Lipoprotein-X: a substrate for lecithin: cholesterol acyltransferase

LCAT Enzyme Replacement Therapy Reduces LpX and Improves Kidney Function in a Mouse Model of Familial LCAT Deficiency

Familial LCAT deficiency (FLD) is due to mutations in lecithin:cholesterol acyltransferase (LCAT), a plasma enzyme that esterifies cholesterol on lipoproteins. FLD is associated with markedly reduced levels of plasma high-density lipoprotein and cholesteryl ester and the formation of a nephrotoxic lipoprotein called LpX. We used a mouse model in which the LCAT gene is deleted and a truncated version of the SREBP1a gene is expressed in the liver under the control of a protein-rich/carbohydrate-low (PRCL) diet-regulated PEPCK promoter. This mouse was found to form abundant amounts of LpX in the plasma and was used to determine whether treatment with recombinant human LCAT (rhLCAT) could prevent LpX formation and renal injury. After 9 days on the PRCL diet, plasma total and free cholesterol, as well as phospholipids, increased 6.1 ± 0.6-, 9.6 ± 0.9-, and 6.7 ± 0.7-fold, respectively, and liver cholesterol and triglyceride concentrations increased 1.7 ± 0.4- and 2.8 ±0.9-fold, respectively, compared with chow-fed animals. Transmission electron microscopy revealed robust accumulation of lipid droplets in hepatocytes and the appearance of multilamellar LpX particles in liver sinusoids and bile canaliculi. In the kidney, LpX was found in glomerular endothelial cells, podocytes, the glomerular basement membrane, and the mesangium. The urine albumin/creatinine ratio increased 30-fold on the PRCL diet compared with chow-fed controls. Treatment of these mice with intravenous rhLCAT restored the normal lipoprotein profile, eliminated LpX in plasma and kidneys, and markedly decreased proteinuria. The combined results suggest that rhLCAT infusion could be an effective therapy for the prevention of renal disease in patients with FLD.

Effect of lipoprotein-X on lipid metabolism in rat kidney

Lipoprotein-X (Lp-X) is found in the plasma of patients with familial lecithin: cholesterol acyltransferase (LCAT) deficiency syndromes. The majority of the patients with this disorder develop progressive glomerulosclerosis. In this study, the effect of Lp-X on lipid metabolism in perfused rat kidney was investigated. Lp-X was isolated from plasma of patients with familial LCAT deficiency by sequential ultracentrifugation and gel filtration column chromatography. Rat kidneys were perfused for 1-2 h with Krebs-Henseleit buffer containing 20 microM [1-(14)C]acetate or 20 microM [Me-3H]choline. In the presence of Lp-X, no significant difference in the incorporation of radioactivity into triglycerides, cholesterol, phosphocholine, CDP-choline and sphingomyelin was observed. However, incorporation of radioactivity into cholesteryl esters and phosphatidylcholine was significantly elevated in Lp-X perfused kidneys. The contents of cholesterol, cholesteryl esters and phosphatidylcholine were also significantly increased in Lp-X perfused kidneys. The increase in lipid content in the Lp-X perfused kidney is attributed to the direct deposition of Lp-X lipids into the organ. The increase in the labelling of cholesteryl esters was attributed to the increase of available substrate (cholesterol) for the acyl-CoA:cholesterol acyltransferase (ACAT) reaction. The increase in phosphatidylcholine labelling was caused by a reduced turnover of the newly synthesized labelled phosphatidylcholine during Lp-X perfusion.

Phospholipid-rich particles in commercial parenteral fat emulsions. An overview

In parenteral nutrition, the infusion of a fat EMU supplies both concentrated energy and covers the essential fatty acid requirements, the basic objective being to mimic as well as possible the input of chylomicrons into the blood. This objective is well met by the TAGRP of the EMU, which behave as true chylomicrons. However, commercial EMU also contain an excess of emulsifier in the form of PLRP. The number of these PLRP depends directly on the PL/TAG ratio of the EMU. They differ from the TAGRP by their composition (PL vs TAG and PL), their structure (PL in bilayer versus monolayer), and their granulometry (mean diameter 70-100 nm for PL vs 200-500 nm). The metabolic fate of the PLRP is similar in several ways to that of the TAGRP: exchanges of PL with the PL of the different cellular membranes and of the lipoproteins; captation of free CH from these same structures; and enrichment in apolipoproteins. However, because the TAGRP are the preferred substrates of the lipolytic enzymes, their clearance is much more rapid (half-life < 1 h) than that of the PLRP. As the infusion is continued, the PLRP end up accumulating and being transformed into LP-X (free CH/PL = 1; half-life of several days). As soon as the EMU is infused, the PLRP enter into competition with the TAGRP, in the lipolysis process as well as for sites of binding and for catabolism. The sites for catabolism of the two types of PAR are not the same: adipose tissues and muscles utilize the fatty acids and monoacylglycerols released by the lipolysis of the TAGRP; hepatocytes take up their remnants; the RES and the hepatocytes participate in the catabolism of the PLRP and the LP-X. Thus, prolonged infusion of EMU rich in PLRP leads to a hypercholesterolemia, or at least a dyslipoproteinemia, due to elevated LP-X, associated with a depletion of cells in CH, stimulating thus tissue cholesterogenesis. However, parenteral nutrition has evolved towards the utilization of EMU with a low PL/TAG ratio (availability of 30% formula) and less rapid delivery. For these reasons, the hypercholesterolemias that used to be observed with the 10% EMU have become much less spectacular or have even disappeared. It is interesting to note that patients on prolonged TPN, in particular those with a short small intestine, have weak cholesterolemia, reflecting a lowering of HDL and LDL not masked by elevated LP-X. At present, it seems difficult to produce sufficiently stable parenteral EMU devoid of PLRP. Notwithstanding, all the observations made since the introduction of the EMU in TPN are in favour of the use of PLRP-poor EMU. It is clear that the 10% formulas, and generally those with a PL/TAG ratio of 12/100, are ill-advised, especially in patients with a retarded clearance of circulating lipids.

Lateral interactions of pig apolipoprotein A-1 with egg yolk phosphatidylcholine and with cholesterol in mixed monolayers at the triolein-saline interface

Interfacial tensions of egg yolk phosphatidylcholine (PC) and cholesterol monolayers adsorbed at the triolein-saline interface were measured in the presence and absence of pig apolipoprotein A-1 (apoA-1) in the saline phase. In the absence of apoA-1, the adsorptions of PC and cholesterol at the interface from the triolein phase are cooperative, showing large lateral attractive interactions between the PC molecules and the cholesterol molecules in the monolayer. In the presence of apoA-1, the PC adsorption is anti-cooperative, indicating strong lateral attractive interactions between the PC and the apoA-1 molecules, i.e., apparently, repulsive lateral interactions between the PC molecules. On the other hand, lateral interactions of very low magnitude are observed between the cholesterol and apoA-1 molecules in the monolayer. Values of the lateral interaction energy are evaluated from the adsorption data by the Defay-Prigogine-Flory theory of monolayers. The large difference in lateral interaction energy with apoA-1 between PC and cholesterol in a mixed monolayer is discussed in connection with current problems in lipoprotein catabolism: reverse cholesterol transport, alterations in affinity of lipid particles to apoA-1, and formation of high-density lipoproteins and abnormal lipoproteins.

Dyslipoproteinaemia of liver disease

Hepatic diseases differ from most other causes of secondary dyslipidaemia in that the circulating lipoproteins are not only present in abnormal amounts but they frequently also have abnormal composition, electrophoretic mobility and appearance. Pre-beta and alpha bands can be absent on electrophoresis in all types of liver disease although material in the VLDL and HDL ranges can be isolated in the ultracentrifuge. Cholestatic liver disease has been the most extensively studied and the hyperlipidaemia can be extreme with marked elevations of free cholesterol and phospholipids. This results largely from the presence of LP-X, an abnormal LDL, with a vesicular structure that appears in rouleaux formation under the electron microscope. It is virtually specific for cholestasis and familial LCAT deficiency. The LDL, however, is heterogeneous and may also contain a large triglyceride-rich particle (LP-Y) as well as more normal-looking particles, which are none the less depleted in cholesteryl esters and rich in triglycerides. Indeed, when patients with cholestasis are hypertriglyceridaemic the excess triglyceride is to be found predominantly in these two LDL fractions rather than in VLDL. HDL in cholestasis may contain disc-like particles, similar to those newly secreted by the liver and intestine, as well as more normal-looking spherical particles. In extrahepatic obstruction concentrations of HDL and its major apolipoproteins, apoAI and apoAII, are frequently reduced, although a subfraction rich in apoE is often found. In all but the latest stages of chronic intrahepatic cholestasis due to primary biliary cirrhosis, however, HDL, especially HDL2, concentrations are increased, probably due to the presence of a circulating inhibitor of HL. Many of these lipoprotein changes found in cholestasis resemble those of familial LCAT deficiency, although the hyperlipidaemia is not usually so severe in the latter condition. Indeed, in patients with cholestasis but well-preserved LCAT activity many of the characteristic lipoprotein changes, such as LP-X, LP-Y and discoidal HDL, may not be seen. In acute hepatocellular disease, such as alcoholic or viral hepatitis, it is not unusual for the patient to go through a cholestatic phase and many of the same lipoprotein changes may be seen. In cirrhosis without cholestasis the patients are not usually significantly hyperlipidaemic and in advanced cases cholesterol and apoB levels may be reduced. Although LCAT activity and the proportion of plasma cholesterol esterified may also be markedly reduced, LP-X is not usually seen, possibly because the flux of free cholesterol and phospholipid (lecithin), the LCAT substrates, is relatively low. Discoidal HDLs are often present.(ABSTRACT TRUNCATED AT 400 WORDS)

Hyperviscosity syndrome in a hypercholesterolemic patient with primary biliary cirrhosis

A 45-yr-old woman with primary biliary cirrhosis, xanthomatosis, and marked hypercholesterolemia developed symptoms of the hyperviscosity syndrome on three separate occasions. On presentation, she had a plasma total cholesterol concentration of 53.40 mM (2065 mg/dl) and a relative serum viscosity of 2.9. Following three courses of plasma exchange in a 5-day period, the total cholesterol level decreased to 6.75 mM (261 mg/dl) and the viscosity to 1.3. The cutaneous xanthomata were markedly diminished 1 wk following plasma exchange. Despite therapy with colestipol (30 g/d), the hyperviscosity syndrome developed 147 days later. This cycle recurred again 137 days after colestipol was discontinued. Serum viscosity and total cholesterol concentration were highly correlated during the postexchange or accumulation phases (R = 0.95, 95% CI: 0.85, 0.98) and during the exchange or interventional phases (R = 0.95, 95% CI: 0.84, 0.99). Serum viscosity was less significantly correlated with total serum protein (R = 0.84; 95% CI: 0.55, 0.95) or with plasma triglyceride (R = 0.63; 95% CI: 0.26, 0.81). There were no significant correlations of red cell mass, plasma fibrinogen levels, or serum bile salts with viscosity. Subfractionation of plasma into lipoprotein classes showed 45% of total cholesterol in the lipoprotein X fraction and a presumptive slow alpha-lipoprotein species. It is postulated that both the hyperviscosity syndrome and rapid resolution of xanthomata in the patient may be attributable to the physiology of her abnormal lipoprotein particles.

Serial measurement of plasma cholesterol and lecithin:cholesterol acyl transferase activity in adults receiving total parenteral nutrition

The effect of total parenteral nutrition (TPN) containing approximately 800 ml Nutralipid daily on plasma cholesterol and lecithin:cholesterol acyl transferase (LCAT) activity was studied in 11 adult hospital patients. LCAT was assayed using an endogenous (S/N) and an artificial (ASA) substrate to differentiate between altered plasma substrate composition (which would influence the S/N method) and enzyme quantity (measured by the exogenous ASA method). Total cholesterol levels increased significantly during TPN, but generally remained within normal range. In comparison to laboratory reference values, free cholesterol was elevated and high-density lipoprotein cholesterol and ASA LCAT activity was reduced in patients before the start of TPN and remained unaltered by the TPN regime. S/N LCAT activity was normal and not altered by TPN. Since changes in plasma high-density lipoprotein and free cholesterol and ASA LCAT were present in patients before TPN, it must be concluded that they resulted from the underlying disease rather than the TPN per se. Longitudinal analyses showed that during the first 21 days of TPN nine patients showed a further fall in ASA LCAT and a rise in free cholesterol, thereafter ASA LCAT activity rose and free cholesterol fell despite continuation of TPN. It is suggested that ASA is a more reliable indicator of cholesterol esterification than S/N and that change in LCAT activity, although not caused by TPN, was related to the altered plasma lipid profile in the patients studied.

The effect of short-term feeding of a high carbohydrate diet on HLD subclasses in normal subjects

Plasma HDL concentrations are effected by several perturbations, including certain dietary manipulations. In this study we have examined the effects of a one week ingestion of an isocaloric, fat-free, high-carbohydrate diet (CHO greater than 80% of calories) and the concentrations and compositions of plasma HDL subclasses. Eleven healthy normolipidemic volunteers (6 females, 5 males) took part in this study. Blood samples for lipoprotein analysis were drawn before and at the end of the dietary period and analyzed for lipoprotein lipid and apoprotein concentrations. Lipoproteins also were characterized by zonal ultracentrifugation. Our results show the following significant changes at the end of the dietary period: plasma concentrations of VLDL-TG, VLDL-cholesterol and total VLDL mass increased, whereas plasma concentrations of LDL-cholesterol, LDL mass and HDL-cholesterol and HDL mass, decreased. Plasma concentrations of apoprotein A1 decreased (from 133.3 +/- 7.7 to 108.1 +/- 8.6; mean +/- S.E.M., p less than 0.0004), and apoprotein A2 concentrations remained unchanged. This resulted in a drop in plasma ratio of ApoA1/ApoA2 (p less than 0.03). Since it has been shown that ApoA1/ApoA2 ratio is higher in HDL2 than HDL3, we examined the concentrations of these two subfractions, employing rate-zonal ultracentrifugation for their isolation. One week of ingestion of the study diet was followed by consistent decreases in HDL2 mass (from 84 +/- 15 to 44 +/- 16 mg/dl, mean +/- S.E.M.), with inconsistent changes in HDL3 mass, (from 254 +/- 18 to 222 +/- 13 mg/dl) resulting in significant decreases in HDL2/HDL3 mass ratio. Lipid analyses of these subfractions did not demonstrate major compositional changes. The alterations noted could be due to decreased HDL production, at least in part, but alterations in the interconversions of lipoproteins also could have played a role. The falls in HDL2 on a diet which should be "antiatherogenic" illustrate the difficulty of assessing the atherogenicity of any given diet solely by the changes it produces in the levels of circulating lipoproteins.

Lipoprotein-X: carbon-13 nuclear magnetic resonance studies on native, reconstituted, and model systems

Lipoprotein-X (LP-X), a lipoprotein isolated from human cholestatic plasma by ethanol--acetate precipitation and zonal ultracentrifugation, has been studied by 13C NMR at 67.9 MHz. Spectra of LP-X and its three subfractions are markedly different from those of normal human high-density lipoprotein3 (HDL3) or low-density lipoprotein (LDL). Spectra of LP-X are characterized by the presence of unusually broad resonance lines, especially those attributable to C6 of unesterified cholesterol (160--260 Hz) and to C beta of phospholipid glyceride (240--290 Hz). In contrast, the CH2O, CH2N, and N(CH3)3 choline resonances have line widths comparable to those of normal LDL and HDL3. For the subfraction LP-X1, spin--lattice relaxation times (T1) of the fatty acyl olefin resonances at 129.8 and 128.0 ppm and of the unesterified cholesterol C6 at 120.1 ppm were measured to be 675, 766, and 162 ms, respectively. These times are comparable to those measured for the corresponding resonances in single bilayer vesicles whose lipid composition approximates that of LP-X. The three LP-X subfractions isolated by zonal ultracentrifugation gave spectra which are identical, within experimental error, as judged qualitatively from their appearance and quantitatively from the line widths of selected resonances. In addition, 13C NMR spectra of sonicated total LP-X lipids are similar to spectra of the intact native lipoprotein. This study suggests (a) that motions of lipids in LP-X as probed by 13C NMR are similar to the motions of lipids found in model vesicular systems, (b) that the motions of the cholesterol rings and phospholipid fatty acyl chains are significantly more restricted in LP-X than in HDL3 and LDL, and (c) that the motions of the phosphoryl moieties in all three systems are similar.

Lipoprotein-X

Lipoprotein-X is an abnormal lipoprotein that appears in the sera of patients with obstructive jaundice, and thus is a sensitive indicator of cholestasis. In patients with familial plasma lecithin, Cholesterol acyltransferase (LCAT) deficiency, there is an inverse relationship between plasma Lp-X levels and LCAT activity. Ultracentrifugation procedures utilized for isolation of Lp-X have shown that it is associated with the low density lipoprotein fraction. Lp-X can be visualized by electrophoresis on either Agar or Agarose. The purity of Lp-X preparations has been documented by immunochemical procedures. The availability of highly purified antisera to Lp-X has served as a basis of one of the assay procedures for this lipoprotein. It's chemical composition has been established. Phospholipids and unesterified cholesterol constitute the bulk of the Lp-X molecule. Electron microscopic studies have demonstrated that Lp-X is a spherical particle which has strong aggregating properties. Membrane bound enzymes have been shown to aggregate with Lp-X. The fact that bile lipoprotein can be converted to Lp-X by the addition of albumin and that Lp-X can be converted to bile lipoprotein by the addition of bile salts offers a possible explanation for the origins of Lp-X. Phospholipases of plasma might play a role in the catabolism of Lp-X. The value and limitations of Lp-X determinations will also be addressed in this review.