Correlation of PU.1 and signal regulatory protein α1 expression in PU.1 transgenic K562 cells.

PU.1 is a key transcription factor for hematopoiesis and the reduction of this protein expression plays important roles in various hematological malignancies. To identify PU.1 downstream target genes, we recently reported dual microarray analyses, using PU.1 knockdown K562 (K562PU.1KD) cells stably expressing short inhibitory RNAs versus control cells and PU.1-overexpressing K562 (K562PU.1OE) cells versus control cells. Several PU.1 candidate target genes, including cell surface receptor, signal regulatory protein (SIRP) α1, were identified. In this study, we revealed that the expression of SIRPα1 is positively correlated with the expression of PU.1 in various K562PU.1KD and K562PU.1OE cells, shown by real-time PCR and flow cytometry analyses. SIRPα1 is a negative regulator of signaling and its reduced expression is considered to play a role in the pathogenesis of hematological malignancies through the activation of downstream signaling pathways. By comparing 3 different clones of K562PU.1KD cells to their controls, we found constitutive phosphorylation of the extracellular signal-regulated kinase (ERK), but not of Akt, in these cells. Taken together, the down-regulation of PU.1 expression suppresses the expression of SIRPα1, which may play a role in the aberrant activation of ERK in these cells.