Regulation of the megakaryocytic glycoprotein IX promoter by the oncogenicEts transcription factor Fli-1

Glycoprotein (GP) IX is a subunit of the von Willebrand receptor, GPIb-V-IX , which mediates adhesion of platelets to the subendothelium of damaged blo od vessels. Previous characterization of the GPIX promoter identified a fun ctional Ets site that, when disrupted, reduced promoter activity. However, the Ets protein(s) that regulated GPIX promoter expression was unknown. In this study, transient cotransfection of several GPIX promoter/reporter cons tructs into 293T kidney fibroblasts with a Fli-1 expression vector shows th at the oncogenic protein Fli-1 can transactivate the GPIX promoter when an intact GPIX Ets site is present. In addition, Fli-1 binding of the GPIX Ets site was identified in antibody supershift experiments in nuclear extracts derived from hematopoietic human erythroleukemia cells. Comparative studie s showed that Fli-1 was also able to transactivate the GPIb alpha and, to a lesser extent, the GPIIb promoter. Immunoblot analysis identified Fli-1 pr otein in lysates derived from platelets. In addition, expression of Fli-1 w as identified immunohistochemically in megakaryocytes derived from CD34(+) cells treated with the megakaryocyte differentiation and proliferation fact or, thrombopoietin. These results suggest that Fli-1 is likely to regulate lineage-specific genes during megakaryocytopoiesis. (C) 1999 by The America n Society of Hematology.