The platelet as a therapeutic target for treating vascular diseases and the role of eicosanoid and synthetic PPARgamma ligands

Profiling the eicosanoid networks that underlie the anti- and pro-thrombotic effects of aspirin

Aspirin prevents thrombosis by inhibiting platelet cyclooxygenase (COX)-1 activity and the production of thromboxane (Tx)A₂, a pro-thrombotic eicosanoid. However, the non-platelet actions of aspirin limit its antithrombotic effects. Here, we used platelet-COX-1-ko mice to define the platelet and non-platelet eicosanoids affected by aspirin. Mass-spectrometry analysis demonstrated blood from platelet-COX-1-ko and global-COX-1-ko mice produced similar eicosanoid profiles in vitro: for example, formation of TxA₂, prostaglandin (PG) F_2α, 11-hydroxyeicosatraenoic acid (HETE), and 15-HETE was absent in both platelet- and global-COX-1-ko mice. Conversely, in vivo, platelet-COX-1-ko mice had a distinctly different profile from global-COX-1-ko or aspirin-treated control mice, notably significantly higher levels of PGI₂ metabolite. Ingenuity Pathway Analysis (IPA) predicted that platelet-COX-1-ko mice would be protected from thrombosis, forming less pro-thrombotic TxA₂ and PGE₂. Conversely, aspirin or lack of systemic COX-1 activity decreased the synthesis of anti-aggregatory PGI₂ and PGD₂ at non-platelet sites leading to predicted thrombosis increase. In vitro and in vivo thrombosis studies proved these predictions. Overall, we have established the eicosanoid profiles linked to inhibition of COX-1 in platelets and in the remainder of the cardiovascular system and linked them to anti- and pro-thrombotic effects of aspirin. These results explain why increasing aspirin dosage or aspirin addition to other drugs may lessen antithrombotic protection.

PPARγ agonists negatively regulate αIIbβ3 integrin outside-in signaling and platelet function through up-regulation of protein kinase A activity

Essentials peroxisome proliferator-activated receptor γ (PPARγ) agonists inhibit platelet function. PPARγ agonists negatively regulate outside-in signaling via integrin αIIbβ3. PPARγ agonists disrupt the interaction of Gα13 with integrin β3. This is attributed to an upregulation of protein kinase A activity.

SUMMARY

Background Agonists for the peroxisome proliferator-activated receptor (PPARγ) have been shown to have inhibitory effects on platelet activity following stimulation by GPVI and GPCR agonists. Objectives Profound effects on thrombus formation led us to suspect a role for PPARγ agonists in the regulation of integrin αIIbβ3 mediated signaling. Both GPVI and GPCR signaling pathways lead to αIIbβ3 activation, and signaling through αIIbβ3 plays a critical role in platelet function and normal hemostasis. Methods The effects of PPARγ agonists on the regulation of αIIbβ3 outside-in signaling was determined by monitoring the ability of platelets to adhere and spread on fibrinogen and undergo clot retraction. Effects on signaling components downstream of αIIbβ3 activation were also determined following adhesion to fibrinogen by Western blotting. Results Treatment of platelets with PPARγ agonists inhibited platelet adhesion and spreading on fibrinogen and diminished clot retraction. A reduction in phosphorylation of several components of αIIbβ3 signaling, including the integrin β3 subunit, Syk, PLCγ2, focal adhesion kinase (FAK) and Akt, was also observed as a result of reduced interaction of the integrin β3 subunit with Gα13. Studies of VASP phosphorylation revealed that this was because of an increase in PKA activity following treatment with PPARγ receptor agonists. Conclusions This study provides further evidence for antiplatelet actions of PPARγ agonists, identifies a negative regulatory role for PPARγ agonists in the control of integrin αIIbβ3 outside-in signaling, and provides a molecular basis by which the PPARγ agonists negatively regulate platelet activation and thrombus formation.

Targeted analysis of omega-6-derived eicosanoids in human serum by SPE-LC-MS/MS for evaluation of coronary artery disease

A targeted approach has been applied to quantitative analysis of eicosanoids derived from omega-6 fatty acids in serum from individuals diagnosed with coronary artery disease (CAD). The target metabolites were series-2 prostaglandins, thromboxane B2, hydroxyeicosatetraenoic acids, and hydroxyoctadecadienoic acids. The method was based on SPELC-MS/MS in selected reaction monitoring mode for highly selective and sensitive determination of the target eicosanoids. The combination of SPE and LC-MS/MS involved the benefits from both direct analysis of serum without a step for protein precipitation and fully automation of the analysis. The method allowed comparison of omega-6-derived eicosanoids in serum from patients diagnosed with CAD and from control individuals. The effect of treatment with aspirin on the profile of the target compounds was evaluated through its incidence on the different pathways. Finally, the serum levels of the target metabolites in patients diagnosed with CAD were also statistically examined according to the severity of the coronary lesion stratified as stable angina, non-ST-elevation acute coronary syndrome, and acute myocardial infarction.

Antiplatelet effect of AMP-activated protein kinase activator and its potentiation by the phosphodiesterase inhibitor dipyridamole

AMP-activated protein kinase (AMPK) activates endothelial nitric oxide synthase (eNOS) via phosphorylation at the activating site. The eNOS-nitric oxide (NO)/soluble guanylate cyclase (sGC)-cGMP/cGMP-dependent protein kinase (PKG) signaling axis is a major antiaggregatory mechanism residing in platelets. Based on the hypothesis that direct activation of AMPK might be a potential strategy to inhibit platelet aggregation, the antiplatelet effect of AMPK activators was investigated. Treatment of isolated platelets with the AMPK activator, 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) resulted in AMPK activation and a decrease in aggregation, which was abolished by pretreatment with the AMPK inhibitors compound C (CC) and ara-A. Such an AMPK-dependent antiaggregatory effect was also observed with other AMPK activators such as A-769662 and PT1. AICAR induced eNOS activation was followed by NO synthesis, cGMP production, and subsequent phosphorylation of vasodilator-stimulated phosphoprotein (VASP), a PKG substrate. All these events were blocked by CC or ara-A pretreatment, and each event was inhibited by the eNOS inhibitor L-NAME, the sGC inhibitor ODQ, and the PKG inhibitor Rp-8-pCPT-cGMPS. Simultaneous treatment of dipyridamole, a phosphodiesterase (PDE) inhibitor, with AICAR potentiated the antiaggregatory effect by enhancing the cGMP elevation. Administration of AICAR increased platelet cGMP and prolonged FeCl3-induced arterial occlusion time in rats, which further increased in combination with dipyridamole. In conclusion, AMPK activators inhibited platelet aggregation by stimulating the eNOS-NO/sGC-cGMP/PKG signaling pathway. The antiplatelet effect of AMPK activators could be potentiated in combination with a PDE inhibitor through the common mechanism of elevating cGMP. Thus, AMPK may serve as a potential target for antiplatelet therapy.

An eicosanoid-centric view of atherothrombotic risk factors

Cardiovascular disease is the foremost cause of morbidity and mortality in the Western world. Atherosclerosis followed by thrombosis (atherothrombosis) is the pathological process underlying most myocardial, cerebral, and peripheral vascular events. Atherothrombosis is a complex and heterogeneous inflammatory process that involves interactions between many cell types (including vascular smooth muscle cells, endothelial cells, macrophages, and platelets) and processes (including migration, proliferation, and activation). Despite a wealth of knowledge from many recent studies using knockout mouse and human genetic studies (GWAS and candidate approach) identifying genes and proteins directly involved in these processes, traditional cardiovascular risk factors (hyperlipidemia, hypertension, smoking, diabetes mellitus, sex, and age) remain the most useful predictor of disease. Eicosanoids (20 carbon polyunsaturated fatty acid derivatives of arachidonic acid and other essential fatty acids) are emerging as important regulators of cardiovascular disease processes. Drugs indirectly modulating these signals, including COX-1/COX-2 inhibitors, have proven to play major roles in the atherothrombotic process. However, the complexity of their roles and regulation by opposing eicosanoid signaling, have contributed to the lack of therapies directed at the eicosanoid receptors themselves. This is likely to change, as our understanding of the structure, signaling, and function of the eicosanoid receptors improves. Indeed, a major advance is emerging from the characterization of dysfunctional naturally occurring mutations of the eicosanoid receptors. In light of the proven and continuing importance of risk factors, we have elected to focus on the relationship between eicosanoids and cardiovascular risk factors.

Non-genomic effects of PPARgamma ligands: inhibition of GPVI-stimulated platelet activation

BACKGROUND

Peroxisome proliferator-activated receptor-(gamma) (PPAR(gamma)) is expressed in human platelets although in the absence of genomic regulation in these cells, its functions are unclear.

OBJECTIVE

In the present study, we aimed to demonstrate the ability of PPAR(gamma) ligands to modulate collagen-stimulated platelet function and suppress activation of the glycoprotein VI (GPVI) signaling pathway.

METHODS

Washed platelets were stimulated with PPAR(gamma) ligands in the presence and absence of PPAR(gamma) antagonist GW9662 and collagen-induced aggregation was measured using optical aggregometry. Calcium levels were measured by spectrofluorimetry in Fura-2AM-loaded platelets and tyrosine phosphorylation levels of receptor-proximal components of the GPVI signaling pathway were measured using immunoblot analysis. The role of PPAR(gamma) agonists in thrombus formation was assessed using an in vitro model of thrombus formation under arterial flow conditions.

RESULTS

PPAR(gamma) ligands inhibited collagen-stimulated platelet aggregation that was accompanied by a reduction in intracellular calcium mobilization and P-selectin exposure. PPAR(gamma) ligands inhibited thrombus formation under arterial flow conditions. The incorporation of GW9662 reversed the inhibitory actions of PPAR(gamma) agonists, implicating PPAR(gamma) in the effects observed. Furthermore, PPAR(gamma) ligands were found to inhibit tyrosine phosphorylation levels of multiple components of the GPVI signaling pathway. PPAR(gamma) was found to associate with Syk and LAT after platelet activation. This association was prevented by PPAR(gamma) agonists, indicating a potential mechanism for PPAR(gamma) function in collagen-stimulated platelet activation.

CONCLUSIONS

PPAR(gamma) agonists inhibit the activation of collagen-stimulation of platelet function through modulation of early GPVI signalling.

Targeting PPAR receptors in the airway for the treatment of inflammatory lung disease

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily. PPARgamma regulates several metabolic pathways by binding to sequence-specific PPAR response elements in the promoter region of genes involved in lipid biosynthesis and glucose metabolism. However, more recently PPARgamma, PPARalpha and PPARbeta/delta agonists have been demonstrated to exhibit anti-inflammatory and immunomodulatory properties thus opening up new avenues for research. The actions of PPARgamma and PPARalpha activation are thought to be due to their ability to down regulate pro-inflammatory gene expression and inflammatory cell functions, and as such makes them an attractive target for novel drug intervention. Interestingly, PPARbeta/delta has been shown to be involved in wound healing, angiogenesis, lipid metabolism and thrombosis. In this review we will focus on the data describing the beneficial effects of these ligands in the airway and in the pulmonary vasculature and in vivo in animal models of allergic and occupational asthma, chronic obstructive pulmonary disease and pulmonary fibrosis. A clinical trial is underway to examine the effect of rosiglitazone in asthma patients and the outcome of this trial is awaited with much anticipation. In conclusion, PPARs are novel targets for lung disease and continued work with these ligands may result in a potential new treatment for chronic inflammatory lung diseases.

15-deoxy-delta12,14-PGJ2 enhances platelet production from megakaryocytes

Thrombocytopenia is a critical problem that occurs in many hematologic diseases, as well as after cancer therapy and radiation exposure. Platelet transfusion is the most commonly used therapy but has limitations of alloimmunization, availability, and expense. Thus, the development of safe, small, molecules to enhance platelet production would be advantageous for the treatment of thrombocytopenia. Herein, we report that an important lipid mediator and a peroxisome proliferator-activated receptor gamma (PPARgamma) ligand called 15-deoxy-Delta(12,14) prostaglandin J(2) (15d-PGJ(2)), increases Meg-01 maturation and platelet production. 15d-PGJ(2) also promotes platelet formation from culture-derived mouse and human megakaryocytes and accelerates platelet recovery after in vivo radiation-induced bone marrow injury. Interestingly, the platelet-enhancing effects of 15d-PGJ(2) in Meg-01 cells are independent of PPARgamma, but dependent on reactive oxygen species (ROS) accumulation; treatment with antioxidants such as glutathione ethyl ester (GSH-EE); or N-acetylcysteine (NAC) attenuate 15d-PGJ(2)-induced platelet production. Collectively, these data support the concept that megakaryocyte redox status plays an important role in platelet generation and that small electrophilic molecules may have clinical efficacy for improving platelet numbers in thrombocytopenic patients.