HSP27 phosphorylation is correlated with ADP-induced platelet granule secretion

The Link Between Heat Shock Proteins, Renin-Angiotensin System, and the Coagulation Cascade in the Pathogenesis of the Coronavirus-19 Disease

INTRODUCTION

Understanding the pathogenesis of COVID-19 is integral for its successful treatment.

METHODS

Available literature on the relationship between COVID-19, heat shock proteins (HSP), and the renin-angiotensin-aldosterone (RAAS) system were searched and used to hypothesize how HSP can be targeted in COVID-19.

RESULTS

During SARS-CoV-2 cellular entry, the ACE-2 receptor is downregulated. This leads to the augmentation of angiotensin-2/AT1 receptor axis along with attenuation of the ACE-2/angiotensin_1-7/Mas axis. Heat shock proteins are key stabilizing molecules in various pathways.In the heart and vessels, HSP-90 and HSP-60 can facilitate angiotensin-2-mediated myocardial injury and endothelial cell activation. HSP-60-TLR4/CD14 complex formation stabilizes IκB-kinase (IKK) potentiating NF-κB activation. HSPs in lungs and kidneys have antioxidant, vasodilatory, and anti-inflammatory actions and may be protective against the effects of RAAS. Stress-induced HSP-70 has a role in complement-mediated microvascular injury such as has been demonstrated in COVID-19. SARS-CoV-2 can induce autophagy via Beclin-1 and ER (endoplasmic reticular) stress via BIP. These two can be potential targets in the HSP environment.

CONCLUSION

Various HSP molecules can modulate the effects of the renin-angiotensin-aldosterone (RAAS) system and thus may have a potential role in the pathogenesis of COVID-19.

Synergistic effect of collagen and CXCL12 in the low doses on human platelet activation

CXCL12, also known as stromal cell-derived factor-1, is a chemokine classified into CXC families, which exerts its function by binding to specific receptors called CXCR4 and CXCR7. Human platelets express CXCR4 and CXCR7 on the plasma membrane. It has been reported that CXCL12 potentiates to induce platelet aggregation in cooperation with agonists including collagen. However, the precise roles and mechanisms of CXCL12 in human platelet activation are not fully elucidated. In the present study, we investigated the effect of simultaneous stimulation with low doses of collagen and CXCL12 on the activation of human platelets. The simultaneous stimulation with collagen and CXCL12 induced the secretion of platelet-derived growth factor (PDGF)-AB and the release of soluble CD40 ligand (sCD40L) from human platelets in addition to their aggregation, despite the fact that the simultaneous stimulation with thrombin receptor-activating peptide (TRAP) or adenosine diphosphate (ADP), and CXCL12 had little effects on the platelet aggregation. The agonist of Glycoprotein (GP) Ⅵ convulxin and CXCL12 also induced platelet aggregation synergistically. The monoclonal antibody against CXCR4 but not CXCR7 suppressed the platelet aggregation induced by simultaneous stimulation with collagen and CXCL12. The phosphorylation of p38 mitogen-activated protein kinase (MAPK), but not p44/p42 MAPK, was induced by the simultaneous stimulation. In addition, the simultaneous stimulation with collagen and CXCL12 induced the phosphorylation of HSP27 and the subsequent release of phosphorylated-HSP27 from human platelets. SB203580, a specific inhibitor of p38 MAPK, attenuated the platelet aggregation, the phosphorylation of p38 MAPK and HSP27, the PDGF-AB secretion, the sCD40L release and the phosphorylated-HSP27 release induced by the simultaneous stimulation with collagen and CXCL12. These results strongly suggest that collagen and CXCL12 in low doses synergistically act to induce PDGF-AB secretion, sCD40L release and phosphorylated-HSP27 release from activated human platelets via p38 MAPK activation.

Olive polyphenol reduces the collagen-elicited release of phosphorylated HSP27 from human platelets

Hydroxytyrosol (HT) and oleuropein (OLE) are natural polyphenols found in extra virgin olive oil. Accumulating evidence indicates that ingestion of olive oil contributes to reduce the risk of cardiovascular diseases and stroke. It has been reported that HT and OLE inhibit human platelet aggregation. We have shown that collagen induces the phosphorylation of heat shock protein 27 (HSP27) in human platelets, resulting in the release of HSP27, an extracellular pro-inflammatory agent. In this study, we investigated the effects of HT and OLE on the collagen-stimulated human platelet activation. The PDGF-AB secretion and the soluble CD40 ligand (sCD40L) release by collagen were reduced by HT or OLE. HT and OLE significantly suppressed the phosphorylation of HSP27 and the release of phosphorylated-HSP27. These findings suggest that olive polyphenol reduces the collagen-stimulated phosphorylation of HSP27 in human platelets and the release. Our results may provide a novel anti- inflammatory effect of olive polyphenol.

Platelet procoagulant phenotype is modulated by a p38-MK2 axis that regulates RTN4/Nogo proximal to the endoplasmic reticulum: utility of pathway analysis

Upon encountering physiological cues associated with damaged or inflamed endothelium, blood platelets set forth intracellular responses to ultimately support hemostatic plug formation and vascular repair. To gain insights into the molecular events underlying platelet function, we used a combination of interactome, pathway analysis, and other systems biology tools to analyze associations among proteins functionally modified by reversible phosphorylation upon platelet activation. While an interaction analysis mapped out a relative organization of intracellular mediators in platelet signaling, pathway analysis revealed directional signaling relations around protein kinase C (PKC) isoforms and mitogen-activated protein kinases (MAPKs) associated with platelet cytoskeletal dynamics, inflammatory responses, and hemostatic function. Pathway and causality analysis further suggested that platelets activate a specific p38-MK2 axis to phosphorylate RTN4 (reticulon-4, also known as Nogo), a Bcl-xl sequestration protein and critical regulator of endoplasmic reticulum (ER) physiology. In vitro, we find that platelets drive a p38-MK2-RTN4-Bcl-xl pathway associated with the regulation of the ER and platelet phosphatidylserine exposure. Together, our results support the use of pathway tools in the analysis of omics data sets as a means to help generate novel, mechanistic, and testable hypotheses for platelet studies while uncovering RTN4 as a putative regulator of platelet cell physiological responses.

Ristocetin induces phosphorylated-HSP27 (HSPB1) release from the platelets of type 2 DM patients: Anti-platelet agent-effect on the release

It has been previously reported that HSP27 is released from platelets in type 2 diabetes mellitus (DM) patients according to phosphorylation. In the present study, we investigated the effect of ristocetin, a glycoprotein (GP)Ib/IX/V activator, on the release of HSP27 and the effect of anti-platelet agents, such as acetylsalicylic acid (ASA), on this release. Forty-six patients with type 2 DM were recruited, and classified into two groups depending on administration of anti-platelet agents, resulting in 31 patients without these agents (control group) and 15 patients with them (anti-platelet group). Ristocetin potently induced the aggregation of platelets in the two groups. Ristocetin-induced changes of the area under the curve of light transmittance and the ratio of the size of platelet aggregates in the anti-platelet group were slightly different from those in the control group. On the other hand, the levels of phosphorylated-HSP27 induced by ristocetin in the platelets from a patient of the anti-platelet group taking ASA were significantly lower than those from a patient of the control group. The levels of HSP27 release from the ristocetin-stimulated platelets were significantly correlated with the levels of phosphorylated-HSP27 in the platelets from patients in the two groups. The levels of HSP27 release and those of platelet-derived growth factor-AB (PDGF-AB) secretion stimulated by ristocetin in the anti-platelet group were lower than those in the control group. In addition, the levels of HSP27 release and those of PDGF-AB secretion stimulated by ADP in the anti-platelet group were lower than those in the control group. These results strongly suggest that anti-platelet agents inhibit the HSP27 release from platelets stimulated by ristocetin but not the aggregation in type 2 DM patients.

Sphingosine 1-phosphate (S1P) suppresses the collagen-induced activation of human platelets via S1P4 receptor

Sphingosine 1-phosphate (S1P) is as an extracellular factor that acts as a potent lipid mediator by binding to specific receptors, S1P receptors (S1PRs). However, the precise role of S1P in human platelets that express S1PRs has not yet been fully clarified. We previously reported that heat shock protein 27 (HSP27) is released from human platelets accompanied by its phosphorylation stimulated by collagen. In the present study, we investigated the effect of S1P on the collagen-induced platelet activation. S1P pretreatment markedly attenuated the collagen-induced aggregation. Co-stimulation with S1P and collagen suppressed collagen-induced platelet activation, but the effect was weaker than that of S1P-pretreatment. The collagen-stimulated secretion of platelet-derived growth factor (PDGF)-AB and the soluble CD40 ligand (sCD40L) release were significantly reduced by S1P. In addition, S1P suppressed the collagen-induced release of HSP27 as well as the phosphorylation of HSP27. S1P significantly suppressed the collagen-induced phosphorylation of p38 mitogen-activated protein kinase. S1P increased the levels of GTP-bound Gαi and GTP-bound Gα13 coupled to S1PPR1 and/or S1PR4. CYM50260, a selective S1PR4 agonist, but not SEW2871, a selective S1PR1 agonist, suppressed the collagen-stimulated platelet aggregation, PDGF-AB secretion and sCD40L release. In addition, CYM50260 reduced the release of phosphorylated-HSP27 by collagen as well as the phosphorylation of HSP27. The selective S1PR4 antagonist CYM50358, which failed to affect collagen-induced HSP27 phosphorylation, reversed the S1P-induced attenuation of HSP27 phosphorylation by collagen. These results strongly suggest that S1P inhibits the collagen-induced human platelet activation through S1PR4 but not S1PR1.

Thrombin Receptor-Activating Protein (TRAP)-Activated Akt Is Involved in the Release of Phosphorylated-HSP27 (HSPB1) from Platelets in DM Patients

It is generally known that heat shock protein 27 (HSP27) is phosphorylated through p38 mitogen-activated protein (MAP) kinase. We have previously reported that HSP27 is released from human platelets associated with collagen-induced phosphorylation. In the present study, we conducted an investigation into the effect of thrombin receptor-activating protein (TRAP) on the release of HSP27 in platelets in type 2 diabetes mellitus (DM) patients. The phosphorylated-HSP27 levels induced by TRAP were directly proportional to the aggregation of platelets. The levels of phosphorylated-HSP27 (Ser-78) were correlated with the levels of phosphorylated-p38 MAP kinase and phosphorylated-Akt in the platelets stimulated by 10 µM TRAP but not with those of phosphorylated-p44/p42 MAP kinase. The levels of HSP27 released from the TRAP (10 µM)-stimulated platelets were correlated with the levels of phosphorylated-HSP27 in the platelets. The released platelet-derived growth factor-AB (PDGF-AB) levels were in parallel with the HSP27 levels released from the platelets stimulated by 10 µM TRAP. Although the area under the curve (AUC) of small aggregates (9–25 µm) induced by 10 µM TRAP showed no significant correlation with the released HSP27 levels, AUC of medium aggregates (25–50 µm), large aggregates (50–70 µm) and light transmittance were significantly correlated with the released HSP27 levels. TRAP-induced phosphorylation of HSP27 was truly suppressed by deguelin, an inhibitor of Akt, in the platelets from a healthy subject. These results strongly suggest that TRAP-induced activation of Akt in addition to p38 MAP kinase positively regulates the release of phosphorylated-HSP27 from human platelets, which is closely related to the platelet hyper-aggregation in type 2 DM patients.

Mechanisms of Nifedipine-Downregulated CD40L/sCD40L Signaling in Collagen Stimulated Human Platelets

The platelet-derived soluble CD40L (sCD40L) release plays a critical role in the development of atherosclerosis. Nifedipine, a dihydropyridine-based L-type calcium channel blocker (CCB), has been reported to have an anti-atherosclerotic effect beyond its blood pressure-lowering effect, but the molecular mechanisms remain unclear. The present study was designed to investigate whether nifedipine affects sCD40L release from collagen-stimulated human platelets and to determine the potential role of peroxisome proliferator-activated receptor-β/-γ (PPAR-β/-γ). We found that treatment with nifedipine significantly inhibited the platelet surface CD40L expression and sCD40L release in response to collagen, while the inhibition was markedly reversed by blocking PPAR-β/-γ activity with specific antagonist such as GSK0660 and GW9662. Meanwhile, nifedipine also enhanced nitric oxide (NO) and cyclic GMP formation in a PPAR-β/-γ-dependent manner. When the NO/cyclic GMP pathway was suppressed, nifedipine-mediated inhibition of sCD40L release was abolished significantly. Collagen-induced phosphorylation of p38MAPK, ERK1/2 and HSP27, matrix metalloproteinase-2 (MMP-2) expression/activity and reactive oxygen species (ROS) formation were significantly inhibited by nifedipine, whereas these alterations were all attenuated by co-treatment with PPAR-β/-γ antagonists. Collectively, these results demonstrate that PPAR-β/-γ-dependent pathways contribute to nifedipine-mediated downregulation of CD40L/sCD40L signaling in activated platelets through regulation of NO/ p38MAPK/ERK1/2/HSP27/MMP-2 signalings and provide a novel mechanism regarding the anti-atherosclerotic effect of nifedipine.

Calpain activity and Toll-like receptor 4 expression in platelet regulate haemostatic situation in patients undergoing cardiac surgery and coagulation in mice

Human platelets express Toll-like receptors (TLR) 4. However, the mechanism by which TLR4 directly affects platelet aggregation and blood coagulation remains to be explored. Therefore, in this study, we evaluated the platelet TLR4 expression in patients who underwent CABG surgery; we explored the correlation between platelet TLR4 expression and the early outcomes in hospital of patients. Additionally, C57BL/6 and C57BL/6-Tlr^LPS−/− mice were used to explore the roles of platelet TLR4 in coagulation by platelet aggregometry and rotation thromboelastometry. In conclusion, our results highlight the important roles of TLR4 in blood coagulation and platelet function. Of clinical relevance, we also explored novel roles for platelet TLR4 that are associated with early outcomes in cardiac surgery.

(-)-Epigallocatechin gallate selectively inhibits adenosine diphosphate‑stimulated human platelet activation: suppression of heat shock protein 27 phosphorylation via p38 mitogen‑activated protein kinase

(‑)‑Epigallocatechin gallate (EGCG) is a major component of green tea. It has been demonstrated that EGCG has an antithrombotic effect by inhibiting platelet aggregation. However, the detailed mechanisms underlying the effects of EGCG remain to be elucidated. The present study examined the effects of EGCG on human platelet activation by various stimulators and the exact underlying mechanisms. EGCG suppressed adenosine diphosphate (ADP)‑stimulated platelet aggregation dose dependently between 30 and 70 µM. By contrast, EGCG failed to affect platelet aggregation stimulated by collagen, U46619 (a TP agonist) or ristocetin (an activator of GPIb/IX/V). EGCG attenuated the ADP‑induced phosphorylation of p38 mitogen‑activated protein (MAP) kinase and heat shock protein 27 (HSP27). The ADP‑stimulated release of platelet‑derived growth factor (PDGF)‑AB and the soluble CD40 (sCD40) ligand was inhibited by EGCG. These findings suggest that EGCG selectively inhibits ADP‑stimulated human platelet activation and that EGCG reduces the release of PDGF‑AB and the sCD40 ligand due to suppressing HSP27 phosphorylation via p38 MAP kinase.

The role of calpain-myosin 9-Rab7b pathway in mediating the expression of Toll-like receptor 4 in platelets: a novel mechanism involved in α-granules trafficking

Toll-like receptors (TLRs) plays a critical role in innate immunity. In 2004, Aslam R. and Shiraki R. first determined that murine and human platelets express functional TLRs. Additionally, Andonegui G. demonstrated that platelets express TLR4, which contributes to thrombocytopenia. However, the underlying mechanisms of TLR4 expression by platelets have been rarely explored until now. The aim of this study was to identify the mechanism of TLR4 expression underlying thrombin treatment. The human washed platelets were used in this study. According to flowcytometry and western blot analysis, the surface levels of TLR4 were significantly enhanced in thrombin-activated human platelets and decreased by TMB-8, calpeptin, and U73122, but not Y27632 (a Rho-associated protein kinase ROCK inhibitor) indicating that thrombin-mediated TLR4 expression was modulated by PAR/PLC pathway, calcium and calpain. Co-immunoprecipitation (co-IP) assay demonstrated that the interaction between TLR4 and myosin-9 (a substrate of calpain) was regulated by calpain; cleavage of myosin-9 enhanced TLR4 expression in thrombin treated platelets. Transmission electron microscope data indicated that human platelets used α-granules to control TLR4 expression; the co-IP experiment suggested that myosin-9 did not coordinate with Rab7b to negatively regulate TLR4 trafficking in thrombin treated platelets. In summary, phospholipase Cγ-calpain-myosin 9-Rab7b axis was responsible for the mechanism underlying the regulation of TLR4 containing α-granules trafficking in thrombin-stimulated platelets, which was involved in coagulation.

Dual role of the p38 MAPK/cPLA2 pathway in the regulation of platelet apoptosis induced by ABT-737 and strong platelet agonists

p38 Mitogen-activated protein (MAP) kinase is involved in the apoptosis of nucleated cells. Although platelets are anucleated cells, apoptotic proteins have been shown to regulate platelet lifespan. However, the involvement of p38 MAP kinase in platelet apoptosis is not yet clearly defined. Therefore, we investigated the role of p38 MAP kinase in apoptosis induced by a mimetic of BH3-only proteins, ABT-737, and in apoptosis-like events induced by such strong platelet agonists as thrombin in combination with convulxin (Thr/Cvx), both of which result in p38 MAP kinase phosphorylation and activation. A p38 inhibitor (SB202190) inhibited the apoptotic events induced by ABT-737 but did not influence those induced by Thr/Cvx. The inhibitor also reduced the phosphorylation of cytosolic phospholipase A2 (cPLA2), an established p38 substrate, induced by ABT-737 or Thr/Cvx. ABT-737, but not Thr/Cvx, induced the caspase 3-dependent cleavage and inactivation of cPLA2. Thus, p38 MAPK promotes ABT-737-induced apoptosis by inhibiting the cPLA2/arachidonate pathway. We also show that arachidonic acid (AA) itself and in combination with Thr/Cvx or ABT-737 at low concentrations prevented apoptotic events, whereas at high concentrations it enhanced such events. Our data support the hypothesis that the p38 MAPK-triggered arachidonate pathway serves as a defense mechanism against apoptosis under physiological conditions.

Thrombopoietin amplifies ADP-induced HSP27 phosphorylation in human platelets: importance of pre-treatment

It has been shown that thrombopoietin (TPO) amplifies agonist-induced platelet activation. However, the precise mechanism of action of TPO has not yet been fully elucidated. We have previously reported that the adenosine diphosphate (ADP)‑induced phosphorylation of heat shock protein 27 (HSP27) via the p38 mitogen-activated protein (MAP) kinase pathway correlates with the ADP-induced platelet-derived growth factor (PDGF)-AB secretion and the release of soluble CD40 ligand (sCD40L) from human platelets. In the present study, we investigated the effects of TPO on platelet activation induced by ADP. We examined the effects of TPO on ADP-induced platelet activation under different treatments: TPO was administered 15 min prior to stimulation with ADP (pre-treatment); TPO and ADP were simultaneously administered (simultaneous treatment); and TPO was administered 2 min following stimulation with ADP (post-treatment). TPO, which alone had no effect on platelet aggregation, synergistically enhanced the ADP (1 mM)-induced platelet aggregation only when it was administered prior to stimulation with ADP. Pre-treatment with TPO significantly increased the secretion of PDGF-AB and the release of sCD40L, and markedly enhanced the ADP-induced phosphorylation of p38 MAP kinase and HSP27 in the platelets. However, simultaneous treatment with TPO or TPO post-treatment failed to affect the ADP-induced platelet aggregation, the secretion of PDGF-AB, the release of sCD40L and the phosphorylation p38 MAP kinase or HSP27. These results strongly suggest that pre-treatment with TPO significantly amplifies ADP-induced HSP27 phosphorylation via the p38 MAP kinase pathway in human platelets.

Significant correlation between the acceleration of platelet aggregation and phosphorylation of HSP27 at Ser-78 in diabetic patients

To clarify the mechanism underlying a high risk of thrombotic complications in diabetic patients, we investigated the relationship between HSP27 phosphorylation and the platelet activation induced by adenosine diphosphate (ADP) in diabetic patients. Platelet-rich plasma was prepared from the blood of type 2 diabetes mellitus (DM) patients. By measuring the dose response of platelet aggregation to ADP, an individual ED50 was determined. Based on the normal range identified in non-DM controls, the subjects were divided into a hyper-aggregate (Group 1) and a normo- or hypo-aggregate group (Group 2). The protein phosphorylation was analyzed by western blotting. The release of PDGF-AB and sCD40 ligand (sCD40L) was measured by ELISA. In both groups, ADP induced HSP27 phosphorylation at Ser-78 and Ser-82. The phosphorylation at Ser-78 and the release of both PDGF-AB and sCD40L induced by a low dose of ADP (1 µM) in Group 1 were significantly higher than these values in Group 2. There was a significant relationship between the ADP-induced HSP27 phosphorylation level at Ser-78 and the ADP ED50 value of platelet aggregation. The ADP (1 µM)-induced phosphorylation of HSP at Ser-78 observed in the platelets from Group 1 was inhibited by PD98059 or SB203580. The use of aspirin ameliorated the accelerated microaggregation of platelets in Group 1, and the low-dose ADP-induced phosphorylation of HSP27 at Ser-78 was no longer observed. These results strongly suggest that the phosphorylation of HSP27 at Ser-78 is correlated with the acceleration of platelet aggregation induced by ADP in type 2 DM patients.

Novel roles of cAMP/cGMP-dependent signaling in platelets

Endothelial prostacyclin and nitric oxide potently inhibit platelet functions. Prostacyclin and nitric oxide actions are mediated by platelet adenylyl and guanylyl cyclases, which synthesize cyclic AMP (cAMP) and cyclic GMP (cGMP), respectively. Cyclic nucleotides stimulate cAMP-dependent protein kinase (protein kinase A [PKA]I and PKAII) and cGMP-dependent protein kinase (protein kinase G [PKG]I) to phosphorylate a broad panel of substrate proteins. Substrate phosphorylation results in the inactivation of small G-proteins of the Ras and Rho families, inhibition of the release of Ca(2+) from intracellular stores, and modulation of actin cytoskeleton dynamics. Thus, PKA/PKG substrates translate prostacyclin and nitric oxide signals into a block of platelet adhesion, granule release, and aggregation. cAMP and cGMP are degraded by phosphodiesterases, which might restrict signaling to specific subcellular compartments. An emerging principle of cyclic nucleotide signaling in platelets is the high degree of interconnection between activating and cAMP/cGMP-dependent inhibitory signaling pathways at all levels, including cAMP/cGMP synthesis and breakdown, and PKA/PKG-mediated substrate phosphorylation. Furthermore, defects in cAMP/cGMP pathways might contribute to platelet hyperreactivity in cardiovascular disease. This article focuses on recent insights into the regulation of the cAMP/cGMP signaling network and on new targets of PKA and PKG in platelets.

Cystic fibrosis transmembrane conductance regulator (CFTR) expression in human platelets: impact on mediators and mechanisms of the inflammatory response

Inflammatory lung disease is a primary cause of morbidity and mortality in cystic fibrosis (CF). Mechanisms of unresolved acute inflammation in CF are not completely known, although the involvement of cystic fibrosis transmembrane conductance regulator (CFTR) in nonrespiratory cells is emerging. Here we examined CFTR expression and function in human platelets (PLTs) and found that they express a biologically active CFTR. CFTR blockade gave an ∼50% reduction in lipoxin A(4) (LXA(4)) formation during PLT/polymorphonuclear leukocytes (PMN) coincubations by inhibiting the lipoxin synthase activity of PLT 12-lipoxygenase. PLTs from CF patients generated ∼40% less LXA(4) compared to healthy subject PLTs. CFTR inhibition increased PLT-dependent PMN viability (33.0±5.7 vs. 61.2±8.2%; P=0.033), suppressed nitric oxide generation (0.23±0.04 vs. 0.11±0.002 pmol/10(8) PLTs; P=0.004), while reducing AKT (1.02±0.12 vs. 0.71±0.007 U; P=0.04), and increasing p38 MAPK phosphorylation (0.650±0.09 vs. 1.04±0.24 U; P=0.03). Taken together, these findings indicate that PLTs from CF patients are affected by the molecular defect of CFTR. Moreover, this CF PLT abnormality may explain the failure of resolution in CF.

p38 mitogen-activated protein kinase activation during platelet storage: consequences for platelet recovery and hemostatic function in vivo

Platelets undergo several modifications during storage that reduce their posttransfusion survival and functionality. One important feature of these changes, which are known as platelet storage lesion, is the shedding of the surface glycoproteins GPIb-alpha and GPV. We recently demonstrated that tumor necrosis factor-alpha converting enzyme (TACE/ADAM17) mediates mitochondrial injury-induced shedding of adhesion receptors and that TACE activity correlates with reduced posttransfusion survival of these cells. We now confirm that TACE mediates receptor shedding and clearance of platelets stored for 16 hours at 37 degrees C or 22 degrees C. We further demonstrate that both storage and mitochondrial injury lead to the phosphorylation of p38 mitogen-activated kinase (MAPK) in platelets and that TACE-mediated receptor shedding from mouse and human platelets requires p38 MAP kinase signaling. Protein kinase C, extracellular regulated-signal kinase MAPK, and caspases were not involved in TACE activation. Both inhibition of p38 MAPK and inactivation of TACE during platelet storage led to a markedly improved posttransfusion recovery and hemostatic function of platelets in mice. p38 MAPK inhibitors had only minor effects on the aggregation of fresh platelets under static or flow conditions in vitro. In summary, our data suggest that inhibition of p38 MAPK or TACE during storage may significantly improve the quality of stored platelets.

Heat shock protein 27 phosphorylation: kinases, phosphatases, functions and pathology

The small heat shock protein Hsp27 or its murine homologue Hsp25 acts as an ATP-independent chaperone in protein folding, but is also implicated in architecture of the cytoskeleton, cell migration, metabolism, cell survival, growth/differentiation, mRNA stabilization, and tumor progression. A variety of stimuli induce phosphorylation of serine residues 15, 78, and 82 in Hsp27 and serines 15 and 86 in Hsp25. This post-translational modification affects some of the cellular functions of Hsp25/27. As a consequence of the functional importance of Hsp25/27 phosphorylation, aberrant Hsp27 phosphorylation has been linked to several clinical conditions. This review focuses on the different Hsp25/27 kinases and phosphatases that regulate the phosphorylation pattern of Hsp25/27, and discusses the recent findings of the biological implications of these phosphorylation events in physiological and pathological processes. Novel therapeutic strategies aimed at restoring anomalous Hsp27 phosphorylation in human diseases will be presented.

Antithrombin III suppresses ADP-induced platelet granule secretion: inhibition of HSP27 phosphorylation

Antithrombin III (AT-III), an anti-coagulant, has recently been reported to directly affect human platelet functions. However, the exact mechanism of AT-III in platelets remains to be clarified. We have previously shown that adenosine diphosphate (ADP)-induced phosphorylation of heat shock protein 27 (HSP27) via p44/p42 mitogen-activated protein kinase (MAPK) and p38 MAPK is correlated with platelet granule secretion. In the present study, we investigated the relationship between AT-III and the ADP-induced platelet granule secretion. The ADP-induced secretion of platelet-derived growth factor (PDGF)-AB and serotonin (5-HT) were significantly suppressed by AT-III. The ADP-induced soluble CD40 ligand (sCD40L) release was inhibited by either PD98059, a MEK inhibitor, or SB203580, a p38 MAPK inhibitor. AT-III also inhibited the sCD40L release. AT-III markedly attenuated the ADP-induced phosphorylation levels of p44/p42 MAPK and p38 MAPK. Furthermore, the ADP-induced HSP27 phosphorylation was suppressed by AT-III. These results strongly suggest that AT-III directly acts on platelets and suppresses ADP-induced platelet granule secretion due to inhibiting HSP27 phosphorylation via p44/p42 MAPK and p38 MAPK.

Collagen-induced p38 MAP kinase activation is a biomarker of platelet hyper-aggregation in patients with diabetes mellitus

AIMS

We developed a novel method for diagnosing platelet hyper-aggregation in patients with type 2 diabetes mellitus (DM).

MAIN METHODS

By measuring the dose response of platelet aggregation to collagen, an individual ED(50) was determined. Based on the normal range identified in non-DM controls (mean+/-two SEM=0.460+/-0.082 microg/ml, n=47), type 2 DM patients were divided into high ED(50) (ED(50)>0.542 microg/ml; n=32: group I) or low ED(50) groups (ED(50)<0.378 microg/ml; n=32; group II). In these patients, collagen-induced levels of phospho-p38 MAPK and phospho-p44/p42 MAPK were measured using Western blots and enzyme-linked immunosorbent assays (ELISA).

KEY FINDINGS

In group II, the collagen (0.3 and 1 microg/ml)-induced levels of both phospho-p38 MAPK and phospho-p44/p42 MAPK measured by western blot analysis were found to be significantly higher than those in group I. The individual ED(50) was found to be significantly correlated with the collagen-induced levels of phospho-p38 MAPK and phospho-p44/p42 MAPK. This correlation was also observed when ELISA was used to measure phospho-p38 MAPK levels in a different population of DM patients (n=90).

SIGNIFICANCE

These results strongly suggest that the phosphorylation levels of collagen-induced p38 MAPK and p44/p42 MAPK represent the hyperaggregability of platelets and that the quantification of phospho-p38 MAPK can be a new and useful diagnostic biomarker of platelet hyper-aggregation in DM patients.

alphaB-crystallin extracellularly suppresses ADP-induced granule secretion from human platelets

alphaB-crystallin, a low-molecular-weight heat shock protein (HSP), has binding sites on platelets. However, the exact role of alphaB-crystallin is not clarified. In this study, we investigated the effect of alphaB-crystallin on platelet granule secretion. alphaB-crystallin attenuated the adenosine diphosphate (ADP)-induced phosphorylation of p44/p42 mitogen-activated protein kinase (MAPK) and p38 MAPK. The ADP-stimulated HSP27 phosphorylation was markedly reduced by alphaB-crystallin. alphaB-crystallin significantly suppressed the ADP-induced secretions of both platelet-derived growth factor (PDGF)-AB and serotonin. Therefore, our results strongly suggest that alphaB-crystallin extracellularly suppresses platelet granule secretion by inhibition of HSP27 phosphorylation via p44/p42 MAPK and p38 MAPK.