Anchorage independent cell cycle start

成纤维细胞需要锚定到细胞外基质才能从G1期过渡到S期，但在致癌转化后不再需要。我们报告了激活复制起点所必需的CDC6蛋白需要锚定或致癌刺激才能执行。在锚定丧失后，尽管E2F依赖的启动子被强制激活，但在大鼠成纤维细胞中，CDC6的转录和转录后表达都是关闭的。然而，用致癌生长因子刺激该细胞可以抑制这种关闭，并同时激活CDK2和CDK6/4，从而覆盖了G1-S转变的锚定要求，从而使细胞能够进行锚定非依赖性的S时相进入。CDC6的增强表达分析表明，G1期细胞周期蛋白依赖的蛋白激酶和CDC6是通过锚定丧失限制G1-S转变的主要细胞周期靶点。

Fibroblasts need anchorage to extracellular matrix to transit from G1 to S phase, but no longer after oncogenic transformation. We report that Cdc6 protein essential for the activation of replication origins requires anchorage or oncogenic stimulation for its execution. Upon anchorage loss, Cdc6 expression is shut of both transcriptionally and post-transcriptionally in a rat fibroblast despite enforced activation of E2F-dependent promoters. However, stimulation of this cell with oncogenic growth factors suppresses this shutoff and concurrently activates Cdk2 and Cdk6/4, thereby overriding the anchorage requirement for the G1-S transition and consequently enabling cells to perform anchorage-independent S phase entry. Analysis with enforced expression of Cdc6 indicates that the G1 cyclin-dependent kinases and Cdc6 constitute major cell cycle targets for the restriction of the G1-S transition by anchorage loss.