Binding of galectin-1 to αIIbβ3 integrin triggers "outside-in" signals, stimulates platelet activation, and controls primary hemostasis

Abstract

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-galactosidebinding 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-αIIbsubunit ofαIIbβ 3 integrin or platelets from patients with Glanzmann's thrombasthenia syndrome (αIIbβ3 deficiency), identified this integrin as a functional Gal-1 receptor in platelets. Binding of Gal-1 to platelets triggered the phosphorylation of β3-integrin, Syk, MAPKs, PI3K, PLCγ2, thromboxane (TXA2) release, and Ca 2+mobilization. Not only soluble but also immobilized Gal-1 promoted platelet activation. Gal-1-deficient (Lgals1-/-) mice showed increased bleeding time (P<0.0002, knockout 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 αIIbβ 3 was also observed in Lgals1-/- platelets, indicating a failure in the "outsidein"signaling through this integrin. This study identifies a noncanonical mechanism, based on galectin-integrin interactions, for regulating platelet activation. © FASEB.