Extensive Incorporation of [2-14C]Mevalonic Acid into Cholesterol Precursors by Human Platelets in Vitro

1H-NMR lipid profiles of human blood platelets; links with coronary artery disease

Blood platelets are closely involved in the early development of atherosclerosis and in the events that lead to thrombosis, both of which are dominating factors in coronary artery disease (CAD). The aim of the present study was to evaluate the platelet lipid profiles of patients suffering from CAD and explore the possibility of a link between platelet lipids and CAD, using high-resolution high-field proton nuclear magnetic resonance spectroscopy as the analytical tool. The total platelet lipid profiles of healthy volunteers were compared with those of patients presenting with chest pain requiring coronary angiography. Two lipid groups changed significantly: cholesterol increased by 16.5% and total diacylglycerophospholipids decreased by 15.7%. There was also a significant decrease of the ethanolamine-containing phospholipids, by 4.7%; the extent of unsaturation of the fatty acid chains, by 0.2, and increase of the linoleate content of the fatty acid chains, by 1.9%. Our results suggest that platelet lipid abnormalities occur in patients with CAD and these changes may predate the development of overt atherosclerosis.

Age-related changes in platelet microviscosity and aggregation in rats

1. To study possible changes in platelet microviscosity in aged animals, 1,6-diphenyl-1,3,5-hexatriene (DPH) was used as a nonpolar probe embedded in thrice-washed platelets from young, adult and aged rats. With the known values of maximum limiting anisotropy and the structural parameter of DPH and by estimating the steady state of fluorescence anisotropy and the average fluorescence of lifetime, we applied the Perrin equation to calculate the microviscosity. 2. We measured platelet aggregation, platelet lipids and platelet polyunsaturated fatty acids to determine any causal relationship between these parameters. Platelet aggregation, the platelet molar ratio of cholesterol to phospholipid ([C]/[PL]) and platelet microviscosity increased with age (P < 0.05) and were correlated with one another (P < 0.05). 3. Age-dependent increases in the steady state of fluorescence anisotropy, order parameters and the short component of fluorescence lifetime of the fluorophore were expressed as functions of variables, such as microviscosity or the [C]/[PL] ratio. 4. Platelet concentrations of arachidonic and eicosapentaenoic acids increased with age, but were not associated with aggregation. Age-related changes in microviscosity and the [C]/[PL] ratio seemed to be determinants affecting biophysical properties of platelet aggregation.

Phosphorylation of farnesol in rat liver microsomes: properties of farnesol kinase and farnesyl phosphate kinase

As farnesol may serve as a nonsterol endogenous regulator of the mevalonate pathway, the possibility that a kinase specific for its phosphorylation is present in the rat liver was investigated. In the 10,000 g supernatant of rat liver, farnesyl monophosphate was synthesized in the presence of ATP. The Km value for farnesol was 2.3 microM. Various detergents inhibited the activity of the enzyme. The farnesol kinase was present in rough and in smooth I microsomes, but not in smooth II microsomes, mitochondria, peroxisomes, Golgi, or plasma membranes. The enzyme was associated with the inner, luminal surface of the vesicles. Further analyses have demonstrated that an enzymatic mechanism exists which catalyzes the phosphorylation of farnesyl-P to farnesyl-PP. Activity of the farnesyl phosphate kinase resulted in the phosphorylation of the monophosphate by CTP but not by ATP, GTP, or UTP. This enzyme is activated by low concentrations of detergents. Treatment with proteases and chemical probes indicate that this second phosphorylation reaction probably takes place on the outer, cytoplasmic surface of microsomal vesicles. These results demonstrate that rat liver microsomes contain two enzymes for the consecutive phosphorylation of farnesol to farnesyl-PP.

Proton nuclear magnetic resonance lipid profiling of intact platelet membranes

A combination of high-resolution one-dimensional and two-dimensional proton nuclear magnetic resonance spectroscopic methods and high pressure liquid chromatography have been used to analyse qualitatively and quantitatively the phospholipids, cholesterol and fatty acid composition of normal human blood platelets. The major phospholipids present were identified and quantified. Information on the nature and composition of the different classes of structural phospholipids and unsaturated fatty acids was obtained and was similar to that reported by others using different analytical procedures. This work showed that the proton nuclear magnetic resonance spectroscopy approach to lipid analysis could provide rapid, comprehensive and detailed information on the roles of lipids in pathological conditions in which platelets have been implicated.

Characterization of the in vitro conversion of dolichol to dolichoate in bovine thyroid

The enzymic conversion of dolichol into dolichoic acid has been studied in bovine thyroid subcellular fractions using [1-3H]dolichol as a substrate. The presence of conversion activity could be demonstrated in both the mitochondrial- and supernatant fractions. Investigation of cofactor requirements revealed that NAD+ was essential for reaching optimal activity. From kinetic studies Km-values of 3.5-4 microM and 0.29 mM could be calculated for, respectively, dolichol and NAD+ using the mitochondrial fraction as an enzyme source. No inhibitory effects from ethanol or pyrazole were detected suggesting that alcohol dehydrogenase is not involved in the dolichol-->dolichoate conversion as observed in a bovine thyroid mitochondrial fraction. From inhibitor studies the conversion system behaves distinctly differently from the NADP(+)-depending microsomal oxidoreductase as well as from catalase. The conversion activity in the supernatant on the other hand must be ascribed, at least partially, to a side activity of alcohol dehydrogenase.

Lipid composition of platelets from patients with atherosclerosis: effect of purified eicosapentaenoic acid ethyl ester administration

Although eicosapentaenoic acid (EPA) has been shown to have beneficial effects in the prevention of atherosclerosis, the mechanism by which these effects occur is not entirely clear. We investigated the lipid composition of platelets in paired subjects with and without atherosclerotic disease, either hypercholesterolemic (low density lipoprotein [LDL] cholesterol [Chol] greater than or equal to 170 mg/dl) or normocholesterolemic (LDL-Chol less than 170 mg/dl). Platelets from patients with atherosclerotic disease had a lower phosphatidylcholine (PC)/Chol ratio, when compared with those from patients without atherosclerotic disease, irrespective of LDL-Chol levels. Eleven patients with atherosclerotic disease were treated with purified EPA ethyl ester (1.8 g/day), and changes in lipid composition of platelets were investigated. Plasma levels of total Chol and LDL-Chol decreased significantly after EPA administration. The phospholipid (PL)/Chol ratio and the PC/Chol ratio in platelets from patients with atherosclerotic disease increased significantly after 4-10 wk EPA treatment. The EPA content in platelets increased, while the arachidonic acid (AA) content decreased. EPA-induced changes in the PL/Chol and the PC/Chol ratios of platelets, as well as fatty acyl chain shifts, may be related to the beneficial effects in preventing atherosclerosis, possibly by increase in the membrane fluidity.

Altered lipid composition and thromboxane A2 formation in platelets from patients affected by IIa hyperlipoproteinemia

Platelets from patients with familial hypercholesterolemia (type IIa hyperlipoproteinemia), a condition associated with high prevalence of atherosclerosis and of its thrombotic complications, are known to be hyperresponsive to aggregating stimuli and to synthesize increased amounts of thromboxane A2 (TxA2) in comparison to platelets from normal subjects. In order to search if these functional alterations are linked to a different platelet lipid composition, we studied a group of young patients affected by IIa hyperlipoproteinemia and a group of suitable controls with similar dietary habits. Both cholesterol and phospholipid content of platelets were higher in patients than in controls with a significant increase of cholesterol/phospholipid molar ratio (at least p less than 0.05). The percent contents of the main platelet phospholipid fractions were not altered, while an increase in saturated fatty acids, both unesterified and esterified in different lipid fractions, was observed. Moreover, an increased TxA2 production by platelets and a significantly increased number of megathrombocytes occur in patients with respect to controls (p less than 0.001). Our results indicates that platelets from patients with IIa hyperlipoproteinemia have an altered lipid composition which could explain, at least in part, the enhanced platelet reactivity reported in these patients.

The partitioning of fatty acid and cholesterol between core and surfaces of phosphatidylcholine-triolein emulsions at pH 7.4

Fatty acids are important intermediate molecules in lipid metabolism. During lipolysis of intracellular lipid droplets or plasma triacylglycerol-rich lipoproteins, fatty acids are generated and may transiently accumulate. We therefore studied the distribution of both fatty acid and free cholesterol between the core and surface of phosphatidylcholine-triolein emulsions at pH 7.4. Nine emulsion systems containing 0.8 to 6.6% cholesterol and 0.16 to 1.02% oleic acid were formed, and core and surface phases were isolated. Phospholipid distributes only to the surface phase. The distribution coefficient of cholesterol surface to core was 23.9 +/- 3.6 S.D., i.e., there was approx. 24-times more cholesterol per unit mass in the surface than in the core phase. This distribution was unchanged by the presence of different quantities of fatty acid in the emulsion particles. The apparent distribution coefficient of fatty acid in surface to core was about 10 at low cholesterol contents and fell to about 7 at high cholesterol contents. However, when the apparent distribution coefficient of fatty acid was related only to the phospholipid component of the surface, the apparent distribution coefficient was constant at about 12.3 +/- 1.1 S.D. Since the fatty acid in the surface phase is about half ionized the true distribution coefficient of unionized fatty acids is about 6.2. The results indicate that fatty acids partition into the phospholipid domains of the surface and not into cholesterol domains and the distribution of fatty acids into surface phospholipid domain is not affected by cholesterol content.

Biological effects of oxysterols on platelet function

We have studied the in vitro exposure to various oxysterols on isolated rat platelets. The oxysterols (1-20 uM) were incubated either dissolved in methanol or as albumin-bound complexes. Aggregation (analyzed by turbidimetry) was measured after stimulation by thrombin or ADP. Serotonin secretion (analyzed by voltammetry) was measured after thombin stimulation. We found that platelet aggregation and serotonin secretion could be either not significantly affected (7B-hydroxy cholesterol) or potentiated (22S-hydroxy cholesterol, 3,5,6-hydroxy cholestan triol) or inhibited (25-hydroxy- and 7-oxo cholesterol), after in vitro incubation with different oxysterols. Our data indicate that a modulation of the platelet behavior occurs after in vitro incubation with different oxysterols, some derivatives acting as inhibitors and others as potentiators. These results provide new interesting information regarding the role of these sterols in cell membrane structure and function in relation to pathology.

Enhanced phospholipase C activity in the vascular wall of spontaneously hypertensive rats

To explore the roles of vascular phospholipase C activity in the development of hypertension, phospholipase C activity was examined in the aortic wall of spontaneously hypertensive rats (SHR). Phospholipase C activity was significantly enhanced (+87%, p less than 0.005) in 14-week-old SHR as compared with normotensive Wistar-Kyoto rats (WKY). The enzymatic activities were positively correlated with the levels of blood pressure in both of the rat strains (r = 0.62, p less than 0.003). Vascular phospholipase C was also significantly activated (+62%; p less than 0.006) in the aortic wall of 4-week-old prehypertensive SHR, as compared with age-matched WKY. In contrast, vascular phospholipase A2 activity was unaffected in the aortic wall of either adult or very young SHR. There was no difference in the cardiac phospholipase C activity between adult SHR and WKY. The vascular phospholipase C of SHR had a lower Michaelis constant (Km) value than that of WKY. Moreover, its pH profile and calcium requirement differed in part from those of WKY. These results indicate that the activation of vascular phospholipase C precedes the development of hypertension and that the enhancement may be induced by both quantitative and qualitative changes in phospholipase C in SHR.

Platelet activation by plasma lipoproteins

Enhanced platelet activity in patients with hypercholesterolemia and moderate hypertriglyceridemia can be attributed to increased LDL and VLDL and/or decreased HDL concentrations. In marked hypertriglyceridemia, where there is an accumulation of chylomicrons, platelet function is reduced. Treatment, whether by diet, medication, or plasmapheresis, that will result in a change in lipoprotein pattern is accompanied by a parallel change in platelet responsiveness. Incubation of lipoproteins with isolated platelets results in enhancement of platelet activation by LDL and VLDL and suppression of activity by HDL and chylomicrons. These findings have in vivo confirmation. They are even more pronounced and sometimes altered when the lipoproteins are derived from hyperlipidemic subjects. The effects of the lipoproteins on platelet activity appear to be dependent on lipoprotein composition and on factors such as cholesterol:protein ratio, apo C-III0:apo C-III2 ratio, apo B concentration, and triglyceride:protein ratio. The lipoproteins interact with platelets at specific receptor sites. Rapid change in platelet composition, particularly with regard to cholesterol, phospholipid, and fatty acid content, might ensue, with consequent alterations in membrane fluidity and enzyme activities and either suppression or activation of platelet function. This review has indicated that lipoproteins have a clear influence on platelet function. This interaction could be well of paramount importance in determining atherogenic risk.

Evidence for the activation of phospholipases during acrosome reaction of human sperm elicited by calcium ionophore A23187

Incubation of washed human sperm with [3H]- or [14C]arachidonic acid allowed a major incorporation of the label into phospholipids, provided that the final concentration of the fatty acid did not exceed 20 microM. A further challenge with calcium ionophore A23187 of spermatozoa suspended in a calcium-containing medium led to phospholipid hydrolysis, which could account for 10-12% of total cell radioactivity. Degradation products were identified as free, unconverted arachidonic acid, occurring with some diacylglycerol. Phospholipid hydrolysis was significant after 15 min of incubation and became maximal after 120 min. It was found to be calcium dependent, diacylglycerol and free arachidonate production occurring maximally at 2 mM and 5 mM CaCl2, respectively. Phosphatidylcholine and phosphatidylinositol were the most significantly degraded phospholipids after 60 min of incubation. Similar incubations conducted with 32P-labeled sperm confirmed the selective hydrolysis of phosphatidylcholine and revealed an increase production of phosphatidic acid probably due to a phosphorylation of diacylglycerol. Under the same conditions, one third of the cells remained motile and electron microscopy revealed that acrosome reaction was completed in 40% of the cells and displayed an intermediary state in 40-50% of the spermatozoa. Furthermore, a good parallelism was observed between the extent of the acrosome reaction and the extent of phospholipid hydrolysis promoted by increasing concentrations of A23187. It is concluded that calcium entry into the cells activates both a phospholipase A2 and a phospholipase C, leading to the production of substances, like lysophospholipid, diacylglycerol or phosphatidic acid, which may or may not be involved in acrosome reaction.

Phospholipase C from human sperm specific for phosphoinositides

Human sperm lysates were incubated in the presence of 1-[14C]stearoyl-2-acyl-sn-glycero-3-phosphocholine, 1-[14C]stearoyl-2-acyl-sn-glycero-3-phosphoethanolamine or 1-[14C]stearoyl-2-acyl-sn-glycero-3-phosphoinositol. Only the latter substrate was hydrolyzed to a significant extent, with a concomitant formation of 1-[14C]stearoyl-2-acyl-sn-glycerol. Furthermore, incubation of phosphatidyl[3H]inositol under the same conditions was accompanied by the formation, in roughly equal amounts, of [3H]inositol 1-phosphate and [3H]inositol 1:2-cyclic monophosphate. Finally [32P]phosphatidylinositol 4-phosphate and [32P]phosphatidylinositol 4,5-bisphosphate were degraded into [32P]inositol 1,4-bisphosphate and [32P]inositol 1,4,5-trisphosphate, respectively. The phosphoinositide-specific phospholipase C was activated by calcium (optimal concentration 5-10 mM) and inhibited by EGTA, although endogenous calcium supported a half-maximal activity. The enzyme displayed an optimal pH of 6.0 and an apparent Km of 0.08 mM. Its specific activity was around 10 nmol/min per mg protein, which is approximately the same as that found in human blood platelets. Subcellular fractionation revealed that 55% of the enzyme was solubilized under conditions where 80% of acrosin appeared in the supernatants. The majority of the particulate phospholipase C activity (37% of total) was found in the 1000 X g pellet, which contained only 8% of total acrosin activity. Further fractionation of spermatozoa into heads and tails indicated no specific enrichment of phospholipase C activity in any of these two fractions. However, owing to a 4-fold higher protein content in the head compared to the tail fraction, it is concluded that about 80% of particulate phospholipase C activity is located in sperm head. The physiological significance of this enzyme is discussed in relation to a possible role in acrosome reaction and (or) in egg fertilization.

Salt-induced plasma factor that inhibits platelet thromboxane A2 release and renal prostaglandin E2 production in rats

This study examined the relationship between a plasma factor (or factors) that inhibits the release of thromboxane A2 from platelets and excessive salt intake in rats. The plasma factor, termed platelet inhibitory factor, was also characterized. The release of thromboxane A2 from thrombin-activated platelets was reduced in Wistar rats that were uninephrectomized and given 2% saline for a week, but not in rats with acute volume expansion. Platelet inhibitory factor was extracted from the plasma of these uninephrectomized and saline-loaded rats and partially purified using membrane sieves, reverse-phase high performance liquid chromatography (HPLC), modified straight-phase HPLC, and gel-permeation column chromatography. The molecular weight of the factor was about 4300 daltons by gel filtration method. The partially purified platelet inhibitory factor decreased the release not only of thromboxane A2, but also of prostaglandin E2 and prostaglandin D2 from thrombin-activated platelets. The factor inhibited the aggregation of human platelets induced by adenosine 5'-diphosphate (ADP), collagen, and thrombin, but not that by arachidonate. The platelet inhibitory factor reduced the activities of phospholipases A2 and C but did not affect the conversion of arachidonate to thromboxane A2. Furthermore, platelet inhibitory factor decreased prostaglandin E2 production in cultured renal cells, and platelet inhibitory factor-like activity was detected in kidney extract from the salt-loaded rats. These results suggest that platelet inhibitory factor is produced by chronic salt intake and involved in the functional alterations of the platelets and probably the kidneys, mainly through its inhibitory action on the liberation of arachidonate.

Pathways of arachidonic acid liberation in thrombin and calcium ionophore A23187-stimulated human endothelial cells: respective roles of phospholipids and triacylglycerol and evidence for diacylglycerol generation from phosphatidylcholine

Cultured endothelial cells from human umbilical vein were incubated for 20 h at 37 degrees C in the presence of [U-14C]arachidonic acid. Around 60-70% of the radioactive fatty acid was incorporated into cell lipids and was predominantly found in phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and triacylglycerol (39%, 33%, 13% and 6.5% of total incorporated radioactivity, respectively). Stimulation of the cells with human thrombin (2 U/ml) or calcium ionophore A23187 (5 microM) promoted the release into supernatants of arachidonic acid, 6-ketoprostaglandin F1 alpha, prostaglandins E2 and F2 alpha, in decreasing order of importance. The amount of secreted material was 4-fold higher with A23187, compared to thrombin. Parallel to the liberation process, phosphatidylcholine underwent a rapid decrease of radioactivity with both agonists, suggesting the involvement of a Ca2+-dependent phospholipase A2. Phosphatidylethanolamine displayed a minor decrease with A23187, whereas some reacylation was observed at 10 min with thrombin. Phosphatidylinositol was non-significantly affected in thrombin-stimulated cells, whereas A23187 promoted an early but minor decrease, followed by resynthesis. In contrast to A23187, thrombin was also able to promote a significant hydrolysis of triacylglycerol, which might thus be implicated in the process of arachidonate liberation. Finally, radioactive phosphatidic acid and diacylglycerol appeared in endothelial cells, in response to the two agonists. However, diacylglycerol formation did not parallel that of phosphatidic acid, especially with A23187. Determination of the 14C/3H ratio of the different lipids upon cell labelling with both [14C]arachidonic acid and [3H]palmitic acid revealed that diacylglycerol and phosphatidic acid are hardly derived from inositol-phospholipid breakdown by phospholipase C. Other possible pathways involving for instance phospholipase C splitting of phosphatidylcholine are discussed.

Intestinal absorption of ester and ether glycerophospholipids in guinea pig. Role of a phospholipase A2 from brush border membrane

In vivo intestinal perfusion was used to follow the absorption of three different choline glycerophospholipids (CGP) in guinea pig. These included 1-[3H]palmitoyl-2-acyl-sn-glycero-3-phosphocholine (diacyl-GPC), 1-[3H]-O-hexadecyl-2-acyl-sn-glycero-3-phosphocholine (alkylacyl-GPC) and 1,2-di-O-hexadecyl-sn-glycero-3-phospho-[3H]-choline (dialkyl-GPC). About 80% of diacyl-GPC was absorbed within 4 hr, compared to 60% of alkylacyl-GPC and 30% of dialkyl-GPC. The radioactivity disappearing from the perfusion fluid was recovered in intestinal lipids, mostly triacylglycerol, free fatty acid and CGP from diacyl-GPC, CGP from alkylacyl-GPC and dialkyl-GPC. These results indicated that the nonhydrolyzable substrate dialkyl-GPC was much less absorbed, whereas diacyl-GPC, which released over 80% of [3H]palmitic acid in the perfusion fluid, displayed the highest absorption rate. The intermediate picture observed for alkylacyl-GPC suggested the possible involvement of a phospholipase A2, which was detected in the entire intestinal tract. This enzyme was further found to concentrate in villus cells, where it is localized in the brush border membrane, as shown using two different subcellular fractionation procedures. These data suggest a possible role of this new enzyme in the digestion of alimentary phospholipids.

Age related changes in platelet lipid composition

Platelet lipid composition was investigated in 52 healthy subjects aged 20 to 68 years with similar dietary habits and living in a narrow geographic area in order to search possible changes referrable to aging. No significant variations were observed when platelet cholesterol, total phospholipids and different phospholipid fractions were considered, whereas cholesterol/phospholipid (C/PL) molar ratio significantly increased with aging (p less than 0.01). Moreover, a significant increase in 16:0 + 16:1 fatty acids was found in phosphatidylcholine (PC) and in sphingomyelin (SP) (r = 0.62, p less than 0.001 and r = 0.30, p less than 0.05 respectively) and a decrease in 18:2 n6 in the phospholipid fractions considered (at least p less than 0.05). These results indicate that modifications in platelet lipid composition occur with aging and that they could affect platelet functions so playing a role in the onset of atherosclerosis and in thrombotic phenomena occurring with increasing frequency in the elderly.

Platelet activation in normo- and hyperlipoproteinemias

In the last few years it became obvious that platelets are involved in the development of atherosclerotic diseases. This involvement of platelets has been taken into account in the "response to injury" hypothesis of atherosclerosis. The hypothesis is based on the assumption that atherosclerotic lesions result from endothelial injury, followed by the interaction of vessel wall constituents with lipoproteins, macrophages, and platelets. In the first part of this review, general aspects of platelet activation are summarized and the pathways of platelet aggregation as well as their involvement in blood coagulation are discussed. The second part of this paper describes the influence of cholesterol, lipoproteins, and apolipoproteins upon the activation and metabolic behavior of platelets. Physiological and pathophysiological processes particularly occurring in different types of hyperlipoproteinemias and atherosclerotic disorders are discussed in this context.

Oral polyunsaturated phosphatidylcholine reduces platelet lipid and cholesterol contents in healthy volunteers

The effects of orally administered polyunsaturated phosphatidylcholine (PPC) on plasma lipids, lipoproteins and platelet function and composition were studied in seven healthy male volunteers. PPC (Nattermann & Cie, GmbH, Cologne, Federal Republic of Germany), 10 g/day, was given for a 6-week period after a 4-week wash out; laboratory tests were repeated after a further 4-week period after the end of treatment. PPC did not appear, during treatment, to modify the levels of plasma total cholesterol and triglycerides. High density lipoprotein (HDL) cholesterol levels were, however, increased after six weeks of PPC. The most dramatic changes occurred in platelet membrane composition: the total lipid/total protein and the cholesterol/protein ratios were reduced significantly, whereas increases of the phospholipid/total lipid ratio and of the linoleic acid membrane content were observed. Platelet function tests, both in whole blood and in platelet rich plasma, were not modified. Similarly, the thromboxane B2 formation after standard stimuli and the sensitivity to exogenous prostaglandin I2 also were unchanged. During the final wash out period following treatment, a reduction of plasma total and low density lipoprotein (LDL) cholesterol levels also was recorded. PPC appears to be capable of modulating lipid exchanges between cell membranes and the plasma compartment.

Plasma lipoprotein levels and in vitro cholesterol synthesis by cells in human blood

This study has measured plasma lipoprotein lipid levels and the in vitro rate of cholesterol synthesis from [2(-14)C]acetate by mononuclear leukocytes in blood from normolipidemic subjects and two patients with Tangier disease. The rate of cholesterol synthesis in blood was related inversely to plasma levels of HDL cholesterol in the normolipidemic subjects. This relationship was mainly due to a similar correlation in the women. The rate of blood cholesterol synthesis was raised in a woman with Tangier disease, which is consistent with the above correlation, but not in a man with this disease. We suggest that this correlation reflects an association between plasma HDL cholesterol levels and whole-body sterol synthesis.

Cholesterol exchange in platelets, erythrocytes and megakaryocytes

Cholesterol exchange between plasma and human platelets and erythrocytes and guinea pig platelets, erythrocytes and megakaryocytes was studied. The characteristics of exchange of cholesterol between [3H]cholesterol-labeled plasma and human platelets and erythrocytes were similar: exchange per cell was independent of cell concentration in whole plasma, decreased only 2-fold over a wide range of cell concentrations in low concentrations of plasma and approached a plateau at 1/3 normal plasma cholesterol concentration, and there was no net change in the cholesterol content of either cell. The activation energy for exchange for both cells was 47 kJ/mol. In all experiments, erythrocyte cholesterol was labeled to approximately twice the specific activity of platelet cholesterol. Guinea pig megakaryocyte cholesterol exchanged at 25-33% of the rate of guinea pig platelet cholesterol in vitro. Similarly, when guinea pigs were fed [3H]cholesterol, erythrocyte cholesterol specific activity after 24 h was 90%, platelet 50-65%, and megakaryocyte 20-26% that of plasma. Guinea pig platelets incubated with plasma radiolabeled in free and esterified cholesterol incorporated radioactivity from free but not esterified cholesterol. The similarity of free cholesterol exchange in platelets and erythrocytes in vitro and in vivo and the apparent inability of platelets to take up cholesterol esters from lipoproteins suggest that the interaction between normal platelets and normocholesterolemic plasma is limited to cholesterol exchange.

The effect of hypercholesterolemia on guinea pig platelets, erythrocytes and megakaryocytes

This study has examined the effect of diet-induced hypercholesterolemia on guinea pig platelets, erythrocytes, megakaryocytes and plasma. The cholesterol/phospholipid ratios of plasma and erythrocytes began to increase after one day on the diet and increased steadily for two weeks and more slowly thereafter until 30 days. In contrast, the cholesterol/phospholipid ratio of platelets remained constant for 4-5 days, then increased until reaching a maximum of about 0.85 in two weeks. Thus, the time-course for increase of the cholesterol/phospholipid ratio is different for platelets than for erythrocytes and plasma. The increase in the cholesterol/phospholipid ratio of megakaryocytes was small and not dependent on the degree of increase in the plasma cholesterol/phospholipid ratio. The cholesterol esters of both platelets and megakaryocytes increased with time for two weeks. The increase in megakaryocyte cholesterol esters appeared to precede that of platelets. The protein content of platelets and megakaryocytes and average megakaryocyte size were increased. Normal platelets incubated in plasma from hypercholesterolemic guinea pigs did not accumulate excess cholesterol, but erythrocyte cholesterol increased 45% in 6 h under the same conditions. Cholesterol synthesis in megakaryocytes was depressed 50-80% by cholesterol feeding and by in vitro incubation of the cells in hypercholesterolemic plasma. The data suggest that the platelets and erythrocytes may accumulate excess cholesterol by different mechanisms. The effects of cholesterol feeding on megakaryocytes and the lag in accumulation of cholesterol in platelets relative to erythrocytes and plasma suggest that a defect in the megakaryocyte may be a primary determinant of accumulation of cholesterol in platelets.

Studies on ether phospholipids. II. Comparative composition of various tissues from human, rat and guinea pig

The ether phospholipid composition of various tissues (brain, heart, lung, liver, kidney, testis, erythrocytes and plasma) has been investigated in human, rat and guinea pig, using a new method of determination (El Tamer, A., Record, M., Fauvel, J., Chap, H. and Douste-Blazy, L. (1984) Biochim. Biophys. Acta 793, 213-220). This is based on the selective removal of diacyl phospholipid species by phospholipase A1 degradation followed by acidolysis of the plasmalogens. Our results fit rather well with other literature data available for human and rat tissues, illustrating the good reliability of the method. Among various differences noted between the three mammalian species, guinea pig is characterized by a relatively higher content of 1-alkyl-2-acyl-sn-glycero-3-phosphocholine (alkylacyl-GPC) and of ethanolamine plasmalogens in blood plasma. Alkylacyl-GPC, a putative precursor of platelet activating factor (PAF-acether or 1-alkyl-2-acetyl-GPC), is also more abundant in guinea pig lung and in human kidney. This study also revealed a striking parallelism between the tissue content of alkylacyl-GPC and alkylacyl-GPE (1-alkyl-2-acyl-sn-glycero-3-phosphoethanolamine). This new observation is discussed in relation to a possible metabolic link between these two phospholipids.

Substrate specificity of two cationic lipases with high phospholipase A1 activity purified from guinea pig pancreas. I. Studies on neutral glycerides

The substrate specificity of two cationic lipases with high phospholipase A1 activity purified from guinea pig pancreas has been tested towards various neutral glycerides. Triolein hydrolysis proceeded in the absence of di- and monoolein accumulation. Optimal conditions for di- and monoolein hydrolysis included an alkaline pH (9-10), a substrate concentration of 10 mM, and the presence of sodium deoxycholate (12 and 24 mM, respectively). Pancreatic colipase (bovine) had no effect on the activity of the two lipases. The comparison between the rates of hydrolysis of various substrates revealed the following order of decreasing enzyme activity: diolein greater than 1(3)-monoolein greater than tributyrin = triacetin greater than or equal to triolein = 2-monoolein. No hydrolysis of p-nitrophenylacetate and cholesteryloleate could be detected. Using 1-[3H]palmitoyl-2-[14C]linoleoyl-sn-glycerol, both enzymes displayed a strong preference for the 1-position, leading to the accumulation of 2-[14C]linoleoyl-sn-glycerol. Identical activities were found for the two lipases. It is concluded that the two cationic lipases from guinea pig pancreas represent a unique group of lipolytic enzymes different from other previously described enzymes, including classical pancreatic lipase, gastric and lingual enzymes, mold lipases and carboxylesterhydrolase.

Platelet interaction with high and low density lipoproteins

Gel-filtered platelets (GFP) from normal human subjects bound both low density lipoproteins (LDL) and high density lipoproteins (HDL). This binding was saturable and 125I-labelled lipoprotein uptake was inhibited by plasma. Platelets are also able to degrade lipoproteins but only to a limited extent. LDL appeared to compete with 125I-labelled HDL for platelet uptake, whereas the ability of HDL to displace 125I-LDL was limited. Cyclohexanedione-treated LDL (CHD-LDL), unlike CHD-HDL, did not compete with [125I]LDL for platelet accumulation, suggesting that arginine residues are necessary for LDL but not HDL binding. Addition of HDL or LDL to GFP did not alter platelet aggregation. However, in the presence of thrombin (0.5 U/ml), 1 mg/ml LDL incubated for 1 h at 23 degrees C enhanced platelet aggregation (215% increase) whereas HDL under similar conditions decreased aggregation by 53%. LDL also shortened the time of maximal aggregation whereas HDL had the opposite effect.

Biosynthesis of platelet lipids in relation to aggregation in women using oral contraceptives

The platelet lipid biosynthesis in relation to platelet aggregation and lipemia was studied by 14C-acetate and mevalonate incorporation into platelets of seventeen women without medication and of eighteen women using a low estrogen oral contraceptive. The lipid biosynthesis was significantly increased by 59% (mevalonate) and 38% (acetate) in women on oral contraceptives. From mevalonate, lipid synthesis was increased mostly in the lanosterol-dihydrolanosterol fraction (p less than .01). From acetate, lipid synthesis was significantly enhanced in all the lipid classes. In the oral contraceptive group, the response of platelets to thrombin aggregation was only slightly higher, but HDL-cholesterol was significantly lower. However, in the women using oral contraceptives, the percentage of abnormal values in HDL-cholesterol, thrombin-aggregation and acetate incorporation into lanosterol was similar. Thus, more than 40% of the women studied here, using low estrogen oral contraceptives, presented an increase in platelet lipid biosynthesis, especially in the lanosterol-dihydrolanosterol fraction, which was significantly correlated (p less than .05) with the response of their platelets to thrombin-induced aggregation.

Specific binding sites on human blood platelets for plasma lipoproteins

Scatchard analysis of the binding of homologous plasma low density lipoproteins (LDL) to human blood platelets shows the existence of a uniform class of saturable specific binding sites. Platelets from healthy donors bound 1470 +/- 640 molecules of LDL per platelet, the constant of association Ka = (6.2 +/- 2.2) X 10-7 l x mol-1. Binding kinetics, temperature dependence, and experiments with formaldehyde-fixed platelets showed that internalization of LDL (at least the labelled apoprotein moiety) did not occur to any considerable degree under the experimental conditions employed. Both very low density lipoproteins (VLDL) and high density lipoproteins 3 (HDL3) markedly inhibited the binding of LDL. In contrast to LDL, HDL3 bound to 3200 +/- 410 binding sites per platelet with a Ka = (9 +/- 1.7) X 10(7) l x mol-1. Additionally, the results of inhibition experiments using both LDL and HDL3 in combination gave evidence, that the sites for HDL3 binding were not identical with those for LDL binding and each inhibited binding of the other noncompetitively with reduction of the binding affinity and the number of available binding sites. VLDL bound to the platelet plasma membrane in a nonspecific-nonsaturable way. Possible significances of the presence of specific LDL receptors on the platelet plasma membrane for recognized functions of these blood elements are discussed.

Oral contraceptive and Platelet lipid biosynthesis in female rats: dose-response relationship

Female rats were treated with different doses of an oral contraceptive (ethinyl estradiol + lynestrenol) and lipid biosynthesis was studied in blood platelets by acetate incorporation into different fractions separated by thin layer chromatography. A marked increase in lipid biosynthesis was observed, especially in the sterol fractions (cholesterol and lanosterol-dihydrolanosterol). It was dose-dependent, observed after a lag-phase, maximal in 3 days and normalized in 8 days. Thus, the oral contraceptive studied here appears to modify platelet lipid biosynthesis for the entire life of the platelets.

Cholesterol and phospholipid biosynthesis in guinea pig megakaryocytes

Megakaryocytes are the bone marrow cells which produce the blood platelets. Platelet lipids are thought to be important determinants of platelet activity in thrombosis and hemostasis. We have investigated lipid synthesis from [U-14C]-acetate in isolated guinea pig megakaryocytes with the goal of elucidating the genesis of platelet lipids. Cholesterol was the major product of megakaryocyte lipid synthesis from [U-14C]acetate. Incorporation of acetate (0.1 mM) into cholesterol per 10(6) megakaryocytes (1.1 mg protein) was 0.14 nmol in 0.5 h, 0.95 nmol in 1.5 h and 3.2 nmol in 4.0 h. Megakaryocytes also synthesized cholesterol from [U-14C]glucose. In contrast, platelet sterol synthesis was negligible. Both megakaryocytes and platelets synthesized phospholipids from [U-14C]acetate. Megakaryocytes incorporated 0.07 nmol acetate per 10(6) cells into phospholipid in 0.5 h, 0.38 nmol in 1.5 h and 1.8 nmol in 4.0 h. Platelets (10(9) cells, 1.3 mg protein) incorporated 0.15 nmol acetate into phospholipids in 1.5 h. Phosphatidylcholine accounted for 62% of the phospholipid radioactive label in megakaryocytes and 78% in platelets. Phospholipid radioactivity was associated with the fatty acids. We hypothesize that the megakaryocyte may synthesize a major portion of platelet cholesterol and that the phospholipid and fatty acid synthetic pathways available to the platelet are derived from the megakaryocytes.

Biosynthesis of cholesterol and cholesterol precursors in platelets of female rats treated with oral contraceptives

Rat platelets were incubated with sodium [U-14C]acetate and labeled lipids were analyzed. The major part of the radioactivity was found in phospholipids and in acylglycerols. When the incubation was performed with platelets of female rats treated with contraceptives, the total incorporation of labeled acetate in lipids was 2-fold higher and 33.6% of the radioactivity was found in lanosterol plus 24-dihydrolanosterol. Moreover, there is labeling of cholesterol. All these compounds were analyzed and identified by gas-liquid chromatography/mass spectrometry. In the incubation with sodium [2-14C]mevalonate the labeling of lanosterol plus 24-dihydrolanosterol and of cholesterol was increased by 30% in platelets of treated rats. The effect of contraceptives on one step of cholesterol biosynthesis is discussed and a possible explanation of the correlation between oral contraceptives and thromboembolic accidents is suggested.

Asymmetric distribution of arachidonic acid in the plasma membrane of human platelets. A determination using purified phospholipases and a rapid method for membrane isolation

1. Non-lytic degradation of human platelet phospholipids have been performed using a combination of bee venom phospholipase A2 (phosphatide 2-acyl-hydrolase, EC 3.1.1.4) and Staphylococcus aureus sphingomyelinase C (sphingomyelin choline phosphohydrolase). Under these conditions, 25.4% of total phospholipds are degraded and 6.4% of total platelet arachidonic acid is released. 2. A new method for rapid isolation of platelet plasma membrane is described, based on the use of [3H]concanavalin A as a membrane marker and of self-generating gradients of Percoll. Plasma membranes are enriched 5.2 fold in lectin marker and 0.43 in N-acetyl-beta-D-glucosaminidase, the main contaminant. This method allows to estimate that 57% of the total cell phospholipids and 61% of the total arachidonic acid content are located in the plasma membrane. 3. The distribution of phospholipids and arachidonic acid between the two leaflets of the plasma membrane has been deduced by using these values and those obtained from non-lytic treatment of intact platelets by phospholipases. It is concluded that 45% of plasma membrane phospholipids, comprising 93% of sphingomyelin, 45% of phosphatidylcholine, 9% of phosphatidylserine, 16% of phosphatidylinositol and 20% of phosphatidylethanolamine form the outer half of the human platelet plasma membrane. The phospholipids appear to bear only 10% of the total membrane arachidonic acid.