Primary keratinocytes have an adhesion dependent S phase checkpoint that is absent in immortalized cell lines

In order to understand the mechanism through which loss of anchorage inhibits growth, we have investigated the events that occur in murine keratinocytes upon substratum detachment utilizing both primary cells and established immortalized cell lines. Our data has revealed that while both primary and immortalized cells undergo growth arrest in suspension, the nature of this arrest is markedly different. Primary cells exhibit a growth arrest that is characterized by rapid cessation of DNA synthesis resulting in a static S phase population. In contrast, an immortalized non-tumorgenic cell line, Balb MK, exhibits growth arrest as measured by thymidine incorporation, but does not prevent cells that have entered S phase from continuing into G2/M, and accumulating as a 4N population. In contrast to both primary and MK cells, the tumorigenic SLC-1 cell line did not accumulate in a specific cell cycle interval and were able to undergo continuous growth in suspension. Examination of cyclin A protein and its associated activity revealed that cyclin A protein levels decreased in primary but not MK cells; suggesting the continued presence of cyclin A may allow continued DNA synthesis observed in MK cells. Furthermore, we demonstrate the accumulation of suspension cultured MK cells as a 4N population correlated with the loss of cyclin A/cdk2 kinase activity, which in turn occurred through the accumulation of p27kip1, whereas neither p27kip1 accumulation nor loss of cyclin A activity was observed in SLC-1 cells. Our results clearly reveal that the process of growth inhibition in suspension cultured cells may occur in several forms with distinct characteristics that are dependent on the status of cyclin/cdk complexes and CKI proteins. Tumor derived cells in suspension did not lose cyclin A dependent kinase activity and thus continued to grow and divide.