Cisplatin sensitivity is related to late DNA damage processing and checkpoint control rather than to the early DNA damage response.

The present study aimed at elucidating mechanisms dictating cell death triggered by cisplatin-induced DNA damage. We show that CL-V5B hamster mutant cells, a derivative of V79B, are hypersensitive to cisplatin-induced apoptotic death. CL-V5B cells are characterized by attenuated cisplatin-induced early (2-6 h) stress response, such as phosphorylation of stress-activated protein kinases (SAPK/JNK), ATM and Rad3-related (ATR) protein kinase, histone H2AX and checkpoint kinase-1 (Chk-1). Human FANCC cells also showed a reduced phosphorylation of H2AX and SAPK/JNK at early time point after cisplatin treatment. This was not the case for BRCA2-defective VC-8 hamster cells, indicating that the FA core complex, rather than its downstream elements, is involved in early damage response. The alleviated early response of CL-V5B cells is not due to a general dysfunction in ATM/ATR-regulated signaling. It is rather due to a reduced formation of primary cisplatin-DNA adducts in the hypersensitive mutant as shown by analysis of DNA platination, DNA intra- and interstrand crosslink formation and DNA replication blockage. Despite of lower initial DNA damage and attenuated early DNA damage response (DDR), CL-V5B cells are characterized by an excessive G2/M arrest as well as an elevated frequency of DNA double-strand breaks (DSB) and chromosomal aberrations (CA) at late times (16-24h) after cisplatin exposure. This indicates that error-prone processing of cisplatin-induced lesions, notably interstrand crosslinks (ICL), and the formation of secondary DNA lesions (i.e. DSB), results in a powerful delayed DNA damage response and massive pro-apoptotic signaling in CL-V5B cells. The data provide an example that the initial level of cisplatin-DNA adducts and the corresponding early DNA damage response do not necessarily predict the outcome of cisplatin treatment. Rather, the accuracy of DNA damage processing and late checkpoint control mechanisms determine the extent of cell death triggered by cisplatin-induced DNA lesions.