Myelosuppresive Effects of Interferon [slpha] and Transforming Growth Factor β Can Be Reversed by Novel p38 MAPK Inhibitor SD-282 through Inhibition of Cell Cycle Arrest.

Cytokines play important roles in the regulation of normal hematopoiesis and a balance between the actions of hematopoietic growth factors and myelosuppressive factors is required for optimal production of cells of different hematopoietic lineages. Even though the effects of Type I Interferons (IFNs α,β) and Transforming Growth Factor βs (TGF βs) as negative regulators of hematopoiesis are well documented, the exact molecular mechanisms by which such effects occur remain unknown. Our previous studies had shown that pharmacological inhibition of the p38 MAPK with commercially available inhibitors SB203580 and SB 202190 was able to reverse the myelosuppresion caused by IFN and TGF β. These inhibitors cannot be used in human studies due to toxicity and are also questioned for their selectivity in inhibiting the p38 MAPK. Thus, to confirm the role of p38 MAPK in regulating hematopoeisis, we conducted experiments with SD-282, a more potent and selective inhibitor of p38 α. SD-282 also performs very similarly in animal and cell models to a p38 inhibitor now in the clinic. Our results show that SD-282 is able to inhibit p38 MAPK selectively in primary human erythroid progenitors (at CFU-E stage of maturation) and suppress activation of downstream kinase MapKapK-2 after IFN α stimulation. In methycellulose clonogenic assays with mobilized CD34+ cells, IFN α treatment resulted in marked suppression of both erythroid (BFU-E) and myeloid (CFU-GM) colonies, which could be reversed in the presence of p38 inhibitor SD-282. In a similar manner TGF-β2 was not able to effectively inhibit both erythroid and myeloid colonies in the presence of p38 blockade by SD-282. In further studies, we demonstrate that the primary mechanism by which the p38 MAPK pathway mediates IFN mediated hematopoietic suppression is by regulation of cell cycle progression and is unrelated to induction of apoptosis. Treatment with p38 inhibitors led to significantly lesser numbers of cells in G0/G1 phase of cell cycle arrest induced by exposure to IFN α. Altogether, these findings confirm that the p38 MAPK signalling pathway is a common effector for type I IFN and TGF beta signaling in human hematopoietic progenitors and plays a critical role in the induction of the suppressive effects of these cytokines on normal hematopoiesis. Our studies also provide a rationale for the use of SD-282 and other p38 inhibitors in cytokine mediated hematological diseases.