Etofibrate increases binding of low and high density lipoprotein to human platelets of patients with type II hyperlipoproteinemia

Platelet function in the postprandial period

Background

Postprandial hyperlipidemia and hyperglycemia have been related to cardiovascular events. Among different underlying mechanisms platelet activation seems to be responsible too. No comparable data between various tests in normo- vs. hyperlipidemics before and at different time intervals are available after a fat meal. We aimed to compare 9 of them within the same patients at several time points in postprandial hyperlipidemia.

Results

For some tests baseline values between the groups were significantly different (TXB₂, platelet sensitivity, sedimentation and WU-test). However, hyperlipidemia revealed a variable influence on the tests examined. Some of the available tests apparently sensitive to show platelet activation reflect the increase in triglycerides (TG), such as the sedimentation index. ADP-induced platelet aggregatory activity in count adjusted washed isolated platelet samples during postprandial hyperlipidemia indicates mildly enhanced platelet activity, but does not seem to induce significant changes in aggregation. In patients with severe hypertriglyceridemia (> 400 mg/dl fasting) changes in platelet function are more pronounced due to delayed decay and may last up to 16 hours paralleling TG reaching the prevalue. The overwhelming majority of platelet function tests do not significantly respond to postprandial hyperlipidemia. The correlation between the tests applied is poor. For standardization purpose, platelet aggregation tests, aimed to examine proaggregatory capacity in atherosclerosis, should only be performed at the same time of the day after a fasting period > 6 hours. The great variation in preanalytical work-up on comparison of various tests, large number of platelet tests available and their respective potential value are discussed.

Conclusions

At present, the suspicion that platelet function is significantly activated in the postprandial period cannot be supported by any of the tests used. The information provided is valuable to know for which test and group of patients a fasting period of which duration is recommendable.

Low-Density Lipoprotein-Lowering Medication and Platelet Function

Elevated low-density lipoprotein (LDL) cholesterol (LDL-C) levels represent one of the most important risk factors for atherosclerosis and therefore cardiovascular morbidity and mortality. LDL-C operates at different levels and through various classic and non-classic mechanisms. In particular, increased or modified LDL enhances platelet function and increases sensitivity of platelets to several naturally occurring agonists. Agents that lower LDL-C in hypercholesterolemic patients have been shown to interfere with platelet function. Several studies, in fact, suggested that statins exert anti-thrombotic effects largely as a result of an anti-platelet activity. Among the other LDL-C-lowering agents those acting by interfering with cholesterol reabsorption from the gut (cholestyramine, colestipol) do not appear to interfere with platelet function, whereas peroxisome proliferator-activated receptor agonists (such as fibrates) can inhibit platelet function. The full potential of these drugs in vascular protection is only just being realized. Further studies are still needed to elucidate the full therapeutic benefits of these agents in plaque stabilization and thrombosis.

Apo A-I binding to platelets detected by flow cytometry

Lipoprotein-platelet interactions are very important in atherosclerosis and thrombosis. Several studies have been carried out on specific binding of various lipoproteins to platelets. But there is considerable disagreement about the details of these binding sites. Although low-density lipoprotein (LDL) receptors of several cells have been studied extensively, there is little datum about high-density lipoprotein (HDL) receptors. Apolipoprotein (apo) A-I may play a major role in the determination of the specificity of HDL receptors. In this study, binding of apo A-I to platelets was investigated by using a flow cytometric method. Citrated blood samples were obtained from five healthy and seven hypercholesterolemic subjects. Apo A-I antibody was incubated with the citrated whole blood before and after activation with ADP or thrombin receptor agonist peptide (TRAP). Then fluorescein isothiocyanate (FITC)-labeled secondary antibodies were added and analyzed on a Becton-Dickinson FACSort flow cytometer. In the hypercholesterolemic group, apo A-I binding to platelets was found to be significantly decreased after activation with TRAP (P<.05), but not after activation with ADP. In the control group, after platelet activation with ADP or TRAP, the apo A-I MFI values were not found to be significantly different from the values of resting platelets (P>.05). In this study, we demonstrated that apo A-I can bind to platelets, and this supports the hypothesis that apo A-I may play a major role in HDL binding to platelets.

Application of 31P NMR spectroscopy to monitor chemotherapy-associated changes of serum phospholipids in patients with malignant lymphomas

31P spectra were obtained from 22 healthy volunteers and 35 patients with malignant lymphomas. Sera from patients were collected at the time of diagnosis and at several time-points during therapy. Long-term follow-up studies showed a good correlation between the 31P NMR spectra of sera and the clinically evident response of the disease to the chemotherapy. During therapy leading to remission resonance from phospholipids increased progressively resulting in spectra similar to those seen in normal sera. By contrast, in patients who did not respond to therapy the intensities of the phospholipid peaks remained relatively low or became progressively reduced as the disease progressed. To understand the source of the spectral differences, we also examined the concentrations of high-density lipoprotein, low-density lipoprotein, cholesterol, and triglycerides. In individuals responding to the treatment, both high-density lipoprotein and cholesterol increased to the point where they were statistically equivalent to those from healthy volunteers.

Gemfibrozil enhances platelet activity in patients with combined hyperlipoproteinemia

A placebo-controlled crossover study was conducted to evaluate whether lipid-lowering with gemfibrozil (10 to 12 weeks) affects platelet function in vivo at rest and during mental stress in 21 men with combined hyperlipoproteinemia. Gemfibrozil lowered plasma triglycerides and total and VLDL cholesterol (P < .001 for all), whereas HDL cholesterol increased (P < .001). Gemfibrozil increased platelet counts by 18% (P < .001) and, according to filtragometry measurements, enhanced overall (means of rest and stress values) platelet aggregability in vivo (P = .014); this effect was more evident during mental stress (P = .027) than at rest (P = .18). Moreover, the urinary excretion of 11-dehydro-thromboxane-B2 was 54% higher during treatment with gemfibrozil (P < .001), whereas the excretion of beta-thromboglobulin in urine was unaltered. Plasma beta-thromboglobulin tended to be slightly elevated during active treatment (P = .15). Mental stress increased heart rate and catecholamine levels and elevated all cholesterol fractions (P < .01) and plasma beta-thromboglobulin (during placebo, P = .02), but platelet aggregability did not increase significantly. A positive correlation was found between 11-dehydrothromboxane-B2 excretion and LDL cholesterol levels during placebo (r = .62, P = .0057). In conclusion, gemfibrozil has beneficial effects on plasma lipoprotein levels but enhances several aspects of platelet activity in vivo and increases platelet counts. These changes might be prothrombotic and thus adverse for the hyperlipidemic patient. Primary preventive effects of gemfibrozil might be enhanced if a platelet inhibitor such as aspirin is administered with gemfibrozil.