Dimri G P, Hara E, Campisi J
Department of Cell and Molecular Biology, Lawrence Berkeley Laboratory, University of California, Berkeley 94720.
J Biol Chem. 1994 Jun 10;269(23):16180-6.
Several mammalian genes expressed in late G1 are positively regulated by E2F, a heterodimeric transcription factor. Genes encoding two E2F proteins, E2F-1 and DP-1, were regulated differently during the cell cycle and replicative senescence of normal human fibroblasts. In presenescent cells, E2F-1 mRNA was cell-cycle regulated, appearing a few hours before S phase. By contrast, DP-1 mRNA was constitutively expressed, independent of position in the cell cycle. After a finite number of divisions, normal cells enter a state of irreversible growth arrest termed senescence. Many genes remain mitogen-inducible in senescent cells; there are, however, exceptions, including several late G1 genes potentially regulated by E2F. Senescent cells expressed DP-1 at the presenescent level, but did not express E2F-1 mRNA. Senescent cells were also markedly deficient in E2F binding activity associated with the dihydrofolate reductase promoter. E2F-1 and DP-1 expression vectors only weakly induced DNA synthesis in quiescent or senescent human cells and immortal murine NIH3T3 cells, although the E2F-1 vector stimulated DNA synthesis in immortal murine A31 cells, and transactivated E2F-responsive promoters in NIH3T3 cells. The results suggest that senescent cells may fail to express late G1 genes due to repression of E2F-1, leading to a deficiency of E2F activity. Furthermore, although E2F-1 stimulates DNA synthesis in some cells, other cells, including normal human fibroblasts, require additional factors.
几种在G1晚期表达的哺乳动物基因受到异源二聚体转录因子E2F的正向调控。编码两种E2F蛋白E2F-1和DP-1的基因在正常人成纤维细胞的细胞周期和复制性衰老过程中受到不同的调控。在衰老前细胞中，E2F-1 mRNA受细胞周期调控，在S期前几小时出现。相比之下，DP-1 mRNA组成性表达，与细胞周期位置无关。经过有限次数的分裂后，正常细胞进入不可逆的生长停滞状态，即衰老。许多基因在衰老细胞中仍可被丝裂原诱导；然而，也有例外，包括几个可能受E2F调控的G1晚期基因。衰老细胞以衰老前水平表达DP-1，但不表达E2F-1 mRNA。衰老细胞中与二氢叶酸还原酶启动子相关的E2F结合活性也明显缺乏。E2F-1和DP-1表达载体在静止或衰老的人类细胞以及永生化的小鼠NIH3T3细胞中仅微弱诱导DNA合成，尽管E2F-1载体在永生化的小鼠A31细胞中刺激DNA合成，并在NIH3T3细胞中转录激活E2F反应性启动子。结果表明，衰老细胞可能由于E2F-1的抑制而未能表达G1晚期基因，导致E2F活性缺乏。此外，尽管E2F-1在某些细胞中刺激DNA合成，但其他细胞，包括正常人成纤维细胞，还需要其他因子。
