Cell cycle regulator E2F1 modulates angiogenesis via p53-dependent transcriptional control of VEGF

Abstract

The transcription factor E2F1 is known to regulate cell proliferation and has been thought to modulate tumorigenesis via this mechanism alone. Here we show that mice deficient in E2F1 exhibit enhanced angiogenesis. The proangiogenic phenotype in E2F1 deficiency is the result of overproduction of vascular endothelial growth factor (VEGF) and is prevented by VEGF blockade. Under hypoxic conditions, E2F1 down-regulates the expression of VEGF promoter activity by associating with p53 and specifically downregulating expression of VEGF but not other hypoxia-inducible genes, suggesting a promoter structure context-dependent regulation mechanism. We found that the minimum VEGF promoter mediating transcriptional repression by E2F1 features an E2F1-binding site with four Sp-1 sites in close proximity. These data disclose an unexpected function of endogenous E2F1: regulation of angiogenic activity via p53-dependent transcriptional control of VEGF expression.E2F ͉ endothelial cell T he coordinated regulation of cell cycle progression and apoptosis is critical for the maintenance of homeostasis in living organisms. In mammals, the intimate relationship between cell cycle and programmed cell death is well illustrated by the evolution of our understanding of the role of the transcription factor E2F1. Initially described as an inducer of cell cycle progression (1-4), the generation of an E2F1 deficient mouse was anticipated to result in a hypoplastic, potentially lethal phenotype. In contrast, these mice exhibit a striking combination of tumors and tissue atrophy (5-7).Vascular homeostasis is also dependent on appropriate regulation of vascular cell cycle and apoptosis. Vascular injury, induced by tissue ischemia or vascular trauma, for example after balloon angioplasty, induces the local proliferation and migration of endothelial cells required to restore micro-and macrovascular integrity. Dysregulation of these processes can have severe consequences. For example, excessive proliferation of endothelial cells is associated with tumor growth and metastasis (8), whereas a deficient endothelial proliferative response to ischemia can result in tissue necrosis. The role of E2F1 in regulating vascular proliferation remains controversial (9-13).We have previously shown that tumor necrosis factor ␣ (TNF-␣), expressed at sites of vascular injury, suppresses endothelial cell (EC) proliferation by repressing E2F1 activity (11). More recently, we demonstrated that ectopic E2F1 overexpression at sites of angioplasty-induced injury promotes EC growth and suppresses apoptosis, thereby promoting functional endothelial recovery (12). In contrast, others (9) have shown that inhibition of E2F signaling in venous bypass grafts prevents the development of occlusive thickening of the vessel wall highlighting the complex role of E2F signaling in vascular biology.To further define the role of E2F1 in EC proliferation and differentiation, we initiated a series of experiments in mice deficient in E2F1 (E2F1 Ϫ/Ϫ ) to evaluate angiogenesis, a process crit...

Loss of autocrine endothelial-derived VEGF significantly reduces hemangiosarcoma development in conditional p53-deficient mice

“…This inhibition is achieved via different mechanisms. 19,20 p21 is a well-known downstream target of p53 and is also a negative regulator of VEGF. 21 Intriguingly, the second most common tumors in p21-null mice are of vascular origin.…”

“…[16][17][18] These findings suggest that alteration in the Trp53 gene could be associated with tumorigenesis of the blood vessel endothelium. In addition, p53 inhibits VEGF expression via SP1, 19 E2F, 20 and p21/ RB-dependent 21 processes. Overall then, we hypothesized that decreased p53 expression and upregulated VEGF levels might act synergistically to transform the vascular endothelium.…”

The transcription factor E2F1 and the SR protein SC35 control the ratio of pro-angiogenic versus antiangiogenic isoforms of vascular endothelial growth factor-A to inhibit neovascularization in vivo

“…However, based on the growing number of studies published in the past decade, it is now clear that E2F1 has numerous other biological roles. Recently, a new function for E2F1 as a negative regulator of angiogenesis has been proposed based on the revelation that mice deficient in E2F1 exhibit enhanced angiogenesis as a result of VEGF overproduction (Qin et al, 2006). In this model, increased angiogenesis occurs during both recovery from ischaemic injury and tumour growth and is dependent on wildtype p53.…”

“…In a murine system, it was previously shown that E2F1 decreases the level of VEGF in a p53-dependent manner (Qin et al, 2006). To test whether E2F1 could also control VEGF expression in human tumour cells deficient in p53, we took advantage of previously established stable E2F1-inducible clones in the H358 cell line derived from a human lung adenocarcinoma.…”

“…Indeed, E2F1 functions in a wide range of biological processes, including DNA replication, mitosis, DNA repair, cell differentiation and apoptosis (Polager and Ginsberg, 2008). In addition, E2F1 downregulates the activity of the VEGF promoter under hypoxic conditions through a p53-dependent mechanism, unravelling an unexpected function of E2F1 in the control of angiogenesis (Qin et al, 2006). Recently, we established the first link between E2F1-and pre-mRNAprocessing events by identifying SC35, a member of the SR (Ser-Rich/Arg) family of proteins that is involved in constitutive and alternative pre-mRNA splicing, as a direct transcriptional target of E2F1 (Merdzhanova et al, 2008).…”