BMI1 is a target gene of E2F-1 and is strongly expressed in primary neuroblastomas

The oncogene BMI1 encodes a polycomb group transcription factor that is required for embryonic development and self-renewal of stem cells. Despite these important functions little is known about the regulation of BMI1 expression. A cDNA microarray based search for target genes of E2F-1 in neuroblastoma cells expressing a 4-OHT-regulated E2F-1-ER fusion protein identified many hitherto unknown E2F-1 regulated genes. A total of 10% of these genes, including BMI1, encode proteins that function primarily in the regulation of gene expression. The BMI1 promoter contains a putative E2F binding site that was required for the activation of a BMI1 promoter-dependent reporter construct by E2F-1. Chromatin immunoprecipitation revealed 4-OHT-dependent binding of E2F-1-ER and binding of endogenous E2F-1 to the BMI1 promoter in tumor cells. We have previously shown activation of the oncogene MYCN by E2F. Thus, in neuroblastomas deregulated E2F-1 can activate two oncogenes, MYCN and BMI1 that are known to co-operate in tumor formation. Consistent with a role of Bmi1 in neuroblastoma tumorigenesis we found strong Bmi1 expression in primary neuroblastomas. Our results reveal a novel link between E2F and polycomb transcription factors and suggest a role of Bmi1 in neuroblastomas.

E2F-1 regulates expression of FOXO1 and FOXO3a

E2F and FOXO transcription factors both play a role in neuronal apoptosis. In addition, both E2F-induced apoptosis and FOXO function are inhibited by the kinase Akt. We therefore tested whether FOXO is downstream of E2F-1 during neuronal apoptosis. We found that expression of endogenous FOXO1 and FOXO3a is induced by E2F-1. The presence of putative E2F binding sites in the promoters of both genes suggested that FOXO genes are direct targets of E2F-1. Indeed, a 4-hydroxytamoxifen activated E2F-1-ER fusion protein induced FOXO expression in the presence of cycloheximide. Moreover, E2F-1 activated the FOXO1 promoter in transient reporter assays, and E2F-1-ER as well as endogenous E2F bound to the FOXO1 promoter in vivo. Yet, E2F-1-mediated apoptosis of differentiated PC12 cells after withdrawal of NGF was not accompanied by changes in FOXO expression, indicating that no transcriptional induction of FOXO occurs during E2F-1-dependent neuronal apoptosis. In summary, our data identify E2F-1 as a first transcription factor regulating FOXO expression, providing a link between E2F and FOXO proteins in the control of cell fate.