Chk1 inhibition induces a DNA damage bystander effect in cocultured tumour cells.

Inhibitors of Chk1 kinase, a key effector of the DNA damage response pathway, are currently undergoing Phase 1 and 2 clinical trials as single agents and in combination with cytotoxic chemotherapy. Understanding the biological effects of Chk1 inhibitors on cancer cells is critical for their continued clinical development. Treatment of adherent HT29 or HCC1937 cancer cells or suspension Jurkat or THP1 cells with a Chk1 inhibitor increased γH2AX in these cells. Chk1i pre-treated HCC1937 or HT29 cells resulted in γH2AX induction in cocultured Jurkat or THP1 cells despite these cells never being treated with a Chk1i. Pre-treatment of HT29 cells with camptothecin or gemcitabine followed by a Chk1i increased the DNA damage bystander effect in naïve cocultured THP1 cells compared to camptothecin or gemcitabine alone. This bystander effect appeared to occur through soluble factors via ATR, ATM, and DNA-PKcs activation in the bystander cells. Chk1 silencing by siRNA in HCC1937 or HT29 cells induced a DNA damage bystander effect in cocultured THP1 cells. However, this bystander effect induced by siRNA appeared mechanistically different to that induced by the Chk1 inhibitor. This work suggests that a Chk1 inhibitor-induced bystander effect may increase the clinical effectiveness of Chk1 inhibitors by inducing additional DNA damage or replication stress in cancer cells not directly exposed to the inhibitor. Conversely, it may also contribute to Chk1 inhibitor toxicity by increasing DNA damage in non-tumour cells.