Cdc6 requires anchorage for its expression

Fibroblasts need anchorage to extracellular matrix to transit from G₁ to S phase, but no longer after oncogenic transformation. Here 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 off 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.