Our understanding of hematopoiesis has evolved from the turn of the last century when early hematopoietic cells were identified by morphologic examination of marrow cells after special stains. During the 1960s and 1970s, Till, McCulloch, Metcalf, and Sachs provided us in vivo and in vitro clonogenic assays to help dissect the hematopoietic pathways. Identification of growth factors and their receptors using these assays markedly propelled our knowledge. This was enhanced by a variety of separation techniques that took advantage of cell surface antigens expressed on various hematopoietic lineages. Over the last several years, focus has turned to identifying transcription factors that could act as the control center to mediate lineage-specific differentiation; their importance has been confirmed by their forced over- or underexpression in hematopoietic cells, both in vitro and in mice. One family of transcription factors is C/EBP, which is composed of 6 members (C/EBP-α, -β, -δ, -γ, -ε, and –ζ). C/EBP-α is robustly expressed in early myelopoiesis and is thought to help mediate nascent myeloid differentiation. C/EBP-β is believed to be especially important in monocyte-macrophage differentiation. C/EBP-ε is important during the later stages of myelopoiesis as suggested by the following: (1) Specific granule deficiency syndrome can be caused by a mutation of C/EBP-ε; affected individuals have incomplete maturation of their granulocytes, are missing key granulocyte-specific proteins, and get frequent microbial infections. (2) C/EBP-ε knockout mice have the same phenotype. (3) Forced overexpression of C/EBP-ε can induce granulocytic differentiation in myeloid leukemia cell lines and cause expression of granulocyte- specific proteins in NIH3T3 fibroblasts.
Using all of the above-mentioned techniques, Nakajima and Ihle (page897) now show that a cytokine (G-CSF) can bind to its cellular receptor and through an undefined secondary signal can induce expression of C/EBP-ε to help mediate myeloid differentiation. They also show that granulocytic differentiation can be induced through more than one signaling pathway (STAT3) and that high levels of C-MYC can block both expression of C/EBP-ε and myeloid differentiation. This block can be bypassed by forced expression of C/EBP-ε. Myelopoiesis is a complicated symphony composed of a myriad of extracellular stimuli, numerous intersecting secondary signaling pathways activating a group of transcription factors that orchestrate expression of a variety of myeloid specific targets. Armed with instruments of the past including those acquired from the genome project, we are now in the era of studying interconnecting, melodious cellular pathways.
