Increase in Membrane Cholesterol Content Enhances Phospholipase A2Activity and Endoperoxide Receptor Response in Human Platelets

A new rapid method to measure human platelet cholesterol: a pilot study

INTRODUCTION

Platelet cholesterol (PC) could be used to assess "tissue" cholesterol of patients with vascular disease. However, the methods available so far to measure PC involve a complex extraction process. We developed a rapid method to measure PC and assessed its correlation with serum total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), LDL-C/HDL-C ratio, triglycerides (TG), and non-HDL-C.

METHODS

We assessed repeatability (20 times, 3 participants) and reproducibility (8 times, 2 participants). A group of 47 healthy participants was studied. Blood was collected to analyze serum TC, LDL-C, HDL-C, and TG. Citrated blood was used to prepare a platelet pellet. A "clear soup" was produced (by disrupting this pellet using freeze-thaw and sonication cycles) and used to measure PC.

RESULTS

Repeatability of PC showed a coefficient of variation (CV) of 4.8%. The reproducibility of PC over a period of 2 months was CV 7.5% and 8.1% (8 measurements for 2 participants). The PC of participants with serum LDL-C >2.6 mmol/L (treatment goal recommended by the National Cholesterol Education Program Adult Treatment Panel III) was 377 ± 120 μmol/10(12) platelets (n = 25). There was a significant correlation (Spearman, correlation coefficient) of PC (n = 25) with serum LDL-C (r(s) = 0.45, P = .02), LDL-C/HDL-C (r(s) = 0.45, P = .02), TG (r(s) = 0.43, P = .03), and non-HDL-C (r(s) = 0.53, P = .007).

CONCLUSION

This technique of measuring PC has the advantage of being reproducible, fast, and simpler than previous methods. Thus, it may be useful for multiple sampling when investigating changes in PC in hypercholesterolemic patients. More extensive evaluation is necessary.

Stimulation by de novo-synthesized ceramide of phospholipase A(2)-dependent cholesterol esterification promoted by the uptake of oxidized low-density lipoprotein in macrophages

The involvement of cytosolic phospholipase A(2) (cPLA(2)) and ceramide in the accumulation of cholesteryl ester induced by the uptake of oxidized low-density lipoproteins (oxLDL) in macrophages was investigated. Uptake of oxLDL by [(3)H]oleic acid-labeled macrophages stimulated the formation of cholesteryl oleate, and this process was completely inhibited by a cPLA(2) inhibitor. Under the conditions, a time-dependent increase in ceramide was observed, while sphingomyelin levels were unaffected. The production of ceramide was completely inhibited by fumonisin B1, an inhibitor of the de novo synthesis of ceramide, and oxLDL-induced cholesteryl oleate formation was inhibited partially. Treatment of the cells with sphingomyelinase accelerated the formation of cholesteryl ester. Furthermore, sphingomyelinase or cell-permeable ceramide induced the release of oleic acid, and this was inhibited by a cPLA(2) inhibitor. These results suggest that activation of cPLA(2) is responsible for the formation of cholesteryl ester induced by the uptake of oxLDL in macrophages, and that de novo-synthesized ceramide is implicated, at least in part, in this process.

Enhanced collagen-induced responses of platelets from rabbits with diet-induced hypercholesterolemia are due to increased sensitivity to TxA2. Response inhibition by chronic ethanol administration in hypercholesterolemia is due to reduced TxA2 formation

The effects of dietary cholesterol and chronic administration of moderate amounts of ethanol on collagen-induced platelet responses were investigated. Three groups of rabbits were fed the following diets for 8 weeks: a normal chow diet, a cholesterol-enriched (0.25% wt/wt) chow diet, and a cholesterol-enriched chow diet plus 6% ethanol in the drinking water for the final week of the dietary period. Cholesterol feeding enhanced collagen-induced responses-aggregation, secretion of [14C]serotonin from prelabeled platelets, and thromboxane formation--of suspensions of washed platelets, and chronic ethanol treatment significantly reduced these enhanced responses. These effects are mediated by thromboxane A2 (TxA2) rather than ADP. Experiments with collagen-stimulated platelets in which feedback amplification of TxA2 was blocked with the prostaglandin H2/TxA2 receptor blocker BM 13.177 and experiments with aspirin-treated platelets stimulated with the stable TxA2 mimetic U46619 showed that cholesterol feeding enhanced platelet sensitivity to TxA2 rather than formation of TxA2 by platelets that had interacted with collagen. Without BM 13.177 or aspirin, TxA2 increased the amount of TxA2 formed by feedback amplification. In contrast, decreased responsiveness to collagen by platelets from cholesterol-fed rabbits given ethanol was due to inhibition of TxA2 formation rather than reduced sensitivity to TxA2. Platelets from cholesterol-fed rabbits given ethanol did not develop tolerance to the acute inhibitory effects of ethanol. Our results indicate that administration of moderate amounts of ethanol to cholesterol-fed rabbits inhibits enhanced collagen-induced responses of platelets by a TxA2-dependent pathway that involves reduction of TxA2 formation rather than reduction of platelet responses to TxA2.

Acute in vitro effects of ethanol on responses of platelets from cholesterol-fed and Watanabe heritable hyperlipidemic rabbits

The effects of ethanol on platelets from rabbits with two different types of hypercholesterolemia, diet-induced and genetically determined, were investigated. There were no differences between the hypercholesterolemic groups and their controls in the extent of (primary) ADP-induced aggregation of washed platelets, and this aggregation was not inhibited by ethanol. Platelets from cholesterol-fed rabbits were more sensitive to aggregation and secretion induced by collagen, whereas platelets from Watanabe heritable hyperlipidemic (WHHL) rabbits were less sensitive. Ethanol inhibited collagen-induced responses of platelets from both hypercholesterolemic groups, but the extent of inhibition of aggregation was not different compared with controls. Because ethanol did not affect U46619-induced responses of aspirin-treated platelets, ethanol does not inhibit aggregation and secretion stimulated by collagen via an effect on thromboxane A2 (TxA2)-induced responses. Platelets from cholesterol-fed rabbits were more sensitive to thrombin even when TxA2 formation was blocked by aspirin, and inhibition of aggregation by ethanol was less in cholesterol-fed rabbits than in controls. However, neither the extent of thrombin-induced responses nor the inhibitory effect of ethanol was different in platelets from WHHL rabbits compared with controls. Thus, different etiologies of hypercholesterolemia produce different changes in platelet function, and ethanol has different effects on the platelets from cholesterol-fed rabbits compared with the platelets from WHHL rabbits. The inhibitory effect of ethanol on the thrombin-induced aggregation of platelets from cholesterol-fed rabbits is attenuated compared with controls, and this finding contrasts with the reported greater inhibitory effect of ethanol on platelets enriched with saturated fats.