Figure 1.Exploring novel putative angiogenic targets of the E2F7/8-HIF complex. (A) HIF binding site (HIF-BS) and E2F binding site (E2F-BS) analysis in promoters of genes contained by the ontology cluster angiogenesis (AmiGO term GO:0001525, 354 genes). The UCSC TFBS function within the DAVID software was used to search in each of these genes for conserved (between human, mouse and rat) binding sites within a region up to 5kb upstream of the transcription start site. (B) Identification of deregulated angiogenesis transcripts in embryos or placentas lacking E2f7/8. Data analysis was performed on a public data set (GSE30488), using Flexarray 1.6.1. After background correction with RMA, Empirical Bayes estimation (Wright&Simon) was performed. Sox2-Cre; E2f7^−/−E2f8^−/− embryos were compared with wild type embryos; and Cyp19-Cre; E2f7^−/−E2f8^−/− placentas with wild type placentas. Gene lists represent transcripts from angiogenesis genes with an adjusted P value < 0.05 vs. wild type, subdivided according to presence of HIF-BS or HIF-BS+E2F-BS as found in (A). Asterisks indicate described HIF target genes.

Figure 2.RB-E2F factors control angiogenesis through regulation of VEGFA and its receptors. In hypoxic cells HIF proteins stimulate VEGFA expression. RB1 and E2F7/8 have both been reported to interact with HIF and co-stimulate transcriptional activation of VEGFA. E2F1 downregulates VEGFA transcription through a yet unknown mechanism. The VEGFA protein is secreted by hypoxic cells, migrates through the intercellular space and binds to VEGFR2 on endothelial cells which in turn activates a downstream signaling cascade leading to the inactivation (hyper-phosphorylation) of pocket proteins such as RB1. As a result, E2F1 is activated and subsequently stimulates expression of VEGFR1/2, thereby regulating endothelial cell function. E2F7/8 on the other hand repress transcription of VEGFR1/2.