Avid Bioservices Inc (CDMO)

[jazzbeerman]

atherosclerotic plaque -


Must read the discussion section of this paper -

Shed Membrane Microparticles With Procoagulant Potential in Human Atherosclerotic Plaques

A Role for Apoptosis in Plaque Thrombogenicity

http://circ.ahajournals.org/cgi/content/full/99/3/348

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Discussion

Apoptosis is a common event that occurs in atherosclerosis.1 2 3 4 5 6 7 However, no direct evidence has been provided for a beneficial or a harmful role for this process in plaque development and progression. In this study, we examined the role of in situ apoptotic cell death in the promotion of plaque thrombogenicity.

TF is a key element in the initiation of the coagulation cascade. In the normal vessel, TF expression is restricted to the adventitia17 and contributes to vessel repair after vessel injury. Enhanced TF expression and activity have been reported in human atherosclerotic plaques,16 17 18 19 20 21 22 and it is now believed that the increase of TF activity is a major determinant of plaque thrombogenicity. However, TF activity is highly dependent on the presence of phosphatidylserine,24 25 26 27 and it has recently been shown that apoptotic cell death results in phosphatidylserine exposure on the cell surface conferring a potent procoagulant activity to the cell.9 10 11 12 13 14 Therefore, we hypothesized that plaque thrombogenicity may be greatly influenced by the occurrence of apoptotic cell death.

We detected substantial expression of TF in TUNEL-positive areas of plaques. Although the TUNEL method, in some instances, may reveal processes other than apoptosis,30 we have adapted the technique to enhance its specificity for the detection of apoptosis. In addition, we have previously shown6 that in the human atherosclerotic plaque, TUNEL-positive cells colocalize with caspase 3, which is characteristic of apoptosis.33

Extracellular expression of TF in the apoptotic areas suggested to us that it was probably released in microparticles during cell death. The finding of a close association between TF expression and apoptosis in acellular and cellular regions of atherosclerotic plaques suggested the possibility of synergy between TF and apoptotic cell membranes, rendering them highly thrombogenic.

To confirm this hypothesis, we searched for the presence of shed membrane particles with procoagulant potential in atherosclerotic plaques. In this study, we show that a large proportion of phosphatidylserine-bearing microparticles is produced in the atherosclerotic plaque. A number of arguments indicate that these microparticles most likely originated from apoptotic cells: (1) Several groups have shown that apoptosis occurs in the atherosclerotic plaque and may contribute to the development of the acellular lipid core.1 2 3 4 5 6 7 30 (2) Stimuli, like pro-inflammatory cytokines, that lead to cell death in the atherosclerotic plaque are potent inducers of apoptosis,34 and TUNEL-positive labeling in the plaque is highly associated with caspase expression,3 6 leading to the suggestion that necrosis in the plaque is a late secondary process due, in part, to the accumulation of unremoved apoptotic bodies. (3) We have previously shown29 that phosphatidylserine-bearing microparticles stem from surface blebs of apoptotic cells and their amount is correlated to the extent of apoptosis. (4) Phosphatidylserine exposure on the external layer of the cell membrane and shedding of membrane microparticles are early events of apoptosis and are produced even when secondary necrosis does not occur.

Interestingly, we found that these plaque-derived apoptotic microparticles account for almost all the TF activity of the plaque extracts, indicating a direct causal relationship between their presence and TF activity. This suggests that shed membrane apoptotic microparticles may play a major role in the initiation of the coagulation cascade. On the other hand, most of these microparticles originated from macrophages and lymphocytes that are known to be abundant at sites of plaque rupture.35 36 37 We therefore propose that apoptotic death of a proportion of these cells may greatly enhance the thrombus formation accelerating vessel occlusion and could lead to acute infarction.

In addition to their direct effect in promotion and amplification of the coagulation cascade, the apoptotic particles bearing CD11a may also act in a variety of intercellular adhesion processes and may be responsible for dissemination of the procoagulant potential to sites remote from the microenvironment of their formation.14 They may also provide the substrate for secretory phospholipase A2, leading to the generation of lysophosphatidic acid, a potent mediator of the inflammatory reaction and a potent platelet agonist.38 Furthermore, such microparticles may play an important role in transcellular lipid metabolism by their concentrated delivery of bioactive lipids.39

Once plaque material is exposed to circulating blood after plaque rupture or erosion, vascular thrombosis may ensue and may culminate in vascular occlusion, precipitating acute ischemic syndromes.40 Procoagulant potential of the exposed material is therefore a key determinant of plaque thrombogenicity and hence of the extent of vascular thrombosis and luminal narrowing. Our results provide strong evidence that apoptotic cell death in the atherosclerotic plaque is a major determinant of plaque thrombogenicity and could offer novel therapeutic strategies for preventing thrombus formation on plaque rupture.



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