The growth factor interleukin-3 (IL-3) promotes the survival and growth of multipotent hematopoietic progenitors and stimulates myelopoiesis. It has also been reported to oppose terminal granulopoiesis and to support leukemic cell growth through autocrine or paracrine mechanisms. While most investigations of the action of hematopoietic cytokines such as IL-3 have focused on cytokine signal transduction leading to transcriptional activation, IL3 has been reported to downregulate proapoptotic Bim in factor-dependent lymphocytes (Matsui et. al., Mol. Cell, 2007). We hypothesized that IL-3 could support the myeloblast phenotype and oppose differentiation by stabilizing transcripts involved in proliferation or differentiation blockade. Kinetic microarray analysis of actinomycin chase experiments in IL-3-dependent 32Dcl3 myeloblasts demonstrated that IL-3 caused stabilization of hundreds of transcripts in pathways relevant to myeloblast function. Control of transcript decay by IL-3 did not require new transcription. Among the most IL-3 stabilized transcripts, 8 out of the top 22 gene ontology categories at a corrected significance level <0.001 were related to cell cycling, and 5 were related to RNA processing. Northern Blotting validated stabilization of transcripts by IL-3, including transcripts associated with proliferation and leukemic transformation (pik3cd, myb, pim-1), hematopoietic development (cited2), differentiation control (cdkn1a) and RNA processing (BRF1, BRF2). A domain was localized in the 3′-utr of IL-6 that mediated IL-3 responsiveness. Proteins known to modulate mRNA stability bound to this domain and to other IL-3 controlled transcripts. The v-Abl oncoprotein (that can support cytokine-independent proliferation of 32Dcl3 myeloblasts) was also shown to stabilize several of the IL-3 targeted transcripts. These findings support a new model of IL-3 action through coordinated mRNA stability control and suggest that leukemic oncoproteins can hijack the ability of IL3 to regulate the decay rate of critical transcripts in myeloblasts.

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