Fibrin-glycoprotein VI interaction increases platelet procoagulant activity and impacts clot structure

BACKGROUND

The glycoprotein VI (GPVI) signaling pathway was previously reported to direct procoagulant platelet activity through collagen binding. However, the impact of GPVI-fibrin interaction on procoagulant platelet development and how it modulates the clot structure are unknown.

OBJECTIVES

To determine the effect of GPVI-fibrin interaction on the platelet phenotype and its impact on the clot structure.

METHODS

Procoagulant platelets in platelet-rich plasma clots were determined by scanning electron microscopy (wild-type and GPVI-deficient murine samples) and confocal microscopy. Procoagulant platelet number, clot density, clot porosity, and clot retraction were determined in platelet-rich plasma or whole blood clots of healthy volunteers in the presence of tyrosine kinase inhibitors (PRT-060318, ibrutinib, and dasatinib) and eptifibatide.

RESULTS

GPVI-deficient clots showed a higher nonprocoagulant vs procoagulant platelet ratio than wild-type clots. The fiber density and the procoagulant platelet number decreased in the presence of Affimer proteins, inhibiting GPVI-fibrin(ogen) interaction and the tyrosine kinase inhibitors. The effect of GPVI signaling inhibitors on the procoagulant platelet number was exacerbated by eptifibatide. The tyrosine kinase inhibitors led to an increase in clot porosity; however, no differences were observed in the final clot weight, following clot retraction with the tyrosine kinase inhibitors, except for ibrutinib. In the presence of eptifibatide, clot retraction was impaired.

CONCLUSION

Our findings showed that GPVI-fibrin interaction significantly contributes to the development of procoagulant platelets and that inhibition of GPVI signaling increases clot porosity. Clot contractibility was impaired by the integrin αIIbβ3 and Btk pathway inhibition. Thus, inhibition of GPVI-fibrin interactions can alleviate structural characteristics that contribute to a prothrombotic clot phenotype, having potential important implications for novel antithrombotic interventions.

Development of resistance to FGFR inhibition in urothelial carcinoma via multiple pathways in vitro

Alterations of fibroblast growth factor receptors (FGFRs) are common in bladder and other cancers and result in disrupted signalling via several pathways. Therapeutics that target FGFRs have now entered the clinic, but, in common with many cancer therapies, resistance develops in most cases. To model this, we derived resistant sublines of two FGFR-driven bladder cancer cell lines by long-term culture with the FGFR inhibitor PD173074 and explored mechanisms using expression profiling and whole-exome sequencing. We identified several resistance-associated molecular profiles. These included HRAS mutation in one case and reversible mechanisms resembling a drug-tolerant persister phenotype in others. Upregulated IGF1R expression in one resistant derivative was associated with sensitivity to linsitinib and a profile with upregulation of a YAP/TAZ signature to sensitivity to the YAP inhibitor CA3 in another. However, upregulation of other potential therapeutic targets was not indicative of sensitivity. Overall, the heterogeneity in resistance mechanisms and commonality of the persister state present a considerable challenge for personalised therapy. Nevertheless, the reversibility of resistance may indicate a benefit from treatment interruptions or retreatment following disease relapse in some patients. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Fibrinogen αC-subregions critically contribute blood clot fibre growth, mechanical stability, and resistance to fibrinolysis

Fibrinogen is essential for blood coagulation. The C-terminus of the fibrinogen α-chain (αC-region) is composed of an αC-domain and αC-connector. Two recombinant fibrinogen variants (α390 and α220) were produced to investigate the role of subregions in modulating clot stability and resistance to lysis. The α390 variant, truncated before the αC-domain, produced clots with a denser structure and thinner fibres. In contrast, the α220 variant, truncated at the start of the αC-connector, produced clots that were porous with short, stunted fibres and visible fibre ends. These clots were mechanically weak and susceptible to lysis. Our data demonstrate differential effects for the αC-subregions in fibrin polymerisation, clot mechanical strength, and fibrinolytic susceptibility. Furthermore, we demonstrate that the αC-subregions are key for promoting longitudinal fibre growth. Together, these findings highlight critical functions of the αC-subregions in relation to clot structure and stability, with future implications for development of novel therapeutics for thrombosis.

Elimination of fibrin γ-chain cross-linking by FXIIIa increases pulmonary embolism arising from murine inferior vena cava thrombi

The onset of venous thromboembolism, including pulmonary embolism, represents a significant health burden affecting more than 1 million people annually worldwide. Current treatment options are based on anticoagulation, which is suboptimal for preventing further embolic events. In order to develop better treatments for thromboembolism, we sought to understand the structural and mechanical properties of blood clots and how this influences embolism in vivo. We developed a murine model in which fibrin γ-chain cross-linking by activated Factor XIII is eliminated (FGG3X) and applied methods to study thromboembolism at whole-body and organ levels. We show that FGG3X mice have a normal phenotype, with overall coagulation parameters and platelet aggregation and function largely unaffected, except for total inhibition of fibrin γ-chain cross-linking. Elimination of fibrin γ-chain cross-linking resulted in thrombi with reduced strength that were prone to fragmentation. Analysis of embolism in vivo using Xtreme optical imaging and light sheet microscopy demonstrated that the elimination of fibrin γ-chain cross-linking resulted in increased embolization without affecting clot size or lysis. Our findings point to a central previously unrecognized role for fibrin γ-chain cross-linking in clot stability. They also indirectly indicate mechanistic targets for the prevention of thrombosis through selective modulation of fibrin α-chain but not γ-chain cross-linking by activated Factor XIII to reduce thrombus size and burden, while maintaining clot stability and preventing embolism.

GPVI (Glycoprotein VI) Interaction With Fibrinogen Is Mediated by Avidity and the Fibrinogen αC-Region

Supplemental Digital Content is available in the text.

Objective:

GPVI (glycoprotein VI) is a key molecular player in collagen-induced platelet signaling and aggregation. Recent evidence indicates that it also plays important role in platelet aggregation and thrombus growth through interaction with fibrin(ogen). However, there are discrepancies in the literature regarding whether the monomeric or dimeric form of GPVI binds to fibrinogen at high affinity. The mechanisms of interaction are also not clear, including which region of fibrinogen is responsible for GPVI binding. We aimed to gain further understanding of the mechanisms of interaction at molecular level and to identify the regions on fibrinogen important for GPVI binding.

Approach and Results:

Using multiple surface- and solution-based protein-protein interaction methods, we observe that dimeric GPVI binds to fibrinogen with much higher affinity and has a slower dissociation rate constant than the monomer due to avidity effects. Moreover, our data show that the highest affinity interaction of GPVI is with the αC-region of fibrinogen. We further show that GPVI interacts with immobilized fibrinogen and fibrin variants at a similar level, including a nonpolymerizing fibrin variant, suggesting that GPVI binding is independent of fibrin polymerization.

Conclusions:

Based on the above findings, we conclude that the higher affinity of dimeric GPVI over the monomer for fibrinogen interaction is achieved by avidity. The αC-region of fibrinogen appears essential for GPVI binding. We propose that fibrin polymerization into fibers during coagulation will cluster GPVI through its αC-region, leading to downstream signaling, further activation of platelets, and potentially stimulating clot growth.

Impact of γ'γ' fibrinogen interaction with red blood cells on fibrin clots

AIM

γ' fibrinogen has been associated with thrombosis. Here the interactions between γ'γ' or γAγA fibrinogen and red blood cells (RBCs), and their role on fibrin clot properties were studied.

MATERIALS & METHODS

Atomic Force microscopy (AFM)-based force spectroscopy, rheological, electron and confocal microscopy, and computational approaches were conducted for both fibrinogen variants.

RESULTS & CONCLUSION

AFM shows that the recombinant human (rh)γ'γ' fibrinogen increases the binding force and the frequency of the binding to RBCs compared with rhγAγA, promoting cell aggregation. Structural changes in rhγ'γ' fibrin clots, displaying a nonuniform fibrin network were shown by microscopy approaches. The presence of RBCs decreases the fibrinolysis rate and increases viscosity of rhγ'γ' fibrin clots. The full length of the γ' chain structure, revealed by computational analysis, occupies a much wider surface and is more flexible, allowing an increase of the binding between γ' fibers, and eventually with RBCs.

A fibrin biofilm covers blood clots and protects from microbial invasion

Hemostasis requires conversion of fibrinogen to fibrin fibers that generate a characteristic network, interact with blood cells, and initiate tissue repair. The fibrin network is porous and highly permeable, but the spatial arrangement of the external clot face is unknown. Here we show that fibrin transitioned to the blood-air interface through Langmuir film formation, producing a protective film confining clots in human and mouse models. We demonstrated that only fibrin is required for formation of the film, and that it occurred in vitro and in vivo. The fibrin film connected to the underlying clot network through tethering fibers. It was digested by plasmin, and formation of the film was prevented with surfactants. Functionally, the film retained blood cells and protected against penetration by bacterial pathogens in a murine model of dermal infection. Our data show a remarkable aspect of blood clotting in which fibrin forms a protective film covering the external surface of the clot, defending the organism against microbial invasion.

Sensing adhesion forces between erythrocytes and γ' fibrinogen, modulating fibrin clot architecture and function

Plasma fibrinogen includes an alternatively spliced γ-chain variant (γ'), which mainly exists as a heterodimer (γAγ') and has been associated with thrombosis. We tested γAγ' fibrinogen-red blood cells (RBCs) interaction using atomic force microscopy-based force spectroscopy, magnetic tweezers, fibrin clot permeability, scanning electron microscopy and laser scanning confocal microscopy. Data reveal higher work necessary for RBC-RBC detachment in the presence of γAγ' rather than γAγA fibrinogen. γAγ' fibrinogen-RBCs interaction is followed by changes in fibrin network structure, which forms an heterogeneous clot structure with areas of denser and highly branched fibrin fibers. The presence of RBCs also increased the stiffness of γAγ' fibrin clots, which are less permeable and more resistant to lysis than γAγA clots. The modifications on clots promoted by RBCs-γAγ' fibrinogen interaction could alter the risk of thrombotic disorders.

Abstract 3145: Mechanisms of resistance to FGFR-targeted therapy in bladder cancer

Fibroblast growth factor receptor 3 (FGFR3) overexpression, point mutations or gene fusions are found in ~80% of non muscle-invasive and ~15% of muscle-invasive bladder cancer. FGFR inhibitors have entered clinical trials in advanced bladder cancer however, as with other targeted therapies, intrinsic and acquired resistance are expected to limit treatment efficacy. We have used an in vitro model to explore possible mechanisms of resistance.

The urothelial cancer cell line RT112 expresses an FGFR3-TACC3 fusion protein and is sensitive to FGFR inhibition. Isogenic resistant cell lines, termed R1, R2 and R3, were derived by long-term culture of RT112 in the presence of the FGFR inhibitor PD173074. Compared to parental RT112, R1 and R2 show reduced proliferation and have a more mesenchymal morphology, decreased expression of FGFR3 and increased expression of N-cadherin. R3 has a faster growth rate, more epithelial morphology and lower N-cadherin expression than R1 and R2.

The changes in morphology and gene expression between parental and resistant derivatives R1 and R2 were reversed when the resistant cells were cultured without PD173074 for 4 passages. Despite this, the cells retained their resistance when re-exposed to PD173074.

Exome sequencing, RNA microarray analysis and phospho-receptor tyrosine kinase array data on the parental cells and resistant derivatives and will be presented. Our data suggests that diverse mechanisms of resistance occur following prolonged FGFR inhibition.

Citation Format: Geoffrey A. Pettitt, Helen R. McPherson, Carolyn D. Hurst, Julie E. Burns, Olivia A. Alder, Matthew C. Dunning, Margaret A. Knowles. Mechanisms of resistance to FGFR-targeted therapy in bladder cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3145. doi:10.1158/1538-7445.AM2017-3145

PIK3CA dependence and sensitivity to therapeutic targeting in urothelial carcinoma

Background

Many urothelial carcinomas (UC) contain activating PIK3CA mutations. In telomerase-immortalized normal urothelial cells (TERT-NHUC), ectopic expression of mutant PIK3CA induces PI3K pathway activation, cell proliferation and cell migration. However, it is not clear whether advanced UC tumors are PIK3CA-dependent and whether PI3K pathway inhibition is a good therapeutic option in such cases.

Methods

We used retrovirus-mediated delivery of shRNA to knock down mutant PIK3CA in UC cell lines and assessed effects on pathway activation, cell proliferation, migration and tumorigenicity. The effect of the class I PI3K inhibitor GDC-0941 was assessed in a panel of UC cell lines with a range of known molecular alterations in the PI3K pathway.

Results

Specific knockdown of PIK3CA inhibited proliferation, migration, anchorage-independent growth and in vivo tumor growth of cells with PIK3CA mutations. Sensitivity to GDC-0941 was dependent on hotspot PIK3CA mutation status. Cells with rare PIK3CA mutations and co-occurring TSC1 or PTEN mutations were less sensitive. Furthermore, downstream PI3K pathway alterations in TSC1 or PTEN or co-occurring AKT1 and RAS gene mutations were associated with GDC-0941 resistance.

Conclusions

Mutant PIK3CA is a potent oncogenic driver in many UC cell lines and may represent a valuable therapeutic target in advanced bladder cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2570-0) contains supplementary material, which is available to authorized users.