Pharmacological modulation of the inflammatory actions of platelets

Platelets and the Cybernetic Regulation of Ischemic Inflammatory Responses through PNC Formation Regulated by Extracellular Nucleotide Metabolism and Signaling

Ischemic events are associated with severe inflammation and are here referred to as ischemic inflammatory response (IIR). Recent studies identified the formation of platelet–neutrophil complexes (PNC) as key players in IIR. We investigated the role of extracellular platelet nucleotide signaling in the context of IIR and defined a cybernetic circle, including description of feedback loops. Cybernetic circles seek to integrate different levels of information to understand how biological systems function. Our study specifies the components of the cybernetic system of platelets in IIR and describes the theoretical progression of IIR passing the cybernetic cycle with positive and negative feedback loops based on nucleotide-dependent signaling and functional regulation. The cybernetic components and feedback loops were explored by cytometry, immunohistological staining, functional blocking antibodies, and ADP/ATP measurements. Using several ex vivo and in vivo approaches we confirmed cybernetic parameters, such as controller, sensor, and effector (VASP phosphorylation, P2Y₁₂, ADORAs and GPIIb/IIIa activity), as well as set points (ADP, adenosine) and interfering control and disturbance variables (ischemia). We demonstrate the impact of the regulated platelet–neutrophil complex (PNC) formation in blood and the resulting damage to the affected inflamed tissue. Taken together, extracellular nucleotide signaling, PNC formation, and tissue damage in IIR can be integrated in a controlled cybernetic circle of platelet function, as introduced through this study.

Platelet Depletion Impairs Host Defense to Pulmonary Infection with Pseudomonas aeruginosa in Mice

Platelets have been implicated in pulmonary inflammatory cell recruitment after exposure to allergic and nonallergic stimuli, but little is known about the role of platelets in response to pulmonary infection with Pseudomonas aeruginosa. In this study, we have investigated the impact of the experimental depletion of circulating platelets on a range of inflammatory and bacterial parameters, and their subsequent impact on mortality in a murine model of pulmonary infection with P. aeruginosa. P. aeruginosa infection in mice induced a mild, but significant, state of peripheral thrombocytopenia in addition to pulmonary platelet accumulation. Increased platelet activation was detected in infected mice through increased levels of the platelet-derived mediators, platelet factor-4 and β-thromboglobulin, in BAL fluid and blood plasma. In mice depleted of circulating platelets, pulmonary neutrophil recruitment was significantly reduced 24 hours after infection, whereas the incidence of systemic dissemination of bacteria was significantly increased compared with non-platelet-depleted control mice. Furthermore, mortality rates were increased in bacterial-infected mice depleted of circulating platelets. This work demonstrates a role for platelets in the host response toward a gram-negative bacterial respiratory infection.

Platelet-targeted pharmacologic treatments as anti-cancer therapy

Platelets act as multifunctional cells participating in immune response, inflammation, allergy, tissue regeneration, and lymphoangiogenesis. Among the best-established aspects of a role of platelets in non-hemostatic or thrombotic disorders, there is their participation in cancer invasion and metastasis. The interaction of many different cancer cells with platelets leads to platelet activation, and on the other hand platelet activation is strongly instrumental to the pro-carcinogenic and pro-metastatic activities of platelets. It is thus obvious that over the last years a lot of interest has focused on the possible chemopreventive effect of platelet-targeted pharmacologic treatments. This article gives an overview of the platelet-targeted pharmacologic approaches that have been attempted in the prevention of cancer development, progression, and metastasis, including the application of anti-platelet drugs currently used for cardiovascular disease and of new and novel pharmacologic strategies. Despite the fact that very promising results have been obtained with some of these approaches in pre-clinical models, with the exclusion of aspirin, clinical evidence of a beneficial effect of anti-platelet agents in cancer is however still largely missing. Future studies with platelet-targeted drugs in cancer must carefully deal with design issues, and in particular with the careful selection of patients, and/or explore novel platelet targets in order to provide a solution to the critical issue of the risk/benefit profile of long-term anti-platelet therapy in the prevention of cancer progression and dissemination.

Ticagrelor promotes atherosclerotic plaque stability in a mouse model of advanced atherosclerosis

Objective

There is increasing evidence supporting the role of platelets in atherosclerotic vascular disease. The G-protein-coupled receptor P2Y₁₂ is a central mediator of platelet activation and aggregation but has also been linked to platelet-independent vascular disease. Ticagrelor is an oral P2Y₁₂ antagonist that is used as a standard treatment in patients after acute myocardial infarction. However, the effects of ticagrelor on advanced atherosclerosis have not been investigated.

Materials and methods

Twenty-week-old apolipoprotein-E-deficient mice received standard chow or standard chow supplemented with 0.15% ticagrelor (approximately 270 mg/kg/day) for 25 weeks. The lesion area was evaluated in the aortic sinus by Movat’s pentachrome staining and lesion composition, thickness of the fibrous cap, and size of the necrotic core evaluated by morphometry. RAW 264.7 macrophages were serum starved and treated with ticagrelor in vitro for the detection and quantification of apoptosis. In addition, oxLDL uptake in RAW 264.7 macrophages was evaluated.

Results

A trend toward the reduction of total lesion size was detected. However, data did not reach the levels of significance (control, n=11, 565,881 μm² [interquartile range {IQR} 454,778–603,925 μm²] versus ticagrelor, n=13, 462,595 μm² [IQR 379,740–546,037 μm²]; P=0.1). A significant reduction in the relative area of the necrotic core (control, n=11, 0.46 [IQR 0.4–0.51] versus ticagrelor, n=13, 0.34 [IQR 0.31–0.39]; P=0.008), and a significant increase in fibrous caps thickness (control, n=11, 3.7 μm [IQR 3.4–4.2 μm] versus ticagrelor, n=13, 4.7 [IQR 4.3–5.5 μm], P=0.04) were seen in ticagrelor-treated mice. In vitro studies demonstrated a reduction in apoptotic RAW 264.7 macrophages (control 0.07±0.03 versus ticagrelor 0.03±0.03; P=0.0002) when incubated with ticagrelor. Uptake of oxLDL in RAW 264.7 was significantly reduced when treated with ticagrelor (control 9.2 [IQR 5.3–12.9] versus ticagrelor 6.4 [IQR 2.5–9.5], P=0.02).

Conclusion

The present study demonstrates for the first time a plaque-stabilizing effect of ticagrelor in a model of advanced vascular disease, potentially induced by a reduction of oxLDL uptake or an inhibition of apoptosis as seen in vitro.

The Multifaceted Clinical Readouts of Platelet Inhibition by Low-Dose Aspirin

Inactivation of platelet cyclooxygenase (COX)-1 by low-dose aspirin leads to long-lasting suppression of thromboxane (TX) A2 production and TXA2-mediated platelet activation and aggregation. This effect is necessary and sufficient to explain aspirin's unique (among other COX-1 inhibitors) effectiveness in preventing atherothrombosis, as well as its shared (with other antiplatelet agents) bleeding liability. However, different mechanisms of action have been suggested to explain other beneficial effects of aspirin, such as prevention of venous thromboembolism, chemoprevention of colorectal (and other) cancers, and reduced risk of dementia. These mechanisms include acetylation of other proteins in blood coagulation, inhibition of COX-2 activity, and other COX-independent mechanisms. The intent of this review is to develop the concept that the multifaceted therapeutic effects of low-dose aspirin may reflect pleiotropic consequences of platelet inhibition on pathophysiological tissue repair processes. Furthermore, the clinical implications of this concept will be discussed in terms of current clinical practice and future research.

Platelet Gi protein Gαi2 is an essential mediator of thrombo-inflammatory organ damage in mice

Platelets are crucial for hemostasis and thrombosis and exacerbate tissue injury following ischemia and reperfusion. Important regulators of platelet function are G proteins controlled by seven transmembrane receptors. The Gi protein Gα(i2) mediates platelet activation in vitro, but its in vivo role in hemostasis, arterial thrombosis, and postischemic infarct progression remains to be determined. Here we show that mice lacking Gα(i2) exhibit prolonged tail-bleeding times and markedly impaired thrombus formation and stability in different models of arterial thrombosis. We thus generated mice selectively lacking Gα(i2) in megakaryocytes and platelets (Gna(i2)(fl/fl)/PF4-Cre mice) and found bleeding defects comparable to those in global Gα(i2)-deficient mice. To examine the impact of platelet Gα(i2) in postischemic thrombo-inflammatory infarct progression, Gna(i2)(fl/fl)/PF4-Cre mice were subjected to experimental models of cerebral and myocardial ischemia/reperfusion injury. In the model of transient middle cerebral artery occlusion stroke Gna(i2)(fl/fl)/PF4-Cre mice developed significantly smaller brain infarcts and fewer neurological deficits than littermate controls. Following myocardial ischemia, Gna(i2)(fl/fl)/PF4-Cre mice showed dramatically reduced reperfusion injury which correlated with diminished formation of the ADP-dependent platelet neutrophil complex. In conclusion, our data provide definitive evidence that platelet Gα(i2) not only controls hemostatic and thrombotic responses but also is critical for the development of ischemia/reperfusion injury in vivo.

The platelet P2 receptors in inflammation

In addition to their well characterized and established role in haemostasis and thrombosis, platelets contribute to the pathogenesis of inflammation. Adenine nucleotides are signalling molecules that regulate the function of virtually every cell in the body, by interacting with P2 receptors. Their important role in inflammation is well established. In the last few years, the pro-inflammatory roles of adenine nucleotides interacting with their platelet P2 receptors has emerged. In particular, it was shown that the platelet P2Y12 receptor for ADP significantly contributed to the pro-inflammatory effects of cysteinyl leukotrienes (CysLT) in experimental models of asthma in mice. More importantly, it was recently shown that P2Y12 variants were associated with lung function in a large family-based asthma cohort and that the P2Y12 antagonist prasugrel tended to decrease bronchial hyper-reactivity to mannitol in patients with allergic bronchial asthma in a randomized, placebo controlled trial.

CONCLUSION

These data strongly suggest that P2Y12 may represent an important pharmacological target for the treatment of patients with allergic bronchial asthma.

Effect of prasugrel in patients with asthma: results of PRINA, a randomized, double-blind, placebo-controlled, cross-over study

BACKGROUND

Although experimental studies have demonstrated that platelets are proinflammatory cells, no randomized studies have tested the anti-inflammatory effect of antiplatelet agents in humans. The platelet P2Y12 receptors mediated bronchial inflammation in a mouse model of asthma, suggesting that P2Y12 represents a pharmacologic target for asthma.

OBJECTIVES

In this proof-of concept, placebo-controlled, randomized, cross-over study, we tested the effects of the P2Y12 antagonist prasugrel on bronchial hyperreactivity of asthmatic patients.

PATIENTS/METHODS

Twenty-six asthmatic patients were randomly and blindly allocated to prasugrel (10 mg once daily) or placebo for 15 days. After a ≥ 15-day wash-out, patients were crossed over to the alternative treatment. Before and after each treatment, patients underwent a bronchial provocation test with mannitol and measurement of fractional exhaled nitric oxide (FeNO). Inhibition of P2Y12 -dependent platelet reactivity (platelet reactivity index [PRI]) was measured with the vasodilator-stimulated phosphoprotein phosphorylation assay.

RESULTS

The provocative dose of mannitol causing a 15% drop in forced expiratory volume in 1 s increased from 142 mg (95% confidence interval [CI] 82-202) to 187 mg (95% CI 113-262) after prasugrel treatment (P = 0.09), and did not change after placebo treatment (136 mg [95% CI 76-196] and 144 mg [95% CI 84-204], P = 0.65). FeNO did not change after either treatment. The PRI decreased from 80% (95% CI 77-83) to 23% (95% CI 7-29) after prasugrel treatment (P < 0.001) and remained unchanged after placebo.

CONCLUSIONS

Our proof-of-concept, randomized, controlled study is the first one to test in vivo the anti-inflammatory effects of platelet inhibition in human patients. The results suggest that pharmacologic inhibition of P2Y12 receptors may slightly reduce the bronchial inflammatory burden, and lay the groundwork for further studies, with clinical endpoints.