogen peroxide and an increased formation of lipid peroxidation products. Before exposure to free radical stress, electron spin resonance studies in intact RBC revealed decreased correlation time of 16-doxyl-stearic acid, confirming a more fluid membrane in RBC from Cu-deficient rats. After in vitro peroxidation, RBC from Cu-deficient rats showed a more ordered state of membrane lipids compared with controls. Together, these studies demonstrate the hemolytic nature of the anemia. The shortened survival of erythrocytes apparently results from changes in membrane fluidity and enhanced susceptibility to peroxidation.

In vitro modulation of rat adipocyte ghost membrane fluidity by cholesterol oxysterols

The effects of cholesterol and cholesterol-derived oxysterols (cholestanone, cholestenone, coprostanone and epicoprostanol) on adipocyte ghost membrane fluidity were studied using a fluorescence depolarization method. The fluorescence anisotropy of the treated membranes was determined using 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). Cholestanone and cholesterol decreased membrane fluidity at both the concentrations tested (10 & 50 microM) while the rest of the sterols did not exert any significant effect on membrane fluidity. In the presence of epinephrine, cholestanone partitioned more towards the lipid core but cholesterol partitioning was not affected. The fusion activation energies (delta E) obtained for membranes preincubated with cholestanone (8.6 kcal/mol) and cholesterol (8.2 kcal/mol) were not significantly different from that of untreated membranes (8.3 kcal/mol). Membranes preincubated with cholestanone and cholesterol did not exhibit any change in lipid phase throughout the temperature range (10-45 degrees C) tested. The sterols were found to inhibit fisetin-induced phospholipid methylation in isolated rat adipocytes in the rank order of cholesterol > epicoprostanol > cholestanone = cholestenone = coprostanone, while basal methylation was unaffected. When adipocytes were preincubated with the sterols before the addition of fisetin, cholestanone and cholestenone showed 74% and 66% inhibition of maximal methylation respectively. These results indicated that cholesterol oxysterols interact differently with rat adipocyte membranes, with cholestanone interacting more with phospholipids located at the inner lipid bilayer (e.g. phosphatidylethanolamine) while cholesterol interacts more with phosphatidylcholine located at the outer lipid bilayer. This differential interaction may cause selective changes in membrane fluidity at different depths of the bilayer and thus may modulate the activities of membrane-bound proteins such as enzymes and receptors.

Hemolysis in primary lipoprotein lipase deficiency

A slight to moderate hemolysis is often present in plasma from patients with primary lipoprotein lipase (LPL) deficiency. To determine the nature of this hemolysis, we measured erythrocyte hypo-osmotic fragility, plasma free hemoglobin, and phospholipid composition in 26 patients with primary LPL deficiency and 21 unrelated controls. In some patients, these investigations were completed by erythrocyte cytoskeletal protein determinations and abdominal echography. Osmotic fragility was similar between control subjects and patients. However, there was a significantly increased concentration of plasma free hemoglobin in primary LPL deficiency (0.282 +/- 0.331 v 0.048 +/- 0.038 g/L in controls, P < .005). In LPL-deficient patients, an increase of plasma lysophosphatidylcholine concentration (12.6% +/- 5.8% v 6.4% +/- 1.9% in controls, P < .0001) was also found. The protein composition of the erythrocyte membrane skeleton was abnormal in some LPL-deficient patients and splenomegaly was present in 12, but these abnormalities did not correlate with plasma free hemoglobin levels. Bilirubin and haptoglobin levels were also within physiologic ranges in these patients, suggesting that the observed hemolysis did not result from hypersplenism. It appears likely that the accumulation of lysophosphatidylcholine was due to an impairment in the reverse metabolic pathway converting lysophosphatidylcholine back to phosphatidylcholine. Collectively, these data, along with a positive correlation between plasma free hemoglobin and lysophosphatidylcholine levels (r = .58, P = .0001), suggest that the hemolysis observed in primary LPL deficiency is mediated to some extent by the abnormally elevated concentration of lysophosphatidylcholine.