Regulation of SCL Expression by the Homeodomain Protein Otx-1 and the Erythroid Transcription Factor GATA-1.

Members of the bicoid homeodomain-containing proteins are important in establishing left-right asymmetry and the antero-posterior axis, suggesting that they could also be involved in asymmetric determination within the hematopoietic system. We have previously shown that Otx1, a member of the bicoid homeodomain-containing proteins, is co-expressed with the SCL transcription factor in hematopoietic pluripotent and erythroid progenitor cells and Otx1-deficiency impairs the erythroid compartment in mice, associated with decreased SCL levels. In the present study, we provide molecular and functional evidence that SCL is a direct transcriptional target of Otx1. First, we show by chromatin immunoprecipitation that Otx1 and GATA-1 are specifically bound to the SCL proximal promoter in erythroid cells. Second, Otx-1 synergizes with GATA-1 to activate transcription from the SCL proximal promoter and this activity depends on the integrity of the proximal GATA site of the SCL promoter 1a. At the molecular level, we show that this synergy occurs via a physical interaction between Otx-1 and GATA-1 in erythroid cells, which maps to the homeodomain of Otx-1. Furthermore, a gain of function of Otx1 in primary hematopoietic cells gives rise to a 6-fold increase in endogenous SCL levels, an increase in TER119-positive erythroid cells and a decrease in the number of CD11b-positive myeloid cells. Finally, a gain of function of SCL rescues the erythroid deficiency in Otx1^−/− mice, consistent with the view that SCL operates downstream of Otx1. Taken together, our observations indicate that Otx1, GATA-1 and SCL operate within the same genetic pathway to specify the erythroid fate during hematopoiesis.