Lee C W, Sørensen T S, Shikama N, La Thangue N B
Division of Biochemistry and Molecular Biology, University of Glasgow, UK.
Oncogene. 1998 May 28;16(21):2695-710. doi: 10.1038/sj.onc.1201818.
Both E2F and p53 are sequence specific transcription factors that regulate early cell cycle progression. The pathway of control mediated through E2F governs the transition from G1 into S phase whereas p53 in response to genotoxic stress can facilitate cell cycle arrest or apoptosis. The mechanisms which influence the outcome of p53 induction are not clear, although transcription of the p53 target gene, encoding the cdk-inhibitor p21(Waf1/Cip1), correlates with p53-mediated cell cycle arrest. Here using a combination of biochemical and functional assays we identify p300 as a co-activator required for p53-dependent transcriptional activation of Waf1/Cip1. Furthermore, we show that the cdk-inhibitor p21(Waf1/Cip1) autoregulates in a positive fashion transcription through modulating the activity of the p53/p300 complex, whilst negatively regulating the activity of E2F by preventing cdk-dependent phosphorylation of pRb. Consistent with a role for p21(Waf1/Cip1) in the autoregulation of p53-dependent transcription, p300 augments the ability of p53 to cause G1 arrest and, conversely, cells undergoing p53-dependent apoptosis are rescued by p300. Thus, our data suggest that the ability of p300 to interact with p53 influences the physiological consequence of p53 activation. From previous studies it is known that cells expressing aberrant levels of E2F-1 can undergo p53-dependent apoptosis. In addition, we find that E2F-1 can cause apoptosis in p53-/- tumour cells and further p300, which also functions as a co-activator for the E2F/DP heterodimer, enhances the apoptotic activity of E2F-1. In conditions where E2F-1 and p53 co-operate in apoptosis E2F-1 can effectively compete for p300, causing a reduction in p53-dependent transcription. Thus, a functional interaction between p300 and either p53 or E2F-1 has a profound impact on early cell cycle progression, specifically in regulating the contrasting outcomes of cell cycle arrest and apoptosis. These results suggest a critical role for p300 in integrating and co-ordinating the functional interplay between the pathways of growth control mediated by E2F and p53.
E2F和p53都是调控细胞周期早期进程的序列特异性转录因子。通过E2F介导的调控途径控制着从G1期到S期的转变，而p53在响应基因毒性应激时可促进细胞周期停滞或凋亡。尽管p53靶基因（编码细胞周期蛋白依赖性激酶抑制剂p21（Waf1/Cip1））的转录与p53介导的细胞周期停滞相关，但影响p53诱导结果的机制尚不清楚。在这里，我们结合生化和功能分析，确定p300是Waf1/Cip1的p53依赖性转录激活所需的共激活因子。此外，我们表明细胞周期蛋白依赖性激酶抑制剂p21（Waf1/Cip1）通过调节p53/p300复合物的活性以正向方式自动调节转录，同时通过阻止pRb的细胞周期蛋白依赖性激酶依赖性磷酸化来负向调节E2F的活性。与p21（Waf1/Cip1）在p53依赖性转录的自动调节中的作用一致，p300增强了p53导致G1期停滞的能力，相反，经历p53依赖性凋亡的细胞可被p300挽救。因此，我们的数据表明p300与p53相互作用的能力影响p53激活的生理后果。从先前的研究可知，表达异常水平E2F-1的细胞可经历p53依赖性凋亡。此外，我们发现E2F-1可在p53基因敲除的肿瘤细胞中诱导凋亡，并且同样作为E2F/DP异二聚体共激活因子的p300可增强E2F-1的凋亡活性。在E2F-1和p53协同诱导凋亡的情况下，E2F-1可有效竞争p300，导致p53依赖性转录减少。因此，p300与p53或E2F-1之间的功能相互作用对细胞周期早期进程有深远影响，特别是在调节细胞周期停滞和凋亡这两种相反结果方面。这些结果表明p300在整合和协调由E2F和p53介导的生长控制途径之间的功能相互作用中起关键作用。
