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Understanding noncanonical mechanisms of platelet activation represents an important challenge for the identification of novel therapeutic targets in bleeding disorders, thrombosis, and cancer. We previously reported that galectin-1 (Gal-1), a β-galactoside-binding protein, triggers platelet activation in vitro. Here we investigated the molecular mechanisms underlying this function and the physiological relevance of endogenous Gal-1 in hemostasis. Mass spectrometry analysis, as well as studies using blocking antibodies against the anti-αIIb subunit of αIIbβε integrin or platelets from patients with Glanzmann's thrombasthenia syndrome (αIIbβε deficiency), identified this integrin as a functional Gal-1 receptor in platelets. Binding of Gal-1 to platelets triggered the phosphorylation of βε-integrin, Syk, MAPKs, PI3K, PLCγ2, thromboxane (TXA2) release, and Ca2+ mobilization. Not only soluble but also immobilized Gal-1 promoted platelet activation. Gal-1-deficient (Lgals1-/-) mice showed increased bleeding time (P<0.0002, knock-out vs. wild type), which was not associated with an abnormal platelet count. Lgals1-/- platelets exhibited normal aggregation to PAR4, ADP, arachidonic acid, or collagen but abnormal ATP release at low collagen concentrations. Impaired spreading on fibrinogen and clot retraction with normal levels of αIIbβε was also observed in Lgals11-/- platelets, indicating a failure in the "outside-in" signaling through this integrin. This study identifies a noncanonical mechanism, based on galectin-integrin interactions, for regulating platelet activation. [ABSTRACT FROM AUTHOR]

BACKGROUND: Transplanted hematopoietic progenitor cells (CD34+) have shown great promise in regenerative medicine. However, the therapeutic potential of transplanted cells is limited by their poor viability. It is well known that the microenvironment in which progenitors reside substantially affects their behavior. Because extracellular acidosis is a common feature of injured tissues or the tumor microenvironment and is a critical regulator of cell survival and activation, we evaluated the impact of acidosis on CD34+ cell biology. STUDY DESIGN AND METHODS: Apoptosis was evaluated by fluorescence microscopy and binding of annexin V, hypodiploid cells, Bcl-xL expression, active caspase-3, and mitochondrial membrane potential was determined by flow cytometry. Colony-forming units were studied by clonogenic assays, and cell cycle was evaluated by flow cytometry. RESULTS: Exposure of CD34+ cells to low pH (7.0-6.5) caused intracellular acidification, decreased cell proliferation, and triggered apoptosis via the mitochondrial pathway. Whereas exposure to thrombopoietin (TPO), stem cell factor (SCF), interleukin (IL)-3 or increases in cyclic adenosine monophosphate (cAMP) levels prevented CD34+ cell death induced by acidic conditions, granulocyte-macrophage-colony-stimulating factor, FMS-like tyrosine kinase 3-ligand, erythropoietin, and vascular endothelial growth factor had no effect. Despite their cytoprotective effect, CD34+ cell expansion triggered by TPO, SCF, or IL-3 was significantly impaired at low pH. However, a cocktail of these three cytokines synergistically supported proliferation, cell cycle progression, and colony formation. DISCUSSION: Our findings indicate that an acidic milieu is deleterious for CD34+ cells and that a combination of certain cytokines and cAMP donors may improve cell viability and function. These data may be useful to develop new therapeutic strategies or to optimize protocols for regenerative medicine. [ABSTRACT FROM AUTHOR]

Platelet activation at sites of vascular injury leads to the formation of a hemostatic plug. Activation of platelets is therefore crucial for normal hemostasis. However, uncontrolled platelet activation may also lead to the formation of occlusive thrombi that can cause ischemic events. Platelets can be activated by soluble molecules including thrombin, TXA [ABSTRACT FROM AUTHOR]

Platelet activation is a critical process during inflammation, thrombosis, and cancer. Here, we show that galectin-1, an endogenous lectin with immunoregulatory properties, plays a key role in human platelet activation and function. Galectin-1 binds to human platelets in a carbohydrate-dependent manner and synergizes with ADP or thrombin to induce platelet aggregation and ATP release. Furthermore, galectin-1 induces F-actin polymerization, up-regulation of P-selectin, and GPIIIa expression; promotes shedding of microvesicles; and triggers conformational changes in GPIIb/IIIa. In addition, exposure to this lectin favors the generation of leukocyte-platelet aggregates. A further mechanistic analysis revealed the involvement of Ca2+ and cyclic nucleotide-dependent pathways in galectin-1-mediated control of platelet activation. Finally, expression of endogenous galectin-1 in human platelets contributes to ADP-induced aggregation. Our study reveals a novel unrecognized role for galectin-1 in the control of platelet physiology with potential implications in thrombosis, inflammation, and metastasis. [ABSTRACT FROM AUTHOR]

We have previously demonstrated that nitric oxide (NO) triggers CD34+-derived megakaryocyte apoptosis. We here show that prostacyclin (PGI2) inhibits PAPA/NO-induced megakaryocyte death detected by fluorescent microscopy and flow cytometry.The cAMP-specific phosphodiesterase inhibitor, Ro 20-1724, and the permeable analog dibutyryl-cAMP also delayed apoptosis. PGI2 effect was fully prevented when adenylyl cyclase activity was suppressed by SQ 22536, and partially reversed by the permeable protein kinase A inhibitor PKI 14-22 amide. ELISA showed that while both PGI2 and NO alone or synergistically raised cAMP, only NO was able to increase intracellular cGMP levels.Treatment of megakaryocytes with PGI2 abolished both basal and NO-raised cGMP levels. Addition of 8-pCPT-cGMP or activation of soluble guanylyl cyclase by BAY 41-2272 induced cell death in a concentration-dependent manner, and ODQ, an inhibitor of guanylyl cyclase, prevented both PAPA/NO- or BAY 41-2272-induced apoptosis. Specific cGMP phosphodiesterase inhibition by Zaprinast or suppression of adenylyl cyclase by SQ 22536 enhanced the PAPA/NO proapoptotic effect.PGI2 completely inhibited NO-mediated generation and the increased activity of the cleaved form of caspase-3.In conclusion, our results demonstrate that contrary to their well-known direct and synergistic inhibitory effects on platelets, PGI2 and NO regulate opposite megakaryocyte survival responses through a delicate balance between intracellular cyclic nucleotide levels and caspase-3 activity control.British Journal of Pharmacology (2005) 145, 283-292. doi:10.1038/sj.bjp.0706200 Published online 21 March 2005 [ABSTRACT FROM AUTHOR]

A letter to the editor depicting the role of a transcriptional factor and a prostaglandin in inducing cell growth arrest and killing of haematopoietic progenitor cells, is presented.