Page 30 - Haematologica Vol. 109 - July 2024

SPOTLIGHT REVIEW ARTICLE - Coagulation factors and endothelial functionality C. Olgasi et al.
that vWF could be an anti-angiogenic factor and this is supported by the demonstration that hypoxia induced an increased vessel formation in the brain microvasculature of both vWF-/- and anti-vWF antibody-treated mice.26 In contrast, another study showed that vWF-/- mice have re- duced angiogenesis after ischemia27 and, in line with these results, vWF was found to play a pro-angiogenic function, binding and recruiting several growth factors thanks to its heparin-binding domain.28 Although it is well-known that vWF binds integrin αvb3 on the EC surface, little is un- derstood about how it regulates vascular stability through this receptor,24 and the downstream mediators activated by vWF in EC have not yet been clarified, with the excep- tion of MAP kinase p38, found activated in EC after vWF treatment, and resulting in an increase in mesenchymal cell adhesion to EC.29
The available data on FVIII controlling EC functions are still not complete, and vWF effects on EC appear, in some cases, controversial, suggesting that it may display both pro- and anti-angiogenic roles (Table 1). Thus, further investigations are needed to elucidate their role in EC homeostasis.
Tissue factor
Following damage to the blood vessel, tissue factor (TF) joins with activated factor VII (FVII) in inducing the acti- vation of FX, leading to fibrin deposition. TF is mainly ex- pressed by platelets, neutrophils, eosinophils, fibroblasts, pericytes, keratinocytes, while monocytes increase its expression after stimulation with lipopolysaccharide (LPS) through activator protein 1 (AP-1) and NF-κB activation.30 This protein has two isoforms: one is full-length (fl-TF), bound to the cell membrane, and the other one is the soluble alternatively spliced (as-TF) isoform. Both iso- forms have been demonstrated to play major roles be- yond coagulation; specifically, they are involved in the regulation of angiogenesis, tumor growth, metastasis, and inflammation.31 Indeed, most of the present data suggest that TF is a pro-metastatic and angiogenic factor,32 and that the main isoform involved in the angiogenic process is the as-TF, which can exert its action through the hy- poxia-inducible factor-1 (HIF1)/VEGF pathway.33 Moreover, as-TF has been shown to increase tubulogenesis, binding integrin α6b1 and activating MAP kinase p42/p44, while it enhances EC migration through integrin αvb3 and MAP kinase p38.34 Congruently, as-TF promotes cancer prolif- eration through integrin b1 signaling, further highlighting the pivotal interconnection of CF with integrins.35 However, as-TF alone is not enough to promote the cor- rect formation of the vasculature during embryogenesis,36 suggesting that both isoforms are necessary to regulate blood vessel formation and maintenance. Indeed, already in 1996, it was demonstrated that fl-TF−/− mice die during embryonic development due to vascular failure.37 More- over, it was described that fl-TF can induce EC migration through PAR2.38
Overall, these data show that both TF isoforms can di- rectly bind integrins and GPCR, regulating EC functionality (Table 1).
Anti-coagulation factors
The intricate system of coagulation within the bloodstream is a finely tuned process and a delicate equilibrium is maintained by the interplay of pro- and anti-CF. These proteins with anti-coagulant activity act as essential regu- lators, counteracting the clot-forming cascade to prevent excessive blood clotting.
One of the major anti-CF is antithrombin (AT), which inhibits most pro-coagulant proteases of the coagulation cascade, with thrombin and activated FX and factor IX (FIX) as main targets of its action.39 It also displays an anti-inflamma- tory role, binding heparan sulfate proteoglycans (HSPG) on the EC surface (especially syndecan-4) and increasing prostacyclin.40 Notably, cleaved and latent forms of AT have been demonstrated to be potent anti-angiogenic factors down-regulating the expression of genes related to extracellular matrix assembly in EC, such as perlecan, byglycan and syndecans.41 Its anti-proliferative function has been demonstrated in EC as well as in tumorigenic cell lines42 and, specifically, its heparin-binding site was shown to inhibit proliferation, migration, capillary-like tube formation, and perlecan expression.41 HSPG have been de- scribed as interacting with integrins to regulate many EC functions; thus, it has been hypothesized that the inter- play between the AT-HSPG complex and the extracellular matrix (ECM) organization could be orchestrated through integrin signaling.40
Interestingly, also thrombomodulin (TM) seems to be im- plicated in angiogenesis regulation. Indeed, TM has been shown to be a pro-angiogenic factor, activating focal ad- hesion kinase and binding fibronectin.43 This observation was strengthened by the demonstration of the in vitro and in vivo activity of recombinant EGF-like domain plus the O-glycosylation site-rich domain of TM.44 In contrast, recombinant lectin-like domain of TM has been shown to interact with Lewis-Y carbohydrate antigen, inhibiting angiogenesis.45 Recently, its role has also been assessed in TM-deficient EC and, several assays have shown it to tightly regulate EC quiescence.46 Taken together, these findings reveal that TM exhibits a dual role as both a pro- and an anti-angiogenic factor.
Importantly, another well-known anti-CF, activated protein C (APC), has been described to protect the endothelial barrier binding angiopoietin-1 receptor (Tie2)47 or PAR1, in- ducing a signaling in which b-arrestin-2 and dishevelled-2 are involved.48 Further investigations have revealed that APC induces anti-apoptotic signaling in EC in a caveo- lin-1-dependent manner,49 and increases phosphorylation of proteins mainly related to gene expression and actin binding, corroborating the role of APC in stabilizing the EC barrier.6
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