Acquired resistance to adriamycin (ADR) in an HL60 cell line is shown to be accompanied by an increase in DNA-dependent protein kinase catalytic subunit (DNA-PKcs) at both the protein and mRNA levels (15-20-fold) and an overall 3-fold increase in DNA-PK enzyme activity. The other components of the DNA-PK Ku autoantigen complex, Ku70 and Ku80, were 3-fold increased and unchanged, respectively. Time dependent repair of ADR-induced DNA damage was measured by the neutral comet assay and found to be more efficient in the drug resistant cell line (HL60/ADR). Antisense RNA transfection reduced the protein expression of DNA-PKcs to 50% in HL60/ADR and partially reversed drug resistance. A fibroblast cell line from a severe combined immunodeficient (SCID) mouse was deficient in functional DNA-PKcs and showed increased sensitivity to ADR and other DNA damaging agents compared to wild type. These studies demonstrate that alteration in DNA-PK can contribute to chronic stress response leading to acquired drug resistance. The overexpression of DNA-PK is thus shown to be a novel cellular adaptation mechanistically contributing to the resistance of cancer cells to the anthracycline drug adriamycin, and as such, may have implications for its therapeutic use.
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