Fli-1, an Ets-related transcription factor, regulates erythropoietin-induced erythroid proliferation and differentiation: Evidence for direct transcriptional repression of the Rb gene during differentiation

Erythropoietin (Epo) is a major regulator of erythropoiesis that alters the survival, proliferation, and differentiation of erythroid progenitor cells . The mechanism by which these events are regulated has not yet been determ ined. Using HB60, a newly established erythroblastic cell line, we show her e that Epo-induced terminal erythroid differentiation is associated with a transient downregulation in the expression of the Ets-related transcription factor Fli-1, Constitutive expression of Fli-1 in HB60 cells, similar to r etroviral insertional activation of Fli-1 observed in Friend murine leukemi a virus (F-MuLV)-induced erythroleukemia, blocks Epo-induced differentiatio n while promoting Epo-induced proliferation. These results suggest that Fli -1 modulates the response of erythroid cells to Epo, To understand the mech anism by which Fli-1 regulates erythropoiesis, we searched for downstream t arget genes whose expression is regulated by this transcription factor. Her e we show that the retinoblastoma (Rb) gene, which was previously shown to be involved in the development of mature erythrocytes, contains a Fli-1 con sensus binding site within its promoter. Fli-1 binds to this cryptic Ets co nsensus site within the Rb promoter and transcriptionally represses Rh expr ession, Both the expression level and the phosphorylation status of Rb are consistent with the response of HB60 cells to Epo-induced terminal differen tiation. We suggest that the negative regulation of Rb by Fli-1 could be on e of the critical determinants in erythroid progenitor cell differentiation that is specifically deregulated during F-MuLV-induced erythroleukemia.