Cholesterol and phospholipid biosynthesis in guinea pig megakaryocytes

The effects of olive oil, hydrogenated palm oil, and omega-3 fatty acid-enriched diets on megakaryocytes and platelets

Unsaturated fatty acids are thought to prevent thrombotic and arteriosclerotic disease, whereas saturated fatty acids are thought to increase the incidence of these disorders. However, the effects of these diets on megakaryocytes and platelets are not well understood. We compared the effects of diets enriched with 8.4% olive oil, 8.4% hydrogenated palm oil, or 10.2% omega-3 fatty acid ethyl esters on guinea pig megakaryocytes and platelets. In plasma, changes in fatty acid composition reflected the composition of each diet. However, in platelets and megakaryocytes, hydrogenated palm oil induced a decrease in 16:0 and an increase in 18:2 while the olive oil diet caused a marked increase in 18:1 and a decrease in most other fatty acids. The differences in the effects of the diets on cellular versus plasma fatty acids suggest that megakaryocytes and platelets have an extensive capacity to regulate their fatty acid composition. Thrombocytosis occurred with the omega-3 fatty acid-enriched diet: 12.9 +/- 1.78 x 10(5) compared with 7.45 +/- 1.08 x 10(5) platelets per microliter of platelet-rich plasma in control animals. There was an increase in megakaryocyte size, ploidy, and morphological stage (cytoplasmic maturation) with the omega-3 fatty acid-enriched diet but not with the other diets. The omega-3 fatty acid-enriched diet decreased platelet thromboxane production while the other diets had no effect. Platelet hypersensitivity was suggested in collagen aggregation studies with olive oil but not with the hydrogenated palm oil diet. Although saturated fatty acid diets are thought to be atherogenic, this diet had no affect on platelet function.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid and membrane fluidity abnormalities in platelets and megakaryocytes of the hereditary macrothrombocytopenic Wistar Furth rat

Biochemical and functional abnormalities of megakaryocytes and platelets were studied in Wistar Furth (WF) rats which have genetically determined macrothrombocytopenia and megakaryocytopenia, and were compared with their counterparts in Sprague-Dawley (SD) rats. Both megakaryocytes and platelets synthesized phospholipids from [14C]acetate. WF and SD megakaryocytes incorporated 0.27 and 0.29 nmol acetate per 10(6) cells, respectively. Phosphatidylcholine (PC) accounted for 64% and 58% of the PL radioactive label in megakaryocytes of SD and WF rats, respectively, (P less than 0.05), while 69% of labeled activity was associated with PC of SD platelets compared to 60% found in PC of WF platelets (P less than 0.01). In WF platelets a significant increase in the levels of lysophosphatidylcholine (6.1% vs. 3.0%) was observed. WF platelets had substantially higher levels of esterified cholesterol, triglycerides, ceramides and a 3-fold increase in the total protein per platelet compared to SD platelets. The fatty acid composition of WF platelet PC showed quantitative abnormalities. Plasma lecithin-cholesterol acyl transferase activity and platelet function monitored by the uptake and release of [14C] serotonin showed nonsignificant variations between SD and WF rats. Compared with the control, platelet membrane fluidity, measured by fluorescence polarization using platelets labeled with 1,6-diphenyl-1,3,5-hexatriene, was significantly decreased in the WF rats.

Erythrocyte membrane fluidity and changes in plasma lipid composition: a possible relationship in childhood obesity

We have studied plasma lipid patterns and erythrocyte membrane fluidity in 60 obese children and 20 normal children. Plasma levels of total cholesterol and associated low-density lipoproteins were significantly increased in 20 obese patients with respect to controls. A significant decrease in membrane fluidity, measured as an increase in the fluorescence polarization value of the probe 1,6-diphenyl-1,3,5-hexatriene, associated with an increase in the cholesterol/protein ratio has been shown in obese patients. The study of the correlation between erythrocyte membrane fluidity and plasma cholesterol has indicated that significant changes in fluidity and membrane lipid composition also occur in erythrocytes of obese patients with normal plasma lipid levels. These findings confirm that the erythrocyte membrane responds very early to modifications of plasma lipoproteins and suggest that in childhood obesity a modified transfer of cholesterol from plasma to erythrocyte membrane may take place.

Phospholipid composition of rat megakaryocytes and its rearrangement in platelets

Rat platelets and their megakaryocyte precursors were examined for phospholipid composition. (1) The phospholipid composition of rat megakaryocytes, which were enriched and prepared from bone marrow cells, was almost identical to that of platelets. (2) The subclass composition of choline-containing glycerophospholipids (CGP) of rat megakaryocytes differed significantly from that of platelets: 1-alkenyl-2-acyl glycerophosphocholine (GPC) in megakaryocytes accounted for 29% of the total, whereas that in platelets was only 7%. (3) Rat platelets contained a larger amount of arachidonic acid than megakaryocytes, especially in ethanolamine-containing glycerophospholipids (EGP). (4) [32P]Phosphoric acid was significantly incorporated into megakaryocytes, whereas platelets showed little incorporation. On the other hand, the uptake of [3H]arachidonic acid into platelet phospholipids was about 15-times higher than that observed with megakaryocytes. (5) As reported previously for other blood cells, such as neutrophils and macrophages, the radioactivity of labeled arachidonic acid incorporated into CGP of platelets decreased, whereas that incorporated into EGP increased during a subsequent chase period. Hardly any such change was observed with megakaryocytes. These results suggest that the phospholipid composition of rat platelets is mainly determined at the time of thrombopoiesis, whereas the composition of molecular species is remodeled during circulation after thrombopoiesis.

Effects of marine oil-enriched diets on guinea pig megakaryocyte and platelet lipids: effects on thromboxane synthesis and platelet function

The effects of marine oil-enriched diets on the fatty acid composition of lipids in guinea pig megakaryocytes (MK) and platelets were studied to obtain a better understanding of the mechanisms for changes in platelet fatty acid composition and platelet function. Animals were fed 2%, 5% and 10% menhaden oil-enriched diets for up to 35 days. Platelets and MK were isolated and MK subpopulations at various stages of development were prepared. The diets did not cause a change in the cholesterol/phospholipid ratio in MK or platelets. The diets induced a dose related incorporation of eicosapentaenoic (20:5) and docosahexaenoic acid (22:6) and an associated decrease in linoleic acid (18:2) in both MK and platelets. However, there was a considerable greater depression of 20:4 in platelets than in MK. These changes were evident with 2% marine oil diets and maximal with 10% diets. Half maximal changes in fatty acid composition occurred after 3 days and maximal changes at 10 days after the initiation of the diets and no further changes occurred up to 35 days. Based on percent of total fatty acids in individual phospholipids, 20:5 had been primarily incorporated into phosphatidylethanolamine (PE) and phosphatidylinositol (PI) and 22:6 into PE and phosphatidylserine (PS) in both MK and platelets. 18:2 was decreased in all phospholipids. 20:4 was decreased only in PI in MK while 20:4 was decreased in PE, PI and PS in platelets. In animals on the 10% marine oil diet, more 20:5 and 22:6 were incorporated into mature than immature MK but the greatest amount of 20:5 and 22:6 had accumulated in platelets. Ingestion of marine oil-enriched diets did not cause thrombocytopenia or affect MK maturation based on the analysis of morphologic stage, ploidy or size. Marine oil-enriched diets caused a decrease in thromboxane synthesis in response to thrombin and calcium ionophore in platelets and MK at all stages of maturation. In platelet-rich plasma, collagen induced platelet aggregation, ATP secretion and thromboxane synthesis were decreased to a greater degree at 35 days than 10 days. Thus, the study indicates that the ingestion of marine oil-enriched diets resulted in the compartmentalization of 20:5 and 22:6 in acidic phospholipids in mature MK and platelets. The observation that marine oil-enriched diet induced maximal changes in lipid composition in MK and platelets within 10 days but caused progressive inhibition of platelet function for up to 35 days indicates that as yet undefined membrane and cellular changes may occur at later time points.

Megakaryocyte function and dysfunction

More than a hundred years have passed since platelets were recognized as cells and their haemostatic functions discovered. However, the process of platelet production is still not understood. The location, the mechanism and the regulation of thrombopoiesis remain elusive. Megakaryocytes are known to be the source of platelets. Investigations of megakaryocytes have revealed their normal functions and some of the abnormalities present in various diseases which affect platelets. In recent years, new techniques of cell isolation and tissue culture have been developed and have made possible advances in characterizing megakaryocyte precursors and differentiation. The primary function of megakaryocytes is to synthesize and assemble platelet components and organelles. Although debated for a long time, new data seems to indicate that the lung may be a central locus of platelet production. The new techniques for megakaryocyte investigations have barely begun to be of use in the study of abnormal platelet production in disease.

Effect of oral treatment with pantethine on platelet and plasma phospholipids in IIa hyperlipoproteinemia

In a single-blind, crossover, completely randomized study, the effects of oral treatment with pantethine or placebo on fatty acid composition of plasma and platelet phospholipids were investigated in 10 IIa hyperlipoproteinemic patients. A significant decrease of total cholesterol and total phospholipids was observed both in plasma and in platelets after a twenty-eight-day treatment. In plasma, pantethine induced a decrease of the ratio sphingomyelin/phosphatidylcholine. Moreover, a relative increase of n3-polyunsaturated fatty acids both in plasma and in platelet phospholipids and a decrease of arachidonic acid in plasma phospholipids were observed. These results indicate that pantethine can affect plasma and platelet lipid composition with possibly favorable influences on the determinants of cell membrane fluidity.

Megakaryopoiesis disturbances in atherosclerotic rabbits

Rabbits given a hypercholesterolemic diet (500 mg/day) for 6 months and then maintained for another 6 months on a normal diet were found to have developed fibrous lipidic lesions in the aorta. Although circulating platelet levels in these animals were normal there was a reduction in mean megakaryocyte ploidy. The high concentrations of megakaryoblasts in all the sedimentation fractions collected by the 'STAPUT' system suggested an increase in megakaryocyte turnover with activation of committed stem cells. In addition, other defects in maturation of megakaryocytes were observed, such as abnormalities in the demarcation membrane system and granule number. These data reveal that defects in megakaryocyte maturation and turnover may occur during the process of reparative fibrosis of the arterial tree following a period of moderate hypercholesterolemic diet in the rabbit.

Inverse relationship between megakaryocyte buoyant density and maturity

We examined the relationship between rat megakaryocyte buoyant density and maturation stage in continuous Percoll density gradients. An average of 88% of megakaryocytes had buoyant densities less than 1.054 g/ml. There was an inverse relationship between megakaryocyte buoyant density and maturation. Morphologically mature forms comprised 90% of the megakaryocytes with buoyant densities of 1.030-1.033 g/ml. In contrast, immature morphology was present in three-quarters of megakaryocytes with buoyant densities of 1.042-1.046 g/ml. These morphological findings were confirmed by [3H]thymidine labelling studies. Cell viability assessed by trypan blue exclusion was highest among more dense megakaryocytes of which the majority were immature. The lowest trypan blue exclusion was found in the less dense, predominantly mature megakaryocytes indicating that these cells are more susceptible to membrane damage during marrow suspension. Megakaryocyte DNA content distributions and platelet antigen levels, determined by two-colour flow cytometry, were also related to megakaryocyte density; the more dense megakaryocytes showed an approximately two-fold higher proportion of 8N cells and less platelet antibody binding than did less dense megakaryocytes. These studies suggest that megakaryocytes can be fractionated according to their buoyant densities into immature and mature populations suitable for molecular studies of differentiation.

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.

Biosynthesis of factor V in isolated guinea pig megakaryocytes

Although platelets contain Factor V, localized primarily in the alpha-granules, the origin of this coagulation cofactor in these cells is not known. We therefore explored whether isolated megakaryocytes could biosynthesize Factor V. Guinea pig plasma Factor V coagulant activity was demonstrated to be neutralized by human monoclonal and rabbit polyclonal antibodies directed monospecifically against human Factor V. These antibodies had been used earlier to purify human Factor V. These antibodies had been used earlier to purify human Factor V and to quantify Factor V antigen concentration, respectively (1983. Chiu, H. C., E. Whitaker, and R. W. Colman. J. Clin. Invest. 72:493-503). As determined by a competitive enzyme-linked immunosorbent assay with guinea pig plasma as a standard, Factor V solubilized from guinea pig megakaryocytes was present at 0.098 +/- 0.018 micrograms/10(5) cells. Each megakaryocyte contained about 500 times as much Factor V as is in a platelet (0.234 +/- 0.180 micrograms/10(8) platelets). The content of Factor V antigen in guinea pig plasma was greater (27.0 +/- 3.0 micrograms/ml) than that of Factor V antigen in human plasma (11.1 +/- 0.4 micrograms/ml). In contrast, human platelets contain ninefold more Factor V antigen (2.01 +/- 1.09 micrograms/10(8) platelets) than do guinea pig were 2.85 +/- 0.30 U/ml plasma, 0.022 +/- 0.012 U/10(8) platelets, and 0.032 +/- 0.03 U/10(5) megakaryocytes, compared with human values of 0.98 +/- 0.02 U/ml plasma and 0.124 +/- 0.064 U/10(8) platelets. Isolated megakaryocytes were found to contain Factor V by cytoimmunofluorescence. The megakaryocytes were incubated with [35S]methionine, and radiolabeled intracellular proteins purified were on a human anti-Factor V immunoaffinity column. The purified protein exhibited Factor V coagulant activity and neutralized the inhibitory activity of a rabbit antihuman Factor V antibody, which suggests that megakaryocyte Factor V is functionally and antigenically intact. These results indicate that Factor V is synthesized by guinea pig megakaryocytes. Nonetheless, megakaryocyte Factor V was more slowly activated by thrombin and in the absence of calcium was more stable after activation than was plasma Factor Va. Electrophoresis in sodium dodecyl sulfate and autoradiography of the purified molecule showed a major band of Mr 380,000 and a minor band of Mr 350,000, as compared with guinea pig and human plasma Factor V, where the protein had an Mr of 350,000. Both forms of Factor V were substrates for thrombin. Possible explanations for the higher molecular weight and different thrombin sensitivity and stability observed are that a precursor of Factor V was isolated or that the megakaryocyte Factor V had not been fully processed before isolation.

Arachidonate synthesis and uptake in isolated guinea-pig megakaryocytes and platelets

Arachidonic acid (20:4) synthesis and uptake were studied in guinea-pig megakaryocytes and platelets. Isolated megakaryocytes and platelets were incubated with [3H]20:4, 8,11,14-[14C]eicosatrienoic acid (gamma-20:3) and [14C]linoleic acid (18:2) and their lipids were analyzed for radioactivity. The study showed that there was 0.153 microM of unesterified 20:3 and 0.237 microM of 20:4 in guinea-pig plasma in nonfasting animals. At these concentrations, 42.6 pmol of gamma-20:3 and 53.3 pmol of 20:4 were taken up per h per 10(5) megakaryocytes in vitro. Megakaryocytes desaturated 27% of the gamma-20:3 to 20:4 but could not desaturate 18:2. Platelets could not desaturate gamma-20:3 or 18:2. In megakaryocytes, the distribution of 20:4 synthesized by the desaturation of gamma-20:3 and 20:4 taken up reflected the endogenous distribution of 20:4 in megakaryocyte phospholipids and 20:4 was predominantly incorporated into phosphatidylethanolamine (PE). In contrast, the distribution of [3H]20:4 taken up into platelets did not reflect the endogenous distribution. 20:4 was primarily incorporated into platelet phosphatidylcholine and phosphatidylinositol. The study showed that megakaryocytes but not platelets possess a delta 5 desaturase and can synthesize 20:4 from gamma-20:3. Neither cell was shown to have a delta 6 desaturase. Megakaryocytes appear to have the capacity to determine the composition of all pools of platelet 20:4 either by uptake or synthesis of 20:4. Platelets, most likely, have a limited capacity to alter structural pools of 20:4 contained primarily in PE and phosphatidylserine (PS).

Platelet function in familial hypercholesterolaemia in South Africa and the effects of probucol

Patients with Familial Hyperlipoproteinaemia Type II showed evidence of increased platelet aggregability when compared to normal controls. This abnormality was not influenced by oral therapy with probucol whether or not the serum cholesterol levels were lowered during the study period. Probucol assays indicated adequate compliance during the study period but did not demonstrate any uptake of the drug by platelets. In vitro studies also failed to demonstrate any inhibition of platelet function at therapeutic drug levels although higher levels were inhibitory.

Lipid composition of guinea pig platelets and megakaryocytes. The megakaryocyte as a probable source of platelet lipids

Platelets are formed by fragmentation of the cytoplasm and plasma membrane of the megakaryocyte in the bone marrow. This study has compared the lipid composition of guinea pig platelets and megakaryocytes. Phospholipids were quantitated by TLC and measurement of lipid phosphorus. Cholesterol and fatty acids were quantitated by GLC. The cholesterol/phospholipid molar distribution in megakaryocytes was: 9.8% phosphatidylserine, 6.7% phosphatidylinositol, 14.2% sphingomyelin, 40.0% phosphatidylcholine and 29.3% phosphatidylethanolamine. Platelets continued 11.2% phosphatidylserine, 5.1% phosphatidylinositol, 16.1% sphingomyelin, 38.6% phosphatidylcholine and 29.0% phosphatidylethanolamine. The major megakaryocyte fatty acids were 20.0% palmitic, 16.4% stearic, 20.6% oleic, 13.2% linoleic and 8.2% arachidonic. The major platelet fatty acids were 17.4% palmitic, 17.5% stearic, 11.6% oleic, 12.4% linoleic and 14.6% arachidonic. The major and minor fatty acid compositions of the individual platelet phospholipids reflected those of the megakaryocyte counterparts. The increased arachidonic acid and decreased oleic acid in platelets relative to megakaryocytes were found in all four glycerophospholipids. The similarity of the phospholipid and fatty acid composition of megakaryocytes and platelets suggests that the lipid composition of the platelet is determined by the megakaryocyte.