C/EBP alpha and G-CSF receptor signals cooperate to induce the myeloperoxidase and neutrophil elastase genes

'To assess cooperation between G-CSF signals and C/EBP alpha, we characteri zed Ba/F3 pro-B cell lines expressing C/EBP alpha WT-ER and the G-CSF recep tor (GCSFR). In these lines, GCSFR signals can be evaluated independent of their effect on C/EBP alpha levels. G-CSF alone did not induce the MPG, NE, LF, or PU.1 RNAs, and C/EBP alpha WT-ER alone stimulated low-level MPO and high-level PU.1 expression. Simultaneous activation of the GCSFR and C/EBP alpha WT-ER markedly increased MPO and NE induction at 24 h, and LF mRNA w as detected at 48 h. G-CSF did not increase endogenous GCSFR, endogenous C/ EBP alpha or exogenous C/EBP alpha WT-ER levels, and C/EBP alpha WT-ER did not induce endogenous or exogenous GCSFR. Several GCSFR mutants were also c o-expressed with C/EBP alpha WT-ER. Mutation of all four cytoplasmic tyrosi nes prevented NE induction but enhanced MPO induction. Mutation of Y704 was required for increased MPO induction. Consistent with this finding, removi ng IL-3 without G-CSF addition enabled MPG, but not NE, induction by C/EBP alpha WT-ER. GCSFR signals or related signals from other receptors may coop erate with C/EBP alpha to direct differentiation of normal myeloid stem cel ls.