Transcriptional repressor E2F-6 regulates apoptosis of hematopoietic stem cells

E2F-6 is a recently-identified member of the E2F family transcription factors. It has been suggested that E2F-6 acts as a dominant-negative transcriptional repressor, since it can bind to the E2F-consensus sites vis its common DNA-binding domain but lacks activation domain. In this study, we investigated the expression of E2F-6 in hematopoietic stem cells and its functional significance. First, we screened the expression of E2F subunits by RT-PCR, and found that E2F-1, E2F-2, E2F-4 and E2F-6 transcripts were present in Cd34+ human bone marrow progenitor cells. The amounts of E2F-6 mRNA decreased along with differentiation of CD34+ cells, whereas the other 3 species were up-regulated. E2F-6 mRNA was barely detectable in CD34− bone marrow mononuclear cells and in mature blood cells such as T-lymphocytes, monocytes, and granulocytes. Overexpression of E2F-65 did not affect the growth of K562 cells but suppressed both spontaneous and E2F-1-induced apoptosis. Among E2F-dependent genes, E2F-6 inhibited promoter activities of ARF, E2F-1, and c-myc but not those of thymidine kinase, DHFR, and cdc2 in transient CAT reporter assays. These results suggest that E2F-6 negatively regulates apoptosis of hematopoitic stem cells through repression of some apoptosis-inducing genes in a dominant-negative manner.