Effect of thrombopoietin receptor agonists on the apoptotic profile of platelets in patients with chronic immune thrombocytopenia

Abstract

Platelet survival depends upon mediators of apoptosis e.g., Bcl-x L, Bax, and Bak, which are regulated by thrombopoietin (TPO)-mediated AKT signaling. Thrombopoietin receptor (TPO-R) signaling might decrease platelet and/or megakaryocyte apoptosis and increase the platelet count. This study therefore explored antiapoptotic effects of TPO-R-agonists in vivo on platelets of patients with immune thrombocytopenia. Patients received eltrombopag or romiplostim for two weeks. Total, immature, and large platelet counts were assessed as were Bcl-x L inhibitor assay; Bcl-x L Western blot; and flow cytometric (FACS) analysis of the AKTsignaling pathway. Eight/ten patients had platelet responses to eltrombopag and all three to romiplostim. Platelet sensitivity to apoptosis by Bcl-xL inhibition was greater in pretreatment patients than controls. This sensitivity normalized after one week of therapy, but surprisingly returned to pretreatment levels at week two. FACS analysis revealed increased AKT-pathway signaling after one week, followed by a decrease at week two. Platelet counts correlated with the Bcl-x L /Bak ratio. Platelet survival may be enhanced by TPO-Ragonists as a transient decrease in platelet sensitivity to apoptosis was accompanied by transient activation of AKT. However, this mechanism has only a short-lived effect. Megakaryocytes and platelets already present at the start of TPO-R-agonist treatment appear to respond differently than those generated de novo.

Romiplostim promotes platelet recovery in a mouse model of multicycle chemotherapy-induced thrombocytopenia

“…It would therefore be interesting to examine whether additional administration of romiplostim later in the cycle would be of benefit. Interestingly, a recent report has found that newly generated platelets from immune thrombocytopenia patients that were treated with TPO-receptor agonists were less prone to apoptosis after the first week than platelets analyzed before treatment [25]. However, this returned to baseline after 2 weeks.…”

“…However, this returned to baseline after 2 weeks. This may be due to direct or indirect action of the TPO-receptor agonists on newly generated platelets to increase prosurvival AKT signaling downstream of the TPO receptor [25]. Although a potential direct action of TPO-receptor agonists on platelets could raise a theoretical concern about platelet activation, TPOreceptor agonists are generally well tolerated.…”

Pharmacological characterization of hetrombopag, a novel orally active human thrombopoietin receptor agonist

“…TPO/TPOR signalling is able to inactivate or induce more than 100 proteins, and generally exerts a prosurvival effect. 32 For the first time, we found that TPOR agonists, including hetrombopag and eltrombopag, as well as rhTPO, effectively up-regulated the G 1 -phase-related proteins p-RB, Cyclin D1 and CDK4/6, and normalized the cell-cycle profile in 32D-MPL cells. To the best of our knowledge, these interesting effects on cell cycling revealed by our investigation of hetrombopag have not been investigated in previous studies on TPOR agonists.…”

“…TPO/TPOR signalling is able to inactivate or induce more than 100 proteins, and generally exerts a prosurvival effect . For the first time, we found that TPOR agonists, including hetrombopag and eltrombopag, as well as rhTPO, effectively up‐regulated the G 1 ‐phase–related proteins p‐RB, Cyclin D1 and CDK4/6, and normalized the cell‐cycle profile in 32D‐MPL cells.…”

“…To the best of our knowledge, these interesting effects on cell cycling revealed by our investigation of hetrombopag have not been investigated in previous studies on TPOR agonists. A previous study showed that TPOR agonists affect the apoptotic profile of platelets in patients with chronic immune thrombocytopenia . TPOR agonists have also been reported to exert antiapoptotic activity in TPOR‐positive cells .…”