Anticancer nucleoside analogs (e.g., ara-C, gemcitabine, fludarabine) induce apoptosis by incorporation into DNA. Removal of incorporated analogs from DNA by 3'-5' exonucleases is presumably a mechanism of drug resistance. Based on our previous observation that the 3'-5' exonuclease activity of wild-type (wt) p53 protein is able to preferentially remove mismatched nucleotides from DNA, in the present study we further investigated the ability of p53 to recognize and remove incorporated therapeutic analogs from DNA and its role in analog-induced apoptosis. We demonstrated that although the 3'-5' exonuclease of wt p53 protein was able to bind and excise the nucleoside analog residues from DNA in vitro, removal of the drug molecules from cellular DNA was slow in whole cells with wt p53 cells, and not detectable in mutant p53 cells. Furthermore, the wt p53 were more sensitive to the cytotoxic effect of the drugs compared to the p53-null or mutant cells. Incubation of ML-1 cells (wt p53) with gemcitabine caused an accumulation of p53 protein in their nuclei and preferentially induced apoptosis in the p53-positive cells, whereas the p53-negative cells remained intact. Transfection of p53-null cells with wt p53 expression vector enhanced the sensitivity of the cells to gemcitabine. Gel mobility shift assay using synthetic DNA containing gemcitabine as the probe suggests that p53 protein is likely to participate in the binding of the analog-containing DNA. Our study suggests that recognition of the incorporated nucleoside analogs in DNA by wt p53 did not confer resistance to the drugs, but it facilitated the apoptotic cell death process.

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