Phosphorothioate oligonucleotides, suramin and heparin inhibit DNA-dependent protein kinase activity

Phosphorothioate oligonucleotides and suramin bind to heparin binding proteins including DNA polymerases, and inhibit their functions. In the present study, we report inhibition of DNA-dependent protein kinase activity by phosphorothioate oligonucleotides, suramin and heparin. Inhibitory effect of phosphorothioate oligonucleotides on DNA-dependent protein kinase activity was increased with length and reached a plateau at 36-mer. The base composition of phosphorothioate oligonucleotides did not affect the inhibitory effect. The inhibitory effect by phosphorothioate oligodeoxycytidine 36-mer can be about 200-fold greater than that by the phosphodiester oligodeoxycytidine 36-mer. The inhibitory effect was also observed with purified DNA-dependent protein kinase, which suggests direct interaction between DNA-dependent protein kinase and phosphorothioate oligonucleotides. DNA-dependent protein kinase will have different binding positions for double-stranded DNA and phosphorothioate oligodeoxycytidine 36-mer because they were not competitive in DNA-dependent protein kinase activation. Suramin and heparin inhibited DNA-dependent protein kinase activity with IC₅₀ of 1.7 μM and 0.27 μg ml^-1 respectively. DNA-dependent protein kinase activities and DNA double-stranded breaks repair in cultured cells were significantly suppressed by the treatment with suramin in vivo. Our present observations suggest that suramin may possibly result in sensitisation of cells to ionising radiation by inactivation of DNA-dependent protein kinase and the impairment of double-stranded breaks repair.