Overexpression of the human HAP1 protein sensitizes cells to the lethal effect of bioreductive drugs

Abasic sites (AP sites) are generated in DNA either directly by DNA-damagin g agents or by DNA glycosylases acting during base excision repair. These s ites are repaired in human cells by the HAP1 protein, which, besides its en donuclease activity, also possesses a redox function. To investigate the ab ility of HAP1 protein to modulate cell resistance to DNA-damaging agents, C HO cells were transfected with HAP1 cDNA, resulting in stable expression of the protein in the cell nuclei. The sensitivity of the transfected cells t o the toxic effect of various agents, e.g. methylmethane sulfonate, bleomyc in and H2O2, was not modified. However, the transfected cells became more s ensitive to killing by mitomycin C, porfiromycin, daunorubicin and aziridin yl benzoquinone, drugs that are activated by reduction. To test whether the redox function of HAP1 protein was involved in this increased cytotoxicity , we have constructed a mutated HAP1 protein endowed with normal AP-endonuc lease activity but deleted for redox function. When this mutated protein wa s expressed in the cells, elevated AP-endonuclease activity was measured, b ut sensitization to the lethal effects of compounds requiring bioreduction was no longer observed. These results suggest that HAP1 protein, besides it s involvement in DNA repair, is able to activate bioreduction of alkylating drugs used in cancer chemotherapy.