Abstract
The efficient treatment of hematological malignancies as Acute Myeloid Leukemia, myelofibrosis and Chronic Myeloid Leukemia, requires the elimination of cancer-initiating cells and the prevention of disease relapse through targeting pathways that stimulate generation and maintenance of these cells. In mammals, inhibition of Smoothened, the key mediator of the Hedgehog signaling pathway, reduces Chronic Myeloid Leukemia progression and propagation. These findings make Smo a candidate target to inhibit maintenance of leukemia-initiating cells. In Drosophila melanogaster the same pathway maintains the hematopoietic precursor cells of the lymph gland, the hematopoietic organ that develops in the larva. Using Drosophila as an in vivo model, we investigated the mode of action of PF-04449913, a small-molecule inhibitor of the human Smo protein. Drosophila larvae fed with PF-04449913 showed traits of altered hematopoietic homeostasis. These include the development of melanotic nodules, increase of circulating hemocytes, the size increase of the lymph gland and accelerated differentiation of blood cells likely due to the exit of multi-potent precursors from quiescence. Importantly, the Smo inhibition can lead to the complete loss of hematopoietic precursors. We conclude that PF-04449913 inhibits Drosophila Smo blocking the Hh signaling pathway and causing the loss of hematopoietic precursor cells. Interestingly, this is the effect expected in patients treated with PF-04449913: number decrease of cancer initiating cells in the bone marrow to reduce the risk of leukemia relapse. Altogether our results indicate that Drosophila comprises a model system for the in vivo study of molecules that target evolutionary conserved pathways implicated in human hematological malignancies.
Highlights
A key challenge in the treatment of solid tumor and hematological malignancies with targeted therapeutic agents is the induction of stable remission and attainment of functional cure
Given that many transcription factors and signaling pathways involved in hematopoiesis are conserved between human and Drosophila [34, 35] and that Hedgehog protein (Hh) secreted from the posterior signaling center (PSC) activates the Hh pathway in blood progenitors and contributes to their maintenance [31], we developed an in vivo genetic model based on Drosophila hematopoiesis to determine the impact at the cellular level of the small molecule PF-04449913, an inhibitor of the human Smo
The average number of Green Fluorescent Protein (GFP) positive hemocytes and lamellocytes were increased in larvae fed with 400 μM PF-04449913 as compared to those fed with Dimethyl sulfoxide (DMSO) (Figure 3A, 3B)
Summary
A key challenge in the treatment of solid tumor and hematological malignancies with targeted therapeutic agents is the induction of stable remission and attainment of functional cure. Given that many transcription factors and signaling pathways involved in hematopoiesis are conserved between human and Drosophila [34, 35] and that Hh secreted from the PSC activates the Hh pathway in blood progenitors and contributes to their maintenance [31], we developed an in vivo genetic model based on Drosophila hematopoiesis to determine the impact at the cellular level of the small molecule PF-04449913, an inhibitor of the human Smo. Here we show that PF-04449913 impacts fly hematopoietic homeostasis, leading to increased and premature hemocyte differentiation and to the formation of melanotic nodules. The accessibility of the fly hematopoietic organ and the preservation of the proximity between the www.impactjournals.com/oncotarget various cell types after the dissection procedure, made possible to show that PF-04449913 leads to accelerated hemocyte differentiation, lymph gland maturation and to CZ size increase at the expense of the multipotent precursor cells Taken together, these results suggest that pharmacological inhibition of Smo in Drosophila alters hematopoietic homeostasis likely by inducing multipotent precursors to exit from quiescence, and accelerating blood cell differentiation. Our body of evidence strongly supports Drosophila as a relatively cheap and fast genetic model to study and validate in vivo, at cell level, the way of action of small molecules targeting evolutionary conserved proteins and pathways implicated in hematological diseases and malignancies
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