In contrast to abdominal aortic aneurysm (AAA), aneurysms of the thoracic ascending aorta (TAA) are not associated with a mural thrombus. However, we have previously shown the presence of prothrombin in the media of TAA.

To determine (i) whether thrombin generation (TG) at the surface of VSMCs is increased in AAA, (ii) its dependence with respect to the etiology of the aneurysm, and (iii) the contribution of avß3 integrins as a receptor for prothrombin (FII) and PAR activation to TG and thrombin-induced proliferation.

Primary cultures of VSMCs were prepared from human biopsies of TAA, AAA and healthy aorta (n=11/group). TG was monitored by thrombography in the presence of healthy plasma or plasma deficient in FII, factor X or VII.

In the presence of prothrombin deficient plasma, similar low levels of thrombin (about 1 nM) are formed on the 3 types of VSMCs indicating the presence of FII within the media confirmed by western blot. There is no TG in plasma deficient for factor VII or X. There was no difference in avß3 integrin expression between the different VSMCs. In the presence of healthy plasma, TG was significantly higher (30% increase) on VSMCs from AAA (accompanied by an increase in tissue factor expression) and lower on VSMCs from TAA compared with healthy VSMCs. Inhibition of TG by an antagonist of PAR-1 was more pronounced in VSMCs from AAA than from TAA a without difference in PAR-1 expression. This effect was associated with increased thrombin-induced proliferation of VSMCs from AAA compared with healthy VSMCs.

The higher prothrombotic effect of VSMCs from AAA is mediated partly by PAR in an auto-amplification loop and suggests the contribution of these cells in the occurrence of thrombotic events. Our results demonstrate also that endogenous synthesis of prothrombin is sufficient to produce nanomolar amounts of thrombin known to exert cell proliferative effect involved in both AAA and TAA progression.