Different S/M checkpoint responses of tumor and non-tumor cell lines to DNA replication inhibition

Cell cycle checkpoint abrogation, especially the inhibition of Chk1 in combination with DNA-damaging treatments, has been proposed as a promising way of sensitizing cancer cells. However, less is known about the possibility to selectively affect tumor cells when they are treated with agents that block DNA synthesis in combination with replication checkpoint inhibitors. Here, we present clear insights in the different responses of tumor and non-transformed cells to the inhibition of DNA replication with hydroxyurea in combination with checkpoint abrogation via inhibition of Ataxia telangiectasia-mutated-(ATM) and Rad3-related/ATM (ATR/ATM) and Chk1 kinases. Interestingly, we find that non-transformed cell lines activate ATR/ATM- and Chk1-independent pathways in response to replication inhibition to prevent mitotic entry with unreplicated DNA. In contrast, tumor cell lines such as HCT116 and HeLa cells rely entirely on Chk1 activity for a proper response to replication inhibitors. Our results show that p38 is activated in response to hydroxyurea treatment and collaborates with Chk1 to prevent mitotic entry in non-transformed cell lines by maintaining cyclin B1/Cdk1 complexes inactive. Furthermore, DNA replication arrest down-regulates cyclin BI promoter activity in non-transformed cells, but not in tumor cells in a Chk1- and p38-independent way. Thus, our data show that non-transformed cells present a more robust DNA replication checkpoint response compared with tumor cells that involves activation of the p38 pathway. We show that some of these responses to replication block can be lost in tumor cells, causing a defective checkpoint and providing a rationale for tumor-selective effects of combined therapies.