DNA-dependent protein kinase (DNA-PK) is involved in non-homologous end joining which repairs DNA double-strand breaks introduced by irradiation and radiomimetic agents. DNA-PK interacts with p53 but may also have p53-independent functions. The present study investigated whether disruption of the gene for the catalytic subunit DNA-PKcs affects chemosensitivity in p53-deficient cells. Drug sensitivity of DNA-PKcs+/+/p53+/+, DNA-PKcs+/+/p53−/−, DNA-PKcs−/−/p53+/+, and DNA-PKcs−/−/p53−− mouse lung-fibroblasts was determined by the MTT assay, the clonogenic assay, and trypan blue exclusion. Susceptibility to apoptosis was determined by DNA fragmentation (TUNEL) and by caspase-3 cleavage. We show that p53-deficient cells were 2 to 3-fold resistant to treatment with doxorubicin, epirubicin, cisplatin, and docetaxel as compared to wild-type cells. We further demonstrate that the additional loss of DNA-PKcs function in p53-deficient cells resulted in a 2-fold increase in sensitivity to doxorubicin and epirubicin as documented by the MTT assay, clonogenic assay, and trypan blue exclusion. Doxorubicin-induced hypersensitivity in these cells correlated with a transient G2/M checkpoint activation but did not seem to correlate with apoptosis. The data indicate that additional loss of DNA-PKcs in p53-deficient cells reverses anthracycline-resistance imposed by p53-deficiency, and that DNA-PKcs modulates p53-independent pathways responding to DNA damage induced by anthracyclines. They also indicate that processes other than apoptosis may contribute to the increased cytotoxicity to anthracyclines. DNA-PKcs may thus be a potential target for functional inhibition, which might increase the efficacy of some anti-tumour agents in the treatment of cancers mutated in the p53 gene.

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