Krüppeling megakaryopoiesis

Erythroid Krüppel-like factor (EKLF) controls the balance between erythroid and megakaryocytic cells.

Until recently, zinc finger transcription factor EKLF has been believed to act largely during late stages of erythropoiesis, where it activates numerous genes encoding β-globin, Eraf (AHSP), and erythroid membrane proteins.^1,2  This notion was supported by the phenotype of EKLF-null mice, which succumb to severe anemia.^3,4  In this issue of Blood, Frontelo and colleagues report a new function for EKLF manifested at earlier stages of hematopoiesis. Specifically, they show that EKLF is expressed at low levels in common myeloid progenitor cells, followed by substantially increased expression in bipotential erythro-megakaryocytic precursors (MEPs). EKLF expression is subsequently maintained at high levels in erythroid cells but is virtually extinguished in megakaryocyte progeny. Notably, conditional overexpression of EKLF in differentiating ES cells reduced the formation of megakaryocytes while increasing the number of erythroid cells. Moreover, the authors defined an early window during which megakaryocyte formation is susceptible to inhibition by EKLF. On the flip side, fetal liver cultures from EKLF-null mice displayed increased numbers and size of megakaryocyte colonies. These findings reveal a previously unsuspected inhibitory role of EKLF during early megakaryopoiesis. Furthermore, they are consistent with EKLF exerting its effects on lineage specification at the MEP stage, although this was not directly examined.

Mechanistic studies show that EKLF-null fetal livers exhibit elevated levels of select megakaryocyte-specific genes, prominently including the ETS family transcription factor Fli-1. Transient expression of EKLF inhibited the activity of a reporter gene driven by the Fli-1 promoter upstream region; moreover, EKLF was detected at the Fli-1 promoter by ChIP, suggesting a direct mechanism of transcriptional repression. The same group had previously shown that EKLF has the po-tential to repress transcription, but the physiological relevance of this finding has been elusive.⁵  Fli-1 might thus be the first bona fide example of a biologically important EKLF-repressed target gene.

Taken together, this work leads to the appealing model of EKLF controlling the fate decision of MEPs by activating erythroid genes and simultaneously inhibiting megakaryocytic ones. How EKLF levels are controlled in differentiating MEPs now becomes an important question.

Most transcription factors that regulate erythroid and megakaryocyte development, including GATA-1, FOG1, NF-E2, and SCL/TAL-1, are expressed in both lineages. The present study provides a contrasting example of a transcription factor that promotes erythroid differentiation, while suppressing the formation of the alternative megakaryocytic lineage. Moreover, the authors identify a regulatory antagonism between 2 nuclear fac-tors that promote erythroid (EKLF) and megakaryocytic (Fli-1) transcription.

How is EKLF recruited to repressed target genes? EKLF levels detected by ChIP at the Fli-1 gene were low, suggesting that EKLF might bind there indirectly via a DNA-bound protein intermediary. Identification of proteins capable of recruiting EKLF to this and other genes would be an important addition to our understanding of the EKLF transcription factor network.

The work by Frontelo et al further suggests that Fli-1 is not the only target for inhibition by EKLF, since Fli-1 is required predominantly for genes expressed during late megakaryocyte maturation. Thus, factors required during early megakaryopoiesis, such as the gene encoding the ETS factor GABPα,⁶  are candidate targets for EKLF-mediated repression.

Finally, it will be important to determine how EKLF discriminates between activating and repressive functions. Several known coregulators and posttranslational modifications influence EKLF activity and are good places to start addressing this question.