The ability of plasma to stimulate fibroblast cholesterol efflux is associated with the -629C-->A cholesteryl ester transfer protein promoter polymorphism: role of lecithin: cholesterol acyltransferase activity

A recent population-based study showed that cholesteryl ester transfer protein (CETP) gene variations, which relate to lower plasma CETP, may predict increased cardiovascular risk, in spite of higher HDL cholesterol. Among other functions, CETP activity contributes to cellular cholesterol efflux, an early step in the anti-atherogenic reverse cholesterol transport (RCT) process. We hypothesized that cellular cholesterol efflux stimulating capacity of plasma could be associated with CETP gene variation. In this study, we tested the extent to which the ability of plasma to promote cholesterol efflux from cultured human fibroblasts is associated with CETP gene variation. In 223 men, the -629C-->A CETP promoter polymorphism, plasma lipids, CETP mass, cholesteryl ester transfer (CET), lecithin:cholesterol acyltransferase (LCAT) activity and the ability of plasma to promote cholesterol efflux from human skin fibroblasts, obtained from a single normolipidemic donor, were determined. In -629CC homozygotes (n=52), cholesterol efflux, plasma CETP mass, CET and LCAT activity were higher, whereas HDL cholesterol was lower compared to -629 AA homozygotes (n=62) and -629CA+AA carriers (n=171) (P<0.05 to P<0.001). Univariate correlation analysis showed that cellular cholesterol efflux was related to CETP genotype (P=0.04), plasma CET (P<0.05), LCAT activity (P<0.001) and apo A-I (P<0.05). Multiple linear regression analysis confirmed the independent association of cellular cholesterol efflux to plasma with CETP genotype. In conclusion, an association of cellular cholesterol efflux with the -629C-->A CETP polymorphism, possibly also involving LCAT activity, could provide a mechanism explaining why CETP gene variation, which relates to lower plasma CETP, does not confer diminished cardiovascular risk.     

Phospholipid transfer protein enhances removal of cellular cholesterol and phospholipids by high-density lipoprotein apolipoproteins.

High-density lipoprotein (HDL) apolipoproteins remove excess cholesterol from cells by an active transport pathway that may protect against atherosclerosis. Here we show that treatment of cholesterol-loaded human skin fibroblasts with phospholipid transfer protein (PLTP) increased HDL binding to cells and enhanced cholesterol and phospholipid efflux by this pathway. PLTP did not stimulate lipid efflux in the presence of albumin, purified apolipoprotein A-I, and phospholipid vesicles, suggesting specificity for HDL particles. PLTP restored the lipid efflux activity of mildly trypsinized HDL, presumably by regenerating active apolipoproteins. PLTP-stimulated lipid efflux was absent in Tangier disease fibroblasts, induced by cholesterol loading, and inhibited by brefeldin A treatment, indicating selectivity for the apolipoprotein-mediated lipid removal pathway. The lipid efflux-stimulating effect of PLTP was not attributable to generation of prebeta HDL particles in solution but instead required cellular interactions. These interactions increased cholesterol efflux to minor HDL particles with electrophoretic mobility between alpha and prebeta. These findings suggest that PLTP promotes cell-surface binding and remodeling of HDL so as to improve its ability to remove cholesterol and phospholipids by the apolipoprotein-mediated pathway, a process that may play an important role in enhancing flux of excess cholesterol from tissues and retarding atherogenesis.     

Phospholipid transfer protein enhances removal of cellular cholesterol and phospholipids by high-density lipoprotein apolipoproteins

High-density lipoprotein (HDL) apolipoproteins remove excess cholesterol from cells by an active transport pathway that may protect against atherosclerosis. Here we show that treatment of cholesterol-loaded human skin fibroblasts with phospholipid transfer protein (PLTP) increased HDL binding to cells and enhanced cholesterol and phospholipid efflux by this pathway. PLTP did not stimulate lipid efflux in the presence of albumin, purified apolipoprotein A-I, and phospholipid vesicles, suggesting specificity for HDL particles. PLTP restored the lipid efflux activity of mildly trypsinized HDL, presumably by regenerating active apolipoproteins. PLTP-stimulated lipid efflux was absent in Tangier disease fibroblasts, induced by cholesterol loading, and inhibited by brefeldin A treatment, indicating selectivity for the apolipoprotein-mediated lipid removal pathway. The lipid efflux-stimulating effect of PLTP was not attributable to generation of preβ HDL particles in solution but instead required cellular interactions. These interactions increased cholesterol efflux to minor HDL particles with electrophoretic mobility between α and preβ. These findings suggest that PLTP promotes cell-surface binding and remodeling of HDL so as to improve its ability to remove cholesterol and phospholipids by the apolipoprotein-mediated pathway, a process that may play an important role in enhancing flux of excess cholesterol from tissues and retarding atherogenesis.     

Reduction in apolipoprotein-mediated removal of cellular lipids by immortalization of human fibroblasts and its reversion by cAMP: lack of effect with Tangier disease cells.

High density lipoprotein (HDL) phospholipids and apolipoproteins remove cellular lipids by two distinct mechanisms, but their relative contribution to reverse cholesterol transport is unknown. Whereas phospholipid-mediated cholesterol efflux from cultured cells reflects the activity of the HDL receptor SR-BI, apolipoprotein-mediated lipid removal is regulated in response to changes in cellular cholesterol content (positive) and cell proliferation rates (negative). Here we show that immortalization of human skin fibroblast lines with the papillomavirus E6/E7 oncogenes increased their proliferation rates and selectively reduced the activity of the apolipoprotein-mediated lipid removal pathway. This reduction was accompanied by a decrease in cellular cAMP levels and was reversed by treatment with a cAMP analog. The stimulatory effect of cAMP was independent of changes in cellular phenotype or activities of cholesteryl ester cycle enzymes. The severely impaired apolipoprotein-mediated lipid removal pathway in Tangier disease fibroblasts, which persisted after immortalization, was not improved by treatment with a cAMP analog, implying that the cellular defect in Tangier disease is upstream from this cAMP-dependent signaling pathway.These results indicate that papillomavirus-induced immortalization of fibroblasts selectively reduces the activity of the apolipoprotein-mediated lipid removal pathway by a cAMP-dependent process, perhaps to prevent loss of cellular lipids needed for continual membrane synthesis.     

Decreased activity of lecithin:cholesterol acyltransferase and hepatic lipase in chronic hypothyroid rats: Implications for reverse cholesterol transport

Chronic hypothyroidism is frequently associated with atherosclerosis due to increased cholesterol plasma levels; nevertheless, the contribution of impaired reverse cholesterol transport (RCT) in this process has not been completely elucidated. The aim of this study was to evaluate the effect of thyroidectomy (Htx) upon the main stages of RCT in rats. Plasma lipid alterations induced by thyroidectomy showed a slight, but significant, reduction of total plasma triglycerides, a 300% increase of LDL-cholesterol and a 25% decrease in HDL-cholesterol compared to control rats. We evaluated the first stage of RCT determining ₃H-cholesterol efflux in Fu5AH cells. The capacity of HDL obtained from Htx rats to promote cholesterol efflux was similar to that of controls. Lecithin:cholesterol acyltransferase (LCAT) activity, the second stage and the driving force of RCT was 30% lower in Htx animals compared to controls, as determined by reconstituted HDL used as an external substrate. Lipoproteins are remodeled by hepatic lipase; the mean activity of this enzyme in postheparin plasma of Htx animals was reduced by 30% compared to controls, thus suggesting an impaired HDL remodeling by this enzyme in the hypothyroid status. In contrast, lipoprotein lipase activity in the Htx group was unchanged. In summary, this study demonstrates that chronic hypothyroidism in the rat induced an impaired RCT mainly at the cholesterol esterification, and HDL remodeling mediated by hepatic lipase. The latter probably results in an abnormal HDL structure, i.e. phospholipid enrichment, which contributes to decrease HDL-apo AI fractional catabolic rates.     

Effect of LCAT on HDL-mediated cholesterol efflux from loaded EA.hy 926 cells

• 1.1. Human endothelial cells (EA.hy 926 line) were loaded with cholesterol, using cationized LDL, and the effect of lecithin:cholesterol acyltransferase (LCAT) on cellular cholesterol efflux mediated by high density lipoproteins (HDL) was measured subsequently.
• 2.2. In plasma, lecithin:cholesterol acyltransferase (LCAT) converts unesterified HDL cholesterol into cholesteryl esters, thereby maintaining the low UC/PL ratio of HDL. It was tested if further decrease in UC/PL ratio of HDL by LCAT influences cellular cholesterol efflux in vitro.
• 3.3. Efflux was measured as the decrease of cellular cholesterol after 24 hr of incubation with various concentrations of HDL in the presence and absence of LCAT. LCAT from human plasma (about 3000-fold purified) was added to the cell culture, resulting in activity levels in the culture media of 60–70% of human serum.
• 4.4. Although LCAT had a profound effect on HDL structure (UC/TC and UC/PL ratio's decreased), the enzyme did not enhance efflux of cellular cholesterol, using a wide range of HDL concentrations (0.05–2.00 mg HDL protein/ml).
• 5.5. The data indicate that the extremely low unesterified cholesterol content of HDL, induced by LCAT, does not enhance efflux of cholesterol from loaded EA.hy 926 cells. It is concluded that the HDL composition (as isolated from plasma by ultracentrifugation) is optimal for uptake of cellular cholesterol.

     

The influence of cellular and lipoprotein cholesterol contents on the flux of cholesterol between fibroblasts and high density lipoprotein

Previous studies indicate that free cholesterol moves passively between high density lipoprotein (HDL) and cell plasma membranes by uncatalyzed diffusion of cholesterol molecules in the extracellular aqueous phase. By this mechanism, the rate constants for free cholesterol influx (Cli) and efflux (ke) should not be very sensitive to the free cholesterol content of cells or HDL. Thus, at a given HDL concentration, the unidirectional influx and efflux of cholesterol mass (Fi, Fe) should be proportional to the cholesterol content of HDL and cells, respectively, and net efflux of cholesterol mass (Fe-Fi greater than 0) should occur when either cells are enriched with cholesterol or HDL is depleted of cholesterol. We have examined the influence of cell and HDL free cholesterol contents on the bidirectional flux of free cholesterol between HDL and human fibroblasts and also attempted to detect some dependence of flux on the binding of HDL to the cells. In the range of HDL concentrations from 1 to 1000 micrograms of protein/ml, ke for cell free cholesterol approximately doubled for every 10-fold increase in HDL concentration, reaching 0.04 h-1 at 1000 micrograms of HDL/ml. ke and Cli were not influenced by the doubling of fibroblast free cholesterol content (from 31 +/- 5 to 62 +/- 13 micrograms of cholesterol/mg of protein). There was an approximate exchange of cholesterol between HDL and the unenriched fibroblasts (e.g. at [HDL] = 100 micrograms/ml, Fe and Fi = 3.2 and 3.0 micrograms of cholesterol/[4 h.mg of cell protein], respectively). In contrast, there was substantial net efflux from the enriched cells (at [HDL] = 100 micrograms/ml, Fe and Fi = 5.5 and 3.1 micrograms of cholesterol/[4 h.mg of cell protein], respectively). The rate constants for cholesterol flux were not influenced by changing the free cholesterol content of HDL, so that there was net efflux of cell cholesterol in the presence of cholesterol-depleted HDL and net influx from cholesterol-rich HDL. The Kd of HDL binding to fibroblasts was reduced from 1.7 to 0.9 micrograms/ml by the enrichment of the cells with free cholesterol; this increase in affinity for HDL was not reflected in enhanced rate constants for cholesterol flux. The inhibition of specific HDL binding by treatment of the lipoprotein with dimethyl suberimidate did not affect cholesterol flux using either control or cholesterol-rich cells at any HDL concentration in the range 1-1000 micrograms/ml. The above results are consistent with the concept that net movement of free cholesterol between cells and HDL occurs by passive, mass-action effects.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

Psoriasis alters HDL composition and cholesterol efflux capacity

Psoriasis, a chronic inflammatory skin disease, has been linked to increased myocardial infarction and stroke. Functional impairment of HDL may contribute to the excess cardiovascular mortality of psoriatic patients. However, data available regarding the impact of psoriasis on HDL composition and function are limited. HDL from psoriasis patients and healthy controls was isolated by ultracentrifugation and shotgun proteomics, and biochemical methods were used to monitor changed HDL composition. We observed a significant reduction in apoA-I levels of HDL from psoriatic patients, whereas levels of apoA-II and proteins involved in acute-phase response, immune response, and endopeptidase/protease inhibition were increased. Psoriatic HDL contained reduced phospholipid and cholesterol. With regard to function, these compositional alterations impaired the ability of psoriatic HDL to promote cholesterol efflux from macrophages. Importantly, HDL-cholesterol efflux capability negatively correlated with psoriasis area and severity index. We observed that control HDL, as well as psoriatic HDL, inhibited dihydrorhodamine (DHR) oxidation to a similar extent, suggesting that the anti-oxidative activity of psoriatic HDL is not significantly altered. Our observations suggest that the compositional alterations observed in psoriatic HDL reflect a shift to a pro-inflammatory profile that impairs cholesterol efflux capacity of HDL and may provide a link between psoriasis and cardiovascular disease.     

Protein kinase C as a mediator of high density lipoprotein receptor-dependent efflux of intracellular cholesterol.

The interaction of high density lipoproteins (HDL) with the HDL receptor stimulates the translocation of cholesterol from intracellular pools to the plasma membrane where the cholesterol becomes available for removal by appropriate acceptors. The role of signal transduction through protein kinase C in HDL receptor-dependent cholesterol translocation and efflux was examined using cholesterol-loaded cultured human skin fibroblasts. Treatment of cells with HDL3 activated protein kinase C, demonstrated by a transient increase in membrane associated kinase activity. Kinase activation appeared to be dependent on binding of HDL3 to the HDL receptor, since tetranitromethane-modified HDL3, which does not bind to the receptor, was without effect. Translocation of intracellular sterol to the plasma membrane was stimulated by treatment of cells with the protein kinase C activators, dioctanoylglycerol and phorbol myristic acetate, and the calcium ionophore A23187. Conversely, treatment of cells with sphingosine, a protein kinase C inhibitor, reduced HDL3-mediated translocation and efflux of intracellular sterols. However, sphingosine had no effect on efflux of labeled cholesterol derived from the plasma membrane. Down-regulation of cellular protein kinase C activity by long term incubation with phorbol esters also inhibited HDL3-mediated efflux of intracellular sterols and abolished the ability of sphingosine to further inhibit HDL3-mediated efflux. These studies support the conclusion that HDL receptor-mediated translocation and efflux of intracellular cholesterol occurs through activation of protein kinase C.     

Efflux of lipid from fibroblasts to apolipoproteins: dependence on elevated levels of cellular unesterified cholesterol.

Earlier work from this laboratory showed that enrichment of cells with free cholesterol enhanced the efflux of phospholipid to lipoprotein acceptors, suggesting that cellular phospholipid may contribute to high density lipoprotein (HDL) structure and the removal of sterol from cells. To test this hypothesis, we examined the efflux of [3H]cholesterol (FC) and [32P]phospholipid (PL) from control and cholesterol-enriched fibroblasts to delipidated apolipoproteins. The percentages of [3H]cholesterol and [32P]phospholipid released from control cells to human apolipoprotein A-I were 2.2 +/- 0.5%/24 h and 0.8 +/- 0.1%/24 h, respectively. When the cellular cholesterol content was doubled, efflux of both lipids increased substantially ([3H]FC efflux = 14.6 +/- 3.6%/24 h and [32P]PL efflux = 4.1 +/- 0.3%/24 h). Phosphatidylcholine accounted for 70% of the radiolabeled phospholipid released from cholesterol-enriched cells. The cholesterol to phospholipid molar ratio of the lipid released from cholesterol-enriched cells was approximately 1. This ratio remained constant throughout an incubation time of 3 to 48 h, suggesting that there was a coordinate release of both lipids. The concentrations of apoA-I, A-II, A-IV, E, and Cs that promoted half-maximal efflux of phospholipid from cholesterol-enriched fibroblasts were 53, 30, 68, 137, and 594 nM, respectively. With apoA-I and A-IV, these values for half-maximal efflux of phospholipid were identical to the concentrations that resulted in half-maximal efflux of cholesterol. Agarose gel electrophoresis of medium containing apoA-I that had been incubated with cholesterol-enriched fibroblasts revealed a particle with alpha to pre-beta mobility. We conclude that the cholesterol content of cellular membranes is an important determinant in the ability of apolipoproteins to promote lipid removal from cells. We speculate that apolipoproteins access cholesterol-phosphatidylcholine domains within the plasma membrane of cholesterol-enriched cells, whereupon HDL is generated in the extracellular compartment. The release of cellular lipid to apolipoproteins may serve as a protective mechanism against the potentially damaging effects of excess membrane cholesterol.     

The effect of variations in lipid composition of high-density lipoprotein on its interaction with receptors on human fibroblasts

To test whether the altered lipid composition of high-density lipoprotein (HDL) particles influences their ability to interact with the HDL receptor on cultured fibroblasts, HDL3 isolated from normal and diabetic donors with different degrees of hypertriglyceridemia was subjected to binding competition, cholesterol efflux, and net cholesterol transport assays. When HDL3 particles from different subjects were incubated with cholesterol-loaded fibroblasts, the initial rates of cholesterol efflux from cells to HDL3 particles appeared to be an exclusive function of the relative ability of HDL3 to interact with the HDL receptor. Variation in lipid composition of HDL3 particles did not appear to have any significant influence on either the receptor-binding or the efflux-promoting abilities of HDL3. When the movement of cholesterol between cells and HDL3 particles was allowed to approach equilibrium, the lipid composition of HDL3 became an important factor in determining the net amount of cholesterol removed from cells, with cholesterol-deficient triacylglycerol-rich HDL3 particles having the best capacity to promote net transport of cholesterol from cells. These results suggest that the ability of HDL to bind to its cell-surface receptor, rather than variations in the lipid composition of the HDL particle, is the major determinant of cholesterol efflux from cells to HDL particles. However, the lipid composition of HDL as well as its receptor-binding activity determine the net amount of cholesterol transported from cells over long-term incubation.     

High density lipoprotein stimulates sterol translocation between intracellular and plasma membrane pools in human monocyte-derived macrophages

Binding of high density lipoprotein (HDL) to its receptor on cultured fibroblasts and aortic endothelial cells was previously shown to facilitate sterol efflux by initiation of translocation of intracellular sterol to the plasma membrane. After cholesterol-loaded human monocyte-derived macrophages were incubated with either [3H]mevalonolactone or lipoprotein-associated [3H]cholesteryl ester to radiolabel intracellular pools of sterol, incubation with HDL3 led to stimulation of 3H-labeled sterol translocation from intracellular sites to the cell surface which preceeded maximum 3H-labeled sterol efflux. A similar pattern was demonstrated for macrophages that were preloaded with cholesterol derived from either low density lipoprotein (LDL), acetyl-LDL, or phospholipase C-modified LDL. However, in macrophages that were not loaded with cholesterol, HDL3 stimulated net movement of 3H-labeled sterol from the plasma membrane into intracellular compartments, the opposite direction from that seen for cholesterol-loaded cells. A similar influx pattern was found in nonloaded macrophages and fibroblasts that were labeled with trace amounts of exogenous [3H]cholesterol. Cholesterol translocation from intracellular pools to the cell surface of cholesterol-loaded macrophages appeared to be stimulated by receptor binding of HDL, since chemical modification of HDL with tetranitromethane (TNM), which abolishes its receptor binding, reduced its ability to stimulate 3H-labeled sterol translocation and efflux. In nonloaded cells, however, the ability of HDL3 to stimulate sterol efflux and movement of sterol from the plasma membrane into intracellular pools was unaffected by TNM modification. Thus, binding of HDL to its receptor on cholesterol-loaded macrophages appears to promote translocation of intracellular cholesterol to the plasma membrane followed by cholesterol efflux into the medium. However, in nonloaded macrophages, HDL stimulates sterol movement from the plasma membrane into intracellular pools by a receptor-independent process.     

SR-BI inhibits ABCG1-stimulated net cholesterol efflux from cells to plasma HDL

This study compares the roles of ABCG1 and scavenger receptor class B type I (SR-BI) singly or together in promoting net cellular cholesterol efflux to plasma HDL containing active LCAT. In transfected cells, SR-BI promoted free cholesterol efflux to HDL, but this was offset by an increased uptake of HDL cholesteryl ester (CE) into cells, resulting in no net efflux. Coexpression of SR-BI with ABCG1 inhibited the ABCG1-mediated net cholesterol efflux to HDL, apparently by promoting the reuptake of CE from medium. However, ABCG1-mediated cholesterol efflux was not altered in cholesterol-loaded, SR-BI-deficient (SR-BI(-/-)) macrophages. Briefly cultured macrophages collected from SR-BI(-/-) mice loaded with acetylated LDL in the peritoneal cavity did exhibit reduced efflux to HDL. However, this was attributable to reduced expression of ABCG1 and ABCA1, likely reflecting increased macrophage cholesterol efflux to apolipoprotein E-enriched HDL during loading in SR-BI(-/-) mice. In conclusion, cellular SR-BI does not promote net cholesterol efflux from cells to plasma HDL containing active LCAT as a result of the reuptake of HDL-CE into cells. Previous findings of increased atherosclerosis in mice transplanted with SR-BI(-/-) bone marrow probably cannot be explained by a defect in macrophage cholesterol efflux.     

Serum activity and hepatic secretion of lecithin:cholesterol acyltransferase in experimental hypothyroidism and hypercholesterolemia

Lecithin:cholesterol acyltransferase (LCAT), the major cholesterol esterifying enzyme in plasma, plays an important role in the removal of cholesterol from peripheral tissues. This study in rat focuses upon the effects of hypothyroidism and cholesterol feeding on serum activity and hepatic LCAT secretion. To obviate the effect that inclusion of high concentrations of cholesterol in the rat serum may have on the proteoliposome used in the assay of LCAT, very low and low density lipoproteins (VLDL and LDL) were removed by ultracentrifugation at d 1.063 g/ml. The molar esterification rate in the euthyroid VLDL + LDL-free serum was found to be 0.94 +/- 0.06 compared to 0.67 +/- 0.05 in hypothyroid rats and 1.56 +/- 0.14 in hypercholesterolemic rats. LCAT secretion by suspension cultures of hepatocytes from hypercholesterolemic rats was found to be significantly depressed when compared to that for euthyroid and hypothyroid animals. Secretion by hepatocytes from hypothyroid rats was depressed for the first 0-4 hr, but rapidly recovered. The depressed secretion of LCAT by hepatocytes from hypercholesterolemic rats correlates with the appearance in the media of apoE-rich, discoidal HDL. Discoidal HDL was six times more effective as a substrate for purified human LCAT than HDL from hypercholesterolemic serum, and twice as effective as serum and nascent HDL from euthyroid animals. It is concluded that the depressed LCAT activity in serum from hypothyroid rats is due to a depressed hepatic secretion of the enzyme and that the elevated serum activity of hypercholesterolemic rats may be related to a defect in LCAT clearance. Finally, the appearance of discoidal HDL in the medium upon culture of hepatocytes from hypercholesterolemic rats appears to be due to an inhibition of LCAT secretion by these cells.     

Lysosomal acid lipase deficiency impairs regulation of ABCA1 gene and formation of high density lipoproteins in cholesteryl ester storage disease

ATP-binding cassette transporter A1 (ABCA1) mediates the rate-limiting step in high density lipoprotein (HDL) particle formation, and its expression is regulated primarily by oxysterol-dependent activation of liver X receptors. We previously reported that ABCA1 expression and HDL formation are impaired in the lysosomal cholesterol storage disorder Niemann-Pick disease type C1 and that plasma HDL-C is low in the majority of Niemann-Pick disease type C patients. Here, we show that ABCA1 regulation and activity are also impaired in cholesteryl ester storage disease (CESD), caused by mutations in the LIPA gene that result in less than 5% of normal lysosomal acid lipase (LAL) activity. Fibroblasts from patients with CESD showed impaired up-regulation of ABCA1 in response to low density lipoprotein (LDL) loading, reduced phospholipid and cholesterol efflux to apolipoprotein A-I, and reduced α-HDL particle formation. Treatment of normal fibroblasts with chloroquine to inhibit LAL activity reduced ABCA1 expression and activity, similar to that of CESD cells. Liver X receptor agonist treatment of CESD cells corrected ABCA1 expression but failed to correct LDL cholesteryl ester hydrolysis and cholesterol efflux to apoA-I. LDL-induced production of 27-hydroxycholesterol was reduced in CESD compared with normal fibroblasts. Treatment with conditioned medium containing LAL from normal fibroblasts or with recombinant human LAL rescued ABCA1 expression, apoA-I-mediated cholesterol efflux, HDL particle formation, and production of 27-hydroxycholesterol by CESD cells. These results provide further evidence that the rate of release of cholesterol from late endosomes/lysosomes is a critical regulator of ABCA1 expression and activity, and an explanation for the hypoalphalipoproteinemia seen in CESD patients.     

ATP-binding cassette transporter A1 mediates cellular secretion of alpha-tocopherol

Alpha-tocopherol (alpha-TOH) is associated with plasma lipoproteins and accumulates in cell membranes throughout the body, suggesting that lipoproteins play a role in transporting alpha-TOH between tissues. Here we show that secretion of alpha-TOH from cultured cells is mediated in part by ABCA1, an ATP-binding cassette protein that transports cellular cholesterol and phospholipids to lipid-poor high density lipoprotein (HDL) apolipoproteins such as apoA-I. Treatment of human fibroblasts and murine RAW264 macrophages with cholesterol and/or 8-bromo-cyclic AMP, which induces ABCA1 expression, enhanced apoA-I-mediated alpha-TOH efflux. ApoA-I lacked the ability to remove alpha-TOH from Tangier disease fibroblasts that have a nonfunctional ABCA1. BHK cells that lack an active ABCA1 pathway markedly increased secretion of alpha-TOH to apoA-I when forced to express ABCA1. ABCA1 also mediated a fraction of the alpha-TOH efflux promoted by lipid-containing HDL particles, indicating that HDL promotes alpha-TOH efflux by both ABCA1-dependent and -independent processes. Exposing apoA-I to ABCA1-expressing cells did not enhance its ability to remove alpha-TOH from cells lacking ABCA1, consistent with this transporter participating directly in the translocation of alpha-TOH to apolipoproteins. These studies provide evidence that ABCA1 mediates secretion of cellular alpha-TOH into the HDL metabolic pathway, a process that may facilitate vitamin transport between tissues and influence lipid oxidation.     

Study of the components of reverse cholesterol transport in lecithin:cholesterol acyltransferase deficiency

Enzymatic and lipid transfer reactions involved in reverse cholesterol transport were studied in healthy and lecithin:cholesterol acyltransferase (LCAT), deficient subjects. Fasting plasma samples obtained from each individual were labeled with [3H]cholesterol and subsequently fractionated by gel chromatography. The radioactivity patterns obtained corresponded to the elution volumes of the three major ultracentrifugally isolated lipoprotein classes (very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL)). In healthy subjects, the LCAT activity was consistently found in association with the higher molecular weight portion of HDL. Similar observations were made when exogenous purified LCAT was added to the LCAT-deficient plasma prior to chromatography. Incubation of the plasma samples at 37 degrees C resulted in significant reduction of unesterified cholesterol (FC) and an increase in esterified cholesterol (CE). Comparison of the data of FC and CE mass measurements of the lipoprotein fractions from normal and LCAT-deficient plasma indicates that: (i) In normal plasma, most of the FC for the LCAT reaction originates from LDL even when large amounts of FC are available from VLDL. (ii) The LCAT reaction takes place on the surface of HDL. (iii) The product of the LCAT reaction (CE) may be transferred to either VLDL or LDL although VLDL appears to be the preferred acceptor when present in sufficient amounts. (iv) CE transfer from HDL to lower density lipoproteins is at least partially impaired in LCAT-deficient patients. Additional studies using triglyceride-rich lipoproteins indicated that neither the capacity to accept CE from HDL nor the lower CE transfer activity were responsible for the decreased amount of CE transferred to VLDL and chylomicrons in LCAT-deficient plasma.     

Up-regulation of high density lipoprotein receptor activity by gamma-interferon associated with inhibition of cell proliferation

High density lipoprotein (HDL) binds to cell surface receptors and promotes selective removal of excess cholesterol from intracellular pools. The activity of this receptor is up-regulated when cells become loaded with cholesterol, but the relative degree of up-regulation depends on the growth state of the cells. The current study demonstrates that treatment of proliferating fibroblasts with gamma-interferon (IFN) increases the activity of the HDL receptor in association with a decrease in the rate of cell proliferation. Addition of IFN during the growth phase reduced the number of cells but had little effect on total cell protein, indicating that IFN inhibited cell proliferation but produced larger cells. IFN treatment increased the number of high affinity receptors for HDL on the surface of cholesterol-loaded fibroblasts, whether receptor binding was expressed per cell or per unit of cell surface area, cell protein, or cell cholesterol. IFN treatment also appeared to increase the amount of 110-kDa HDL binding protein in fibroblast membranes that has been postulated to represent the HDL receptor molecule. The IFN-induced increase in HDL receptor activity was associated with an enhanced ability of HDL3 to remove cholesterol from intracellular pools. These results are consistent with the hypothesis that inhibition of cell proliferation increases HDL receptor-mediated transport of excess cholesterol from cells, possibly to rid cells of cholesterol that accumulates in response to a reduced rate of membrane synthesis.     

The role of lecithin:cholesterol acyltransferase in the modulation of cardiometabolic risks — A clinical update and emerging insights from animal models

Lecithin cholesterol acyltransferase (LCAT) is the key enzyme in mediating the esterification of cholesterol on circulating lipoproteins. It has long been suggested that LCAT plays a crucial role in reverse cholesterol transport, a process depicting the removal of cellular cholesterol through efflux to high density lipoproteins (HDL) and its delivery to the liver for eventual excretion from the body. Although loss-of-function LCAT mutations invariably result in profound HDL deficiency, the role of LCAT in atherogenesis continues to be clouded with controversy. Increasing number of large scale, population-based studies failed to detect an elevated cardiac risk with reduced blood levels of LCAT, suggesting that reduced LCAT activity may not be a risk factor nor a therapeutic target. More recent studies in human LCAT gene mutation carriers tend to suggest that atherogenicity in LCAT deficiency may be dependent on the nature of the mutations, providing plausible explanations for the otherwise contradictory findings. Genetic models of LCAT excess or deficiency yielded mixed findings. Despite its known profound effects on HDL and triglyceride metabolism, the role of LCAT in metabolic disorders, including obesity and diabetes, has not received much attention. Recent studies in LCAT deficient mouse models suggest that absence of LCAT may protect against insulin resistance, diabetes and obesity. Coordinated modulation of a number of anti-obesity and insulin sensitizing pathways has been implicated. Further studies to explore the role of LCAT in the modulation of cardiometabolic disorders and the underlying mechanisms are warranted.