Thrombopoietin primes human platelet aggregation induced by shear stress and by multiple agonists

Abstract

Recombinant thrombopoietin has been reported to stimulate megakaryocytopoiesis and thrombopoiesis and it may be quite useful to treat patients with low platelet counts after chemotherapy. As little is known regarding the possible activation of platelets by thrombopoietin, we examined the effects of thrombopoietin on platelet aggregation induced by shear stress and various agonists in native plasma. Using hirudin as an anticoagulant, thrombopoietin (1 to 100 ng/mL) enhanced platelet aggregation induced by 2 micromol/L adenosine- diphosphate (ADP) in a dose dependent fashion. The enhancement was not affected by treatment of platelets with 1 mmol/L aspirin plus SQ-29548 (a thromboxane antagonist, 1 micromol/L) but was inhibited by a soluble form of the thrombopoietin receptor, suggesting that the enhancement was mediated by the specific receptors and does not require thromboxane production. Epinephrine (1 micromol/L), which does not induce platelet aggregation in hirudin platelet rich plasma (PRP), did so in the presence of thrombopoietin (10 ng/mL). Thrombopoietin (10 ng/mL) also enhanced or primed platelet aggregation induced by collagen (0.5 micron.mL),. thrombin, serotonin, and vasopressin. Thrombopoietin does not induce any rise in cytosolic ionized calcium concentration nor activation of protein kinase C, as estimated by phosphorylation of preckstrin, indicating that the priming effects of thrombopoietin does not require those processes. The ADP- or thrombin-induced rise in cytosolic ionized calcium concentration was not enhanced by thrombopoietin (100 ng/mL). Further, shear (ca. 90 dyn/cm2)-induced platelet aggregation was also potentiated by thrombopoietin. The priming effect on epinephrine-induced platelet aggregation in hirudin PRP was unique to thrombopoietin, with no effects seen using interleukin-6 (IL-6), IL-11, IL-3, erythropoietin, granulocyte-colony stimulating factor, granulocyte macrophage-colony stimulating factor, or c-kit ligand. These data indicate that monitoring of platelet functions may be necessary in the clinical trials of thrombopoietin.

Interactions Between Megakaryocytes and Tumour Cells at the Bone Marrow Vascular Stem Cell Niche Promote Tumour Growth and Metastasis.

“…Similarly, the ultrastructure of platelets is unchanged, and platelet activation in response to agonist stimulation ex vivo has been shown to be normal (Bunting et al, 1997). This is in keeping with data that TPO does not induce platelet activation, although it appears to potentiate platelet response to physiological agonists such as ADP and thrombin (Oda et al, 1996).…”

Apoptosis and megakaryocytic differentiation during ex vivo expansion of human cord blood CD34+ cells using thrombopoietin

“…TPO supports the differentiation and proliferation of MK progenitor cells and has an essential role in the complete maturation of MKs (Zeigler et al , 1994; Hunt et al , 1995). TPO also supports the formation and function of platelets in vitro (Choi et al , 1995; Oda et al , 1996). Accordingly, the clinical use of TPO for this purpose has been proposed.…”

Elevated thrombopoietin and platelet indices confirm active thrombopoiesis but fail to predict clinical severity of puumala hantavirus infection

“…[22] Moreover, high TPO has a priming effect on platelet aggregation in response to different agonists. [23] In this study, the effect of low platelet count on platelet aggregation was not compensated by the simultaneously occurring elevated TPO level and high proportion of young platelets. As major bleeding problems were avoided and no thromboembolic complications were noted, hemostasis appears clinically balanced in spite of laboratory findings reflecting impairment of primary hemostasis.…”