Extrahepatic cell membrane lipid abnormalities and cellular dysfunction in liver disease

Liver disease is associated with characteristic changes in the lipid composition of the surface coat of plasma lipoprotein particles. Cholesterol and lecithin molecules accumulate as hepatic secretion of lecithin-cholesterol acyltransferase decreases, and the arachidonate content, the precursor for eicosanoid production, is also reduced. By exchange and equilibration processes, such abnormal circulating lipoproteins should tend to induce corresponding changes in cell membrane lipid composition; studies in both human and experimental liver disease confirm that this does occur and that it is wide-spread. The correct functioning of membrane proteins, which serve as receptors or are responsible for enzymatic and transport processes, is most commonly dependent on the fluidity of their lipid bilayer matrix. Because cholesterol enrichment of biomembranes reduces bulk lipid fluidity, it can be predicted that extrahepatic membrane dysfunction might be a general feature of severe liver disease. This concept is supported by increasing experimental evidence and, as a consequence, it is proposed that many of the cellular disturbances and metabolic abnormalities accompanying hepatic disease result from, or are exacerbated by, lipoprotein-induced changes in membrane lipid composition and function. Importantly, this mechanism also suggests that drugs which can fluidise membranes, such as S-adenosyl-L-methionine (SAMe), might help ameliorate cellular dysfunction.

Binding of apoE-rich high density lipoprotein particles by saturable sites on human blood platelets inhibits agonist-induced platelet aggregation

High density lipoproteins (HDL, d 1.063-1.21 g/ml) are reported to stimulate, to have no effect on, or to inhibit agonist-induced platelet aggregation. We have hypothesized that these conflicting reports might be explained by opposing effects of individual HDL subclasses on platelet aggregability. Physiologic concentrations of HDL3 had little effect on ADP-induced aggregation of washed platelet suspensions, although higher levels were stimulatory. Normal concentrations of HDL2 (0.2-0.4 mg of protein/ml) inhibited aggregation; further fractionation by heparin-Sepharose chromatography identified the particles rich in apolipoprotein E, termed HDL-E, as the major anti-aggregatory subclass. Washed platelets bound radioiodinated HDL-E to a uniform class of saturable sites; they numbered 4,200 per platelet and the KD was 7.9 x 10(-7) M. Binding of HDL-E by platelets, and its anti-aggregatory action, showed a similar rapidity and both occurred within the physiologic concentration range. Moreover, the two processes were independent of the presence of divalent ions and were impaired by chemical modification of the apolipoprotein constituents of HDL-E. We conclude that occupation of cell-surface receptors by HDL-E particles impairs platelet responsiveness to exogenous agonists and that platelet aggregability in the presence of whole HDL may reflect the relative concentrations of the individual subclasses in the particular sample.

Inhibition of platelet aggregation by abnormal high density lipoprotein particles in plasma from patients with hepatic cirrhosis

ADP-induced aggregation of normal washed platelets was measured by nephelometry in the presence of plasma high density lipoprotein (HDL) from normal subjects and from 30 patients with hepatic cirrhosis. HDL, at one-eighth of its plasma concentration, inhibited platelet aggregation; the effect of cirrhotic HDL (40% [SD 29%] inhibition) was significantly greater than that of normal HDL (16% [11%]). The mean apolipoprotein E content of cirrhotic HDL was significantly higher than that of normal HDL, and strongly inhibitory HDL contained twice as many apolipoprotein-E-rich particles as weakly inhibitory HDL. Inhibition of platelet aggregation was correlated with the apolipoprotein E content of HDL from patients with cirrhosis.

The secondary structure of apolipoproteins in human HDL3 particles after chemical modification of their tyrosine, lysine, cysteine or arginine residues. A Fourier transform infrared spectroscopy study

Fourier transform infrared spectra of apolipoprotein E-depleted human HDL3 have been obtained in H2O and 2H2O buffers. The absorption bands in the protein amide I and amide II regions (1700-1500 cm-1) were assigned to alpha-helical, disordered and beta-strand/beta-turn structures of apolipoproteins A-I and A-II (apoA-I and apoA-II), the apolipoprotein constituents of HDL3. Modification of HDL3 by tetranitromethane (TNM) treatment, acetylation, reduction plus alkylation and 1,2-cyclohexanedione treatment derivatised tyrosine, lysine, cysteine and arginine residues, respectively, and caused alteration of the secondary structure of the HDL3 apolipoproteins to different extents. Each of the chemical modifications caused changes in the frequency of bands associated with beta-strands/beta-turns, but only TNM treatment of HDL3, as judged by the second- and fourth-derivative spectra, resulted in a shift of the band assigned to the alpha-helical structure of the proteins. In agreement with other workers, only TNM treatment of HDL3 particles was found to inhibit their binding by high-affinity cell membrane receptors. It is proposed, therefore, that receptor recognition of HDL3 particles is dependent on conservation of the alpha-helix structures within apoA-I and apoA-II, and that beta-strand/beta-turn structures are not involved. This conclusion is consistent with the predominance of amphipathic alpha-helical structures in both apolipoproteins and with the relaxed specificity of the receptors which are thought to recognise both apoA-I and apoA-II.

A study of the structure of the gene for lecithin: cholesterol acyltransferase in four unrelated individuals with familial lecithin: cholesterol acyltransferase deficiency

1. We have used polyclonal antibodies and a complementary DNA clone for human lecithin:cholesterol acyltransferase (LCAT) to study LCAT protein and the structure of the LCAT gene, respectively, in patients with familial LCAT deficiency from Norway, Ireland, Germany and Italy. 2. The patients had low levels of non-functional LCAT protein in their serum as measured by rocket immunoelectrophoresis; its mol. wt. of approximately 68,000 was identical with that of LCAT in normal plasma, as judged by immunoblotting. 3. Enzymatic digestion of DNA samples from the patients produced LCAT gene fragments which were indistinguishable from those found in normal individuals. 4. We conclude that LCAT deficiency in these patients is not caused by a large deletion or rearrangement of the LCAT gene sequences.

Hereditary lecithin-cholesterol acyltransferase deficiency. Case report of a German patient

Lecithin-cholesterol acyltransferase (LCAT) deficiency was first described in a Norwegian family as an inborn error of metabolism. Altogether, 35 patients in 18 families have been identified. The authors report the first German patient, who presented with the characteristic clinical features of corneal opacity, proteinuria, and mild anemia. Renal biopsy revealed foam cells and an increased mesangial matrix in the glomeruli. Confirmation of the clinical diagnosis of LCAT deficiency was obtained by plasma enzyme and lipid analyses. Functional LCAT activity was not detected in incubated plasma by chemical or radiochemical methods, although rocket immunoelectrophoresis indicated that the patient had about one-third of normal LCAT mass. In keeping with other reports of LCAT deficiency, apoE-rich discoidal particles were seen in the patient's high-density lipoprotein fraction by electron microscopic examination.

Stomatocytosis, abnormal platelets and pseudo-homozygous hypercholesterolaemia

A 13-yr-old girl with congenital haemolytic anaemia associated with pseudo-homozygous hypercholesterolaemia is described. The erythrocyte morphology showed 50-80% stomatocytes, but no abnormality of membrane lipid or protein composition or of cation transport was detected. The platelets were reduced in number, abnormally large and showed reduced adhesion. Successful treatment of the hypercholesterolaemia did not influence the stomatocytosis.

Membrane lipid changes in erythrocytes, liver and kidney in acute and chronic experimental liver disease in rats

Lipid molecules in lipoprotein surfaces exchange with their counterparts in cell plasma membranes. In human or experimental liver disease, plasma lipoprotein surfaces are enriched in cholesterol and deficient in arachidonate; corresponding alterations occur in membrane lipids of erythrocytes. To determine whether similar changes take place in membranes of nucleated cells, the lipid content of plasma and of erythrocyte, liver and kidney membranes was measured in rats with acute (3-day) galactosamine-induced hepatitis or chronic (3-week) biliary obstruction. In both models of liver injury the cholesterol:phospholipid ratio in plasma and in erythrocytes was significantly increased (P less than 0.001). Although this ratio was also elevated in liver and kidney microsomes, only in liver microsomes of obstructed rats was the increase significant (P less than 0.001). However, the cholesterol:phospholipid ratio of kidney brush-border membranes, was significantly higher in bile-duct-ligated rats; presumably, compensating mechanisms limit cholesterol accumulation in intracellular membranes. Kidney brush-border membranes from obstructed rats were deficient in arachidonate as were plasma and erythrocytes. However, arachidonate levels were unchanged in kidney microsomes; renal delta 6-desaturase, the rate-limiting enzyme in the conversion of linoleic acid to arachidonic acid, was increased by 50% (P less than 0.001) and may have counteracted a reduced supply of exogenous lipoprotein arachidonate. We conclude that in experimental liver disease lipoprotein-induced lipid abnormalities can occur in renal membranes, although compensatory mechanisms may operate; the alterations seen, cholesterol accumulation and arachidonate depletion, would be expected to interfere with sodium transport and prostaglandin production, respectively. Our findings support the hypothesis that lipid abnormalities in kidney membranes contribute to the renal dysfunction which is a frequent complication of human liver disease.

Platelet aggregation and plasma lipoproteins in alcoholics during alcohol withdrawal

Platelet aggregation, platelet lipid composition and plasma lipoprotein concentrations were measured each week in a group of seventeen alcoholics, without overt liver disease, for one month, following acute, total alcohol withdrawal. The platelets were initially hypoaggregable but, within 1-2 weeks of cessation of drinking, they became hyperaggregable and then gradually returned towards normal values. Hyperaggregability could not be explained by increases in either the cholesterol or the arachidonic acid content of the platelets. Plasma very-low-density lipoprotein cholesterol levels remained high throughout the study, but the initially raised levels of high-density lipoprotein (HDL) cholesterol fell by 26%. Low-density lipoprotein (LDL) cholesterol concentration rose by 10% after two weeks of withdrawal but then returned to about the starting level. The resulting changes in the plasma LDL-cholesterol:HDL-cholesterol ratio, which had increased by more than 50% after two weeks of abstinence, essentially paralleled the time course of enhanced platelet reactivity in all but four of the alcoholics. These findings suggest that alterations in plasma lipoprotein concentrations during acute alcohol withdrawal may be a contributory factor to the haemostatic disorders present in such patients.

Erythrocyte echinocytosis in liver disease. Role of abnormal plasma high density lipoproteins

Echinocytes were frequently found in patients with liver disease when their blood was examined in wet films, but rarely detected in dried, stained smears. When normal erythrocytes (discocytes) were incubated with physiologic concentrations of the abnormal high density lipoproteins (HDL) from some jaundiced patients, echinocytosis developed within seconds. Other plasma fractions were not echinocytogenic. There was a close correlation between the number of echinocytes found in vivo and the ability of the corresponding HDL to induce discocyte-echinocyte transformation. On incubation with normal HDL, echinocytes generated in vitro rapidly reverted to a normal shape, and echinocytes from patients showed a similar trend. Echinocytosis occurred without change in membrane cholesterol content, as did its reversal, and was not caused by membrane uptake of lysolecithin or bile acids. Abnormal, echinocytogenic HDL showed saturable binding to approximately 5,000 sites per normal erythrocyte with an association constant of 10(8) M-1. Nonechinocytogenic patient HDL and normal HDL showed only nonsaturable binding. Several minor components of electrophoretically separated erythrocyte membrane proteins bound the abnormal HDL; pretreatment of the cells with trypsin or pronase reduced or eliminated binding. Echinocytosis by abnormal HDL required receptor occupancy, rather than transfer of constituents to or from the membrane, because cells reversibly prefixed in the discoid shape by wheat germ agglutinin, and then exposed to abnormal HDL, did not become echinocytes when the HDL and lectin were successively removed. Binding did not cause dephosphorylation of spectrin. We conclude that the echinocytes of liver disease are generated from discocytes by abnormal HDL, and we infer that the shape change is mediated by cell-surface receptors for abnormal HDL molecules.

Abnormal high density lipoproteins from patients with liver disease regulate cholesterol metabolism in cultured human skin fibroblasts

Apolipoprotein B (apoB) of plasma low density lipoproteins (LDL) binds to high affinity receptors on many cell types. A minor subclass of high density lipoproteins (HDL), termed HDL1, which contains apoE but lacks apoB, binds to the same receptor. Bound lipoproteins are engulfed, degraded, and regulate intracellular cholesterol metabolism and receptor activity. The HDL of many patients with liver disease is rich in apoE. We tested the hypothesis that such patient HDL would reduce LDL binding and would themselves regulate cellular cholesterol metabolism. Normal HDL had little effect on binding, uptake, and degradation of 125I-labeled LDL by cultured human skin fibroblasts. Patient HDL (d 1.063-1.21 g/ml) inhibited these processes, and in 15 of the 25 samples studied there was more than 50% inhibition at 125I-labeled LDL and HDL protein concentrations of 10 micrograms/ml and 25 micrograms/ml, respectively. There was a significant negative correlation between the percentage of 125I-labeled LDL bound and the apoE content of the competing HDL (r = -0.54, P less than 0.01). Patient 125I-labeled HDL was also taken up and degraded by the fibroblasts, apparently through the LDL-receptor pathway, stimulated cellular cholesterol esterification, increased cell cholesteryl ester content, and suppressed cholesterol synthesis and receptor activity. We conclude that LDL catabolism by the receptor-mediated pathway may be impaired in liver disease and that patient HDL may deliver cholesterol to cells.

The aggregation of isolated human platelets in the presence of lipoproteins and prostacyclin

Addition of prostacyclin (PGI2) temporarily inhibits platelet aggregation and permits the isolation of platelets free from plasma proteins, which have the same sensitivity as those in plasma [Moncada, Radomski & Vargas (1982) Br. J. Pharmacol. 75, 165P]. By using a modification of this technique we have established that platelets isolated from normal subjects aggregate more readily in response to ADP and adrenaline when physiological concentrations of low-density lipoproteins (LDL) are present. At high LDL concentrations spontaneous aggregation occurs. High-density lipoproteins (HDL) and very-low-density lipoproteins (VLDL) had no effect on agonist-induced platelet aggregation at normal concentrations, but HDL sensitized at higher concentrations. These effects by lipoproteins are not accompanied by changes in platelet lipid content. Cyclohexanedione treatment of LDL to modify apolipoproteins appeared to abolish the sensitization effect, indicating that binding to receptors was essential for the effects of LDL. LDL, but not HDL, overcame the inhibitory effect of PGI2 on platelet aggregation, except at very high concentrations of PGI2. PGI2 raised the cyclic AMP content of isolated platelets, but LDL only partially prevented this rise. These results suggest that LDL may have a greater role in platelet aggregation than previously recognized and may also regulate effects of PGI2. These findings may be of relevance to an understanding of cardiovascular diseases.

Decreased erythrocyte membrane fluidity and altered lipid composition in human liver disease

Abnormal plasma lipoproteins in patients with liver disease are associated with characteristic changes in erythrocyte membrane lipid composition. The membranes are enriched in cholesterol and phosphatidylcholine and both the cholesterol/phospholipid and phosphatidylcholine/sphingomyelin molar ratios are increased. Phospholipid fatty acid composition is also abnormal; the proportions of arachidonic acid and stearic acid are decreased and that of palmitic acid raised. In this study we have examined the effects of these membrane lipid abnormalities on membrane fluidity. Erythrocyte membrane fluidity was assessed in 30 patients with a variety of liver diseases and in 25 normal subjects using the hydrophobic, fluorescent probe 1,6-diphenylhexa-1,3,5-triene and the values were related to their lipid composition. Membrane fluidity was significantly decreased in the patient erythrocytes (lipid order parameter, S(v)[37 degrees C] = 0.713 +/- 0.018, mean +/- S.D. compared to 0.686 +/- 0.008 in the normal subjects, P < 0.001) and correlated significantly with the cholesterol/phospholipid ratio (r = 0.88, P < 0.001). The fluidity of lipid extracts from the membranes of patient erythrocytes was also decreased, suggesting that decreased membrane fluidity was mainly a consequence of altered lipid composition rather than protein abnormalities. Incubation of patient erythrocytes for 20 hr with normal, heated plasma removed the excess cholesterol without affecting the phosphatidylcholine/sphingomyelin ratio or phospholipid fatty acid composition; following incubation the fluidity of these membranes was similar to that of normal membranes. We conclude that in liver disease changes in the composition of the phospholipid bilayer matrix in the erythrocyte membrane have little influence on its fluidity; the reduced fluidity is predominantly a result of increases in cholesterol relative to phospholipid.-Owen, J. S., K. R. Bruckdorfer, R. C. Day, and N. McIntyre. Decreased erythrocyte membrane fluidity and altered lipid composition in human liver disease.

Hydrogenation of plasma lipoproteins by a water-soluble catalyst; its use as a structural probe

Hydrogenation of double bonds of fatty acyl chains in the lipids of individual, intact plasma lipoproteins has been accomplished using a water-soluble, homogenous catalyst. Up to 40% of the double bonds can be hydrogenated. The pattern of hydrogenation is similar for each of the lipoprotein fractions; the phospholipids are most extensively hydrogenated but small amounts of cholesteryl ester and triacylglycerol are also hydrogenated. This results is consistent with a surface location of phospholipids and with the majority of the apolar lipids being in a hydrophobic inner core since the catalyst was shown not to be specific for an individual lipid class. It also suggests the some cholesteryl ester and triacylglycerol may be near the lipoprotein surface, accessible to the catalyst. If this is the case, it is speculated that the known exchange of these neutral lipids between the lipoprotein classes and their degradation by lipolytic enzymes may thus be facilitated.

Ophthalmoplegia and ptosis in congenital fiber type disproportion

Bilateral ophthalmoplegia and ptosis is reported for the first time in a patient with a rare neuromuscular disorder, congenital fiber type disproportion (CFTD). The importance of limb muscle biopsy in the diagnostic evaluation is emphasized. A summary is presented of other congenital neuromuscular diseases which may have associated ophthalmoplegia.

Platelet lipid composition and platelet aggregation in human liver disease

Abnormal plasma lipoproteins in patients with liver disease are associated with an increase in erythrocyte cholesterol concentration and a raised erythrocyte cholesterol/phospholipid molar ratio. We hypothesized that their platelets would also have an increased cholesterol/phospholipid ratio and that this might affect aggregation in vitro. Platelet aggregates by adrenaline and ADP was measured in 34 patients with a variety of liver diseases and in 20 normal subjects and the values were related to platelet lipid composition. The platelet cholesterol/phospholipid ratio was 13% higher in the patients and correlated closely with erythrocyte cholesterol/phospholipid ratio. Platelet aggregation was reduced in most of the patients and inversely correlated with the cholesterol/phospholipid ratio. Cross-incubation and hemostasis studies indicated that there were no inhibitory factors present in the plasma; the defect was in the platelets. In contrast, other workers have shown that cholesterol-rich platelets, either from patients with Type IIa hyperlipoproteinemia or prepared in vitro, aggregate more readily than normal platelets. However, the phospholipid and fatty acid compositions of our patient platelets were also abnormal: the lecithin/sphingomyelin ratio was increased and was inversely correlated with aggregation; the proportion of arachidonic acid was decreased and positively correlated with the aggregation. In our patients with liver diseases the effects of the altered phospholipid and fatty acid composition presumably overrode those of the increased cholesterol content so that instead of enhanced aggregation, only reduced or normal aggregation was seen. We conclude that the reduced platelet aggregation seen in liver disease may reflect a decrease in arachidonic acid availability for prostaglandin and/or thromboxane production.

Evidence for active phospholipase A in mouse plasma

Mouse, rat and human plasma were exposed to minimum concentrations of disulphide or minimum pre-incubation at 55 degrees C in order to inhibit lecithin : cholesterol acyltransferase activity completely. The plasma samples were subsequently incubated at 37 degrees C and changes in individual phospholipid concentrations determined. Significant utilization of phosphatidylcholine and formation of lysophosphatidylcholine occurred only in disulphide-treated mouse plasma and this was accompanied by a decrease in total phospholipid concentration. When disulphide-treated mouse plasma was incubated with [U-14C]phosphatidylcholine radioactivity was additionally recovered in the lysophosphatidylcholine, non-esterified fatty acid and glycero-3-phosphocholine fractions; maximum conversion occurred at close to physiological pH. These observations suggest that phospholipase A and lysophosphatidylcholine hydrolase enzymes are active in mouse plasma but that phospholipase A is either absent or inactive in rat and human plasma.