Hypoxia-induced alterations of G2 checkpoint regulators

Hypoxia promotes an aggressive tumor phenotype with increased genomic instability, partially due to downregulation of DNA repair pathways. However, in addition to DNA repair, genome stability is also controlled by cell cycle checkpoints. An important issue is therefore whether hypoxia also can alter the DNA damage cell cycle checkpoints. Here, we show that hypoxia (24h 0.2% O2) alters the expression of several G2 checkpoint regulators, as examined by microarray gene expression analysis and immunoblotting of U2OS cells. While some of the changes reflected hypoxia-induced inhibition of cell cycle progression, flow cytometric bar-coding analysis of individual cells showed that the levels of several G2 checkpoint regulators were reduced in G2 phase cells after hypoxic exposure, in particular cyclin B1. These effects were accompanied by decreased Cyclin dependent kinase (CDK) activity in G2 phase cells after hypoxia. Furthermore, cells pre-exposed to hypoxia showed a longer G2 checkpoint arrest upon treatment with ionizing radiation. Similar results were found following other hypoxic conditions (~0.03 % O2 20h and 0.2% O2 72h). These results demonstrate that the DNA damage G2 checkpoint can be altered as a consequence of hypoxia, and we propose that such alterations may influence the genome stability of hypoxic tumors. We measured gene expression changes in U2OS cells after treatment with 0.2% hypoxia for 24 hours. The data were used in the exploration of hyopxia induced alterations in the G2 checkpoint.