Differential Activation of Functionally Distinct STAT5 Proteins by IL-5 and GM-CSF During Eosinophil and Neutrophil Differentiation from Human CD34^+ Hematopoietic Stem Cells

Abstract

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