Genetic evidence for a predominant role of PI3Kβ catalytic activity in ITAM- and integrin-mediated signaling in platelets

Platelet endothelial cell adhesion molecule-1 regulates collagen-stimulated platelet function by modulating the association of phosphatidylinositol 3-kinase with Grb-2-associated binding protein-1 and linker for activation of T cells

BACKGROUND

Platelet activation by collagen depends on signals transduced by the glycoprotein (GP)VI-Fc receptor (FcR)γ-chain collagen receptor complex, which involves recruitment of phosphatidylinositol 3-kinase (PI3K) to phosphorylated tyrosines in the linker for activation of T cells (LAT). An interaction between the p85 regulatory subunit of PI3K and the scaffolding molecule Grb-2-associated binding protein-1 (Gab1), which is regulated by binding of the Src homology 2 domain-containing protein tyrosine phosphatase-2 (SHP-2) to Gab1, has been shown in other cell types to sustain PI3K activity to elicit cellular responses. Platelet endothelial cell adhesion molecule-1 (PECAM-1) functions as a negative regulator of platelet reactivity and thrombosis, at least in part by inhibiting GPVI-FcRγ-chain signaling via recruitment of SHP-2 to phosphorylated immunoreceptor tyrosine-based inhibitory motifs in PECAM-1.

OBJECTIVE

To investigate the possibility that PECAM-1 regulates the formation of the Gab1-p85 signaling complexes, and the potential effect of such interactions on GPVI-mediated platelet activation in platelets.

METHODS

The ability of PECAM-1 signaling to modulate the LAT signalosome was investigated with immunoblotting assays on human platelets and knockout mouse platelets.

RESULTS

PECAM-1-associated SHP-2 in collagen-stimulated platelets binds to p85, which results in diminished levels of association with both Gab1 and LAT and reduced collagen-stimulated PI3K signaling. We therefore propose that PECAM-1-mediated inhibition of GPVI-dependent platelet responses result, at least in part, from recruitment of SHP-2-p85 complexes to tyrosine-phosphorylated PECAM-1, which diminishes the association of PI3K with activatory signaling molecules, such as Gab1 and LAT.

Regulation and roles of PI3Kβ, a major actor in platelet signaling and functions

Phosphoinositide 3-kinases (PI3Ks) are important signaling enzymes involved in the regulation of a number of critical cell functions. Significant progress has been made during the last few years in defining the implication of individual PI3K isoforms. The role of the class IA PI3Kβ in different cell types has only been recently uncovered by the use of isoform-selective inhibitors and the development of mouse models harboring p110β catalytic subunit knock-out or germline knock-in of a kinase-dead allele of p110β. Although it is classically admitted that class IA PI3Ks are activated by receptor tyrosine kinases through recruitment of the regulatory subunits to specific tyrosine phosphorylated motifs via their SH2 domains, PI3Kβ is activated downstream of G protein-coupled receptors, and by co-operation between heterotrimeric G proteins and tyrosine kinases. PI3Kβ has been extensively studied in platelets where it appears to play an important role downstream of ITAM signaling, G protein-coupled receptors and aIIbβ3 integrin. Accordingly, mouse exhibiting p110β inactivation selectively in megakaryocyte/platelets are resistant to thromboembolism induced by carotid injury. The present review summarizes recent data concerning the mechanisms of PI3Kβ regulation and the roles of this PI3K isoform in blood platelet functions and other cell types.

Src family kinases are essential for primary aggregation by G(i) -coupled receptors

INTRODUCTION AND BACKGROUND

Adrenaline stimulates biphasic aggregation in plasma through the G(i) -coupled α(2A) -adrenoreceptor. In the present study, we demonstrate that both primary and secondary wave aggregation induced by adrenaline in plasma is blocked by two structurally distinct inhibitors of Src family kinases, dasatinib and PD0173952.

METHODS AND RESULTS

In contrast, primary aggregation is partially inhibited or unaffected in the presence of inhibitors of cyclo-oxygenase, phosphoinositide (PI) 3-kinases, and P2Y(1) and P2Y(12) ADP receptors, although secondary aggregation is abolished. The ability of adrenaline to inhibit adenylyl cyclase and to synergize with platelet agonists in mediating platelet activation in plasma is retained in the presence of Src family kinase inhibition. Moreover, adrenaline does not activate Src family kinases, as determined by western blotting of their regulatory tyrosines, suggesting that constitutive signaling from Src family kinases may underlie their role in activation. Adrenaline is widely used in clinical laboratories for investigation of patients with suspected bleeding disorders. In a group of 90 unrelated patients with a clinically diagnosed platelet bleeding disorder, we identified four who did not exhibit primary wave aggregation in response to adrenaline, although the catecholamine potentiated the response to other agonists, and five who failed to undergo secondary wave aggregation. In contrast, adrenaline stimulated biphasic aggregation in 60 controls. All of the patients with a defective response to adrenaline had impaired ADP-induced platelet activation.

CONCLUSIONS

The present results indicate a previously unappreciated role for Src family kinases in mediating G(i) signaling in plasma, and demonstrate heterogeneity in response to adrenaline in patients with a clinically diagnosed platelet disorder.

Role of phosphoinositide 3-kinase beta in platelet aggregation and thromboxane A2 generation mediated by Gi signalling pathways

PI3Ks (phosphoinositide 3-kinases) play a critical role in platelet functional responses. PI3Ks are activated upon P2Y12 receptor stimulation and generate pro-aggregatory signals. P2Y12 receptor has been shown to play a key role in the platelet aggregation and thromboxane A2 generation caused by co-stimulation with Gq or Gz, or super-stimulation of Gi pathways. In the present study, we evaluated the role of specific PI3K isoforms alpha, beta, gamma and delta in platelet aggregation, thromboxane A2 generation and ERK (extracellular-signal-regulated kinase) activation. Our results show that loss of the PI3K signal impaired the ability of ADP to induce platelet aggregation, ERK phosphorylation and thromboxane A2 generation. We also show that Gq plus Gi- or Gi plus Gz-mediated platelet aggregation, ERK phosphorylation and thromboxane A2 generation in human platelets was inhibited by TGX-221, a PI3Kbeta-selective inhibitor, but not by PIK75 (a PI3Kalpha inhibitor), AS252424 (a PI3Kgamma inhibitor) or IC87114 (a PI3Kdelta inhibitor). TGX-221 also showed a similar inhibitory effect on the Gi plus Gz-mediated platelet responses in platelets from P2Y1-/- mice. Finally, 2MeSADP (2-methyl-thio-ADP)-induced Akt phosphorylation was significantly inhibited in the presence of TGX-221, suggesting a critical role for PI3Kbeta in Gi-mediated signalling. Taken together, our results demonstrate that PI3Kbeta plays an important role in ADP-induced platelet aggregation. Moreover, PI3Kbeta mediates ADP-induced thromboxane A2 generation by regulating ERK phosphorylation.

Genome-wide meta-analyses identifies seven loci associated with platelet aggregation in response to agonists

Platelet function mediates both beneficial and harmful effects on human health, but few genes are known to contribute to variability in this process. We tested association of 2.5 million SNPs with platelet aggregation responses to three agonists (ADP, epinephrine and collagen) in two cohorts of European ancestry (N Collapse

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MESH Headings

• Adenosine Diphosphate/pharmacology
• Adrenergic alpha-Agonists/pharmacology
• Cohort Studies
• Collagen/pharmacology
• Epinephrine/pharmacology
• Female
• Genome-Wide Association Study
• Genotype
• Humans
• Male
• Meta-Analysis as Topic
• Phenotype
• Platelet Aggregation/drug effects
• Platelet Aggregation/genetics
• Polymorphism, Single Nucleotide
• Prospective Studies

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Grants

• R01 HL087698 NHLBI NIH HHS
• R01 HL087698-03 NHLBI NIH HHS
• M01 RR000052-46 NCRR NIH HHS
• N01 HC025195 NHLBI NIH HHS
• N02 HL64278 NHLBI NIH HHS
• R01 HL048157 NHLBI NIH HHS
• M01-RR000052 NCRR NIH HHS
• M01 RR000052 NCRR NIH HHS
• U01 HL72518 NHLBI NIH HHS
• U01 HL072518 NHLBI NIH HHS
• N02-HL-6-4278 NHLBI NIH HHS
• N01-HC-25195 NHLBI NIH HHS
• R01 HL087698-01 NHLBI NIH HHS
• N01HC25195 NHLBI NIH HHS
• Z99 HL999999 Intramural NIH HHS
• U01 HL072518-06 NHLBI NIH HHS
• M01 RR005280-08 NCRR NIH HHS

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Affiliation(s)

Andrew D Johnson National Heart, Lung, and Blood Institute's The Framingham Heart Study, Framingham, Massachusetts, USA.
Lisa R Yanek
Ming-Huei Chen
Nauder Faraday
Martin G Larson
Geoffrey Tofler
Shiow J Lin
Aldi T Kraja
Michael A Province
Qiong Yang
Diane M Becker
Christopher J O'Donnell
Lewis C Becker

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A constitutively activated form of the p110beta isoform of PI3-kinase induces prostatic intraepithelial neoplasia in mice

Recent work has shown that ablation of p110beta, but not p110alpha, markedly impairs tumorigenesis driven by loss of phosphatase and tensin homolog (PTEN) in the mouse prostate. Other laboratories have reported complementary data in human prostate tumor lines, suggesting that p110beta activation is necessary for tumorigenesis driven by PTEN loss. Given the multiple functions of PTEN, we wondered if p110beta activation also is sufficient for tumorigenesis. Here, we report that transgenic expression of a constitutively activated p110beta allele in the prostate drives prostate intraepithelial neoplasia formation. The resulting lesions are similar to, but are clearly distinct from, the ones arising from PTEN loss or Akt activation. Array analyses of transcription in multiple murine prostate tumor models featuring PI3K/AKT pathway activation allowed construction of a pathway signature that may be useful in predicting the prognosis of human prostate tumors.

Salvianolic acid A inhibits platelet activation and arterial thrombosis via inhibition of phosphoinositide 3-kinase

BACKGROUND AND OBJECTIVE

Salvianolic acid A (SAA) is a water-soluble component from the root of Salvia miltiorrhiza Bunge, a herb that is widely used for atherothrombotic disease treatment in Asian medicine. As platelets play pivotal roles in atherothrombogenesis, we studied the effect of SAA on platelet activation and its underlying mechanisms.

METHODS AND RESULTS

SAA dose-dependently inhibited platelet aggregation induced by ADP, thrombin, collagen and U46619. It reduced ADP-enhanced platelet P-selectin expression and fibrinogen binding, which consequently hampered ADP-induced platelet-leukocyte aggregation. SAA also inhibited platelet spreading on fibrinogen, a process mediated by outside-in signaling. Under an arterial shear rate of 1000 s(-1), SAA decreased platelet adhesion on collagen surfaces by approximately 40%. Western blot analysis showed that SAA, like the phosphoinositide 3-kinase (PI3K) inhibitors LY294002 and TGX-221, potently inhibited PI3K, as shown by reduced Akt phosphorylation. The in vitro findings were further evaluated in the mouse model of arterial thrombosis, in which SAA prolonged the mesenteric arterial occlusion time in wild-type mice (35 + or - 2 min without SAA and 56 + or - 4 min with SAA; P < 0.01). Interestingly, SAA could even counteract the shortened arterial occlusion time in Ldlr(tm1Her) mutant mice (21 + or - 2 min without SAA and 45 + or - 4 min with SAA; P < 0.01).

CONCLUSIONS

SAA inhibits platelet activation via the inhibition of PI3K, and attenuates arterial thrombus formation in vivo. Our data suggest that SAA may be developed as a novel therapeutic agent for the prevention of thrombotic disorders.

PI3K inhibition in inflammation: Toward tailored therapies for specific diseases

In the past decade, the availability of genetically modified animals has enabled the discovery of interesting roles for phosphatidylinositol 3-kinase-gamma (PI3Kgamma) and -delta (PI3Kdelta) in different cell types orchestrating innate and adaptive immune responses. Therefore, these PI3K isoforms appear to be attractive drug targets for the treatment of diseases caused by unrestrained immune reactions. Currently, pharmacological targeting of PI3Kgamma and/or PI3Kdelta represents one of the most promising challenges for companies interested in the development of novel safe treatments for inflammatory diseases. In this review we provide a general outline of PI3Kgamma- and PI3Kdelta-specific functions in distinct subsets of inflammatory cells. We also discuss the therapeutic impact of novel compounds targeting PI3Kgamma, PI3Kdelta or both, in mouse models of autoimmune disorders (systemic lupus erythematosus (SLE) and rheumatoid arthritis), respiratory diseases (allergic asthma and chronic obstructive pulmonary disease) and cardiovascular dysfunctions (atherosclerosis and myocardial infarction).

Deletion of the p110β isoform of phosphoinositide 3-kinase in platelets reveals its central role in Akt activation and thrombus formation in vitro and in vivo

During platelet activation, phosphoinositide 3-kinases (PI3Ks) produce lipid second messengers participating in the regulation of functional responses. Here, we generated a megakaryocyte-restricted p110β null mouse model and demonstrated a critical role of PI3Kβ in platelet activation via an immunoreceptor tyrosine-based activation motif, the glyco-protein VI-Fc receptor γ-chain complex, and its contribution in response to G-protein–coupled receptors. Interestingly, the production of phosphatidylinositol 3,4,5-trisphosphate and the activation of protein kinase B/Akt were strongly inhibited in p110β null platelets stimulated either via immunoreceptor tyrosine-based activation motif or G-protein–coupled receptors. Functional studies showed an important delay in fibrin clot retraction and an almost complete inability of these platelets to adhere onto fibrinogen under flow condition, suggesting that PI3Kβ is also acting downstream of αIIbβ3. In vivo studies showed that these mice have a normal bleeding time and are not protected from acute pulmonary thromboembolism but are resistant to thrombosis after FeCl3 injury of the carotid, suggesting that PI3Kβ is a potential target for antithrombotic drugs.

PI3Kgamma-dependent signaling in mouse olfactory receptor neurons

Phosphatidylinositol 3-kinase (PI3K)-dependent signaling couples to receptors for many different ligands in diverse cellular systems. Recent findings suggest that PI3K-dependent signaling also mediates inhibition of odorant responses in rat olfactory receptor neurons (ORNs). Here, we present evidence that murine ORNs show PI3K-dependent calcium responses to odorant stimulation, they express 2 G protein-coupled receptor (GPCR)-activated isoforms of PI3K, PI3Kbeta and PI3Kgamma, and they exhibit odorant-induced PI3K activity. These findings support our use of a transgenic mouse model to begin to investigate the mechanisms underlying PI3K-mediated inhibition of odorant responses in mammalian ORNs. Mice deficient in PI3Kgamma, a class IB PI3K that is activated via GPCRs, lack detectable odorant-induced PI3K activity in their olfactory epithelium and their ORNs are less sensitive to PI3K inhibition. We conclude that odorant-dependent PI3K signaling generalizes to the murine olfactory system and that PI3Kgamma plays a role in mediating inhibition of odorant responses in mammalian ORNs.

Essential role of the p110beta subunit of phosphoinositide 3-OH kinase in male fertility

Phosphoinositide 3-kinases (PI3K) are key molecular players in male fertility. However, the specific roles of different p110 PI3K catalytic subunits within the spermatogenic lineage have not been characterized so far. Herein, we report that male mice expressing a catalytically inactive p110beta develop testicular hypotrophy and impaired spermatogenesis, leading to a phenotype of oligo-azoospermia and defective fertility. The examination of testes from p110beta-defective tubules demonstrates a widespread loss in spermatogenic cells, due to defective proliferation and survival of pre- and postmeiotic cells. In particular, p110beta is crucially needed in c-Kit-mediated spermatogonial expansion, as c-Kit-positive cells are lost in the adult testis and activation of Akt by SCF is blocked by a p110beta inhibitor. These data establish that activation of the p110beta PI3K isoform by c-Kit is required during spermatogenesis, thus opening the way to new treatments for c-Kit positive testicular cancers.

PI 3-Kinase p110β regulation of platelet integrin α(IIb)β3

Hemopoietic cells express relatively high levels of the type I phosphoinositide (PI) 3-kinase isoforms, with p110δ and γ exhibiting specialized signaling functions in neutrophils, monocytes, mast cells, and lymphocytes. In platelets, p110β appears to be the dominant PI 3-kinase isoform regulating platelet activation, irrespective of the nature of the primary platelet activating stimulus. Based on findings with isoform-selective p110β pharmacological inhibitors and more recently with p110β-deficient platelets, p110β appears to primarily signal downstream of G(i)- and tyrosine kinase-coupled receptors. Functionally, inhibition of p110β kinase function leads to a marked defect in integrin α(IIb)β₃ adhesion and reduced platelet thrombus formation in vivo. This defect in platelet adhesive function is not associated with increased bleeding, suggesting that therapeutic targeting of p110β may represent a safe approach to reduce thrombotic complications in patients with cardiovascular disease.

Non-redundant roles of phosphoinositide 3-kinase isoforms alpha and beta in glycoprotein VI-induced platelet signaling and thrombus formation

Platelets are activated by adhesion to vascular collagen via the immunoglobulin receptor, glycoprotein VI (GPVI). This causes potent signaling toward activation of phospholipase Cgamma2, which bears similarity to the signaling pathway evoked by T- and B-cell receptors. Phosphoinositide 3-kinase (PI3K) plays an important role in collagen-induced platelet activation, because this activity modulates the autocrine effects of secreted ADP. Here, we identified the PI3K isoforms directly downstream of GPVI in human and mouse platelets and determined their role in GPVI-dependent thrombus formation. The targeting of platelet PI3Kalpha or -beta strongly and selectively suppressed GPVI-induced Ca(2+) mobilization and inositol 1,4,5-triphosphate production, thus demonstrating enhancement of phospholipase Cgamma2 by PI3Kalpha/beta. That PI3Kalpha and -beta have a non-redundant function in GPVI-induced platelet activation and thrombus formation was concluded from measurements of: (i) serine phosphorylation of Akt, (ii) dense granule secretion, (iii) intracellular Ca(2+) increases and surface expression of phosphatidylserine under flow, and (iv) thrombus formation, under conditions where PI3Kalpha/beta was blocked or p85alpha was deficient. In contrast, GPVI-induced platelet activation was insensitive to inhibition or deficiency of PI3Kdelta or -gamma. Furthermore, PI3Kalpha/beta, but not PI3Kgamma, contributed to GPVI-induced Rap1b activation and, surprisingly, also to Rap1b-independent platelet activation via GPVI. Together, these findings demonstrate that both PI3Kalpha and -beta isoforms are required for full GPVI-dependent platelet Ca(2+) signaling and thrombus formation, partly independently of Rap1b. This provides a new mechanistic explanation for the anti-thrombotic effect of PI3K inhibition and makes PI3Kalpha an interesting new target for anti-platelet therapy.

Phosphoinositide 3-kinase p110 beta regulates integrin alpha IIb beta 3 avidity and the cellular transmission of contractile forces

Phosphoinositide (PI) 3-kinase (PI3K) signaling processes play an important role in regulating the adhesive function of integrin alpha(IIb)beta(3), necessary for platelet spreading and sustained platelet aggregation. PI3K inhibitors are effective at reducing platelet aggregation and thrombus formation in vivo and as a consequence are currently being evaluated as novel antithrombotic agents. PI3K regulation of integrin alpha(IIb)beta(3) activation (affinity modulation) primarily occurs downstream of G(i)-coupled and tyrosine kinase-linked receptors linked to the activation of Rap1b, AKT, and phospholipase C. In the present study, we demonstrate an important role for PI3Ks in regulating the avidity (strength of adhesion) of high affinity integrin alpha(IIb)beta(3) bonds, necessary for the cellular transmission of contractile forces. Using knock-out mouse models and isoform-selective PI3K inhibitors, we demonstrate that the Type Ia p110 beta isoform plays a major role in regulating thrombin-stimulated fibrin clot retraction in vitro. Reduced clot retraction induced by PI3K inhibitors was not associated with defects in integrin alpha(IIb)beta(3) activation, actin polymerization, or actomyosin contractility but was associated with a defect in integrin alpha(IIb)beta(3) association with the contractile cytoskeleton. Analysis of integrin alpha(IIb)beta(3) adhesion contacts using total internal reflection fluorescence microscopy revealed an important role for PI3Ks in regulating the stability of high affinity integrin alpha(IIb)beta(3) bonds. These studies demonstrate an important role for PI3K p110 beta in regulating the avidity of high affinity integrin alpha(IIb)beta(3) receptors, necessary for the cellular transmission of contractile forces. These findings may provide new insight into the potential antithrombotic properties of PI3K p110 beta inhibitors.

PI3Kbeta goes to the head of its class

In this issue of Blood, Canobbio and colleagues report on the unique and redundant contributions of 2 isoforms of PI3K, PI3Kbeta and PI3Kgamma, to platelet activation by GPCRs and ITAM-coupled receptors.

Role of phosphoinositide 3-kinase beta in glycoprotein VI-mediated Akt activation in platelets

Glycoprotein (GP) VI is a critical platelet collagen receptor. Phosphoinositide 3-kinase (PI3K) plays an important role in GPVI-mediated platelet activation, yet the major PI3K isoforms involved in this process have not been identified. In addition, stimulation of GPVI results in the activation of Akt, a downstream effector of PI3K. Thus, we investigated the contribution of PI3K isoforms to GPVI-mediated platelet activation and Akt activation. A protein kinase C inhibitor GF 109203X or a P2Y(12) receptor antagonist AR-C69931MX partly reduced GPVI-induced Akt phosphorylation. Platelets from mice dosed with clopidogrel also showed partial Akt phosphorylation, indicating that GPVI-mediated Akt phosphorylation is regulated by both secretion-dependent and -independent pathways. In addition, GPVI-induced Akt phosphorylation in the presence of ADP antagonists was completely inhibited by PI3K inhibitor LY294002 and PI3Kbeta inhibitor TGX-221 indicating an essential role of PI3Kbeta in Akt activation directly downstream of GPVI. Moreover, GPVI-mediated platelet aggregation, secretion, and intracellular Ca(2+) mobilization were significantly inhibited by TGX-221, and less strongly inhibited by PI3Kalpha inhibitor PIK75, but were not affected by PI3Kgamma inhibitor AS252424 and PI3Kdelta inhibitor IC87114. Consistently, GPVI-induced integrin alpha(IIb)beta(3) activation of PI3Kgamma(-/-) and PI3Kdelta(-/-) platelets also showed no significant difference compared with wild-type platelets. These results demonstrate that GPVI-induced Akt activation in platelets is dependent in part on G(i) stimulation through P2Y(12) receptor activation by secreted ADP. In addition, a significant portion of GPVI-dependent, ADP-independent Akt activation also exists, and PI3Kbeta plays an essential role in GPVI-mediated platelet aggregation and Akt activation.