Evaluation of targeting c-Src by the RGT-containing peptide as a novel antithrombotic strategy

Thrombotic events associated with immune checkpoint inhibitors and novel antithrombotic strategies to mitigate bleeding risk

Although immunotherapy is expanding treatment options for cancer patients, the prognosis of advanced cancer remains poor, and these patients must contend with both cancers and cancer-related thrombotic events. In particular, immune checkpoint inhibitors are associated with an increased risk of atherosclerotic thrombotic events. Given the fundamental role of platelets in atherothrombosis, co-administration of antiplatelet agents is always indicated. Platelets are also involved in all steps of cancer progression. Classical antithrombotic drugs can cause inevitable hemorrhagic side effects due to blocking integrin β3 bidirectional signaling, which regulates simultaneously thrombosis and hemostasis. Meanwhile, many promising new targets are emerging with minimal bleeding risk and desirable anti-tumor effects. This review will focus on the issue of thrombosis during immune checkpoint inhibitor treatment and the role of platelet activation in cancer progression as well as explore the mechanisms by which novel antiplatelet therapies may exert both antithrombotic and antitumor effects without excessive bleeding risk.

Advances of nanomedicine in treatment of atherosclerosis and thrombosis

Atherosclerosis (AS) is a chronic inflammatory vascular disease. Myocardial ischemia originated from AS is the main cause of cardiovascular diseases, one of the major factors contributing to the global disease burden. AS is typically quiescent until occurrence of plaque rupture and thrombosis, leading to acute coronary syndrome and sudden death. Currently, clinical diagnostic techniques suffer from major pitfalls including lack of accuracy and specificity, which makes it rather difficult for drugs to directly target plaques to achieve therapeutic effect. Therefore, how to accurately diagnose and effectively intervene vulnerable AS plaques to achieve accurate delivery of drugs has become an urgent and evolving clinical problem. With the rapid development of nanomedicine and nanomaterials, nanotechnology has shown unique advantages in monitoring vulnerable plaques and thrombus and improving drug efficacy. Recent studies have shown that application of nanoparticle drug delivery system can booster the safety and effectiveness of drug therapy, and molecular imaging technology and nanomedicine also exhibit high clinical application potentials in disease diagnosis. Therefore, nanotechnology provides another promising avenue for diagnosis and treatment of AS and thrombosis, and has shown excellent performance in the development of targeted drug therapy and biomaterials. In this review, the research progress, challenges and prospects of nanotechnology in AS and thrombosis are discussed, expecting to provide new ideas for the prevention, diagnosis and treatment of AS and thrombosis.

Repurposing of rilpivirine for preventing platelet β3 integrin-dependent thrombosis by targeting c-Src active autophosphorylation

BACKGROUND

HIV-infected individuals are known to be at higher risk for thrombotic cardiovascular disease (CVD), which may also be differentially affected by components of anti-HIV drugs. To identify the effects of a series of FDA-approved anti-HIV drugs on platelet aggregation in humans, focusing on the novel pharmacological effects of rilpivirine (RPV), a reverse transcriptase inhibitor, on platelet function both in vitro and in vivo and the mechanisms involved.

METHODS AND RESULTS

In vitro studies showed that RPV was the only anti-HIV reagent that consistently and efficiently inhibited aggregation elicited by different agonists, exocytosis, morphological extension on fibrinogen, and clot retraction. Treatment of mice with RPV significantly prevented thrombus formation in FeCl3-injured mesenteric vessels, postcava with stenosis surgery, and ADP -induced pulmonary embolism models without defects in platelet viability, tail bleeding, and coagulation activities. RPV also improved cardiac performance in mice with post-ischemic reperfusion. A mechanistic study revealed that RPV preferentially attenuated fibrinogen-stimulated Tyr773 phosphorylation of β3-integrin by inhibiting Tyr419 autophosphorylation of c-Src. Molecular docking and surface plasmon resonance analyses showed that RPV can bind directly to c-Src. Further mutational analysis showed that the Phe427 residue of c-Src is critical for RPV interaction, suggesting a novel interaction site for targeting c-Src to block β3-integrin outside-in signaling.

CONCLUSION

These results demonstrated that RPV was able to prevent the progression of thrombotic CVDs by interrupting β3-integrin-mediated outside-in signaling via inhibiting c-Src activation without hemorrhagic side effects, highlighting RPV as a promising reagent for the prevention and therapy of thrombotic CVDs.

Structure, signal transduction, activation, and inhibition of integrin αIIbβ3

Integrins are heterodimeric receptors comprising α and β subunits. They are expressed on the cell surface and play key roles in cell adhesion, migration, and growth. Several types of integrins are expressed on the platelets, including αvβ3, αIIbβ3, α2β1, α5β1, and α6β1. Among these, physically αIIbβ3 is exclusively expressed on the platelet surface and their precursor cells, megakaryocytes. αIIbβ3 adopts at least three conformations: i) bent-closed, ii) extended-closed, and iii) extended-open. The transition from conformation i) to iii) occurs when αIIbβ3 is activated by stimulants. Conformation iii) possesses a high ligand affinity, which triggers integrin clustering and platelet aggregation. Platelets are indispensable for maintaining vascular system integrity and preventing bleeding. However, excessive platelet activation can result in myocardial infarction (MI) and stroke. Therefore, finding a novel strategy to stop bleeding without accelerating the risk of thrombosis is important. Regulation of αIIbβ3 activation is vital for this strategy. There are a large number of molecules that facilitate or inhibit αIIbβ3 activation. The interference of these molecules can accurately control the balance between hemostasis and thrombosis. This review describes the structure and signal transduction of αIIbβ3, summarizes the molecules that directly or indirectly affect integrin αIIbβ3 activation, and discusses some novel antiαIIbβ3 drugs. This will advance our understanding of the activation of αIIbβ3 and its essential role in platelet function and tumor development.

Mechanism of Abnormal Coagulation Induced by Tigecycline in Cancer Patients

Tigecycline is a broad-spectrum active intravenous antibiotic that is active against methicillin-resistant staphylococcus aureus. In Phase 3 and 4 clinical trials, increased all-cause mortality was observed in patients treated with tigecycline compared to patients in the control group. The reason for the increase is unclear. In this study, we found that tigecycline cause abnormal coagulation in tumor patients, especially in patients with hematological malignancies. The main manifestations were decreased fibrinogen and prolonged activated prothrombin time (APTT), thrombin time (TT), and D-dimer. In addition, through functional studies, we found that tigecycline inhibit platelet adhesion and aggregation, and the coagulation function of patients gradually recover after discontinuation. Gene sequencing results suggested that tigecycline significantly regulate the expression of genes related to platelet function pathways and increase the incidence of single nucleotide polymorphisms and the number of alternative splices in the Chinese hamster ovary (CHO) cells treated with tigecycline. An abnormal function and low numbers of platelets are common in patients with hematological malignancies. Our study can explain the mechanism of abnormal coagulation caused by tigecycline. Additionally, doctors who apply tigecycline to cure infections in tumor patients should be warned.

Targeting the RT loop of Src SH3 in Platelets Prevents Thrombosis without Compromising Hemostasis

Conventional antiplatelet agents indiscriminately inhibit both thrombosis and hemostasis, and the increased bleeding risk thus hampers their use at more aggressive dosages to achieve adequate effect. Blocking integrin αIIbβ3 outside-in signaling by separating the β3/Src interaction, yet to be proven in vivo, may nonetheless resolve this dilemma. Identification of a specific druggable target for this strategy remains a fundamental challenge as Src SH3 is known to be responsible for binding to not only integrin β3 but also the proteins containing the PXXP motif. In vitro and in vivo mutational analyses show that the residues, especially E97, in the RT loop of Src SH3 are critical for interacting with β3. DCDBS84, a small molecule resulting from structure-based virtual screening, is structurally validated to be directed toward the projected target. It specifically disrupts β3/Src interaction without affecting canonical PXXP binding and thus inhibits the outside-in signaling-regulated platelet functions. Treatment of mice with DCDBS84 causes a profound inhibition of thrombosis, equivalent to that induced by extremely high doses of αIIbβ3 antagonist, but does not compromise primary hemostasis. Specific targets are revealed for a preferential inhibition of thrombosis that may lead to new classes of potent antithrombotics without hemorrhagic side effects.

Ibrutinib Suppresses Early Megakaryopoiesis but Enhances Proplatelet Formation

Ibrutinib, an irreversible inhibitor of Bruton's tyrosine kinase, has a favorable safety profile in patients with B cell-related malignancies. A primary adverse effect of ibrutinib is thrombocytopenia in the early stages of treatment, but platelet counts increase or recover as treatment continues. Currently, the effects of ibrutinib on megakaryopoiesis remain unclear. In this study, we investigated the mechanism by which ibrutinib induces thrombocytopenia using cord blood CD34+ hematopoietic stem cells (HSCs), a human megakaryoblastic cell line (SET-2), and C57BL/6 mice. We show that treatment with ibrutinib can suppress CD34+ HSC differentiation into megakaryocytes (MKs) and decrease the number of colony-forming unit-MKs (CFU-MKs). The ibrutinib-dependent inhibition of early megakaryopoiesis seems to mainly involve impaired proliferation of progenitor cells without induction of apoptosis. The effects of ibrutinib on late-stage megakaryopoiesis, in contrast to early-stage megakaryopoiesis, include enhanced MK differentiation, ploidy, and proplatelet formation in CD34+ HSC-derived MKs and SET-2 cells. We also demonstrated that MK adhesion and spreading, but not migration, were inhibited by ibrutinib. Furthermore, we revealed that integrin αIIbβ3 outside-in signaling in MKs was inhibited by ibrutinib. Consistent with previous clinical observations, in C57BL/6 mice treated with ibrutinib, platelet counts decreased by days 2 to 7 and recovered to normal levels by day 15. Together, these results reveal the pathogenesis of ibrutinib-induced transient thrombocytopenia. In conclusion, ibrutinib suppresses early megakaryopoiesis, as evidenced by inhibition of MK progenitor cell proliferation and CFU-MK formation. Ibrutinib enhances MK differentiation, ploidy, and proplatelet formation, while it impairs integrin αIIbβ3 outside-in signaling.

Characterization of Procoagulant COAT Platelets in Patients with Glanzmann Thrombasthenia

Patients affected by the rare Glanzmann thrombasthenia (GT) suffer from defective or low levels of the platelet-associated glycoprotein (GP) IIb/IIIa, which acts as a fibrinogen receptor, and have therefore an impaired ability to aggregate platelets. Because the procoagulant activity is a dichotomous facet of platelet activation, diverging from the aggregation endpoint, we were interested in characterizing the ability to generate procoagulant platelets in GT patients. Therefore, we investigated, by flow cytometry analysis, platelet functions in three GT patients as well as their ability to generate procoagulant collagen-and-thrombin (COAT) platelets upon combined activation with convulxin-plus-thrombin. In addition, we further characterized intracellular ion fluxes during the procoagulant response, using specific probes to monitor by flow cytometry kinetics of cytosolic calcium, sodium, and potassium ion fluxes. GT patients generated higher percentages of procoagulant COAT platelets compared to healthy donors. Moreover, they were able to mobilize higher levels of cytosolic calcium following convulxin-plus-thrombin activation, which is congruent with the greater procoagulant activity. Further investigations will dissect the role of GPIIb/IIIa outside-in signalling possibly implicated in the regulation of platelet procoagulant activity.

SARS-CoV-2 binds platelet ACE2 to enhance thrombosis in COVID-19

BACKGROUND

Critically ill patients diagnosed with COVID-19 may develop a pro-thrombotic state that places them at a dramatically increased lethal risk. Although platelet activation is critical for thrombosis and is responsible for the thrombotic events and cardiovascular complications, the role of platelets in the pathogenesis of COVID-19 remains unclear.

METHODS

Using platelets from healthy volunteers, non-COVID-19 and COVID-19 patients, as well as wild-type and hACE2 transgenic mice, we evaluated the changes in platelet and coagulation parameters in COVID-19 patients. We investigated ACE2 expression and direct effect of SARS-CoV-2 virus on platelets by RT-PCR, flow cytometry, Western blot, immunofluorescence, and platelet functional studies in vitro, FeCl3-induced thrombus formation in vivo, and thrombus formation under flow conditions ex vivo.

RESULTS

We demonstrated that COVID-19 patients present with increased mean platelet volume (MPV) and platelet hyperactivity, which correlated with a decrease in overall platelet count. Detectable SARS-CoV-2 RNA in the blood stream was associated with platelet hyperactivity in critically ill patients. Platelets expressed ACE2, a host cell receptor for SARS-CoV-2, and TMPRSS2, a serine protease for Spike protein priming. SARS-CoV-2 and its Spike protein directly enhanced platelet activation such as platelet aggregation, PAC-1 binding, CD62P expression, α granule secretion, dense granule release, platelet spreading, and clot retraction in vitro, and thereby Spike protein enhanced thrombosis formation in wild-type mice transfused with hACE2 transgenic platelets, but this was not observed in animals transfused with wild-type platelets in vivo. Further, we provided evidence suggesting that the MAPK pathway, downstream of ACE2, mediates the potentiating role of SARS-CoV-2 on platelet activation, and that platelet ACE2 expression decreases following SARS-COV-2 stimulation. SARS-CoV-2 and its Spike protein directly stimulated platelets to facilitate the release of coagulation factors, the secretion of inflammatory factors, and the formation of leukocyte-platelet aggregates. Recombinant human ACE2 protein and anti-Spike monoclonal antibody could inhibit SARS-CoV-2 Spike protein-induced platelet activation.

CONCLUSIONS

Our findings uncovered a novel function of SARS-CoV-2 on platelet activation via binding of Spike to ACE2. SARS-CoV-2-induced platelet activation may participate in thrombus formation and inflammatory responses in COVID-19 patients.

Platelet integrin αIIbβ3: signal transduction, regulation, and its therapeutic targeting

Integrins are a family of transmembrane glycoprotein signaling receptors that can transmit bioinformation bidirectionally across the plasma membrane. Integrin αIIbβ3 is expressed at a high level in platelets and their progenitors, where it plays a central role in platelet functions, hemostasis, and arterial thrombosis. Integrin αIIbβ3 also participates in cancer progression, such as tumor cell proliferation and metastasis. In resting platelets, integrin αIIbβ3 adopts an inactive conformation. Upon agonist stimulation, the transduction of inside-out signals leads integrin αIIbβ3 to switch from a low- to high-affinity state for fibrinogen and other ligands. Ligand binding causes integrin clustering and subsequently promotes outside-in signaling, which initiates and amplifies a range of cellular events to drive essential platelet functions such as spreading, aggregation, clot retraction, and thrombus consolidation. Regulation of the bidirectional signaling of integrin αIIbβ3 requires the involvement of numerous interacting proteins, which associate with the cytoplasmic tails of αIIbβ3 in particular. Integrin αIIbβ3 and its signaling pathways are considered promising targets for antithrombotic therapy. This review describes the bidirectional signal transduction of integrin αIIbβ3 in platelets, as well as the proteins responsible for its regulation and therapeutic agents that target integrin αIIbβ3 and its signaling pathways.

Functional Effect of the Mutations Similar to the Cleavage during Platelet Activation at Integrin β3 Cytoplasmic Tail when Expressed in Mouse Platelets

Previous studies in Chinese hamster ovary cells showed that truncational mutations of β3 at sites of F⁷⁵⁴ and Y⁷⁵⁹ mimicking calpain cleavage regulate integrin signaling. The roles of the sequence from F⁷⁵⁴ to C-terminus and the conservative N⁷⁵⁶ITY⁷⁵⁹ motif in platelet function have yet to be elaborated. Mice expressing β3 with F⁷⁵⁴ and Y⁷⁵⁹ truncations, or NITY deletion (β3-ΔTNITYRGT, β3-ΔRGT, or β3-ΔNITY) were established through transplanting the homozygous β3-deficient mouse bone marrow cells infected by the GFP tagged MSCV MigR1 retroviral vector encoding different β3 mutants into lethally radiated wild-type mice. The platelets were harvested for soluble fibrinogen binding and platelet spreading on immobilized fibrinogen. Platelet adhesion on fibrinogen- and collagen-coated surface under flow was also tested to assess the ability of the platelets to resist hydrodynamic drag forces. Data showed a drastic inhibition of the β3-ΔTNITYRGT platelets to bind soluble fibrinogen and spread on immobilized fibrinogen in contrast to a partially impaired fibrinogen binding and an almost unaffected spreading exhibited in the β3-ΔNITY platelets. Behaviors of the β3-ΔRGT platelets were consistent with the previous observations in the β3-ΔRGT knock-in platelets. The adhesion impairment of platelets with the β3 mutants under flow was in different orders of magnitude shown as: β3-ΔTNITYRGT>β3-ΔRGT>β3-ΔNITY to fibrinogen-coated surface, and β3-ΔTNITYRGT>β3-ΔNITY>β3-ΔRGT to collagen-coated surface. To evaluate the interaction of the β3 mutants with signaling molecules, GST pull-down and immunofluorescent assays were performed. Results showed that β3-ΔRGT interacted with kindlin but not c-Src, β3-ΔNITY interacted with c-Src but not kindlin, while β3-ΔTNITYRGT did not interact with both proteins. This study provided evidence in platelets at both static and flow conditions that the calpain cleavage-related sequences of integrin β3, i.e. T⁷⁵⁵NITYRGT⁷⁶², R⁷⁶⁰GT⁷⁶², and N⁷⁵⁶ITY⁷⁵⁹ participate in bidirectional, outside-in, and inside-out signaling, respectively and the association of c-Src or kindlin with β3 integrin may regulate these processes.