Eosinophilopoiesis: To be or not to be (an eosinophil)? That is the question: transcriptional mechanisms regulating eosinophil genes and development

Abstract

The molecular controls for eosinophil development from multipotential myeloid progenitors, and the mechanisms by which eosinophil-specific genes are expressed and regulated during eosinophil terminal differentiation, are incompletely understood. Recent studies of avian Myb-ets transformed multipotent haematopoietic progenitors suggest that eosinophil development proceeds via a common neutrophil/ macrophage/eosinophil myeloid precursor that is dependent on PU.1 expression, with the decision to enter the eosinophil program specified by expression of C/EBPs (C/ EBPa and/or b) in the context of GATA-1 (1). Impaired eosinophil development occurs in both C/EBPa and C/ EBPe deficient mice (2,3), but PU.1 null mice have not been studied. To assess the activities of C/EBPs (a, b, e) in eosinophil differentiation, we have analysed their roles in regulating the IL-5 receptor a subunit gene (IL-5Ra), expressed early in eosinophil commitment, and the major basic protein gene (MBP), expressed during terminal maturation, in human committed (AML14) promyelocyte (AML14.eos) and fully differentiated (AML14?3D10) eosinophil cell lines. Both IL-5Ra (P1) and MBP (P2) promoters (Fig. 1) possess functional C/EBP sites that bind C/EBPb and C/EBPe in nuclear extracts of eosinophildifferentiated AML14?3D10 cells. In contrast, C/EBPa complexes were detected only in the parental AML14 and AML14.eos cell lines, but not in AML14?3D10. Both C/ EBPe mRNA and binding activity was identified in all three cell lines, with AML14?3D1044AML14.eos>AML14? Human IL-5Ra promoter constructs were transactivated by C/EBPa, but not by C/EBP b, e-long (32?2 kDa) or eshort (14?3 kDa) isoforms. However, the MBP promoter was equally transactivated by C/EBPa or b, less so by e-