The Role of p27 in Controlling the Oxygen-dependent Checkpoint of Mammalian Cells in Late G₁

Abstract

We have studied hypoxia-induced cell cycle arrest in human cells where the retinoblastoma tumour suppressor protein (pRb) is either functional (T-47D cells) or abrogated by expression of the HPV18 E7 oncoprotein (NHIK 3025 cells). Cells of both types are arrested in a restriction point in late G₁, here denoted as the oxygen-dependent restriction point in late G₁. This arrest seems to occur under extreme hypoxia in all types of mammalian cells so far tested. During an 18-h exposure to extreme hypoxia, the p27 protein level increased in G₁-phase in both cells lines investigated and was followed by a binding between p27 and CDK2. This was observed both in the pRb-positive T-47D cells and in the pRb-negative NHIK 3025 cells. We, therefore, believe that p27 and not pRb is the mediator of this oxygen-dependent checkpoint in late G₁. Our results also suggest that p27 regulates the restart of cell cycle progression of these arrested cells after reoxygenation.