Thrombopoietin receptor–independent stimulation of hematopoietic stem cells by eltrombopag

Abstract

Eltrombopag (EP), a small-molecule thrombopoietin receptor (TPO-R) agonist and potent intracellular iron chelator, has shown remarkable efficacy in stimulating sustained multilineage hematopoiesis in patients with bone marrow failure syndromes, suggesting an effect at the most immature hematopoietic stem and multipotent progenitor level. Although the functional and molecular effects of EP on megakaryopoiesis have been studied in the past, mechanistic insights into its effects on the earliest stages of hematopoiesis have been limited. We investigated the effects of EP treatment on hematopoietic stem cell (HSC) function using purified primary HSCs in separation-of-function mouse models, including a TPO-R-deficient strain, and stem cells isolated from patients undergoing TPO-R agonist treatment. Our mechanistic studies showed a stimulatory effect on stem cell self-renewal independently of TPO-R. Human and mouse HSCs responded to acute EP treatment with metabolic and gene expression alterations consistent with a reduction of intracellular labile iron pools that are essential for stem cell maintenance. Iron preloading prevented the stem cell stimulatory effects of EP. Moreover, comparative analysis of stem cells in the bone marrow of patients receiving EP showed a marked increase in the number of functional stem cells compared to patients undergoing therapy with romiplostim, another TPO-R agonist lacking an iron-chelating ability. Together, our study demonstrates that EP stimulates hematopoiesis at the stem cell level through iron chelation-mediated molecular reprogramming and indicates that labile iron pool-regulated pathways can modulate HSC function.

Eltrombopag maintains human hematopoietic stem and progenitor cells under inflammatory conditions mediated by IFN-γ

“…Because eltrombopag activates c-MPL by association with a specific amino acid found only in humans and chimpanzees, but not in rodents, 45 it has been postulated that the iron-binding properties of eltrombopag are independent of c-MPL, perhaps via direct diffusion into cells. 49,54 Although data from our study confirm the established dependence of eltrombopag on c-MPL expression in HSPCs and activation of downstream signaling for its activity in target cells, they do not exclude that some of the beneficial effects of eltrombopag may be independent of c-MPL stimulation and restoration of signal transduction cascades in HSPCs. Thus, both mechanistic models are likely complementary rather than exclusionary.…”

Efficacy and safety of romiplostim in refractory aplastic anaemia: a Phase II/III, multicentre, open‐label study

“…Eltrombopag has been shown to stimulate haematopoiesis at the stem cell level through iron chelationmediated molecular re-programming, whereas romiplostim does not exhibit iron-chelating activity. 15 Furthermore, the agents have different locations for TPO-receptor binding: romiplostim binds to the extracellular region of the receptor and eltrombopag to the transmembrane region. 16 At the end of the study, week 53, 12 (39%) patients had achieved a trilineage response and 20 (65%) patients had achieved a platelet response.…”

“…G-banding analysis revealed 45, XX, À7 [4]/45, idem, del(5)(q?) [1]/46, XX [15]. FISH analysis of BM detected monosomy 7.…”

Multifaceted roles of thrombopoietin in hematopoietic stem cell regulation

“…47 Recently, it was reported that, in addition to its effect on activating Thpo-Mpl signaling in HSCs, eltrombopag enhances the stem cell potential of human HSCs through the reduction of intracellular iron, due to its structure containing a chelator motif. 48 Eltrombopag effectively lowered cellular labile iron concentration, as analyzed by enhanced staining to calcein AM in HSCs. These data suggest that the action of Thpo mimetic drugs on HSCs may differ.…”

“…Despite high endogenous Thpo levels, the effect of eltrombopag on HSCs can be sustained even after the termination of the drug in aplastic anemia patients . Recently, it was reported that, in addition to its effect on activating Thpo–Mpl signaling in HSCs, eltrombopag enhances the stem cell potential of human HSCs through the reduction of intracellular iron, due to its structure containing a chelator motif . Eltrombopag effectively lowered cellular labile iron concentration, as analyzed by enhanced staining to calcein AM in HSCs.…”