Loss of anchorage in checkpoint-deficient cells increases genomic instability and promotes oncogenic transformation

Mammalian cells generally require both mitogens and anchorage signals in order to proliferate. An important characteristic of many tumour cells is that they have lost this anchorage-dependent cell-cycle checkpoint, allowing them to proliferate without signals provided by their normal microenvironment. In the absence of anchorage signals from the extracellular matrix, many cell types arrest cell-cycle progression in G1 phase as a result of Rb-dependent checkpoints. However, despite inactivation of p53 and Rb proteins, SV40LT-expressing cells retain anchorage dependency, suggesting the presence of an uncharacterised cell-cycle checkpoint, which can be overridden by coexpression of oncogenic Ras. We report here that, although cyclin-CDK complexes persisted in suspension, proliferation was inhibited in LT-expressing cells by the CDK inhibitor p27(Kip1) (p27). Interestingly, this did not induce a stable arrest, but aberrant cell-cycle progression associated with stalled DNA replication, rereplication and chromosomal instability, which was sufficient to increase the frequency of oncogenic transformation. These results firstly indicate loss of anchorage in Rb- and p53-deficient cells as a novel mechanism for promotion of genomic instability; secondly suggest that anchorage checkpoints that protect normal cells from inappropriate proliferation act deleteriously in Rb- and p53-deficient cells to promote tumourigenesis; and thirdly indicate caution in the use of CDK inhibitors for cancer treatment.