We have previously demonstrated that homeodomain proteins, MEIS1 and PBXs, transactivate the PF4 gene through the novel regulatory element termed TME. This study focuses on Pbx regulating protein 1 (PREP1), a MEIS1 homolog protein, for its transcriptional activity in the PF4 promoter. PREP1 binds to the TME in HEL cells. PREP1 was expressed in human megakaryocytes that differentiated from CD34(+) cells. EMSA shows that either PREPI by itself or PREP1/PBX complexes bind to the two TGACAG motifs in the TME and activate the PF4 promoter. Furthermore, PREPI and PREP1/PBX complexes synergistically activate the PF4 promoter with GATA-1 and ETS-1. These data demonstrate that PREPI is also an important transcription factor that regulates PF4 gene expression such as MElS1. Additionally, these data imply functional similarities and differences between PREPI and MEIS1 in the regulation of PF4 gene expression. (C) 2003 Elsevier Science (USA). All rights reserved.