Cyclic AMP Responsive Element Binding Proteins Are Involved in 'Emergency' Granulopoiesis through the Upregulation of CCAAT/Enhancer Binding Protein β

In contrast to the definitive role of the transcription factor, CCAAT/Enhancer binding protein alpha (C/EBP alpha), in steady-state granulopoiesis, previous findings have suggested that granulopoiesis during emergency situations, such as infection, is dependent on C/EBP beta. In this study, a novel lentivirus-based reporter system was developed to elucidate the molecular switch required for C/EBP beta-dependency. The results demonstrated that two cyclic AMP responsive elements (CREs) in the proximal promoter region of C/EBP beta were involved in the positive regulation of C/EBP beta transcription during granulocyte-macrophage colony-stimulating factor (GM-CSF)-induced differentiation of bone marrow cells. In addition, the transcripts of CRE binding (CREB) family proteins were readily detected in hematopoietic stem/progenitor cells. CREB was upregulated, phosphorylated and bound to the CREs in response to GM-CSF stimulation. Retroviral transduction of a dominant negative CREB mutant reduced C/EBP beta mRNA levels and significantly impaired the proliferation/differentiation of granulocyte precursors, while a constitutively active form of CREB facilitated C/EBP beta transcription. These data suggest that CREB proteins are involved in the regulation of granulopoiesis via C/EBP beta upregulation.