RNA polymerase I transcription in confluent cells: Rb downregulates rDNA transcription during confluence-induced cell cycle arrest
RNA polymerase I transcription in confluent cells: Rb downregulates rDNA transcription during confluence-induced cell cycle arrest
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DOI：
10.1038/sj.onc.1203690
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发表时间：
2000-07-20
期刊：
影响因子：
8
通讯作者：
Rothblum, LI
中科院分区：
文献类型：
--
作者：
Hannan, KM;Kennedy, BK;Rothblum, LI
当 3T6 细胞汇合时，它们退出细胞周期。伴随细胞周期停滞，观察到 RNA 聚合酶 I 转录显着减少（100% 汇合时减少 80%）。在本研究中，我们研究了核糖体基因转录与细胞密度诱导的细胞周期停滞耦合的机制。有趣的是，随着细胞密度的增加（从 3% 汇合到 43%），观察到过度磷酸化 Rb 的细胞含量显着积累。随着细胞密度进一步增加，低磷酸化形式的 Rb 变得占主导地位并在核仁中积累，免疫共沉淀实验表明，与 rDNA 转录因子 UBF 相关的低磷酸化 Rb 数量也显着增加，这增加了 Rb 和 UBF 之间的相互作用，与 rDNA 转录速率的降低相关。此外，重组 Rb 的过表达抑制了汇合或指数细胞中共转染 rDNA 报告基因的 UBF 依赖性转录激活。我们检查的 rDNA 转录成分的数量或活性并没有随着细胞周期停滞而显着变化。尽管聚合酶相关因子PAF53的含量略有改变（减少38%），但减少的时间过程和幅度与rDNA转录率的降低无关。所呈现的结果支持这样一种模型，其中UBF与Rb的结合的调节以及PAF53的细胞含量可能是细胞周期和rDNA转录关联机制的组成部分。
When 3T6 cells are confluent, they withdraw from the cell cycle. Concomitant with cell cycle arrest significant reduction in RNA polymerase I transcription (80% decrease at 100% confluence) is observed. In the present study, we examined mechanism(s) through which transcription of the ribosomal genes is coupled to cell cycle arrest induced by cell density. Interestingly with an increase in cell density (from 3-43% confluence), a significant accumulation in the cellular content of hyperphosphorylated Rb was observed. As cell density increased further, the hypophosphorylated form of Rb became predominant and accumulated in the nucleoli, Co-immunoprecipitation experiments demonstrated there was also a significant rise in the amount of hypophosphorylated Rb associated with the rDNA transcription factor UBF, This increased interaction between Rb and UBF correlated with the reduced rate of rDNA transcription. Furthermore, overexpression of recombinant Rb inhibited UBF-dependent activation of transcription from a cotransfected rDNA reporter in either confluent or exponential cells. The amounts or activities of the rDNA transcription components we examined did not significantly change with cell cycle arrest. Although the content of PAF53, a polymerase associated factor was altered marginally (decreased 38%), the time course and magnitude of the decrease did not correlate with the reduced rate of rDNA transcription. The results presented support a model wherein regulation of the binding of UBF to Rb and, perhaps the cellular content of PAF53, are components of the mechanism through which cell cycle and rDNA transcription are linked.