Impact of osteopontin on platelet adhesion and activation in intraluminal thrombus formation and abdominal aortic aneurysm progression

Objective Abdominal aortic aneurysm (AAA) is frequently accompanied by the formation of an intraluminal thrombus (ILT) that is found in more than 75 % of all AAAs. However, it is still not known how the ILT develops and how it affects the progression of AAA. One protein of interest for developing such a thrombus could be osteopontin (OPN). It is already shown that OPN in serum of AAA patients is increased. Moreover, OPN can be cleaved by enzymes like thrombin which plays a pivotal role in thrombus formation. OPN has an arginine-glycine-aspartic acid (RGD) motif and is able to bind to integrins which are abundant on the platelets surface. Currently, the influence of full length and cleaved OPN in platelet adhesion and activation is only partially understood.

Material and Methods Analysis of platelet activation and adhesion after stimulation with OPN and its fragments were performed. Histological sections of the vessel wall from AAA patients were stained for N - and C - terminal OPN.

Results Platelet activation with different agonist showed no alteration in integrin IIb3 (fibrinogen receptor) activation or degranulation after preincubation with full length or cleaved OPN. Furthermore, platelet adhesion was performed on immobilized full length and cleaved OPN coated surfaces. After stimulation with ADP (adenosine diphosphate) or CRP (collagen related peptide) platelets showed an increased adhesion on full length and cleaved OPN. The inhibition of integrin IIb3 with tirofiban or eptifibatid resulted in increased platelet adhesion on OPN. However, inhibition of integrin v3 (vitronectin receptor) on platelets showed decreased but not fully abrogated platelet adhesion on OPN. Adhesion experiments on collagen coated surfaces were performed to investigate the influence of soluble OPN. After platelets were preincubated with soluble full length and cleaved OPN they showed no alteration in adhesion. In addition, stained histological sections showed more N – than C – terminal OPN in the vessel wall of AAA patients.

Conclusion Taken together, platelet activation is not affected by OPN. However, OPN plays an important role in platelet adhesion mediated –at least in part- by integrin v3. This mechanism might be of great relevance for platelet adhesion into the ILT and migration of platelets into the vessel wall that all could affect ILT formation and AAA progression.

Publikationsverlauf

Artikel online veröffentlicht:
18. Juni 2021