The Influence Of E2F1 On MDM2-p53 Feedback Loop

MDM2 expression is down-regulated upon E2F1 over-expression, but the mechanism is not well defined. In the current study, we found that E2F1 inhibits MDM2 expression by suppressing its promoter activity. Although E2F1 binds to the MDM2 promoter, the inhibitory effect of E2F1 on the MDM2 promoter does not require the direct binding. We demonstrate that E2F1 inhibits MDM2 promoter activity in a p53-dependent manner. Knockdown of p53 in U2OS cells impairs the inhibitory effect of E2F1 on the MDM2 promoter. Consistent with this observation, E2F1 does not inhibit MDM2 promoter activity in p53-deficient H1299 cells, and the inhibition is restored when p53 is expressed exogenously. Both E2F1 and p53 are upregulated after DNA damage stimulation. We show that such stimulation induces E2F1 to inhibit MDM2 promoter activity and promote p53 accumulation. Furthermore, inhibition of MDM2 by E2F1 promotes E2F1 induced apoptosis. These data suggest that E2F1 regulates the MDM2-p53 pathway by inhibiting p53 induced up-regulation of MDM2. Hypoxia stimulation can activate P53 and induce apoptosis, but P53 activation pattern is different from other DNA damage stimulation, P53 lost its transactivation capability. In our study we found that P53 binds with MYH9 and this binding is disrupted by hypoxia treatment. MYH9 motor domain binds with P53 in nuclear and this binding is regulated by P53 photosphorylation level. Prokaryotic expressed P53 protein shows no difference binding activity with MYH9 treated normoxia or hypoxia. CIAP de-photosphorylate P53 and gain P53 capbility in binding with MYH9 after hypoxia treatment. After knocking down MYH9 with siRNA-MYH9 in HCT116 p53+/+ cells P53 lost its transactivation activiy on some of its target genes. As P53 lost its binding activity with MYH9 under hypoxia treatment and also its transactivation capability, we perform MYH9 as a new P53 co-activator and its activity is repressed by hypoxia treatment.