Accelerated atherogenesis occurs following platelet destruction and increases in megakaryocyte size and DNA content

Modifying platelets at their birth: anti-thrombotic therapy without haemorrhage

Cardiovascular disease is a leading cause of death. The current approach to the prevention of arterial thrombosis in cardiovascular disease is dependent on the use of therapies which inhibit the activation of platelets. Predictably these are associated with an increased risk of haemorrhage which causes significant morbidity. The thrombotic potential of an activated platelet is modifiable; being determined before thrombopoiesis. Increased megakaryocyte ploidy is associated with larger and more active platelets carrying an increased risk of thrombosis. The reduction in the ploidy of megakaryocytes is therefore a novel area of therapeutic interest for reducing thrombosis. We propose a new therapeutic approach for the prevention and treatment of thrombosis by targeting the reduction in ploidy of megakaryocytes. We examine the role of a receptor mediated event causing megakaryocytes to increase ploidy, the potential for targeting the molecular mechanisms underpinning megakaryocyte endomitosis and the existence of two separate regulatory pathways to maintain haemostasis by altering the thrombotic potential of platelets as targets for novel therapeutic approaches producing haemostatically competent platelets which are not prothrombotic.

The causal role of megakaryocyte–platelet hyperactivity in acute coronary syndromes

Platelets are causally involved in coronary artery obstruction in acute coronary syndromes (ACS). This cell type is unique to mammals and its production, which is unlike that of any other mammalian cell, involves polyploid nuclear change in the mother cell (megakaryocyte) and the production of anucleate cells with a log Gaussian distribution of volume. Platelets vary more in cellular volume than any other circulating blood element in mammals. Larger platelets are denser, contain more secretory granules, and are more reactive than their smaller counterparts. A causal relationship between the presence of large, dense, reactive platelets in the circulation and ACS is supported by many clinical studies. Furthermore, the results of two large, prospective, epidemiological studies have demonstrated that mean platelet volume was the strongest independent predictor of outcome in patients with acute myocardial infarction. Notably, evidence indicates that an increase in mean platelet volume in the pathogenesis of ACS can potentially overwhelm current therapeutics. The control system for the physiological and pathophysiological production of large platelets should, therefore, be researched. An understanding of this system might give rise to new therapeutics that could control platelet reactivity and thereby comprehensively prevent ACS.

Cigarette smoking increases the mean platelet volume in elderly patients with risk factors for atherosclerosis

To study the effects of cigarette smoking and atherosclerosis on platelet size, we measured the mean platelet volume (MPV) and other platelet parameters in 142 elderly smokers and nonsmokers with or without atherosclerotic risk factors. The MPV and the platelet count were highest and their inverse correlation was strongest in the atherosclerotic smokers (r = 0.54, P < 0.05) when compared with the nonsmoking and non-atherosclerotic groups. A 10% decrease of MPV was found in 8 smoking subjects in the atherosclerotic group, who successfully discontinued smoking (P < 0.05). These results suggest that smoking may increase platelet consumption in atherosclerotic vessels and that subsequently megakaryocytes are activated to produce larger platelets, which are more active. Thus, an increase in MPV due to smoking may also contribute to the acceleration of atherosclerosis and should be considered as a risk factor for atherosclerotic disease.

Thrombopoietin and mean platelet volume in coronary artery disease

BACKGROUND

Large platelets are shown to be hemostatically more active. It has been suggested that mean platelet volume (MPV) is increased during acute myocardial infarction (AMI) and unstable angina pectoris (USAP). However, the underlying mechanism of the phenomenon remains unclear.

HYPOTHESIS

In this study, platelets, MPV, and thrombopoietin (TP) levels were investigated in patients with coronary artery disease (CAD) and healthy controls.

METHODS

Twenty patients with AMI and 20 patients with USAP were included in this study. Seventeen healthy adult subjects served as controls. Venous blood samples of the subjects were drawn within 12 h after admission. Thrombopoietin levels were measured by ELISA and platelet counts and MPV were assayed by autoanalyzer.

RESULTS

Patients with AMI and USAP had higher platelet counts than those in the control group. Although the platelet counts were slightly higher in AMI than in USAP, this did not reach statistical significance. Mean platelet volume and levels of TP were found to be elevated in patients with AMI and USAP compared with control subjects (p < 0.001). Thrombopoietin levels were higher in AMI than USAP, but this was not statistically significant. There was a positive correlation between TP levels and MPV values (p < 0.05).

CONCLUSION

Increased TP levels may increase both platelet counts and platelet size, resulting in hemostatically more active platelets, which may contribute to the development and progression of CAD.

Polyploidy: occurrence in nature, mechanisms, and significance for the megakaryocyte-platelet system

OBJECTIVE

Polyploidy, the state of having greater than the diploid content of DNA, has been recognized in a variety cells. Among these cell types, the megakaryocytes are classified as obligate polyploid cells, developing a polyploid DNA content regularly during the normal life cycle of the organism, while other cells may become polyploid only in response to certain stimuli. The objective of this review is to briefly describe the different cell cycle alterations that may lead to high ploidy, while focusing on the megakaryocyte and the importance of high ploidy to platelet level and function.

MATERIALS AND METHODS

Relevant articles appearing in scientific journals and books published in the United States and in Europe during the years 1910-1999 were used as resources for this review. We selected fundamental studies related to cell cycle regulation as well as studies relevant to the regulation of the endomitotic cell cycle in megakaryocytes. Also surveyed were publications describing the relevance of high ploidy to high platelet count and to platelet reactivity, in normal situations and in a disease state.

RESULTS

Different cells may achieve polyploidy through different alterations in the cell cycle machinery.

CONCLUSIONS

While upregulation of cyclin D3 further augments ploidy in polyploidizing megakaryocytes in vivo, future investigation should aim to explore how normal megakaryocytes may initiate the processes of skipping late anaphase and cytokinesis associated with high ploidy. In humans, under normal conditions, megakaryocyte ploidy correlates with platelet volume, and large platelets are highly reactive. This may not apply, however, to the disease state.

Megakaryocytes and platelets in vascular disease

Platelets are anucleate cells with no DNA. They are derived from their precursor, the megakaryocyte (MK), whose differentiation is characterized by nuclear polyploidization through a process called endomitosis. Changes in the MK-platelet-haemostasis axis may precede acute thrombotic events. Changes in MK ploidy distribution may be associated with the production of large platelets. Mean platelet volume (MPV) is an important biological variable as it is a determinant of platelet reactivity. Large platelets are denser and more active haemostatically. MPV is increased in patients after myocardial infarction (MI) and is a predictor of a further ischaemic event and death when measured after MI. It has been suggested that changes not only in platelets but also in the parental MK are associated with chronic and acute vascular events. The regulation of megakaryocytopoiesis depends on several haematopoietic factors such as thrombopoietin. An understanding of the signalling system that controls platelet number and size might give insight into a role of platelet production in thrombogenesis and atherogenesis.

Naftidrofuryl inhibits the release of 5-hydroxytryptamine and platelet-derived growth factor from human platelets

Angioplasty and bypass-grafting are associated with restenosis which limits their efficacy. Platelet-rich thrombus formation is the predominant cause of acute occlusion whereas platelet release products with proliferating properties, e.g. 5-hydroxytryptamine (5-HT) and platelet-derived growth factor (PDGF), may contribute to late restenosis. Naftidrofuryl (NAF), a drug for the treatment of peripheral vascular disease, was shown previously to inhibit platelet shape change and aggregation. This study establishes whether NAF inhibits the release of 5-HT and PDGF from platelets obtained from healthy subjects. Platelets stimulated with agonists aggregated less and released less 5-HT/PDGF when pre-incubated with NAF. Indomethacin (INDO), a cyclooxygenase inhibitor, alone inhibited aggregation and PDGF/5-HT release; NAF enhanced the inhibitory effects of INDO. The effect of NAF, on its own or in combination with a cyclooxygenase inhibitor, may therefore confer protection against graft occlusion.

The megakaryocyte platelet system and vascular disease

Platelets form a heterogeneous population of cells produced from the uniquely large polyploid cell found in the bone marrow, the megakaryocyte. The platelet megakaryocyte axis forms a dynamic equilibrium varying in normal biology and in disease. Prolonged platelet destruction leads to the production of large platelets from large, high ploidy megakaryocytes. In vivo and ex vivo studies show that such platelets have more haemostatic potential than smaller less dense platelets. The evidence suggesting that prothrombotic changes in the megakaryocyte platelet axis precede coronary artery thrombosis and the importance of platelet reactivity in atherosclerosis will be reviewed.

Influence of platelet size before coronary angioplasty on subsequent restenosis

Platelet size has been shown to reflect platelet activity. We prospectively measured the mean platelet volume (MPV) in 47 patients undergoing single vessel angioplasty for symptomatic angina. The patients underwent repeat angiography 4-8 months later irrespective of symptomatic status. Restenosis was assessed quantitatively by hand held calliper measurements of the lesion and qualitatively by a return of angina, ST segment changes on an exercise test and visual analysis of the lesion severity by two experienced angiographers. Twenty-four patients developed recurrent angina during the follow-up period, the MPV in the group with chest pain was 8.54 +/- 0.60 fl compared to 8.1 +/- 0.69 fl in the asymptomatic group (P = 0.04). Twenty two patients had significant ST segment changes at exercise. In this group the MPV was 8.6 +/- 0.56 fl compared to 8.0 +/- 0.70 fl for the group with a negative test (P = 0.002). Similarly visually assessed angiographic stenosis showed a significant increase in the restenotic group (8.6 +/- 0.56 vs. 8.0 +/- 0.61 fl, P = 0.001). The relative odds for developing clinically defined restenosis were 10.2 times greater if the pre-procedural MPV lay in the upper compared to the lowest quartile. There was a positive correlation between MPV and change in minimal luminal diameter between post angioplasty and follow-up angiography, assessed quantitatively, r = +0.56, P = 0.016. There was no association between clinical or angiographic definitions of restenosis and haemoglobin, red cell count, mean corpuscular volume, white cell count or platelet count. Platelet size may influence the development of restenosis after successful coronary angioplasty.

The regulation of megakaryocyte polyploidization and its implications for coronary artery occlusion

Polyploidization is a distinctive feature of megakaryocyte differentiation. The physiological meaning and the regulation of this process are obscure. Megakaryocyte ploidy varies in normal biology and in disease. Here we review the evidence suggesting that ploidy changes may have a role in the determination of platelet reactivity and in the aetiology of coronary artery occlusion. We also present a hypothesis that may serve as a framework to explore the regulation of megakaryocyte polyploidization at the molecular level and also may provide a rational basis to explain the occurrence of ploidy changes in ischaemic heart disease.

Oral cicaprost reduces platelet and neutrophil activation in experimental hypercholesterolemia

Oral treatment of cholesterol-fed rabbits with the PGI2 mimetic cicaprost largely reduces hypercholesterolemia-induced platelet and neutrophil hyperreactivity. In addition, cicaprost prevents atherosclerosis-induced platelet desensitization for PGI2. These effects persist after cicaprost treatment is withdrawn. Since platelets and leukocytes are supposed to contribute to atherogenesis, this suggests a favourable effect of long-term oral PGI2 substitution in hypercholesterolemia.

Arterial wall hypoxia following thrombosis of the vasa vasorum is an initial lesion in atherosclerosis

Pressure on the outside of arteries can cause physical and biochemical changes in the vessel wall of rabbits which are characteristic of atherosclerosis. It is hypothesized that occlusion of the vasa vasorum causes ischaemia of the arterial media which results in smooth muscle cell proliferation and cellular accumulation of cholesteryl esters. Hypoxia increases mRNA for platelet-derived growth factor in arterial wall cells and increases the activity of acyl CoA:cholesterol acyltransferase (ACAT). Such a mechanism may explain many of the anatomical, actuarial and environmental risk factors for atherosclerosis. Hypoperfusion may follow thrombosis of the vasa vasorum.

Arterial wall hypoxia following hyperfusion through the vasa vasorum is an initial lesion in atherosclerosis

Pressure on the outside of arteries can cause physical and biochemical changes in the vessel wall of rabbits which are characteristic of atherosclerosis. It is hypothesized that occlusion of the vasa vasorum causes ischaemia of the arterial media which results in smooth muscle cell proliferation and cellular accumulation of cholesteryl esters. Hypoxia increases mRNA for platelet-derived growth factor in arterial wall cells and increases the activity of acyl CoA: cholesterol acyltransferase (ACAT). Such a mechanism may explain many of the anatomical, actuarial and environmental risk factors for atherosclerosis. Hyperfusion of the vasa vasorum may follow thrombosis.