Ionizing radiation down-regulates p53 protein in primary Egr-1-/- mouse embryonic fibroblast cells causing enhanced resistance to apoptosis

被引:54
作者
Das, A
Chendil, D
Dey, S
Mohiuddin, M
Mohiuddin, M
Milbrandt, J
Rangnekar, VM
Ahmed, MM
机构
[1] Univ Kentucky, Dept Radiat Med, Lexington, KY 40536 USA
[2] Univ Kentucky, Markey Canc Ctr, Lexington, KY 40536 USA
[3] Washington Univ, Dept Pathol, St Louis, MO 63130 USA
关键词
D O I
10.1074/jbc.M008454200
中图分类号
Q5 [生物化学]; Q7 [分子生物学];
学科分类号
071010 ; 081704 ;
摘要
In this study, we sought to investigate the mechanism of the proapoptotic function of Egr-1 in relation to p53 status in normal isogenic cell backgrounds by using primary MEF cells established from homozygous (Egr-1(-/-)) and heterozygous (Egr-1(+/-)) Egr-1 knock-out mice. Ionizing radiation caused significantly enhanced apoptosis in Egr-1(+/-) cells (22.8%; p < 0.0001) when compared with Egr-1(-/-) cells (3.5%). Radiation elevated p53 protein in Egr-1(+/-) cells in 3-6 h, However, in Egr-1(-/-) cells, the p53 protein was down-regulated 1 h after radiation and was completely degraded at the later time points. Radiation elevated the p53-CAT activity in Egr-1(+/-) cells but not in Egr-1(-/-) cells. Interestingly, transient overexpression of EGR-1 in p53(-/-) MEF cells caused marginal induction of radiation-induced apoptosis when compared with p53(+/+) MEF cells. Together, these results indicate that Egr-1 may transregulate p53, and both EGR-1 and p53 functions are essential to mediate radiation-induced apoptosis, Rb, an Egr-1 target gene, forms a trimeric complex with p53 and MDM2 to prevent MDM2-mediated p53 degradation. Low levels of Rb including hypophosphorylated forms were observed in Egr-1(-/-) MEF cells before and after radiation when compared with the levels observed in Egr-1(+/-) cells. Elevated amounts of the p53-MDM2 complex and low amounts of Rb-MDM-2 complex were observed in Egr-1(-/-) cells after radiation. Because of a reduction in Rb binding to MDM2 and an increase in MDM2 binding with p53, p53 is directly degraded by MDM2, and this leads to inactivation of the p53-mediated apoptotic pathway in Egr-1(-/-) MEF cells. Thus, the proapoptotic function of Egr-1 may involve the mediation of Rb protein that is essential tot overcome the antiapoptotic function of MDM2 on p53.
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页码:3279 / 3286
页数:8
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