Myelodysplastic syndromes (MDS) are characterized by ineffective hematopoiesis that leads to peripheral cytopenias. TGF-β is a myelosuppressive cytokine that has been indirectly linked to the pathogenesis of some subsets of MDS and acute leukemias. We demonstrate by immunohistochemistry that smad2, a component of the TGF-β signaling pathway, is constitutively activated in MDS bone marrows. This activation was seen in both low and high grade cases of MDS when compared to anemic controls and provides the first direct evidence of activation of TGF-β signaling in MDS. To demonstrate the functional role of TGF-β signaling in MDS, a GFP expressing lentiviral siRNA construct was designed to knockdown TGF receptor I (TBRI) expression. Stable expression of this lentivirus in a variety of hematopoietic cells resulted in >70% inhibition of TBRI mRNA by qPCR. Lentiviral siRNA Knockdown of TBRI in primary CD34+ cells resulted in an increase in GFP+ erythroid colony formation, indicating an inhibitory role of TGF-β in human hematopoiesis. To further test the efficacy of TBRI inhibition in stimulating hematopoiesis, we used a specific and potent inhibitor of TBRI (ALK5) kinase, SD-208. SD-208 was effectively able to inhibit TGF-β mediated activation of smad-2 and potently inhibit TGF-β mediated gene expression in bone marrow stromal cells. SD-208 treatment also led to reversal of TGF-β induced inhibition of primary CD34 cell proliferation and both erythroid and myeloid colony formation in short term assays. Most importantly, treatment with SD-208 led to significant dose dependent increases in erythroid and myeloid colony formation from primary MDS bone marrow derived hematopoietic progenitors. Furthermore, we tested the efficacy and specificity of SD-208 in a TGF-overexpressing transgenic mouse. This mouse constitutively expresses TGF-β through an albumin promoter, develops progressive anemia and dysplasia and thus serves as a novel model of human bone marrow failure. Treatment with SD-208 by oral lavage at 30mg/kg/d for two weeks led to significant increases in both myeloid and erythroid colony formation from murine bone marrows and led to a trend of increasing hemoglobin and hematocrit. Taken together, these studies demonstrate a role of TBRI inhibition in stimulating hematopoiesis in human bone marrow failure and should lead to future studies with SD-208 and other inhibitors of TGF signaling pathways in MDS.

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Disclosure: No relevant conflicts of interest to declare.