Phosphoinositide 3-kinase p110α negatively regulates thrombopoietin-mediated platelet activation and thrombus formation.

Phosphoinositide 3-kinase (PI3K) plays an important role in platelet function and contributes to platelet hyperreactivity induced by elevated levels of circulating peptide hormones, including thrombopoietin (TPO). Previous work established an important role for the PI3K isoform; p110ß in platelet function, however the role of p110α is still largely unexplored. Here we sought to investigate the role of p110α in TPO-mediated hyperactivity by using a conditional p110α knockout (KO) murine model in conjunction with platelet functional assays. We found that TPO-mediated enhancement of collagen-related peptide (CRP-XL)-induced platelet aggregation and adenosine triphosphate (ATP) secretion were significantly increased in p110α KO platelets. Furthermore, TPO-mediated enhancement of thrombus formation by p110α KO platelets was elevated over wild-type (WT) platelets, suggesting that p110α negatively regulates TPO-mediated priming of platelet function. The enhancements were not due to increased flow through the PI3K pathway as phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) formation and phosphorylation of Akt and glycogen synthase kinase 3 (GSK3) were comparable between WT and p110α KO platelets. In contrast, extracellular responsive kinase (ERK) phosphorylation and thromboxane (TxA2) formation were significantly enhanced in p110α KO platelets, both of which were blocked by the MEK inhibitor PD184352, whereas the p38 MAPK inhibitor VX-702 and p110α inhibitor PIK-75 had no effect. Acetylsalicylic acid (ASA) blocked the enhancement of thrombus formation by TPO in both WT and p110α KO mice. Together, these results demonstrate that p110α negatively regulates TPO-mediated enhancement of platelet function by restricting ERK phosphorylation and TxA2 synthesis in a manner independent of its kinase activity.

Identification of roles for the SNARE-associated protein, SNAP29, in mouse platelets.

Platelets are critical for maintaining vascular hemostasis, but also play a major role in the formation of occlusive cardiovascular and cerebrovascular thrombi under disease conditions. Secretion of platelet alpha and dense granules is a requirement for efficient thrombus formation. Understanding and targeting the mechanisms of secretion is important to aid the development of effective antithrombotics. SNAP29 is a tSNARE found in platelets, but whose role has not been defined. Using a platelet-specific SNAP29 knockout mouse model, we assessed the role of SNAP29 in platelet secretion and function under standardized conditions and also in in vitro and in vivo thrombosis. The data showed no major defects in SNAP29-null platelets, but revealed a minor defect in α-granule secretion and a significant increase in embolization rate of thrombi in vivo. These data suggest that SNAP29 contributes to the regulation of platelet α-granule secretion and thrombus stability, possibly partially masked by functional redundancy with other tSNAREs, such as SNAP23.

Circulating primers enhance platelet function and induce resistance to antiplatelet therapy.

BACKGROUND: Aspirin and P2Y12 antagonists are antiplatelet compounds that are used clinically in patients with thrombosis. However, some patients are 'resistant' to antiplatelet therapy, which increases their risk of developing acute coronary syndromes. These patients often present with an underlying condition that is associated with altered levels of circulating platelet primers and platelet hyperactivity. Platelet primers cannot stimulate platelet activation, but, in combination with physiologic stimuli, significantly enhance platelet function. OBJECTIVES: To explore the role of platelet primers in resistance to antiplatelet therapy, and to evaluate whether phosphoinositide 3-kinase (PI3K) contributes to this process. METHODS AND RESULTS: We used platelet aggregation, thromboxane A2 production and ex vivo thrombus formation as functional readouts of platelet activity. Platelets were treated with the potent P2Y12 inhibitor AR-C66096, aspirin, or a combination of both, in the presence or absence of the platelet primers insulin-like growth factor-1 (IGF-1) and thrombopoietin (TPO), or the Gz-coupled receptor ligand epinephrine. We found that platelet primers largely overcame the inhibitory effects of antiplatelet compounds on platelet functional responses. IGF-1-mediated and TPO-mediated, but not epinephrine-mediated, enhancements in the presence of antiplatelet drugs were blocked by the PI3K inhibitors wortmannin and LY294002. CONCLUSIONS: These results demonstrate that platelet primers can contribute to antiplatelet resistance. Furthermore, our data demonstrate that there are PI3K-dependent and PI3K-independent mechanisms driving primer-mediated resistance to antiplatelet therapy.

Cytohesin-2 phosphorylation by protein kinase C relieves the constitutive suppression of platelet dense granule secretion by ADP-ribosylation factor 6.

BACKGROUND: Protein kinase C (PKC) is a major regulator of platelet function and secretion. The underlying molecular pathway from PKC to secretion, however, is poorly understood. By a proteomics screen we identified the guanine nucleotide exchange factor cytohesin-2 as a candidate PKC substrate. OBJECTIVES: We aimed to validate cytohesin-2 as a PKC substrate in platelets and to determine its role in granule secretion and other platelet responses. METHODS AND RESULTS: Immunoprecipitation was performed with a phosphoserine PKC substrate antibody followed by mass spectrometry, leading to the identification of cytohesin-2. By western blotting we showed that different agonists induced cytohesin-2 phosphorylation by PKC. Protein function was investigated using a pharmacological approach. The cytohesin inhibitor SecinH3 significantly enhanced platelet dense granule secretion and aggregation, as measured by lumi-aggregometry. Flow cytometry data indicate that α-granule release and integrin αII b ß3 activation were not affected by cytohesin-2 inhibition. Lysosome secretion was assessed by a colorimetric assay and was also unchanged. As shown by western blotting, ARF6 interacted with cytohesin-2 and was present in an active GTP-bound form under basal conditions. Upon platelet stimulation, this interaction was largely lost and ARF6 activation decreased, both of which could be rescued by PKC inhibition. CONCLUSIONS: Cytohesin-2 constitutively suppresses platelet dense granule secretion and aggregation by keeping ARF6 in a GTP-bound state. PKC-mediated phosphorylation of cytohesin-2 relieves this inhibitory effect, thereby promoting platelet secretion and aggregation.

Protein kinase C and P2Y12 take center stage in thrombin-mediated activation of mammalian target of rapamycin complex 1 in human platelets.

BACKGROUND: Rapamycin, an inhibitor of mammalian target of rapamycin complex-1 (mTORC1), reduces platelet spreading, thrombus stability, and clot retraction. Despite an important role of mTORC1 in platelet function, little is known about how it is regulated. The objective of this study was to determine the signaling pathways that regulate mTORC1 in human platelets. METHODS: Mammalian target of rapamycin complex-1 activation was assessed by measuring the phosphorylation of its downstream substrate ribosomal S6 kinase 1 (p70S6K). RESULTS: Thrombin or the protein kinase C (PKC) activator phorbal 12-myristate 13-acetate stimulated activation of mTORC1 in a PKC-dependent, Akt-independent manner that correlated with phosphorylation of tuberin/tuberous sclerosis 2 (TSC2) (Ser939 and Thr1462). In contrast, insulin-like growth factor 1 (IGF-1)-stimulated TSC2 phosphorylation was completely dependent on phosphoinositide 3 kinase (PI3 kinase)/Akt but did not result in any detectable mTORC1 activation. Early (Ser939 and Thr1462) and late (Thr1462) TSC2 phosphorylation in response to thrombin were directly PKC dependent, whereas later TSC2 (Ser939) and p70S6K phosphorylation were largely dependent on paracrine signaling through P2Y(12). PKC-mediated adenosine diphosphate (ADP) secretion was essential for thrombin-stimulated mTORC1 activation, as (i) ADP rescued p70S6K phosphorylation in the presence of a PKC inhibitor and (ii) P2Y(12) antagonism prevented thrombin-mediated mTORC1 activation. Rescue of mTORC1 activation with exogenous ADP was completely dependent on the Src family kinases but independent of PI3 kinase/Akt. Interestingly, although inhibition of Src blocked the ADP rescue, it had little effect on thrombin-stimulated p70S6K phosphorylation under conditions where PKC was not inhibited. CONCLUSION: These results demonstrate that thrombin activates the mTORC1 pathway in human platelets through PKC-mediated ADP secretion and subsequent activation of P2Y(12), in a manner largely independent of the canonical PI3 kinase/Akt pathway.

Insulin-like growth factor-II is produced by, signals to and is an important survival factor for the mature podocyte in man and mouse.

Podocytes are crucial for preventing the passage of albumin into the urine and, when lost, are associated with the development of albuminuria, renal failure and cardiovascular disease. Podocytes have limited capacity to regenerate, therefore pro-survival mechanisms are critically important. Insulin-like growth factor-II (IGF-II) is a potent survival and growth factor; however, its major function is thought to be in prenatal development, when circulating levels are high. IGF-II has only previously been reported to continue to be expressed in discrete regions of the brain into adulthood in rodents, with systemic levels being undetectable. Using conditionally immortalized human and ex vivo adult mouse cells of the glomerulus, we demonstrated the podocyte to be the major glomerular source and target of IGF-II; it signals to this cell via the IGF-I receptor via the PI3 kinase and MAPK pathways. Functionally, a reduction in IGF signalling causes podocyte cell death in vitro and glomerular disease in vivo in an aged IGF-II transgenic mouse that produces approximately 60% of IGF-II due to a lack of the P2 promoter of this gene. Collectively, this work reveals the fundamental importance of IGF-II in the mature podocyte for glomerular health across mammalian species.

Insulin/IGF-1 hybrid receptor expression on human platelets: consequences for the effect of insulin on platelet function.

OBJECTIVES: As platelets express both insulin and insulin-like growth factor-1 (IGF-1) receptors, their subunits may randomly heterodimerize to form insulin/IGF-1 receptor hybrids, which avidly bind IGF-1, but not insulin. This study investigated the possibility that platelets express hybrid receptors, which may affect insulin action on platelet function. METHODS: Platelets were incubated with insulin and IGF-1. Expression and phosphorylation of insulin/IGF-1 receptors was determined by western blotting of immunoprecipitates, and compared with platelet functional responses. Relative expression of insulin and IGF-1 receptors was estimated by competitive ligand binding and quantitative polymerase chain reaction. RESULTS: We demonstrated the presence of insulin/IGF-1 hybrid receptors on human platelets by detecting both insulin and IGF-1 receptor beta subunits in coimmunoprecipitation studies. Stimulation of platelets with insulin (1-100 nm) resulted in tyrosine phosphorylation of insulin receptors, but not of hybrid receptors. High insulin concentrations (50-100 nm) stimulated weak phosphorylation of IGF-1 receptors and protein kinase B (Akt), and correlated with moderately increased aggregation and fibrinogen binding, whereas low insulin concentrations (1-10 nm) had no effect. In contrast, IGF-1 (1-100 nm) induced strong phosphorylation of both hybrid and IGF-1 receptors, and potentiated platelet aggregation and fibrinogen binding. Specific binding of [(125)I]IGF-1 (1.08% +/- 0.16%) was significantly higher than that of [(125)I]insulin (0.15% +/- 0.03%). Accordingly, IGF-1 receptor mRNA was more abundant than insulin receptor mRNA (IGF-1 receptor/insulin receptor ratio 69 +/- 3.8). CONCLUSIONS: Insulin has minimal effects on platelet function, which can be explained by the relatively low insulin receptor expression levels resulting in the majority of insulin receptor subunits being expressed as insulin/IGF-1 hybrids.

The PKB inhibitor Akti-1/2 potentiates PAR-1-mediated platelet function independently of its ability to block PKB.

BACKGROUND: The role of PKB in platelet function is poorly defined due to the lack of genuinely selective small-molecule inhibitors and limiting genetic models. Recently, a selective, non-ATP-competitive PKB inhibitor, Akti-1/2 has been reported, but the efficacy and specificity against PKB activation in platelet function is unknown. OBJECTIVE AND METHODS: To determine the effect of the PKB inhibitor Akti-1/2 on PKB activation and platelet function by Western blotting and aggregometry/flow cytometry, respectively. RESULTS: Akti-1/2 potently inhibited thrombin-mediated PKB phosphorylation on Thr(308) and Ser(473) and phosphorylation of its downstream substrate GSK3beta, with a negligible effect on the phosphorylation of pleckstrin, p38, ERK and JNK. Surprisingly, Akti-1/2 strongly potentiated PAR-1-mediated platelet aggregation. This effect persisted in the presence of PI3 kinase inhibitors, indicating a mechanism of action that is independent of PKB. Potentiation of aggregation by Akti-1/2 was associated with increased [Ca(++)](i), PKC activation and degranulation and was ablated by agents that antagonized this pathway. CONCLUSIONS: The PKB inhibitor Akti-1/2 increases PAR-1-mediated platelet responses in a PKB-independent, Ca(++)/PKC-dependent manner. This effect is strong and rapid and may impact on the therapeutic application of Akti-1/2 and structurally related compounds.

Regulation of small GTP-binding proteins by insulin.

Several members of the extensive family of small GTP-binding proteins are regulated by insulin, and have been implicated in insulin action on glucose uptake. These proteins are themselves negatively regulated by a series of specific GAPs (GTPase-activating proteins). Interestingly, there is increasing evidence to suggest that PKB (protein kinase B)-dependent phosphorylation of some GAPs may relieve this negative regulation and so lead to the activation of the target small GTP-binding protein. We review recent evidence that this may be the case, and place specific emphasis on the role of these pathways in insulin-stimulated glucose uptake.

Role of protein kinase B in insulin-regulated glucose uptake.

The activation of protein kinase B (or Akt) plays a central role in the stimulation of glucose uptake by insulin. Currently, however, numerous questions remain unanswered regarding the role of this kinase in bringing about this effect. For example, we do not know precisely where in the GLUT4 trafficking pathway this kinase acts. Nor do we know which protein substrates are responsible for mediating the effects of protein kinase B, although two recently identified proteins (AS160 and PIKfyve) may play a role. This paper addresses these important questions by reviewing recent progress in the field.

Evaluation of soil gas sampling and analysis techniques at a former petrochemical plant site.

Methods for soil gas sampling and analysis are evaluated as part of a research study on soil vapour intrusion into buildings, conducted at a former petro-chemical plant site ("Chatterton site"). The evaluation process was designed to provide information on reliability and selection of appropriate methods for soil gas sampling and analysis, and was based on a literature review of data and methods, and experiments completed as part of the research study. The broader context of this work is that soil gas characterization is increasingly being used for input into risk assessment of contaminated sites, particularly when evaluating the potential intrusion of soil vapour into buildings. There are only a limited number of research studies and protocols addressing soil gas sampling and analysis. There is significant variability in soil gas probe design and sample collection and analysis methods used by practitioners. The experimental studies conducted to evaluate soil gas methods address the permeation or leakage of gases from Tedlar bags, time-dependent sorption of volatile organic compound (VOC)-vapours onto probe surfaces and sampling devices, and analytical and quality control issues for light gas and VOC analyses. Through this work, common techniques for soil gas collection and analysis are described together with implications for data quality arising from the different methods used. Some of the potential pitfalls that can affect soil gas testing are identified, and recommendations and guidance for improved protocols are provided.

The use of indoor air measurements to evaluate intrusion of subsurface VOC vapors into buildings.

The implementation of a risk-based corrective action approach often requires consideration of soil vapor migration into buildings and potential inhalation exposure and risk to human health. Due to the uncertainty associated with models for this pathway, there may be a desire to analyze indoor air samples to validate model predictions, and this approach is followed on a somewhat frequent basis at sites where risks are considered potentially significant. Indoor air testing can be problematic for a number of reasons. Soil vapor intrusion into buildings is complex, highly dependent on site-specific conditions, and may vary over time, complicating the interpretation of indoor air measurements when the goal is to deduce the subsurface-derived component. An extensive survey of indoor air quality data sets highlights the variability in indoor volatile organic compound (VOC) concentrations and numerous sources that can lead to elevated VOC levels. The contribution from soil vapor is likely to be small relative to VOCs from other sources for most sites. In light of these challenges, we discuss how studies that use indoor air testing to assess subsurface risks could be improved. To provide added perspective, we conclude by comparing indoor air concentrations and risks arising from subsurface VOCs, predicted using standard model equations for soil vapor fate and intrusion into buildings, to those associated with indoor sources.

Fyn and Lyn phosphorylate the Fc receptor gamma chain downstream of glycoprotein VI in murine platelets, and Lyn regulates a novel feedback pathway.

Activation of platelets by collagen is mediated by the complex glycoprotein VI (GPVI)/Fc receptor gamma (FcR gamma chain). In the current study, the role of 2 Src family kinases, Fyn and Lyn, in GPVI signaling has been examined using murine platelets deficient in one or both kinases. In the fyn(-/-) platelets, tyrosine phosphorylation of FcR gamma chain, phopholipase C (PLC) activity, aggregation, and secretion are reduced, though the time of onset of response is unchanged. In the lyn(-/-) platelets, there is a delay of up to 30 seconds in the onset of tyrosine phosphorylation and functional responses, followed by recovery of phosphorylation and potentiation of aggregation and alpha-granule secretion. Tyrosine phosphorylation and aggregation in response to stimulation by collagen-related peptide is further attenuated and delayed in fyn(-/-)lyn(-/-) double-mutant platelets, and potentiation is not seen. This study provides the first genetic evidence that Fyn and Lyn mediate FcR immune receptor tyrosine-based activation motif phosphorylation and PLC gamma 2 activation after the ligation of GPVI. Lyn plays an additional role in inhibiting platelet activation through an uncharacterized inhibitory pathway. (Blood. 2000;96:4246-4253)

Inhibition of platelet integrin alpha(IIb)beta(3) by peptides that interfere with protein kinases and the beta(3) tail.

alpha-Thrombin stimulation of human platelets initiates inside-out signaling to integrin alpha(IIb)beta(3) (glycoprotein IIb/IIIa), resulting in the exposure of ligand binding sites. In the present study, the regulation of alpha(IIb)beta(3) via protein kinases was investigated in platelets permeabilized with streptolysin O by introducing peptides that interfere with these enzymes and with possible regulatory domains in the cytosolic tail of the beta(3) subunit. Compared with intact platelets, the permeabilized platelets preserved >80% of the aggregation, secretion, and alpha(IIb)beta(3) ligand binding capacity. The peptide YIYGSFK, a substrate for Src kinases, inhibited alpha-thrombin-induced ligand binding to alpha(IIb)beta(3), but a reversed peptide with Y-->F substitutions (KFSGFIF) had no effect. Ligand binding to alpha(IIb)beta(3) was also inhibited by the peptide RKRCLRRL, which binds irreversibly to the catalytic domain of protein kinase C. Peptides corresponding to parts of the protein C inhibitor and beta(2)-glycoprotein I were used as negative controls and failed to interfere with ligand binding. Possible target domains for protein kinases are present in the cytoplasmic tail of the beta(3) subunit. The LLITIHDR peptide, matching the membrane-proximal domain of beta(3) (residues 717 to 724), had no effect, but NNPLYKEA (residues 743 to 750), EATSTFTN (residues 749 to 756), and TNITYRGT (residues 755 to 762), which mimicked overlapping domains of the carboxy-terminal part of beta(3), reduced alpha-thrombin-induced ligand binding by 60+/-4%, 97+/-1%, and 97+/-2% (n=3) at 500 micromol/L peptide, respectively. These observations indicate that Src kinases and protein kinase C take part in inside-out signaling to integrin alpha(IIb)beta(3) and identify target domains in beta(3) that contribute to the regulation of this integrin.

Evidence against a direct role of the integrin alpha2beta1 in collagen-induced tyrosine phosphorylation in human platelets.

In the present study we have investigated whether the collagen receptor alpha2beta1 (GPIa-IIa; GP, glycoprotein) regulates protein tyrosine phosphorylation in platelets directly through activation of tyrosine kinases or indirectly through modification of the response to GPVI. The interaction of collagen with alpha2beta1 was inhibited in two distinct ways, using the metalloprotease jararhagin, which cleaves the beta1 subunit, or the antibody P1E6 which competes with binding of collagen to the integrin. The two inhibitors caused a shift to the right in the collagen concentration response curves for protein tyrosine phosphorylation and platelet activation consistent with a causal relationship between the two events. There was no change in the overall pattern of tyrosine phosphorylation in response to high concentrations of collagen in the presence of alpha2beta1 blockade demonstrating that the integrin is not required for this event. In contrast, jararhagin and P1E6 had a small, almost negligible inhibitory effect against responses to the GPVI-selective agonist collagen-related peptide (CRP) and the G protein-coupled receptor agonist thrombin. Crosslinking of alpha2beta1 in solution or by adhesion to a monolayer using a variety of antibodies to either subunit of the integrin did not induce detectable protein tyrosine phosphorylation in whole cell lysates. The snake venom toxin trimucytin-stimulated a similar pattern of tyrosine phosphorylation to that induced by crosslinking of GPVI which was maintained in the presence of jararhagin. Trimucytin may therefore induce activation via GPVI rather than alpha2beta1 as previously thought. These observations show that the integrin alpha2beta1 is not required for regulation of tyrosine phosphorylation by collagen.

The protein kinase C inhibitors bisindolylmaleimide I (GF 109203x) and IX (Ro 31-8220) are potent inhibitors of glycogen synthase kinase-3 activity.

Here we report that the widely used protein kinase C inhibitors, bisindolylmaleimide I and IX, are potent inhibitors of glycogen synthase kinase-3 (GSK-3). Bisindolylmaleimide I and IX inhibited GSK-3 in vitro, when assayed either in cell lysates (IC(50) 360 nM and 6.8 nM, respectively) or in GSK-3beta immunoprecipitates (IC(50) 170 nM and 2.8 nM, respectively) derived from rat epididymal adipocytes. Pretreatment of adipocytes with bisindolylmaleimide I (5 microM) and IX (2 microM) reduced GSK-3 activity in total cell lysates, to 25.1+/-4.3% and 12.9+/-3.0% of control, respectively. By contrast, bisindolylmaleimide V (5 microM), which lacks the functional groups present on bisindolylmaleimide I and IX, had little apparent effect. We propose that bisindolylmaleimide I and IX can directly inhibit GSK-3, and that this may explain some of the previously reported insulin-like effects on glycogen synthase activity.

Differential involvement of tyrosine and serine/threonine kinases in platelet integrin alphaIIbbeta3 exposure.

The relative contributions of protein tyrosine kinases (PTKs) and protein kinase C isoenzymes (PKCs), a family of serine/threonine kinases, in integrin alpha(IIb)beta3 (glycoprotein IIb/IIIa) exposure are the subject of much controversy. In the present study we measured the effect of the PTK inhibitor herbimycin A and the PKC inhibitor bisindolylmaleimide I on 125I-fibrinogen binding to alpha(IIb)beta3 and on aggregation/secretion induced by different agonists. Dose-response studies showed complete inhibition of alpha(IIb)beta3 exposure by 30 micromol/L (ADP stimulation) and 35 to 40 micromol/L (alpha-thrombin stimulation) herbimycin A. In contrast, inhibition of exposure by bisindolylmaleimide I varied from none (for ADP and epinephrine), to 30% (for platelet-activating factor), and to approximately 80% (for alpha-thrombin). Studies with a submaximal dose of herbimycin A (approximately 50% inhibition of the ADP-response) and a maximal dose of bisindolylmaleimide I showed that optical aggregation had a similar sensitivity to the inhibitors as alpha(IIb)beta3 exposure with minimal interference by secreted ADP. Thus, the relative contributions of tyrosine and serine/threonine kinases in alpha(IIb)beta3 exposure and aggregation differ among the different agonists, with an exclusive role for PTKs in ADP- and epinephrine-induced responses and a role for both PTKs and PKCs in responses induced by platelet-activating factor and alpha-thrombin.

Exposure of ligand-binding sites on platelet integrin alpha IIB/beta 3 by phosphorylation of the beta 3 subunit.

The exposure of ligand-binding sites for adhesive proteins on platelet integrin alpha IIB/beta 3 (glycoprotein IIB/IIIA) by platelet-activating (PAF) is transient, whereas sites exposed by alpha-thrombin remain accessible. The same difference is seen in the phosphorylation of the beta 3 subunit. Inhibition of protein kinases (1 microM staurosporine) and protein kinase C (10 microM bisindolylmaleimide) closes binding sites exposed by both agonists and induces dephosphorylation of beta 3. Inhibition of Tyr-kinases (20 microM Herbimycin A) has only a slight effect. Inhibition of Ser/Thr-phosphatases (1 microM okadaic acid, 30 s preincubation) changes the transient exposure and beta phosphorylation by PAF into the 'permanent' patterns induced by alpha-thrombin. Inhibition of Tyr-phosphatases (100 microM vanadate) has little effect. Preincubation with okadaic acid makes exposed binding sites and phosphorylated beta 3 insensitive to staurosporine, resulting in exposed alpha IIB/beta 3 independent of concurrent phosphorylation/dephosphorylation. The stoichiometry of beta 3 phosphorylation by alpha-thrombin is 0.80+/-0.10. Thus, one of the mechanisms that regulates exposure and closure of ligand-binding sites on the alpha IIb/beta 3 is phosphorylation/dephosphorylation of a Ser/Thr-residue in the beta 3 subunit.