Complex formation with pentraxin-2 regulates factor X plasma levels and macrophage interactions

Abstract

Recently, we have identified scavenger receptor class A member I (SR-AI) as a receptor for coagulation factor X (FX), mediating the formation of an FX reservoir at the macrophage surface. Here, we demonstrate that the FX/SR-AI-complex comprises a third protein, pentraxin-2 (PTX2). The presence of PTX2 is essential to prevent internalization of FX by SR-AI, and the presence of FX is needed to interfere with internalization of PTX2. Binding studies showed that FX, SR-AI, and PTX2 independently bind to each other (: 0.2-0.7 μM). Surprisingly, immunoprecipitation experiments revealed that FX and PTX2 circulate as a complex in plasma, and complex formation involves the FX activation peptide. No binding of PTX2 to other vitamin K-dependent proteins was observed. Short hairpin RNA-mediated inhibition of PTX2 levels in mice resulted not only in reduced levels of PTX2, but also in similarly reduced FX levels. Moreover, PTX2 and FX levels were correspondingly reduced in SR-AI-deficient mice. Analysis of 71 human plasma samples uncovered a strong correlation between FX and PTX2 plasma levels. Furthermore, plasma samples of patients with reduced FX levels (congenital/acquired FX deficiency or after anti-vitamin K treatment) were characterized by concomitantly decreased PTX2 levels. In conclusion, we identified PTX2 as a novel partner for FX, and both proteins cooperate to prevent their SR-AI-mediated uptake by macrophages. Interestingly, their respective plasma levels are interdependent. These findings seem of relevance in perspective of ongoing clinical trials, in which plasma depletion of PTX2 is used as a therapeutical approach in the management of systemic amyloidosis.

Macrophage scavenger receptor SR-AI contributes to the clearance of von Willebrand factor

“…Briefly, human macrophages were differentiated from the THP1 acute monocytic leukemia cell line using phorbol 12-myristate 13-acetate, human macrophage colony-stimulating factor and human granulocyte-macrophage colony-stimulating factor as described elsewhere. 18 , 19 Non-transfected and stable HEK293 cell lines expressing human SR-AI were cultured as described presviously. 19 Murine macrophages were obtained from CD115 + cells as described by Breslin et al .…”

“…We previously observed that the majority of VWF is targeted to macrophages, and that chemical depletion of macrophages results in a 2- to 3-fold increase in VWF levels. 17 , 18 We therefore explored the hypothesis that macrophages express one or more additional receptors that contribute to VWF clearance. Here, we present data that are compatible with the macrophage-specific receptor Scavenger receptor class A member I (SR-AI) being a clearance-receptor for VWF.…”

The long pentraxin PTX3: A prototypical sensor of tissue injury and a regulator of homeostasis

“…CRP binding to PC‐expressing cells leads to the activation of the classical pathway of the complement, with the subsequent opsonization and phagocytosis . Recently, SAP has been demonstrated to contribute to the maintenance of the plasmatic levels of coagulation factor X (FX), mediating its uptake and internalization by scavenging macrophages …”

“…53 Recently, SAP has been demonstrated to contribute to the maintenance of the plasmatic levels of coagulation factor X (FX), mediating its uptake and internalization by scavenging macrophages. 54 PTX3 is the first long pentraxin to be identified. 46 Human PTX3 gene is located on chromosome 3q25 locus and it is composed by 3 exons, encoding for the leader peptide, the long N-terminal domain (amino acids 18-178) and the C-terminal pentraxin-like domain (amino acids .…”

Emerging Therapeutic Strategies in the Treatment of Hemophilia A

“…Moreover, similarly to zymogenlike FXa variants, the half-life of FX FpA in vivo is probably seriously reduced compared with wild-type FX, as it lacks the natural FX activation peptide sequence known to be essential to maintain circulating levels of FX. 48,49 This approach deserves further investigations to evaluate the usefulness and safety of thrombin-activatable FX in hemophilia.…”