Defective Angiogenesis Delays Thrombus Resolution

Abstract

Objective Restoration of patency is a natural target of vascular remodeling following venous thrombosis that involves vascular endothelial cells and smooth muscle cells as well as leukocytes. Acute pulmonary emboli usually resolve within six months. However, in some instances, thrombi transform into fibrous vascular obstructions, resulting in occlusion of the deep veins, or in chronic thromboembolic pulmonary hypertension (CTEPH). We proposed that dysregulated thrombus angiogenesis may contribute to thrombus persistence. Approach and Results Mice with an endothelial-cell-specific conditional deletion of vascular endothelial growth factor receptor 2/kinase insert domain protein receptor (VEGF-R2/Kdr) were utilized in a model of stagnant flow venous thrombosis closely resembling human deep vein thrombosis. Biochemical and functional analyses were performed on pulmonary endarterectomy specimens from patients with CTEPH, a human model of non-resolving venous thromboembolism. Endothelial cell-specific deletion of Kdr and subsequent ablation of thrombus vascularization delayed thrombus resolution. In accordance with these findings, organized human CTEPH thrombi were largely devoid of vascular structures. Several vessel-specific genes such as KDR, vascular endothelial cadherin and podoplanin were expressed at lower levels in white CTEPH thrombi than in organizing deep vein thrombi and organizing thrombi from aortic aneurysms. In addition, red CTEPH thrombi attenuated the angiogenic response induced by VEGF. Conclusions In the present work, we propose a mechanism of thrombus non-resolution demonstrating that endothelial cell-specific deletion of Kdr abates thrombus vessel formation, misguiding thrombus resolution. Medical conditions associated with the development of CTEPH may be compromising early thrombus angiogenesis.

Update on Chronic Thromboembolic Pulmonary Hypertension

“…70 Recently, the sequence of events that promote deep vein thrombosis was analyzed with the use of variations of the murine vena cava ligation model. 3 In the same model, genetic deletion of platelet endothelial cell adhesion molecule 1 4 illustrated the crucial roles of leukocyte migration and angiogenesis 71 in resolution of deep vein thrombosis and resolution of thrombus in CTEPH. Despite all of this evidence, one must bear in mind the limitations of studying thrombosis and thrombus resolution in animals or in in vitro models.…”

Involvement of inflammatory markers in pathogenesis of venous thromboembolism

“…In an animal model, neutropenia was associated with larger thrombi in the second and seventh day and was correlated with higher degrees of fibrosis in the thrombus and significantly lower serum levels of plasminogen activators such as urokinase and MMP-9. Early collagenolysis seems to take place in the first 7 days, as suggested by a high rigidity in the vein wall and persists up to 14 days, being accompanied by high activity of MMP-2 and MMP-9 [35].…”

“…In a comparative study between low weight heparins and oral P selectin inhibitors administered 2 days after the thrombosis, it was revealed that P selectin inhibitors significantly reduce the injury of the vein wall, independently of the size of thrombus [34,35].…”

Is inflammation a potential therapeutic target in chronic thromboembolic pulmonary hypertension?

“…A bundle of converging evidence further supports the hypothesis that IP-10 and IFN-c signalling pathways are potential key players in the pathogenesis of CTEPH ( fig. 1a) and not simply innocent bystanders: 1) medical conditions with an inflammatory component, including staphylococcal infection, splenectomy, inflammatory bowel disease and osteomyelitis, are risk factors for CTEPH [22], and are associated with IP-10 secretion and/or elevated circulating IP-10 [23][24][25]; 2) IFN-c signalling could determine the relative pathogenicity of Staphylococcus aureus strains [26]; 3) venous thrombus resolution is misguided in the presence of S. aureus infection [27]; 4) IP-10 is an inhibitor of angiogenesis; and 5) defective angiogenesis delaying thrombus resolution is potentially involved in CTEPH [28], and could be promoted by splenectomy [29]. In our hands, CRP also seemed to contribute to the disease process by activating the nuclear factor (NF)-kB pathway, thereby inducing endothelial cell dysfunction in CTEPH ( fig.…”

“…Prospective long-term follow-up studies gathering large numbers of acute pulmonary embolism patients, and combining clinical, biological and environmental parameters, are necessary to validate these pathogenic hypotheses. Similarly, development of accurate and relevant animal models clearly mimicking CTEPH pathobiology is needed, in addition to the recently developed models of abnormal venous thrombus resolution [28,29] or of large vessel mechanical obstruction [34,35].…”