Thrombopoietin increases platelet adhesion under flow and decreases rolling

Abstract

Summary. Thrombopoietin (TPO) is known to sensitize platelets to other agonists at 20 ng/ml, and above 100 ng/ml it is an independent activator of aggregation and secretion. In studies with a perfusion chamber, TPO, between 0AE01 ng/ml and 1 ng/ml, increased platelet adhesion to surface-coated fibrinogen, fibronectin and von Willebrand Factor (VWF) but not to a collagen-coated surface. Increased adhesion was observed at shear rates of 300/s and 800/s in perfusions with whole blood as well as in suspensions of platelets and red blood cells reconstituted in plasma. The by the cyclooxygenase inhibitor, indomethacin, and the thromboxane A 2 -receptor blocker, SQ30741, abolished the stimulation by TPO. The effect of TPO was mimicked by a very low concentration (10 nmol/l) of the thromboxane TxA 2 analogue, U46619. Real-time studies of platelet adhesion to a VWF-coated surface at a shear of 1000/s showed that about 20% of the platelets were in a rolling phase before they became firmly attached. TPO (1 ng/ml) pretreatment reduced this number to < 5%, an effect again abolished by indomethacin. Thus, TPO potentiates the direct and firm attachment of platelets to surface-coated ligands for a IIb b 3 , possibly by increasing the ligand affinity of the integrin.

Megakaryocyte development and platelet production

“…In platelets, TPO enhances a-granule secretion and aggregation that is induced by thrombin in a phosphoinositide-3 kinase (PI3K)-dependent fashion (Kojima et al, 2001). This powerful cytokine also affects mature platelets, reducing the level of ADP, collagen, or thrombin necessary for aggregation (Oda et al, 1996;Pasquet et al, 2000), and stimulates platelet adhesion (Van Os et al, 2003).…”

The molecular mechanisms that control thrombopoiesis

“…In suspension culture, thrombopoietin stimulates the formation of large, highly polyploid megakaryocytes that can form proplatelet processes that then fragment into immature and mature platelets (39). The hormone also affects mature platelets, reducing the level of ADP, collagen, or thrombin needed to induce aggregation (40,41), and enhances platelet adhesion to fibrinogen, fibronectin, and vWF in parallel plate perfusion chamber assays (42).…”

Autosomal Dominant Hereditary Essential Thrombocythemia due to a Gain of Function Mutation in the Thrombopoietin (TPO) and JAK2 Gene as the Cause of Congenital Aspirin-Responsive Sticky Platelet Syndrome: Personal Experiences and Review of the Literature

“…Pre-incubation of platelet with 20 ng/ml TPO for 5 minutes increases the amount of serotonin secretion by low a low dose of the agonist thrombin (0.1 U/ mL) [23]. Activation of platelets induced by TPO and thrombin leads to the subsequent activation of the enzyme cPLA2, which liberates arachidonic acid from membrane phospholipids, which is the source for platelet cyco-oxygenase (COX-1), thereby causing spontaneous platelet aggregation with formation of platelet thrombi in the endarterial circulation at places of high shear rate consistent with aspirin responsive platelet sticky syndrome (SPS) [17,23,[30][31][32][33]. In addition to the activation of platelets free in suspension (plasma) by soluble agonist such as thrombin, platelets bind to each other mediated by the von Willebrand factor (VWF) as well as to subendotheliam and specific surface bound VWF protein that become accessible in the damaged vessel wall [23].…”