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Sin1 (Stress-Activated Protein Kinase-Interacting Protein) Regulates Ischemia-Induced Microthrombosis Through Integrin αIIbβ3-Mediated Outside-In Signaling and Hypoxia Responses in Platelets

Originally publishedhttps://doi.org/10.1161/ATVBAHA.118.311822Arteriosclerosis, Thrombosis, and Vascular Biology. 2018;38:2793–2805

Abstract

Objective—

Microthrombosis as a serious consequence of myocardial infarction, impairs the microvascular environment and increases the occurrences of heart failure, arrhythmia, and death. Sin1 (stress-activated protein kinase-interacting protein) as an essential component of mTORC2 (mammalian target of rapamycin complex 2) is required for cell proliferation and metabolism in response to nutrients, stress, and reactive oxygen species and activates Akt and PKC (protein kinase C). However, the activation and function of Sin1/mTORC2 in ischemia-induced microthrombosis remain poorly understood.

Approach and Results—

The phosphorylation of the mTORC2 target Akt at S473 (serine 473) was significantly elevated in platelets from the distal end of left anterior descending obstructions from patients who underwent off-pump coronary artery bypass grafting compared with platelets from healthy subjects. Consistent with this finding, phosphorylation of T86 in Sin1 was also dramatically increased. Importantly, the augmented levels of phosphorylated Sin1 and Akt in platelets from 61 preoperative patients with ST-segment—elevation myocardial infarction correlated well with the no-reflow phenomena observed after revascularization. Platelet-specific Sin1 deficiency mice and Sin1 T86 phosphorylation deficiency mice were established to explore the underlying mechanisms in platelet activation. Mechanistically, Sin1 T86 phosphorylation amplifies mTORC2-mediated downstream signals; it is also required for αIIbβ3-mediated outside-in signaling and plays a role in generating hypoxia/reactive oxygen species through NAD+/Sirt3 (sirtuin 3)/SOD2 (superoxide dismutase 2) pathway. Importantly, Sin1 deletion in platelets protected mice from ischemia-induced microvascular embolization and subsequent heart dysfunction in a mouse model of myocardial infarction.

Conclusions—

Together, the results of our study reveal a novel role for Sin1 in platelet activation. Thus, Sin1 may be a valuable therapeutic target for interventions for ischemia-induced myocardial infarction deterioration.

Highlights

  • The augmented phosphorylation levels of Sin1 (stress-activated protein kinase-interacting protein) T86 and Akt S473 (serine 473) in platelets from ST-segment—elevation myocardial infarction patients correlated well with no-reflow after revascularization after percutaneous coronary intervention.

  • Sin1 is a susceptible sensor of hypoxia/reactive oxygen species signaling in ischemic environments after myocardial infarction.

  • Sin1 may serve as an efficient and safe therapeutic target for antithrombotic therapy avoiding bleeding complications.

  • Sin1 is important for ischemia-induced microvascular embolization and will help establish the most specific and sensitive prognostic biomarkers of unfavorable outcomes after myocardial infarction.

Introduction

Acute myocardial infarction (MI) is a severe ischemic cardiovascular disease with an increasing morbidity.1 Percutaneous coronary intervention (PCI) has greatly improved the survival of patients with acute MI. However, a state of microvascular thrombus-induced myocardial perfusion deficiency in the epicardial coronary artery often occurs after PCI, is a strong predictor of poor prognosis, and is associated with higher mortality.2 A large number of platelets become activated during acute MI, and abnormal platelet activation exacerbates the myocardial injury.3 Although reactive oxygen species (ROS)-induced platelet activation in the ischemic microvasculature has been proposed to be an important contributor to inadequate myocardial perfusion and dysfunction,4,5 the mechanism by which abnormal platelet activation functions in ischemic microvascular embolization and its relationship with clinical outcomes remains unclear.

Akt activation is critical for multiple cellular processes, such as oxidative stress, metabolism, proliferation, and migration.6 Akt activation is regulated by the phosphorylation of its T308 in the activation loop (A-loop) of the kinase domain by PDK1 (3-phosphoinositide-dependent protein kinase 1) and S473 (serine 473; also called HM [hydrophobic motif]) in the C terminus by rapamycin-insensitive mTORC2 (mammalian target of rapamycin complex 2).7,8 mTORC2 is a key regulator of Akt S473 phosphorylation but also selectively regulates Akt1 and Akt2 isoforms in platelets.9 mTORC2 comprises multiple subunits that include mTOR, DEPTOR (DEP domain-containing mTOR-interacting protein), GβL (G-protein beta subunit-like), rapamycin-insensitive companion of mTOR (Rictor), and Sin1 (stress-activated protein kinase-interacting protein).7,10,12 Although Akt activation requires Sin1 and mTORC2, its activation in turn also leads to Sin1 phosphorylation at T86, which may further positively regulate mTORC2 activity.10,11

PI3Ks (phosphoinositide 3-kinases) activate diverse substrates and facilitate the recruitment of downstream effectors to the plasma membrane.13 Akt is the primary enzyme that is activated by PI3Ks and plays critical roles in platelet activation induced by αIIbβ3,13 GP (glycoprotein) Ib/IX/V,14,15 the collagen receptor GPVI16 and GPCRs (G-protein–coupled receptors).17 As shown in our previous study, PTEN (phosphatase and tensin homolog), a negative regulator of the PI3K/Akt pathway, suppresses Akt phosphorylation and collagen-induced platelet activation.18 Moreover, PDK1 phosphorylates Akt at T308 in platelets to regulate receptor-mediated platelet activation and thrombosis.8 In the present study, we identify the mechanism by which the Sin1-Akt axis is activated in platelets from patients with ST-segment—elevation MI (STEMI) and show that the activation of the Sin1-mTORC2-Akt axis activation plays a critical role in microthrombosis.

Materials and Methods

The authors declare that all supporting data are available within the article and its online-only Data Supplement.

Antibodies and Reagents

α-thrombin was from Enzyme Research Laboratories (South Bend, IN). ADP, apyrase, prostaglandin E1, fibrinogen, and thromboxane A2 analog U46619 were purchased from Sigma-Aldrich (St Louis, MO). Collagen was from Chrono-log (Havertown, PA). The anti-Akt, anti-phospho-Akt (S473), anti-phospho-Akt (T308), anti-phospho-Sin1 (T86), anti-Rictor, anti-mTOR, and anti-Gapdh antibodies were from Cell Signaling Technology (Danvers, MA). Anti-Sin1, anti-SOD2 (superoxide dismutase 2), and anti-Sirt3 (sirtuin 3) were from Abcam (Cambridge, MA). Anti-CD42c antibody was from LifeSpan BioSciences, Inc (Seattle, WA). Alexa 647-conjugated fibrinogen and rhodamine-conjugated phalloidin were from Life Technologies (Gaithersburg, MD). Akt inhibitor MK2206 and PDK1 inhibitor OSU03012 were from Selleck Chemicals (Houston, TX).

Mouse

Generation of Rosa26mT/mGPf4-cre+ Mice

The Rosa26mT/mG (Jax007676) mice on a C57BL/6 genetic background were obtained from Nanjing Biomedical Research Institute of Nanjing University. Rosa26mT/mG mice were crossed with Pf4-cre+mice19 to obtain Rosa26mT/mGPf4-cre+ mice. Further mating produced Rosa26mT/mGPf4-cre+ mice with GFP+ platelets. The mice were genotyped by polymerase chain reaction (PCR).

Generation of Megakaryocyte/Platelet-Specific Sin1 Knockout Mice

The mouse embryonic stem cell line containing the Sin1 target allele was obtained from The European Conditional Mouse Mutagenesis Program. The targeted allele contains exons 2, 3, 4, and 5, as well as the lacZ and neomycin resistance genes (Neo) flanked by FRT (flippase recognition target) sites. Additionally, exon 4 of Sin1 is also flanked by LoxP recombination sites, enabling the deletion of the lacZ and Neo genes and independent conditional deletion of Sin1 exon 4. This deletion causes a frameshift that generates a premature stop codon during the translation of the corresponding mRNA. After selection, the targeted embryonic stem cell clones were identified by PCR, and 1 PCR-identified embryonic stem cell clone was injected into blastocysts, which were then implanted into pseudo-pregnant mice. First, the F1 generation mice were identified by PCR to obtain chimeras with the Sin1 target allele. Second, the chimeric mice were crossed with wild-type mice to generate F2 generation mice containing a Sin1 target allele in each cell. Third, F2 generation mice were crossed with Flp-expressing mice (B6.SJL-Tg (ACTFLPe) 9205Dym/J) to acquire Sin1fl/+ mice in which the lacZ and Neo genes were deleted by the Flp enzyme. Subsequently, the mice with the Sin1-floxed allele were backcrossed 9× onto the C57BL/6 background. Mice with a specific knockout in megakaryocytes/platelets were produced by breeding Sin1-floxed mice with transgenic mice that expressed Pf4 (platelet factor 4) promoter-driven Cre recombinase (008535, Pf4-Cre; The Jackson Laboratory). All knockout lines were produced on the C57BL/6J background. Genotyping was performed by PCR, and the Sin1 deficiency was confirmed by immunoblotting.

Generation of the Sin1-R81T Mutant Mice

The Sin1-R81T mutant mice were generated on a C57BL/6J genetic background using the clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein (Cas) system. Mouse strains carrying the Sin1-R81T mutation were created with the CRISPR-Cas system through the injection of Cas9 mRNA and guide RNA into embryos, and Cas9-induced gene editing was used to produce mutations.20–22 First, 5’-TAGAAGACGCTCAAACACAGG-3’ was selected as the target sequence and synthesized into guide RNA templates. Second, an oligo DNA was constructed as the donor template for homologous recombination. Because the target sequences should not be synthesized in the template DNA, synonymous mutations near the R81 site were generated to avoid recutting the templates. The donor oligo sequences containing the Sin1-R81T mutation were catatacgcccagtctgttgatattacatcgagctgggactttggtattACGCgCAgAtcTaaTacaggtaaactccgccttctagttgtgtcttctt. Third, pseudo-pregnant recipient mice were generated to acquire the 1-cell embryos. Next, the donor templates and Cas9 mRNA and guide RNA were microinjected into the pronuclei of embryos for homologous recombination. Finally, Sin1-R81T mutant founders were selected and hybridized with wild-type mice to obtain mutant strains, and the genotypes of the pups were identified by gene sequencing.

Platelet Isolation and Aggregation

Human and mouse washed platelets were prepared and stimulated as described.18 Inhibitor was incubated with the platelets for 3 minutes at 37°C before stimulation. Institutional Review Board approval was obtained from the Shanghai Jiao Tong University School of Medicine, and informed consent was obtained from volunteers in accordance with the Declaration of Helsinki.

Fibrinogen Binding Assay

Isolated platelets were incubated with 40 μg/mL Alexa 647-conjugated fibrinogen and 0.05 U/mL α-thrombin in a final volume of 50 μL Tyrode’s buffer containing 1 mmol/L CaCl2 for 20 minutes at room temperature. Fibrinogen binding was measured by a flow cytometer (LSR Fortessa/FACS Calibur, BD Biosciences).

Platelet Spreading on Immobilized Fibrinogen

Analysis of platelet spreading on immobilized fibrinogen was done as described.23 Images of spreading platelet stained by rhodamine-conjugated phalloidin were captured with a microscope, and platelet size was quantified using the Image J software (National Institutes of Health, Bethesda, MD). Statistical significance was evaluated by the Student t test.

Ferric Chloride-Induced Carotid Artery Injury

A ferric chloride-induced carotid artery injury murine thrombosis model was processed as described.24 Monitoring of carotid artery blood flow was initiated at the time of FeCl3 treatment and continuously monitored for 20 minutes. Carotid artery blood flow <0.06 mL/min was scored as occlusion, allowing the time to first occlusion to be determined.

Tail Bleeding Time Assay

The bleeding time assay was performed as previously described.25 Briefly, the anesthetized 8-week-old mice were maintained at 37°C and 3 mm of the tail tip was amputated with a sharp scalpel. The injured tail was immediately placed into PBS maintained at 37°C, and the bleeding time was measured from the start to the end until no rebleeding occurs within 1 minute.

mRNA Expression of SOD2 and Sirt3 in Mouse Platelets

Total mRNA was extracted from washed mouse Sin1f/fand Sin1−/− platelets. The mRNA expression levels of SOD2 and SIRT3 in platelet were measured using real-time quantitative PCR (ABI 7500 Real-Time PCR system). Relative mRNA expression levels were normalized to GAPDH expression level. The sequences of PCR primers were as follows. For GAPDH: forward 5’-TGTGTCCGTCGTGGATCTGA-3’ and reverse 5’-TTGCTGTTGAAGTCGCAGGAG-3’. For SOD226: forward 5’-CACCGAGGAGAAGTACCA-3’, reverse 5’-ACACATCAATCCCCAGCA-3’; Sirt327: forward 5’-GCTTGAGAGAGCATCTGGGAT-3’ and reverse 5’-CCTGTCCGCCATCACATCAG-3’.

ROS Measurement

ROS were examined in the platelets pretreated with 6-carboxy-2’, 7’- dichlorodihydrofluorescein diacetate, and the process was described before.28 Briefly, platelets were incubated with 10 mmol/L dichlorodihydrofluorescein diacetate 37°C for 15 minutes, and the platelets were washed with Tyrode’s buffer 3×. Platelets were then treated with hypoxia (5% oxygen) for 60 minutes or with normoxia for 60 minutes. The DCF (dichlorodihydrofluorescein) fluorescence intensity of the platelets was detected by flow cytometry.

Mouse Model of MI

The left anterior descending (LAD) coronary artery ligation was a well-established murine MI model as previously described.29 The mouse was anesthetized and undertaken thoracotomy at the fourth intercostal space to expose the heart. LAD artery was ligated 2 mm from its ostial origin with 7-0 silk suture. The murine chest was closed with a 6-0 coated vicryl, and the skin was sewed with 6-0 nylon suture. The anesthesia was stopped, and the mouse was kept oxygen uptaken until recovered consciousness. A sham group was carried on the same procedures except the LAD coronary artery ligation.

Echocardiography

Echocardiographic analysis of sham and MI mice was performed using Vevo2100 echocardiography machine (Visual Sonics, Toronto, Canada) and a linear-array 30-MHz transducer (MS-550D). M-mode was performed to evaluate murine heart functions, including left ventricular (LV) ejection fraction, fractional shortening, lower LV volume (LV end-systolic volume and LV end-diastolic volume ), and less dilated LV cavity (LVEIDd [left ventricular internal diameter end diastole] and LVEIDs [left ventricular internal diameter end systole]).

Intracellular NAD+ and NADH Measurement

Washed platelets were prepared and 3×108 platelets were immediately frozen by liquid nitrogen and saved at −80°C until use. Platelets were resuspended in 190 μL H2O and quenched by adding 80 μL methyl alcohol. Phenylamine was added as an internal control. Supernatants were centrifuged at 14000g for 15 minutes at 4°C and dried using vacuum pump. The metabolite was dissolved in 60 μL 50% acetonitrile. The chromatographic analysis was performed using Agilent 6470 Triple Quad liquid chromatography-mass spectrometry.

Operation to Obtain Distal Blood From Obstructed Coronary Arteries

Although patients with severe 3-vessel coronary disease were being treated with off-pump coronary artery bypass grafting, the artery was cut open at the distal end of the LAD artery obstruction, and 10 mL of blood were obtained in a syringe before the shunt was placed. The study protocol was approved by Renji Hospital Ethical Committee, Shanghai Jiao Tong University School of Medicine, and written informed content was obtained from all subjects.

Patient Population

Patients with a first STEMI who were admitted to and treated in the Ninth People’s Hospital Affiliated with the Shanghai Jiao Tong University School of Medicine from May to November in 2016 were enrolled in the study. The inclusion criteria were as follows: a time of persistent chest pain >30 minutes, >2 adjacent leads exhibiting ST-segment—elevation on ECGs, and a PCI that was accomplished within 12 hours of the initiation of pain. Patients with a history of coronary interventions or coronary artery bypass, and patients with active infections, cardiogenic shock at admission, systemic autoimmune disease, tumors, or liver or kidney dysfunction were excluded from the study. Overall, 61 patients were admitted into the study and included 29 males and 11 females aged 32 to 88 years old. During the process of PCI, STEMI patients were applied antiplatelet therapy by continuous intravenous administration of tirofiban (GP IIb/IIIa inhibitor) by micropump. When there were several coronary arteries blocking or the coronary was seriously obstructed, tirofiban will be directly injected into the patient’s coronary before the insertion of stents. Heparin was also used for anticoagulants for STEMI patients who underwent PCI. Antiplatelet therapy, anticoagulants, coronary angiography, and PCI were administered according to the current standard guidelines. Peripheral blood was obtained from patients with STEMI before PCI surgery. No-reflow was defined as a final TIMI (Thrombolysis In Myocardial Infarction) flow grade ≤2 without residual epicardial coronary stenosis, dissection or spasm. Finally, 36 healthy subjects with no history of chest pain and angiographically normal coronary arteries were also included. The study protocol was approved by the Ninth People’s Hospital Ethical Committee, Shanghai Jiao Tong University School of Medicine, and written informed content was obtained from all subjects.

Statistical Analysis

Data from the healthy subjects and patients with STEMI are presented as the means±SD, and comparisons between groups were performed using Mann-Whitney U tests, as indicated. Correlation analyses were performed using Spearman tests, and the frequencies of the normal reflow and no-reflow conditions post-PCI were compared using the χ2 test. A logistic regression analysis was performed to evaluate whether Sin1 and Akt activation were independent predictors of no-reflow. Variables with a significant or marginal association with no-reflow (P≤0.1) and some previously reported potential predictors (thrombus aspiration, culprit LAD, heart rate, etc) were included in the multivariate model. Statistical analyses were implemented with SPSS 20 (SPSS, Chicago, IL).

Results

Akt Activation Is Increased in Peripheral Platelets From STEMI Patients

A persistent LAD-ligation mouse model was created in Rosa26mT/mGPf4-cre+ mice to study the platelet response during ischemia-induced microembolization. Microthrombosis in the distal end of the LAD was assessed on day 3 post-LAD ligation (Figure 1A). GFP+ platelets were activated and tightly adhered to the vessel wall. This type of thrombus was probably because of the exposure of subendothelial matrix proteins, such as collagen and VWF (von Willebrand factor), and the activation of platelet adhesion receptors. Akt is a critical kinase for platelet activation. Akt activity was measured in peripheral platelets from the Rosa26mT/mGPf4-cre+ mice on day 3 post-LAD ligation (Figure 1B). Elevated levels of Akt phosphorylated at S473 and T308 (compared with Rosa26mT/mGPf4-cre+ mice with the sham operation) in the peripheral platelets reflected LAD occlusion-induced platelet activation. Phosphorylation of Akt at S473 in platelets from patients with STEMI at the onset of MI was also significantly increased compared with that in healthy subjects (Figure 1C and 1D; Figure IA and IB in the online-only Data Supplement). Statistically, patients with STEMI who presented with no-reflow (which refers to coronary distal embolization and microvascular obstruction after PCI) displayed higher levels of Akt phosphorylated at S473 than patients with normal reflow (Table I in the online-only Data Supplement). The occurrence of no-reflow in patients with STEMI is associated with high levels of Akt phosphorylated at S473 in platelets compared with the level observed in patients with STEMI presenting normal reflow (Tables II and III in the online-only Data Supplement). Therefore, high levels of Akt phosphorylated at S473 in platelets may be a valuable predictor of the clinical prognosis of patients with STEMI.

Figure 1.

Figure 1. Akt activation is increased in peripheral platelets from patients with ST-segment—elevation myocardial infarction (STEMI) and megakaryocyte/platelet (MP)-Sin1−/− mice exhibited improved heart functions post-MI. A, Immunofluorescence staining of the heart sections from Rosa26mT/mGPf4-cre+ mice on day 3 after Sham or left anterior descending (LAD) ligation surgery (n=3, males). B, Akt activity was measured in peripheral platelets from the Rosa26mT/mGPf4-cre+ mice on day 3 post-LAD ligation (n=3, males, *P<0.05, ***P<0.001). C and D, Relative levels of Akt phosphorylated at S473 (serine 473) and T308 in the platelets from 61 patients with STEMI and 36 healthy subjects. The data were analyzed using the Mann-Whitney U test (61 STEMI patients: males=49, *P<0.05, ***P<0.001). E, Levels of Akt phosphorylated at S473 and T308 in the presence of dimethyl sulfoxide (DMSO) and PP242 (n=4, ***P<0.001, males). F, Levels of Akt, Sin1, Rictor, and mTOR in the Sin1f/f and Sin1−/− platelets. G, Platelet counts in the Sin1−/− and Sin1f/f mice (n=15, males=9; P>0.05). H, Levels of Akt phosphorylated at S473 and T308 were measured in peripheral platelets from control Sin1f/f and MP-Sin1−/− mice on day 3 after Sham or LAD-ligation surgery (n=4, males; **P<0.01, ***P<0.001). I, Less CD42c (platelet) staining was observed in the infarct areas in MP-Sin1−/− mouse hearts than in the control Sin1f/f mice on day 3 after LAD-ligation surgery (n=5, males; ***P<0.001). J, The MP-Sin1−/− mice displayed smaller infarct areas in the heart on day 7 after LAD-ligation surgery (reduced Masson’s trichrome staining of the heart sections compared with the control Sin1f/f and MP-Sin1−/− mice, mean % of the whole ventricle area, n=7, males=4; ***P<0.001). K, Echocardiographic quantifications of the left ventricular ejection fraction (LVEF), fractional shortening (FS), LV end-diastolic volume (EDV), LV end-systolic volume (ESV), LV internal dimension at end diastole (IDd), LV internal dimension at end systolic (IDs) in the control Sin1f/f and MP-Sin1−/− mice on day 7 after Sham (n=5, males=3; P>0.05) or LAD-ligation (n=10, males=6; *P<0.05, **P<0.01) surgery. The independent samples comparisons were performed by standard Student t test. And pairwise comparisons were carried out using the paired t test. ANOVA was used for multigroup comparison. DAPI indicates 4’,6-diamidino-2-phenylindole; GFP, green fluorescent protein; and ns, not significant.

Protective Roles of Sin1 Deficiency in Platelets in Post-MI Cardiac Dysfunction

Because LAD-ligation activated Akt in mouse platelets and high level of Akt phosphorylated at S473 were detected in platelets from patients with STEMI presenting with no-reflow and mTORC2 inhibitor blocked thrombin-induced Akt phosphorylation (Figure 1E), we reasoned that Akt phosphorylation at S473 might be involved in ischemia-induced microvascular obstruction and potentially indicated that mTORC2 and Sin1 played roles in this process.

Megakaryocyte/platelet-specific Sin1-deficient mice (MP-Sin1−/−) were established to study the role of Sin1/mTORC2 in ischemia-induced platelet activation and thrombosis (Figure IIA in the online-only Data Supplement). Sin1 deficiency disrupts the integrity of the mTORC2 complex in platelets (Figure 1F) but has no significant effects on peripheral platelets counts (Figure 1G). First, the LAD mouse model was used to analyze the roles of Sin1 in ischemia-induced distal microvascular embolization and cardiac dysfunction. LAD ligation-induced transmural ischemia, as evidenced by the ST-segment—elevations on ECGs from control wild-type Sin1f/f and MP-Sin1−/− mice (data not shown). LAD ligation significantly enhanced Akt phosphorylation at S473 and T308, and Sin1 deficiency greatly suppressed LAD ligation-induced Akt activation (Figure 1H). The platelet-induced microthrombi in the coronary capillaries were immunohistochemically stained using the platelet-specific marker CD42c. The platelet-rich thrombi were less prevalent in the infarct area of MP-Sin1−/− mouse hearts than in the Sin1f/f mice on 3 days post-LAD ligation (Figure 1I).

Based on the results of Masson’s trichrome staining of mouse hearts, the anterior walls of the LV exhibited signs of extensive infarction in the control Sin1f/f mice, whereas the degree of damage was clearly reduced in the hearts of the MP-Sin1−/− mice (Figure 1J). Furthermore, Sin1 deficiency in platelets improved heart function on day 7 after LAD ligation, based on the following findings: (1) a higher LV ejection fraction, (2) improved fractional shortening, (3) a similar LV internal dimension at end diastole, (4) an abridged LV internal dimension systole, and (5) a similarly dilated LV end-diastolic volume and a smaller LV end-systolic volume (Figure 1K; Figure IIB and Movies I and II in the online-only Data Supplement). Therefore, Sin1 deletion in platelets played a vital cardioprotective role in mice after MI.

Sin1 Regulates Platelet Activation and Arterial Thrombus Formation via Integrin αIIbβ3 Outside-In Signaling

Hemostasis and thrombus formation were evaluated in the Sin1f/f and MP-Sin1−/− mice in vivo using a tail bleeding time assay and the FeCl3-induced carotid artery thrombosis model. The average bleeding times for the Sin1f/f and MP-Sin1−/− mice were not different, suggesting that the Sin1 deficiency in platelets did not affect hemostasis (Figure 2A). The average first occlusion time in the MP-Sin1−/− mice was prolonged (Figure 2B), indicating that platelet Sin1 was an important regulator of arterial thrombus formation in vivo. Based on these results, mTORC2 inhibition may be an efficient and safe antithrombotic target.

Figure 2.

Figure 2. Sin1 (stress-activated protein kinase-interacting protein) regulates platelet activation and arterial thrombus formation via integrin αIIbβ3 outside-in signaling and amplifies αIIbβ3-mediated Akt phosphorylation at S473 (serine 473) and T308. A, Tail bleeding times of the control Sin1f/f and megakaryocyte/platelet (MP)-Sin1−/− mice (n=10, males=6; P>0.05). B, The mouse carotid artery was injured with 10% FeCl3. Traces of blood flow in the carotid arteries of the control Sin1f/f and MP-Sin1−/− were recorded, and the times to first occlusion were measured (n=11, males=6; ***P<0.001). C, The aggregation of Sin1−/− platelets was diminished in response to α-thrombin, collagen, ADP, and U46619 (n=4, males; *P<0.05, **P<0.01). D, Binding of Alexa 647-fibrinogen to washed control Sin1f/f and MP-Sin1−/− mouse platelets stimulated with a low dose of α-thrombin. E, Spreading of Sin1f/f and Sin1−/− platelets on immobilized fibrinogen in the presence of dimethyl sulfoxide (DMSO,) OSU03012, or MK2206. Quantification of the areas (pixel numbers) of 3 random fields of Sin1f/f and Sin1−/− platelets (n=3, males; **P<0.01, ***P<0.001). F, Spreading of human platelets on immobilized fibrinogen in the presence of DMSO, the PDK1 (3-phosphoinositide-dependent protein kinase 1) inhibitor OSU03012 or the Akt inhibitor MK2206. Quantification of the areas (pixel numbers) of 3 random fields of human platelets (n=3, males=2; **P<0.01). G and H, Phosphorylation of Akt at S473 and T308 and PKC (protein kinase C) α at T638 in Sin1f/f and Sin1−/− platelets (left) and human platelets (right) spreading on fibrinogen in the presence of DMSO, OSU03012, or MK2206 (n=3, males; **P<0.01, ***P<0.001). Throughout the figure, the experiment was repeated at least 3× and the data are presented as the means±SEM. The independent samples comparisons were performed by standard Student t test. And pairwise comparisons were carried out using the paired t test. ANOVA was used for multigroup comparison. ns indicates not significant.

Receptor-mediated platelet activation plays a vital role in atherothrombosis induced by subendothelial matrix exposure.30 We measured the aggregation of in response to low doses of many agonists to investigate the roles of Sin1 in mediating receptor signaling. Compared with the wild-type platelets, the aggregation of the MP-Sin1−/− platelets was reduced in response to multiagonists (Figure 2C). The Sin1 deficiency did not affect α-thrombin-induced fibrinogen binding to platelets, indicating that Sin1 is not involved in αIIbβ3 activation (Figure 2D). However, Sin1−/− platelets exhibited insufficient spreading (Figure 2E), suggesting that Sin1 may regulate platelet activation through effects on αIIbβ3-mediated outside-in signaling.

Sin1 Amplifies αIIbβ3-Mediated mTORC2 Activation

Akt is activated by the PDK1-mediated phosphorylation of T308 and the mTORC2-mediated phosphorylation of S473.7,8 The spreading of human platelets and wild-type mouse platelets, but not Sin1−/− mouse platelets, was inhibited by the PDK1 inhibitor OSU03012 and the Akt inhibitor MK2206 (Figure 2E and 2F), indicating that Sin1 and PDK1 are required for αIIbβ3-mediated platelet activation.

Levels of Akt phosphorylated at S473 and T308 were greatly diminished in the Sin1−/− platelets, suggesting that Sin1 is required for the phosphorylation of both residues. Interestingly, the phosphorylation of Akt at S473 and T308 was greatly suppressed during platelet spreading on fibrinogen in the presence of OSU03012 and MK2206 (Figure 2G and 2H; Figure IIC and IID in the online-only Data Supplement), indicating the presence of a positive regulatory loop between PDK1, Sin1, and Akt. In the presence of PI3K, PDK1 is initially activated, enabling the transport of Akt to the plasma membrane in close proximity to Sin1-mTORC2 and subsequently promoting Akt phosphorylation at T308 and S473. And presumably activated Akt in turn upregulate the mTORC2 activity.

PKC (protein kinase C) belongs to the AGC kinase family and participates in integrin-mediated intracellular signal transduction, platelet activation, and thrombus formation.31 PKCα activation requires the phosphorylation of T638 in TM and S657 in HM.32 Interestingly, mTORC2 is important for PKCα phosphorylation at T638 and is critical for PKCα maturation and signaling.32 In the present study, phosphorylation of PKCα at T638 was diminished in Sin1−/− platelets, suggesting that Sin1 regulates PKCα phosphorylation in the TM domain. PKCα phosphorylation was suppressed during platelet spreading on fibrinogen in the presence of OSU03012 and MK2206 (Figure 2G and 2H), indicating that the Sin1-mTORC2-Akt axis also controls PKCα activity to more broadly influence intracellular signaling.

Sin1 Regulates Hypoxia-Mediated ROS Generation and ROS-Induced Platelet Activation

The persistent ischemic hypoxia induced by a reduction in or interruption of the coronary blood flow and oxygen supply is the major cause of poor outcomes post-MI.33 Although hypoxia has been shown to enhance platelet reactivity,34 the regulators and effectors of hypoxia-induced platelet activation are unclear. Figure 3A shows a significant increase in Akt phosphorylation in platelets obtained from the distal end of LAD obstructions from patients who underwent off-pump coronary artery bypass grafting compared with platelets from healthy subjects. Thus, Akt activation was increased in platelets in the ischemic vessels. Levels of Akt phosphorylated at S473 and T308 were significantly increased in the hypoxia-exposed human and wild-type mouse platelets, indicating that hypoxia was sufficient to activate the Sin1-mTORC2-Akt axis in platelets (Figure 3B and 3C). Moreover, increased ROS levels were detected in the human and Sin1f/f mouse platelets in response to hypoxia (Figure 3D). Sin1 deficiency significantly suppressed the hypoxia-induced phosphorylation of Akt at S473 and T308 and ROS generation in platelets (Figure 3C and 3D), implying that hypoxia-induced platelet activation and ROS generation depend on the activation of the Sin1-mTORC2-Akt axis activation.

Figure 3.

Figure 3. Sin1 (stress-activated protein kinase-interacting protein) regulates hypoxia-mediated platelet reactive oxygen species (ROS) generation and ROS-induced platelet activation in ischemic myocardial infarction. A, Schematic describing the operation used to obtain platelets from the distal end of the left anterior descending (LAD) obstruction during off-pump coronary artery bypass grafting (OPCABG). Levels of Akt phosphorylated at S473 (serine 473) and T308 were increased in platelets obtained from the distal end of the LAD obstruction. Relative levels of Akt phosphorylated at S473 and T308 in platelets obtained from the distal end of the LAD obstruction and peripheral platelets from healthy subjects were analyzed (n=3, males; *P<0.05, **P<0.01). B and C, Human, Sin1f/f and Sin1−/− platelets were exposed to normoxic conditions or 5% oxygen and the phosphorylation levels of Akt were detected (n=4, males; ***P<0.001). D Human, Sin1f/f and Sin1−/− platelets were exposed to normoxic conditions or 5% oxygen for 60 min, and the ROS levels were measured by flow cytometry. The DCF (dichlorodihydrofluorescein) fluorescence intensity was analyzed, and the ROS levels were standardized to the percentage observed in the normoxic human and Sin1f/f mouse platelets (n=4, males; **P<0.01). E, Aggregation of human, Sin1f/f and Sin1−/− platelets in the presence of 0.05 U/mL thrombin and 50 μmol/L H2O2 (n=4, males; *P<0.05, ***P<0.001). F and G, Levels of phosphorylated Akt in human, Sin1f/f and Sin1−/− platelets treated with 50 μmol/L H2O2 for 0 to 10 min (n=4, males; *P<0.05, ***P<0.001). The independent samples comparisons were performed by standard Student t test. And pairwise comparisons were carried out using the paired t test. ANOVA was used for multigroup comparison.

H2O2 promotes low-level thrombin-induced human and Sin1f/f mouse platelet aggregation, further confirming that ROS is an important factor that facilitates platelet activation in the ischemic microenvironment (Figure 3E). Levels of Akt phosphorylated at S473 were increased in the human and Sin1f/f mouse platelets in response to H2O2 in a time- and dose-dependent manner (Figure 3F and 3G; Figure IIE and IIF in the online-only Data Supplement). The Sin1 deficiency significantly impaired the potential of ROS to facilitate platelet aggregation and Akt phosphorylation at S473 and T308 (Figure 3E and 3G; Figure IIF in the online-only Data Supplement), indicating that the activation of the Sin1-mTORC2-Akt axis may promote ischemia by inducing platelet hyperactivation.

Based on these findings, ischemic MI promoted ROS accumulation and the activation of the Sin1-mTORC2-Akt axis in platelets, and the inhibition of this axis may have a cardioprotective effect on subjects after MI.

Sin1 Phosphorylation at T86 Positively Regulates Platelet Activation

Sin1 phosphorylation at T86 by Akt regulates mTORC2 signaling.11 However, levels of Akt phosphorylated at T308 and S473 and levels of Sin1 phosphorylated at T86 were substantially increased in human platelets stimulated with multiagonists (Figure 4A; Figure IIIG in the online-only Data Supplement), and the phosphorylation of Sin1 at T86 was significantly increased in platelets obtained from the distal end of LAD obstructions from patients who underwent off-pump coronary artery bypass grafting compared with platelets from healthy subjects (Figure 4B). The levels of Sin1 phosphorylated at T86 in platelets obtained from patients with STEMI at the onset of MI were also significantly increased (Figure 4C; Figure IB in the online-only Data Supplement). Patients with STEMI presenting no-reflow also displayed higher levels of Sin1 phosphorylated at T86 (Table I in the online-only Data Supplement). And the occurrence of no-reflow in patients with STEMI was associated with higher levels of Sin1 phosphorylated at T86 in platelets (Tables II and III in the online-only Data Supplement). Therefore, Sin1 phosphorylation at T86 may be positively correlated with platelet activation and a high level of Sin1 phosphorylated at T86 in platelets is probably another predictor of a poor clinical prognosis for patients with STEMI.

Figure 4.

Figure 4. Sin1 (stress-activated protein kinase-interacting protein) phosphorylation at T86 positively regulates Sin1 and Akt activation in αIIbβ3-mediated signaling and reactive oxygen species (ROS)-induced platelet activation. A, Levels of Sin1 phosphorylated at T86 and Akt phosphorylated at S473 (serine 473) and T308 in human platelets in response to α-thrombin. B, Levels of Sin1 phosphorylated at T86 were increased in the platelets from the distal end of the left anterior descending (LAD) obstruction from patients who underwent off-pump coronary artery bypass grafting (OPCABG; n=3, males; *P<0.05). C, Relative levels of Sin1 phosphorylated at T86 (phospho-Sin1 T86, ***P<0.001) in the platelets from 61 patients with ST-segment—elevation myocardial infarction (STEMI) and 36 healthy subjects. The data were analyzed using the Mann-Whitney U test (61 STEMI patients: males=49; ***P<0.001). D, Platelet counts in Sin1 WT and Sin1-R81T mice (n=7, males=4; P>0.05). E, The aggregation of Sin1-R81T platelets was diminished in response to α-thrombin, collagen, ADP, and U46619 (n=3, males; *P<0.05). F, Spreading of Sin1 WT and Sin1-R81T platelets on fibrinogen. Quantification of the areas (pixel numbers) of 3 random fields of human platelets (n=3, males; **P<0.01). G, Levels of Akt phosphorylated at S473 and T308 in Sin1 WT and Sin1-R81T mouse platelets in response to α-thrombin (n=3, males; ***P<0.001). H, The integrity of mTORC2 (mammalian target of rapamycin complex 2) in resting and activated Sin1 WT and Sin1-R81T mouse platelets was detected in an IP experiment. (I) Phosphorylation of Sin1 at T86 in Sin1f/f and Sin1−/− platelets spreading on fibrinogen in the presence of dimethyl sulfoxide (DMSO), OSU03012, or MK2206 (n=3, males; ***P<0.001). J, Levels of phosphorylated Sin1 in human platelets treated with 50 μmol/L H2O2 for 0 to 10 min (n=4, males=2; **P<0.01, ***P<0.001). K, The phosphorylation levels of Akt at S473 and T308 in Sin1-WT and Sin1-R81T mouse platelets stimulated by H2O2. L, The phosphorylation level of Sin1 at T86 was measured in peripheral platelets from control Sin1f/f and MP-Sin1−/− mice on day 3 post-LAD ligation (n=4, males=3; **P<0.01). The independent samples comparisons were performed by standard Student t test. And pairwise comparisons were performed using the paired t test. ANOVA was used for multigroup comparison. WT indicates wild type.

The Sin1-R81T mutation impairs Sin1 phosphorylation at T86 without interfering with mTORC2 integrity.11,12Sin1-R81T knockin mice were established to verify the role of Sin1 phosphorylation at T86 in platelet activation (Figure IIH and III in the online-only Data Supplement). The Sin1-R81T mutation inhibited multiagonist-induced platelet activation by suppressing integrin αIIbβ3-mediated outside-in signaling and suppressed the Akt phosphorylation at S473 and T308 by preventing Sin1 phosphorylation at T86 in activated platelets (Figure 4E through 4G; Figures IIJ and IIK and IIIA in the online-only Data Supplement). Importantly, mTORC2 remained intact in stimulated and unstimulated Sin1-WT and Sin1-R81T platelets (Figure 4H; Figure IIIB and IIIC in the online-only Data Supplement). The phosphorylation level of Sin1 at T86 was greatly suppressed during platelet spreading on fibrinogen in the presence of the PDK1 inhibitor OSU03012 and the Akt inhibitor MK2206 (Figure 4I), indicating the phosphorylation of Sin1 at T86 and upregulating mTORC2 activity require activation of PDK1-Akt axis. These results confirm the presence of a positive regulatory loop between PDK1, Sin1, and Akt.

Sin1 activation is the earliest event occurring in human and mouse platelets in response to H2O2 (Figure 4J; Figure IID and IIE in the online-only Data Supplement). Additionally, the Sin1-R81T mutation inhibited H2O2-induced Akt phosphorylation at S473 and T308 (Figure 4K; Figure IIID in the online-only Data Supplement), indicating that Sin1 phosphorylation at T86 may also regulate H2O2-induced Akt activation. Moreover, LAD ligation/ischemia enhanced Sin1 phosphorylation at T86 in platelets from Sin1f/f mice on day 3 postligation (Figure 4L). Therefore, Sin1 phosphorylation at T86 reflects an activated status of mTORC2 and is critical for mTORC2-mediated Akt activation in platelets.

Sin1 Regulates Hypoxia Responses in Platelets Through NAD+/Sirt3/SOD2 Pathway

Mitochondria are important organelles responsible for the generation of cellular ROS.35 Mitochondrial dysfunction contributes to augmented ROS production.36 Sirt3 is a member of the Sirtuin deacetylase family and regulates the acetylation of mitochondrial enzymes.37 SOD2 is the primary antioxidant enzyme responsible for the clearance of unbalanced mitochondrial ROS levels.38 It has been reported that acetylation of SOD2 at K68 ablate SOD2 activity.39 Sirt3 deacetylates ac-SOD2 at K68 and enhances SOD2 activity.39,40 We found that the levels of SOD2 acetylation were markedly upregulated in Sin1f/f platelets under hypoxia condition, and Sin1 deficiency significantly suppressed the acetylation of SOD2, suggesting that Sin1 regulates SOD2 activity thereby ROS generation (Figure 5A). Levels of the SOD2 and Sirt3 proteins, but not mRNAs, were also significantly elevated in Sin1−/− platelets (Figure 5A and 5B). These results indicated that the expression and activation of SOD2 were upregulated in Sin1 deficient platelets, which probably promoted the clearance of unbalanced ROS.

Figure 5.

Figure 5. Sin1 (stress-activated protein kinase-interacting protein) regulates NAD+/Sirt3 (sirtuin 3)/SOD2 (superoxide dismutase 2) pathway to promote reactive oxygen species (ROS) production and platelet activation. A, Sin1f/f and Sin1−/− platelets were exposed to normoxic conditions or 5% oxygen for 30 min, and the protein levels of ac-SOD2 at K68, SOD2, and Sirt3 were examined (n=4, males; *P<0.05, ***P<0.001). B, The mRNA levels of the Sod2 and Sirt3 were examined in Sin1f/f and Sin1−/− platelets. C, Sin1f/f and Sin1−/− platelets were exposed to normoxic conditions or 5% oxygen for 30 min, and the intracellular levels of NAD+/NADH were measured by liquid mass spectrometry (n=7, male=4; *P<0.05). D, Schematic described the αIIbβ3-mediated Sin1/Akt/PKC (protein kinase C) α signaling and ROS-mediated Sin1/NAD+/Sirt3/SOD2 network in platelet activation. The independent samples comparisons were performed by standard Student t test. And pairwise comparisons were performed using the paired t test. ANOVA was used for multigroup comparison.

SIRT3 as a NAD-dependent deacetylase regulates the process of ROS generation and mitochondrial function.40,41 We found that hypoxia significantly enhanced the intracellular ratios of NAD+/NADH in both Sin1f/f and Sin1−/− platelets, and Sin1 deficiency greatly raised the ratios of NAD+/NADH in platelets (Figure 5C). These results demonstrated that platelet Sin1 deficiency significantly inhibited ROS production by upregulating NAD+/NADH ratio and the NAD+-dependent deacetylase activity of Sirt3 to enhance SOD2 activity.

Discussion

STEMI is a serious type of MI that results from the rupture of a vulnerable atherosclerotic plaque and the platelet-mediated complete occlusion of a coronary artery that is characterized by elevation of the ST segment. Although PCI is the most expeditious strategy for the recovery of the blood supply in the ischemic myocardium in patients with STEMI, it often fails to restore myocardial reperfusion primarily because of no-reflow.42 Most reported clinical predictors of no-reflow were defined based on data from retrospective studies that were limited in terms of the subjects’ ages, blood pressure values, and peak levels of CK-MB (creatine kinase-myocardial band isoenzyme), CRP (C-reactive protein), Killip class, etc Currently, a reliable indicator of the occurrence of no-reflow is not available.2,43 In the present study, the Sin1-Akt pathway was hyperactive in platelets from preoperative patients with STEMI, and the occurrence of no-reflow in patients with STEMI was statistically associated with higher levels of Akt phosphorylated at S473 and Sin1 phosphorylated at T86 in platelets. Therefore, levels of Akt phosphorylated at S473 and Sin1 phosphorylated at T86 are potentially predictors of the no-reflow phenomenon.

The intracellular PI3K/Akt pathway has emerged as a crucial player in receptor-initiated platelet activation, hemostasis, and thrombosis.44 Although the PI3K/Akt pathway is involved in bidirectional integrin αIIbβ3 signaling,17,44 the Sin1 deficiency inhibited PAR, Tp, and GPVI receptor-initiated platelet activation by suppressing the αIIbβ3-mediated outside-in signaling pathways in the present study. As shown in our previous study, a PDK1 deficiency completely blocks Akt phosphorylation at T308 but only partially inhibits Akt phosphorylation at S473 in platelets in response to agonists.8 The Sin1 deficiency greatly inhibited Akt phosphorylation at T308 in the activated platelets, suggesting that Akt phosphorylation at T308 absolutely depends on PDK1 function and Sin1-mediated Akt phosphorylation at S473. Furthermore, the inhibition of PDK1 and Akt greatly suppressed Sin1 phosphorylation at T86, implying that Sin1 activation depends on the PDK1/Akt T308 pathway. Moreover, the efficient phosphorylation of Akt at T308 in turn requires the Sin1/Akt S473 pathway. Based on these results, Sin1 and PDK1 amplify αIIbβ3-mediated platelet activation. PKCα is probably the functional export of the positive regulatory loop comprising PDK1, Sin1, and Akt. Furthermore, PKCα activation further amplifies αIIbβ3-mediated intracellular signal transduction.

According to Gong et al,45 strategies that selectively target integrin outside-in signaling suppress thrombosis without disturbing hemostasis. Because platelet Sin1 amplified αIIbβ3-mediated outside-in signaling, we were not surprised that the Sin1 deficiency did not cause a bleeding disorder. Although Liu et al11 reported that Sin1 phosphorylated at T86 and T398 suppresses mTORC2 activity by disturbing mTORC2 integrity, Yang et al10 found that Akt phosphorylates Sin1 at T86, which enhances mTORC2 activity. In the present study, Sin1 phosphorylation at T86 regulated Akt activation instead of mTORC2 disintegration. Therefore, Sin1-mediated amplification of αIIbβ3-initiated outside-in signaling in platelets is probably a major mechanism of microthrombosis in the post-MI ischemic environment.

The prolonged ischemia caused by ROS-rich environments is postulated facilitate the progression of no-reflow.5,46 Although hypoxia was reported to be associated with increased platelet reactivity in several studies, the underlying signaling events remain unclear.47 Hypoxia significantly enhanced the endogenous ROS levels in platelets in vitro, and the Sin1 deficiency decreased ROS levels in platelets under normoxic and hypoxic conditions, suggesting that Sin1 is a major regulator of platelet ROS generation. The mechanism by which Sin1 promotes ROS generation in platelets in response to hypoxia is to regulate NAD+/NADH ratio and the NAD+-dependent deacetylase activity of Sirt3 to control SOD2 activity. However, the mechanism by which Sin1 suppresses NAD+/NADH ratio requires further study. Accumulated ROS facilitate platelet activation in patients with ischemic cardiovascular diseases.28 Sin1 probably functioned as a ROS sensor in platelets under hypoxic conditions in the present study. Consistent with these findings, in the LAD-ligation MI mouse model, myocardial ischemia increased Sin1 and Akt phosphorylation. Moreover, the platelet Sin1 deficiency suppressed microvascular thrombosis and improved the outcomes of myocardial ischemia. Therefore, Sin1-mediated sensing and facilitation of hypoxia/ROS signaling in platelets is probably another major mechanism of microthrombosis in ischemic environments observed after MI.

Overall, Sin1 is an amplifier of αIIbβ3-mediated outside-in signaling and a susceptible sensor of hypoxia/ROS signaling in ischemic environments after MI. The high level of activated Sin1 probably predicts poor clinical prognoses of patients with STEMI. Moreover, Sin1 serves as an efficient and safe target for antithrombotic therapy and the prevention of unfavorable outcomes after MI.

Nonstandard Abbreviations and Acronyms

CRISPR

clustered regularly interspaced short palindromic repeats

CRP

C-reactive protein

GP

glycoprotein

GPCRs

G-protein–coupled receptors

LAD

Left anterior descending

LV

left ventricle

MI

myocardial infarction

mTORC2

Mammalian target of rapamycin complex 2

PCI

percutaneous coronary intervention

PCR

polymerase chain reaction

PF4

platelet factor 4

PKC

protein kinase C

PTEN

phosphatase and tensin homolog

ROS

reactive oxygen species

S473

serine 473

Sin1

Stress-activated protein kinase-interacting protein

SIRT3

Sirtuin 3

SOD2

Superoxide dismutase 2

STEMI

ST-segment—elevation MI

TIMI

Thrombolysis In Myocardial Infarction

VWF

von Willebrand factor

Acknowledgments

J. Liu and Y. Xu designed the experiments, analyzed data, and wrote the article; Y. Xu, M. Zhang, and J. Gu performed the experiments; X. Fan, Z. Hu, X. Ouyang, L. Yan, J. Zhang, L. Zhang., G. Chen, S. Xue, and B. Su helped with the experiments.

Footnotes

The online-only Data Supplement is available with this article at https://www.ahajournals.org/doi/suppl/10.1161/ATVBAHA.118.311822.

Correspondence to Junling Liu, Department of Biochemistry and Molecular Cell Biology, Shanghai Jiao Tong University School of Medicine, 280 S Chongqing Rd, Shanghai 200025, China, Email
Bing Su, Department of Immunology and Microbiology and Molecular Cell Biology, Shanghai Jiao Tong University School of Medicine, 280 S Chongqing Rd, Shanghai 200025, China, Email

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