E2F1 and E2F3 activate ATM through distinct mechanisms to promote E1A-induced apoptosis

Deregulation of the Rb-E2F pathway occurs in many cancers and results in aberrant cell proliferation as well as an increased propensity to undergo apoptosis. In most cases, apoptosis in response to Rb inactivation involves the activation of p53 but the molecular details of the signaling pathway connecting Rb loss to p53 are poorly understood. Here we demonstrate that the E1A oncoprotein, which binds and inhibits Rb family members, induces the accumulation and phosphorylation of p53 through the DNA damage-responsive ATM kinase. As a result, E1A-induced apoptosis is significantly impaired in cells lacking ATM. In contrast, inactivation of ARF, which is widely believed to activate p53 in response to oncogenic stress, has no effect on p53 induction and only a modest effect on apoptosis in response to E1A. Both E2F1 and E2F3 contribute to ATM-dependent phosphorylation of p53 and apoptosis in cells expressing E1A. However, deregulated E2F3 activity is implicated in the DNA damage caused by E1A while E2F1 stimulates ATM- and NBS1-dependent p53 phosphorylation and apoptosis through a mechanism that does not involve DNA damage.     

E2F1 and E2F2 are differentially required for homeostasis-driven and antigen-induced T cell proliferation in vivo

Homeostasis-driven T cell proliferation occurs in response to a lymphopenic environment and is mediated by TCR and IL-7 signaling. In this report, we demonstrate a defect in the proliferation of murine naive and memory T cells lacking both E2F1 and E2F2 in response to lymphopenic conditions, suggesting that E2F1 and E2F2 function redundantly downstream of TCR and/or IL-7 signaling during homeostasis-driven proliferation. In contrast, T cell proliferation in response to antigenic stimulation is either unaffected (in vivo) or potentiated (ex vivo) by loss of E2F1 and E2F2, indicating divergent requirements for these E2F factors in T cell proliferation mediated by distinct stimuli. E2F1/E2F2 double knockout (DKO) T cells enter S phase in response to homeostatic signaling, but fail to divide, suggesting that S phase progression is either incomplete or defective. In addition, E2F1/E2F2 DKO mice do not recover normal T cell numbers following exposure to a sublethal dose of radiation, indicating that this defect in homeostasis-driven proliferation is physiologically relevant. Consistent with their failure in cell cycle progression, the differentiation of DKO T cells into memory T cells in response to homeostatic signals is significantly reduced. These observations support the idea that proliferation is required for memory T cell formation and also have implications for the development of clinical strategies to minimize the occurrence of lymphopenia-induced autoimmunity.     

FATE/BJ-HCC-2基因表达促进细胞增殖及肿瘤形成

FATE/BJ-HCC-2是本实验室鉴定的一个新的肿瘤-睾丸抗原基因,定位于Xq28染色体.为探讨FATE/BJ-HCC-2 对细胞生物学行为的影响及裸鼠体内肿瘤形成能力的改变,分别采用脂质体转染真核表达载体及逆转录病毒感染两种方式,获得了FATE/BJ-HCC-2 表达的Bel-7402单克隆细胞株及NIH-3T3细胞克隆,并通过RT-PCR和Western 印迹在基因和蛋白水平鉴定了FATE/BJ-HCC-2 表达情况.通过3H-TdR参入法,检测细胞的增殖能力;通过稀释铺板克隆形成率测定和soft agar克隆形成实验,检测细胞的克隆形成能力;通过裸鼠体内细胞注射成瘤,检测细胞成瘤性和肿瘤生长速度.结果表明,FATE/BJ-HCC-2基因表达能明显提高细胞体外增殖、克隆形成能力和成瘤性.提示其对肿瘤的发生发展起到促进作用.     

Specificity of E2F1, E2F2, and E2F3 in mediating phenotypes induced by loss of Rb.

The Rb/E2F pathway plays a critical role in the control ofcellular proliferation. Here, we report that E2F1, E2F2, and E2F3 make major individual contributions toward the in vivo phenotypic consequences of Rb deficiency. In the developing lens of Rb(-/-) embryos, loss of E2F1, E2F2, or E2F3 reduces the unscheduled proliferation of fiber cells, with the loss of E2F3 having the most pronounced effect. In Rb-deficient retinas, all three E2Fs contribute equally to the ectopic proliferation of postmitotic neuronal cells. In contrast, E2F1 is unique in mediating apoptosis in both Rb(-/-) lenses and retinas. In the central nervous system, loss of E2F1 or E2F3 can almost completely eliminate the ectopic DNA replication and apoptosis observed in Rb(-/-) embryos, and loss of E2F2 partially reduces the unscheduled DNA replication and has no effect on apoptosis. These results provide clear evidence for functional specificity among E2Fs in the control of Rb-dependent proliferation and apoptosis in a tissue-specific manner.     

E2F7 and E2F8 keep the E2F family in balance

An article by Li and colleagues (in this issue of Developmental Cell) shows that the atypical E2Fs, E2F7 and E2F8, are critical for mouse development. One of the important functions of these family members stems from a negative feedback loop in which E2F7 and E2F8 limit the expression of E2F1 and prevent E2F1-dependent apoptosis.     

靶向E2F1的shRNA抑制眼内视网膜母细胞瘤生长

视网膜母细胞瘤(retinoblastoma,RB)中由于Rb基因缺失或失活,转录调控因子E2F1始终处于活化状态,细胞不断增殖,E2F1活性增加是恶性肿瘤的普遍现象．根据人E2F1基因设计了3段shRNA,构建RNA干扰表达载体pSilencer-shE2F1,瞬时转染人视网膜母细胞瘤细胞株HXO-Rb-44和人肝癌细胞株SMMC-7721,通过荧光定量PCR检测E2F1表达水平的变化,PI (碘化丙锭)染色检测细胞周期各时相细胞数量以检测其对细胞周期的影响．根据筛选出的有效shRNA序列构建腺病毒载体并包装出腺病毒Ad-shE2F1,在裸鼠前房模型中,体外按50 moi的感染系数感染稳定表达绿色荧光蛋白(GFP)的人视网膜母细胞瘤细胞株48 h后,收集细胞,将2×10⁵细胞注射到裸鼠眼前房内,连续观察眼内肿瘤细胞生长情况并拍照记录．通过转染肿瘤细胞株检测E2F1转录水平,筛选到1个能明显抑制肿瘤细胞内E2F1表达的shRNA序列,并使细胞周期中S期细胞数减少．经Ad-shE2F1感染的RB肿瘤细胞与对照相比较,其在裸鼠眼前房内肿瘤生长减缓．结果说明,有效抑制hE2F1表达的shRNA具有抑制体内外人视网膜母细胞瘤生长的作用．