The Bowman-Birk protease inhibitor enhances clonogenic cell survival of ionizing radiation-treated nucleotide excision repair-competent cells but notof xeroderma pigmentosum cells

Purpose: The radioprotective effect of the Bowman-Birk protease inhibitor ( BBI) was previously shown to result from a TP53 dependent mechanism. Whethe r this effect involves specific DNA repair mechansims is now tested. Material and methods: Normal human fibroblasts were pre-treated with BBI be fore exposure to X-rays, UVB or to chemical agents (bleomycin, N-methyl=N'- nitro-N-nitrosoguanidine (MNNG), cisplatin). These agents were chosen becau se of their ability to induce different spectra of DNA damage. The radiomet ric agent bleomycin primarily induces double-strand breaks (dsb), which are repaired by recombination; MNNG results in alkylated bases which are repai red by base excision repair (BER); cisplatin results in DNA-crosslinks whic h are repaired mainly by nucleotide excision repair (NER); and finally UVB generates thymine dimers and thymine-cytosine-6-4 products which are also r epaired by NER. Cell survival was analysed by colony formation assay and DN A dsb by constant field gel electrophoresis. The combined effect of BBI and X-rays was also tested for XP-fibroblasts, which are defective in NER. Results: For normal human fibroblasts the radioprotective effect of BBI was clearly found by using a delayed plating procedure. The radioprotective ef fect was found to be unrelated to an altered induction or repair of radiati on-induced DNA dsb. Pretreatment with BBI did not affect cell killing after exposure to bleomycin or MNNG, but resulted in a significant protection of cells exposed to cisplatin or UVB. These results indicate that pre-treatme nt with BBI did not alter recombination repair or BER, but was able to modi fy NER. The latter finding was supported by the observation made for XP-cel ls, where pretreatment with BBI failed to result in radioprotection after e xposure to ionizing radiation. Conclusions: On the basis of these data it is proposed that the radioprotec tive effect of BBI is the result of an improved nucleotide excision repair mechanism.