Differential response of normal and malignant urothelial cells to CHK1 and ATM inhibitors

This study focuses on the apparently distinct roles of various checkpoint associated kinases including ATM, ATR, and CHK1 in bladder cancer compared with normal human urothelial (NHU) cells. The primary signalling cascade that responds to replication stress in bladder cancer cells (ATR-CHK1) is deficient in normal urothelial cells after treatment with a replication inhibitor and these cells do not depend upon CHK1 for protection from apoptosis during replication stress. This is very different from the situation in bladder cancer cells, where ATR-CHK1 plays a prominent role. Instead ATM signalling is more rapidly activated under these conditions. Intriguingly, an ATM inhibitor suppressed S-phase checkpoint activation after exposure to replication inhibitors and stopped entry of hTERT-NHU cells into S-phase with G1 accumulation of cells suggesting activation of a G1 checkpoint. Consistent with this, cells treated with the ATM inhibitor and thymidine showed increased levels of cyclin-dependent kinase inhibitor p19INK4D, reduced levels of cyclin D1 and CDK4, and reduced phosphorylation of the retinoblastoma (RB) protein. In contrast, a bladder cancer cell line treated with the ATM inhibitor progressed more slowly through S-phase and showed a dramatic increase in apoptosis following co-treatment with ATM and replication inhibitors. Taken together,
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these findings suggest that ATM and CHK1 signalling cascades play different roles in tumour and normal epithelial cells, confirming these as promising therapeutic targets.