Interleukin-5 (IL-5) and granulocyte macrophage-colony\ud stimulating factor (GM-CSF) are important\ud cytokines for the proliferation, differentiation, and acti-vation\ud of myeloid lineages. The JAK/STAT pathway is\ud one of the signaling pathways implicated in mediating\ud biological responses induced by these cytokines. Previous\ud studies have demonstrated that these cytokines predomi-nantly\ud activate an 80 kDa STAT5 isoform in mature\ud granulocytes. To better understand the role of STAT pro-teins\ud during growth and differentiation of granulocytes,\ud we evaluated differentiation of human CD34^+ hematopoi-etic\ud stem cells ex vivo toward eosinophils and neutrophils.\ud Bandshift experiments showed that in an early stage of\ud both differentiation pathways (14 days), the 94 kDa\ud STAT5B protein was activated by both IL-5 (eosino-phil\ud lineage) and GM-CSF (neutrophil lineage). How-ever,\ud during maturation of both lineages (days 21 and\ud 28), increased expression of a functionally distinct 80\ud kDa STAT5 isoform was observed, resulting in het-erodimer\ud DNA-binding complexes containing both the\ud 94 and 80 kDa STAT5 proteins. The finding that\ud functionally distinct isoforms of STAT5 are activated\ud during the early and late differentiation stages of\ud granulocytes suggests that they might be involved in\ud regulating different biological functions in these cells
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