Sáenz-Robles M Teresa, Markovics Jennifer A, Chong Jean-Leon, Opavsky Rene, Whitehead Robert H, Leone Gustavo, Pipas James M
Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260, USA.
J Virol. 2007 Dec;81(23):13191-9. doi: 10.1128/JVI.01658-07. Epub 2007 Sep 12.
The simian virus 40 large T antigen contributes to neoplastic transformation, in part, by targeting the Rb family of tumor suppressors. There are three known Rb proteins, pRb, p130, and p107, all of which block the cell cycle by preventing the transcription of genes regulated by the E2F family of transcription factors. T antigen interacts directly with Rb proteins and disrupts Rb-E2F complexes both in vitro and in cultured cells. Consequently, T antigen is thought to inhibit transcriptional repression by the Rb family proteins by disrupting their interaction with E2F proteins, thus allowing E2F-dependent transcription and the expression of cellular genes needed for entry into S phase. This model predicts that active E2F-dependent transcription is required for T-antigen-induced transformation. To test this hypothesis, we have examined the status of Rb-E2F complexes in murine enterocytes. Previous studies have shown that T antigen drives enterocytes into S phase, resulting in intestinal hyperplasia, and that the induction of enterocyte proliferation requires T-antigen binding to Rb proteins. In this paper, we show that normal growth-arrested enterocytes contain p130-E2F4 complexes and that T-antigen expression destroys these complexes, most likely by stimulating p130 degradation. Furthermore, unlike their normal counterparts, enterocytes expressing T antigen contain abundant levels of E2F2 and E2F3a. Concomitantly, T-antigen-induced intestinal proliferation is reduced in mice lacking either E2F2 alone or both E2F2 and E2F3a, but not in mice lacking E2F1. These studies support a model in which T antigen eliminates Rb-E2F repressive complexes so that specific activator E2Fs can drive S-phase entry.
猿猴病毒40大T抗原部分通过靶向肿瘤抑制因子Rb家族来促成肿瘤转化。已知有三种Rb蛋白，即pRb、p130和p107，它们均通过阻止由转录因子E2F家族调控的基因转录来阻断细胞周期。T抗原在体外和培养细胞中均直接与Rb蛋白相互作用，并破坏Rb-E2F复合物。因此，T抗原被认为是通过破坏Rb家族蛋白与E2F蛋白的相互作用来抑制其转录抑制作用，从而使E2F依赖的转录以及进入S期所需的细胞基因得以表达。该模型预测，T抗原诱导的转化需要活跃的E2F依赖的转录。为了验证这一假设，我们研究了小鼠肠上皮细胞中Rb-E2F复合物的状态。先前的研究表明，T抗原可驱使肠上皮细胞进入S期，导致肠道增生，且肠上皮细胞增殖的诱导需要T抗原与Rb蛋白结合。在本文中，我们表明正常生长停滞的肠上皮细胞含有p130-E2F4复合物，而T抗原的表达会破坏这些复合物，很可能是通过刺激p130降解来实现的。此外，与正常的肠上皮细胞不同，表达T抗原的肠上皮细胞含有大量的E2F2和E2F3a。同时，在仅缺乏E2F2或同时缺乏E2F2和E2F3a的小鼠中，T抗原诱导的肠道增殖减少，但在缺乏E2F1的小鼠中并未减少。这些研究支持了一个模型，即T抗原消除Rb-E2F抑制复合物，以便特定的激活因子E2F能够驱动进入S期。
