Maturation Stage-Specific Regulation of Megakaryocytic Genes by Pointed-Domain Ets Proteins.

The pointed-domain Ets transcription factor Fli-1 has a critical role during megakaryocyte-specific gene expression. Previously, we demonstrated that Fli-1 occupies the early megakaryocyte-specific gene αIIb in vivo. Moreover, our work suggested a mechanism for Fli-1 function by showing that Fli-1 facilitates GATA-1/FOG-1 dependent expression of the αIIb gene. However, studies by others with a targeted disruption of the Fli-1 gene in mice showed that while Fli-1 is essential for normal megakaryocyte maturation, αIIb mRNA levels were not significantly reduced in the resulting megakaryocytes, suggesting that a related Ets factor(s) might compensate for the loss of Fli-1. Here we show that the widely expressed pointed domain Ets protein GABPα specifically binds in vitro to Ets elements from two early megakaryocyte-specific genes, αIIb and c-mpl. Chromatin immunoprecipitation (ChIP) experiments using primary murine fetal liver-derived megakaryocytes reveal that GABPα associates with αIIb and c-mpl in vivo. Moreover, GABPα is capable of mediating GATA-1/FOG-1 synergy in the context of αIIb promoter constructs. These results suggest that GABPα contributes to megakaryocyte-restricted gene expression and is capable of at least partially compensating for the loss of Fli-1. However, loss of Fli-1 leads to a pronounced decrease in the expression of the late megakaryocyte-specific gene GPIX, indicating that compensation by GABPα is incomplete. Consistent with this observation, ChIP experiments fail to detect significant levels of GABPα at the regulatory region of GPIX while Fli-1 is readily detected there. Together, these results point to a model in which Fli-1 and GABPα serve overlapping, but distinct roles, during the development of megakaryocytes. GABPα may be important during early megakaryopoiesis, but Fli-1 exerting an essential role during late stages of maturation.